A PROGRESS REPORT ON OPPORTUNITIES FOR …nrri.umn.edu/egg/REPORTS/CAES/CAESMidTermReport.pdf ·...

Natural Resources Research Institute December 2013 University of Minnesota Duluth 5013 Miller Trunk Highway Duluth MN 55811‐1442 1 Natural Resources Research Institute, University of Minnesota Duluth. 2 Humphrey School of Public Affairs, University of Minnesota. 3 St. Anthony Falls Laboratory, University of Minnesota. 4 Department of Civil Engineering, University of Minnesota Duluth.

by

Donald R. Fosnacht1, Principal Investigator Elizabeth J. Wilson2, Economic & Policy Team Leader

Jeffrey D. Marr2, Facilities Team Leader Carlos Carranza‐Torres4, Geotechnical Engineering Team Leader

Steven A. Hauck1, Geology Team Leader Rebecca L. Teasley4, Environmental Team Leader

A PROGRESS REPORT ON OPPORTUNITIES FOR COMPRESSED AIR

ENERGY STORAGE IN MINNESOTA

Cover image: CAES location map. Recommended citation: Fosnacht, D.R., Wilson, E.J., Marr, J.D., Carranza‐Torres, C., Hauck, S.A., and Teasley, R.L., 2013, A Progress Report on Opportunities for Compressed Air Energy Storage in Minnesota: Natural Resources Research Institute, University of Minnesota Duluth, 106 p. Natural Resources Research Institute University of Minnesota, Duluth 5013 Miller Trunk Highway Duluth, MN 55811‐1442 Telephone: 218‐720‐4272 Fax: 218‐720‐4329 e‐mail: [email protected] Web site: http://www.nrri.umn.edu/egg ©2013 by the Regents of the University of Minnesota All rights reserved. The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.

This publication is accessible from the home page of the Economic Geology Group of the Center for Applied Research and Technology Development at the Natural Resources Research Institute, University of Minnesota Duluth (http://www.nrri.umn.edu/egg) as a PDF file readable with Adobe Acrobat 6.0.

Date of release: December 2013

CAES Progress Report – Table of Contents i

TABLE OF CONTENTS

LIST OF TABLES............................................................................................................................................ iiv

LIST OF FIGURES............................................................................................................................................ v

LIST OF APPENDICES .................................................................................................................................... vi

EXECUTIVE SUMMARY .................................................................................................................................. 1

Policy, Economics, and Past Attempts ...................................................................................................... 1

Facilities and Basic Operational Characteristics ....................................................................................... 1

Location Analyses...................................................................................................................................... 2

Air Leakage Control and Structure Integrity ............................................................................................. 3

Environmental and Permitting.................................................................................................................. 4

1. CAES PROGRESS REPORT – ECONOMIC AND POLICY TEAM ..................................................................... 5

1.1. WORK SUMMARY................................................................................................................................... 5

1.2. KEY FINDINGS......................................................................................................................................... 5

1.3. INTERIM REPORT ON ECONOMIC AND POLICY TEAM TASKS ................................................................ 5

1.3.1. Task 1: Policy and Economic Environment for CAES Development................................................ 6

1.3.2. Task 2: Develop Preliminary Regulatory And Socio‐Political Criteria For Assessment Guidelines ................................................................................................................................................. 7

1.3.3. Task 3: Assess Socio‐Political Factors Affecting CAES Implementation ..........................................7

1.3.4. Task 4: Identify Life Cycle Parameters ............................................................................................ 8

1.3.5. Task 5: Analyze Economic Factors Affecting CAES Implementation...............................................8

1.3.5.1. Literature Review on Compressed Air Energy Storage............................................................ 8

1.3.5.2. Key economic issues affecting CAES implementation ............................................................. 9

1.3.6. Task 6: Compare First Order Economic Estimates of a CAES Project with the Previously Developed PHES Analysis. .......................................................................................................................10

1.4. REFERENCES .........................................................................................................................................10

2. CAES PROGRESS REPORT – FACILITIES TEAM .........................................................................................23

2.1. REVIEW OF THE FACILITIES TEAM SCOPE ............................................................................................23

2.2. ANNUAL PROGRESS SUMMARY: FACILITY ASSESSMENT TEAM ..........................................................23

2.2.1. Task 1: Technology assessment and review of published literature and evaluation of existing/planned facilities .......................................................................................................................23

2.2.2. Task 2: Evaluation of conventional and advanced compressed air energy storage technologies............................................................................................................................................23

CAES Progress Report – Table of Contents ii

2.2.3. Task 3: CAES feasibility assessment and preliminary schematic of surface and subsurface facilities configurations ........................................................................................................24

2.2.4. Task 4: Assess plant operational characteristics (daily, annual, design life timescales)...............24

2.2.5. Task 5: Development of Deliverables: “CAES Assessment guidelines,” “CAES Site Screening methodology,” and “Screening Assessment of CAES Potential” ...........................................24

2.3. ADVANCED COMPRESSED AIR STORAGE TECHNOLOGIES...................................................................24

2.4. CALENDAR YEAR 2014, QUARTER 1 ACTIVITIES...................................................................................25

3. CAES PROGRESS REPORT – GEOTECHNICAL ENGINEERING TEAM .........................................................51

3.1. INTRODUCTION....................................................................................................................................51

3.2. LITERATURE REVIEW OF CAES SYSTEMS WITH FOCUS ON GEOTECHNICAL ENGINEERING ASPECTS ......................................................................................................................................................51

3.3. GEOTECHNICAL CONSIDERATIONS FOR THE DESIGN OF UNDERGROUND EXCAVATIONS FOR A CAES PLANT..............................................................................................................................................58

3.4. REFERENCES .........................................................................................................................................64

3.5. POWERPOINT PRESENTATIONS (DRAFT VERSIONS) DEVELOPED FOR DISCUSSION OF GEOTECHNICAL ENGINEERING ASPECTS OF UNDERGROUND EXCAVATIONS FOR CAES PLANTS..............65

4. CAES PROGRESS REPORT – GEOLOGY TEAM ..........................................................................................76

4.1. OVERVIEW............................................................................................................................................76

4.2. INTRODUCTION....................................................................................................................................76

4.3. OBJECTIVE 1 .........................................................................................................................................78

4.3.1. Progress/Findings to Date.............................................................................................................78

4.3.1.1. Mesabi Range.........................................................................................................................78

4.3.1.2. Cuyuna Range/Emily District..................................................................................................79

4.3.1.3. Vermilion Range.....................................................................................................................81

4.4. OBJECTIVE 4 .........................................................................................................................................85

4.4.1. Progress/findings to Date .............................................................................................................85

4.5. ADDITIONAL PROJECT WORK...............................................................................................................85

4.5.1. Presentations (Appendix 4‐A) .......................................................................................................85

4.5.2. Bibliographies................................................................................................................................86

4.6. REFERENCES .........................................................................................................................................86

5. CAES PROGRESS REPORT – GROUNDWATER/ENVIRONMENTAL TEAM.................................................99

5.1. GROUNDWATER MODELING................................................................................................................99

CAES Progress Report – Table of Contents iii

5.1.1. Model Configuration.....................................................................................................................99

5.1.2. Model Parameters ......................................................................................................................100

5.1.3. Importance of Hydraulic Conductivity ........................................................................................101

5.1.4. Boundary Conditions...................................................................................................................101

5.1.5. Remaining Questions ..................................................................................................................103

5.2. ENVIRONMENTAL PERMITTING ISSUES .............................................................................................103

5.2.1. Geology and Soil..........................................................................................................................103

5.2.1.1. Soil Erosion...........................................................................................................................103

5.2.1.2. Seismicity .............................................................................................................................104

5.2.2. Water Quality..............................................................................................................................104

5.2.3. Groundwater...............................................................................................................................104

5.2.4. Surface Water .............................................................................................................................104

5.2.5. Biological Resources....................................................................................................................105

5.2.6. Impacts Common to Large Facility Construction ........................................................................105

5.2.6.1. Agricultural Resources .........................................................................................................105

5.2.6.2. Cultural Resources ...............................................................................................................105

5.2.6.3. Aesthetic Resources.............................................................................................................105

5.2.6.4. Recreational Resources........................................................................................................105

5.2.6.5. Population and Housing.......................................................................................................105

5.2.6.6. Air Quality and Noise ...........................................................................................................105

5.2.6.7. Greenhouse Gas Emissions ..................................................................................................105

5.2.6.8. Hazardous materials ............................................................................................................106

5.2.6.9. Environmental Permitting....................................................................................................106

5.3. TIMELINE ............................................................................................................................................106

5.4. REFERENCES .......................................................................................................................................106

CAES Progress Report – List of Tables iv

LIST OF TABLES

Table 5‐1. MODFLOW Layer Properties. ...................................................................................................100

Table 5‐2. Typical Value for Hydraulic Conductivity used for Modeling...................................................100

Table 5‐3. Recorded historical earthquakes affecting Minnesota (adapted from USGS, 2009)...............104

CAES Progress Report – List of Figures v

LIST OF FIGURES

Figure 3‐1. Constant pressure CAES storage with surface reservoir and compensating water column (after Succar and Williams, 2008). .................................................................................................54

Figure 3‐2. Diagram summarizing the relationship between generated energy, storage volume, upper and lower storage pressure and operation case (Cases 1, 2 or 3) described in the main text. The insert (point P and associated text) represents an example of use of the diagram as described also in the main text. After Succar and Williams (2008). ...........................................................56

Figure 3‐3. Same diagram as in Figure 2, including the position of the two existing CAES plants (Huntorf and McIntosh) and one of the planned plants (Seneca), described in the main text..................57

Figure 3‐4. Cross sectional area of a mining shaft of the abandoned Cuyuna mine in central Minnesota. Drawing provided by J. Oreskovich. ........................................................................................59

Figure 3‐5. Possible shapes of cross sections for CGES (Compressed Gas Energy Storage) openings – adapted from Kovari (1993). ....................................................................................................60

Figure 3‐6. Determination of depth of emplacement of a shallow lined cavern for gas/air storage. .......................................................................................................................................................62

Figure 3‐7. Determination of depth of emplacement of an unlined cavern for gas/air storage................63

Figure 3‐8. a) Cross sections of unlined caverns with two water curtains configurations: umbrella (left) and circumventing (right) configurations – from Kovari (1993); and b) plan view of air cushion surge unlined chamber with water infiltration – from Blindheim et al. (2004). ...........................64

Figure 4‐1. CAES location map. ...................................................................................................................77

Figure 4‐2. Cuyuna Iron Range....................................................................................................................78

Figure 4‐3. Utica Extension Mine................................................................................................................80

Figure 4‐4. Armour No. 1, Armour No. 2, Bonnie Bell, and Ironton Mines ................................................82

Figure 4‐5. Partial rendering of the Armour No. 1 Mine ............................................................................83

Figure 4‐6. Map of Cuyuna cities and mining features ...............................................................................84

Figure 5‐1. Maximum Model Head with Ratio of Hydraulic Conductivity to Depth.................................101

Figure 5‐2. CAES Cross‐Section with Water Curtain (Bauer et al., 2012)..................................................102

Figure 5‐3. Cavern Model with Multiple AE Wells for a Water Curtain....................................................103

CAES Progress Report – List of Appendices vi

LIST OF APPENDICES

APPENDIX 1‐A – ENVIRONMENTAL PERMITS AND APPROVALS MATRIX ...................................................12

APPENDIX 1‐B – ECONOMIC MODELING BASED ON REVENUE FROM ARBITRAGE....................................21

APPENDIX 2‐A – DRAFT SUMMARY OF THE PHYSICS OF CAES ...................................................................26

APPENDIX 2‐B – DRAFT ANNOTATED BIBLIOGRAPHY.................................................................................35

APPENDIX 2‐C – DRAFT FACILITY DESIGN SUMMARY.................................................................................47

APPENDIX 3‐A – POWERPOINT PRESENTATIONS .......................................................................................67

APPENDIX 4‐A – GEOLOGY TEAM PRESENTATION......................................................................................87

CAES Progress Report – Executive Summary 1

EXECUTIVE SUMMARY

Various activities undertaken by the team have included preliminary assessments for cost of operation, types of equipment associated with compressed air energy storage technologies (CAES), the current treatment of electricity discharged from a CAES storage facility, and the volume of air required over a 10 hour period for allowing 100 MW of electricity to be produced. Additional work has been undertaken to assess potential underground mine workings that might be used for a future CAES facility. In addition, analysis has been undertaken to determine the potential means of minimizing of air leakage to the rock structures associated with a given facility. The following are preliminary findings from the initial studies. The focus for the next year of study will refine the findings and potentially identify some sites that can be analyzed in the future in a more detailed manner.

Policy, Economics, and Past Attempts

• Economic modeling considering only revenue from arbitrage suggests that a CAES project on a potential site can operate profitably only if variable operating costs of electricity production are $3/MWh.

• Understanding how MISO market rule changes would affect CAES value is more difficult. Most literature suggests that without revenue from participating in the reserve market, and that without these additional funds, CAES remains uneconomic. Stored Energy Resource (SER) rule changes have recently made it possible for CAES to participate in reserve and regulation markets in MISO.

• Conventional CAES economic feasibility is sensitive to high investment costs and increasing natural gas prices. Both lead to high operation costs, which decrease opportunities for arbitrage and threaten the feasibility of CAES plants.

• To benefit from the incentives related with renewable energy and complement the intermittent renewable energy, coupling CAES plants with a wind farm should be considered. However, current sites examined by the facility team do not support the co‐location of wind and CAES plant.

• The geological characteristics of the CAES site will decide the feasibility, operational specifics, and financial risk. For example, the Iowa CAES plant was abandoned after geological surveys of the selected sandstone structure proved permeable, and the Colorado site was also abandoned. Site selection of the CAES reservoir adds significant financial risk due to possibilities of leakage.

Facilities and Basic Operational Characteristics

• Research was conducted on identifying prototyped and/or constructed CAES facilities by focusing on their location, size, components, layout, and other elements that provide physical characterization of the facility. Once identified, a study of each site was conducted using any available published information and by conducting interviews with equipment manufacturers, vendors and facility operators to gain deeper understanding for facility design, operation, and costs.

• Annotated bibliography of relevant papers on conventional and advanced CAES was compiled. Advanced CAES include isothermal and adiabatic approaches.


• During the initial stages of the project, it became clear that the emergence of advanced CAES technologies (isothermal and adiabatic CAES) should be considered in this project. The facilities team expanded the scope of the literature and technology review to include this technology in our feasibility analysis. In general, isothermal and adiabatic compressed air storage offers greater round trip efficiencies to energy storage and also may reduce the capital equipment costs and allow a decrease in the size of an underground cavern through use of higher pressure storage. Isothermal CAES is also directly integrated into the study through consideration of commercial technologies, including General Compression, Lightsail Energy, SustainX, and the NSF‐supported CAES for wind turbines project at the University of Minnesota.

• There are two types of CAES possibilities, namely: 1) surface storage; and 2) underground storage, i.e., use of existing underground cavities, e.g., mining rooms, drifts, and shafts, or cavities created with this specific purpose, e.g., in salt formations, or new cavities that can be easily created by dissolution. There are no salt formations in Minnesota.

• The target power for a Minnesota CAES plant is 100 MW that would need to be sustained for at least 10 hours and would suggest the need of approximately 300,000 m3 of air storage capacity. Therefore, for 300,000 m3 of air storage using Minnesota shafts or tunnels, a total length of shaft (or tunnel) of 22,160 meters (or 72,703 feet) would be required, assuming a shaft cross‐sectional area of 13.74 m2. Given the amount of volume involved, it seems worth exploring the possibility of using not only existing mining excavations, but new openings that could be created with the objective of air storage.

• Another fundamental aspect to consider in the conception of a CAES system is whether the caverns will be lined or unlined. With the understanding that lining refers to that layer of material, e.g., steel, concrete or plastic, installed on the periphery of the cavern to avoid leakage of air/gas. The selection of the cavern depth also depends on whether the cavity will be lined or unlined.

• In the case of unlined caverns, leakage can be prevented by choosing a depth such that the hydrostatic pressure of the water surrounding the cavern is equal or greater than the pressure at which the gas is to be stored, i.e., a water curtain.

Location Analyses

• Underground workings on the Mesabi Range have been compiled from previous work done at MNDNR Lands and Minerals in Hibbing, MN. Additional maps for a potential site, the Utica Extension Mine, were obtained from Great Northern Iron Ore Properties in Hibbing.

• The Cuyuna Range and Emily District offer potentially better siting opportunities for CAES in existing underground workings. Folded rocks here, as opposed to relatively flat‐lying strata on the Mesabi Range, led to linear ore bodies of more limited width.

• A series of mine shafts located north of the cities of Ironton and Crosby present potential for CAES. Armour No. 1 mine shaft is the deepest at 243.8 m (800 feet). It is located outside of the iron‐formation in potentially competent rock. Haulage drifts extend over 121.9 m (400 feet) from the shaft before reaching the ore body. The Armour No. 2 shaft is the next deepest at 160 m (525 feet).


• To date, maps have been obtained showing evidence for underground workings at 28 mines in Crow Wing County.

• The Vermilion Range was briefly investigated. The Soudan Mine has competent rock and large rooms that could conceivably host CAES. However, the Soudan Mine is a State Park that provides public tours of the underground mine. It also contains a large underground physics laboratory. These factors likely preclude its use for CAES. At least 20 underground mines were started on the Vermilion Range, 12 of which amounted to little more than a shaft and exploratory drifts and thus do not hold much promise. Based on distance from major electric transmission lines, the mines of the Vermilion District were given a lower priority for study.

• An on‐line search led to discovery of a document detailing the geological examinations, physical tests, and design plans for a proposed underground liquid propane storage cavern in Minnesota (Fenix and Scisson, Inc., 1960). Such a cavern was constructed near Erskine, Pope County, MN. It has been in operation for decades. This document has application for the geotechnical team in terms of evaluating sites for a potential newly constructed CAES cavern.

• The Emily District north of the Cuyuna Range has a significant manganese deposit. Cooperative Mineral Resources (CMR), a subsidiary of Crow Wing Power, is working to develop the best project plan for extracting manganese from an 80‐acre parcel north of the City of Emily. One method that has been tested on site is extraction by solution mining via a borehole. This method produces a cavern that could potentially be used for CAES.

• Current open pit mines on the Mesabi Range may make it difficult to use nearby existing underground mines for CAES because of blasting concerns. Southward (down dip) expansion of open pit taconite mining poses an additional impediment. Abandoned underground mines on the Cuyuna Range may be a more realistic option, although many were converted to open pit mines or are now flooded. Future precious metal mining operations in the Duluth Complex may be another option to consider converting into CAES storage facilities after the mining operation is complete. It may also be possible to couple future underground iron mining operations with a potential CAES facility. This will be considered relative to iron‐formations that cannot be reached with open pit techniques.

Air Leakage Control and Structure Integrity

• At this time, the major design questions related to impacts on groundwater are: 1) dewatering of the abandoned mines; 2) leakage of air from the cavern; and 3) the potential use of a water curtain for unlined caverns and reservoirs to maintain constant pressure in the cavern. Changes in pressure in the cavern from dewatering or air leakage could, also, alter the surrounding water table.

• A design decision that has to be made in a CAES operation is how to confine the caverns. Theoretically, if the cavern is deep enough, water pressure could be high enough to keep air contained. Gas escape may not occur as long as the water pressure along all possible escape paths increases for some small distance in the direction of potential gas escape (Goodall et. al, 1988). Alternatively, a water curtain could be built to contain air with increased water pressure or the cavern could be lined with an airtight material. Lining materials need to be able to withstand fatigue from the cyclic compression and decompression of CAES operation. Thermal stresses in the lining would also be an issue with adiabatic compression.


• For effective air storage, water at the cavern boundary should flow toward the cavern or have no flow. Outwards movement of water is possible at the bottom of the cavern, so to prevent air from leaking; the cavern bottom should be saturated with water or lined.

Environmental and Permitting

• A number of environmental and permitting issues will need to be answered by selection of the site: 1) soil erosion; 2) seismicity; 3) water quality; 4) groundwater; 5) surface water; 6) biological resources; 7) agricultural resources; 8) cultural resources; 9) aesthetic resources; 10) recreational resources; 11) population and housing; 12) air quality and noise; 13) greenhouse gases; 14) hazardous materials; and 15) environmental permitting.

CAES Progress Report – Economic and Policy Team 5

1. CAES PROGRESS REPORT – ECONOMIC AND POLICY TEAM

Nahyeon Bak

1.1. WORK SUMMARY

Economic and Policy research in the period from January to November 2013 focused on three areas:

1. Operational and economic issues related to the integration of CAES with the electrical grid; 2. Challenge and opportunity for CAES in policy; and 3. Site selection criteria.

The Economic and Policy Team took part in three meetings: Jan. 9, 2013 kick‐off meeting, May 30th meeting, and August 12th meeting in Hinckley, MN. Our team has also coordinated with the Facility Team through several meetings in July and August to share wind farm and technical information. Our team also contacted the Department of Commerce for energy facility permitting information.

1.2. KEY FINDINGS

• Economic modeling considering only revenue from arbitrage suggests that a CAES project on the potential site can operate profitably only if variable operating costs of electricity production are $3/MWh.

• Understanding how MISO market rule changes would affect CAES value is more difficult. Most literature suggests that without revenue from participating in the reserve market, and that without these additional funds, CAES remains uneconomic. Stored Energy Resource (SER) rule changes have recently made it possible for CAES to participate in reserve and regulation markets in MISO.

• CAES economic feasibility is sensitive to high investment costs and increasing natural gas prices. Both lead to high operation costs, which decrease opportunities for arbitrage and threaten the feasibility of CAES plants.

• To benefit from the incentives related with renewable energy and complement the intermittent renewable energy, coupling CAES plants with a wind farm should be considered. However, current sites examined by the facility team do not support the co‐location of wind and CAES plant.

• The geological characteristics of the CAES site will decide the feasibility, operational specifics, and financial risk. For example, the Iowa CAES plant was abandoned after geological surveys of the selected sandstone structure proved permeable, and the Colorado site was also abandoned. Site selection of the CAES reservoir adds significant financial risk due to possibilities of leakage.

1.3. INTERIM REPORT ON ECONOMIC AND POLICY TEAM TASKS

Work on each of the Economic and Policy team tasks is described in the following sections, along with results and a summary of the work still remaining.


1.3.1. Task 1: Policy and Economic Environment for CAES Development

Characterization of the economic and policy environment for CAES has included research into relevant laws and regulations, criteria for potential site decision and incentives in ISO, land and mineral ownership, and potential land use conflicts. This characterization falls into federal and state regulation as well as incentives in MISO, the regional transmission organization. We have examined relevant state and federal legislation and reviewed incentives for storage technology development and CAES relevant legislation specifically.

The need for energy storage is reiterated in many pieces of legislation. The Energy Policy Act of 2005, Energy Independence and Security Act of 2007, and the American Recovery and Reinvestment Act of 2009 all underscore the critical role that energy storage can play in the electrical transmission system. Additionally, many efforts have been made to promote legislation supporting investment in storage technologies, many proposing Investment Tax Credits, much like the Production Tax Credit that has been so helpful for supporting wind power. While legislation has been proposed in Congress, e.g., the STORAGE Act of 2011 or the Clean Energy Standard Act of 2012, no legislation has yet been passed, and the current political gridlock in Washington appears to make passage unlikely.

At the state level, renewable portfolio standards require generation from renewable sources of electricity, but the role of energy storage in renewable portfolio standards is varied; most focus on generation technologies, not storage capabilities. For example, California mandates that by 2024, California’s three investor‐owned utilities must invest in 1.325 GW of energy storage capacity. In Minnesota, there has not been specific legislation promoting storage. However, if a CAES project were to be developed in conjunction with wind development, applicable policies related with renewable energy would also be important. In Minnesota, the Renewable Energy Production Incentive (Minn. Statute 216C.41 Subdivision 1‐Definitions): “Qualified wind energy conversion facility” 1.0 cent per kw‐h until December 31, 2018, the Renewable Energy Standard (Minn.Stat.216B.1691) and the Energy Policy Goal (Minn. Statute 216C.05 Subd.2): 25% of total energy used in the state should be derived from renewable energy resources by 2025, are all important pieces of legislation shaping the policy environment.

Another important policy consideration is regional de facto policy through regional transmission organizations (RTOs), both through their planning activities and in market design. RTOs also recognize the benefits from storage plants and FERC Order 755 helps to address this issue by rewarding speed and accuracy through pay‐for‐performance requirements.

MISO defines Stored Energy Resource (SER) as eligible to provide Regulating Reserves. In the regulation reserves market, a storage plant deserves to receive payment for mileage because of its fast ramping rate. In Oct 2011, the Federal Energy Regulatory Commission (FERC) issued Order 755 that required the organized wholesale power markets to also provide compensation for generation movement in response to regulation dispatch. In December 2012, MISO added a regulation “mileage” product to financially compensate generators for providing regulation capacity. This is important for the value of CAES projects.

Additionally, CAES projects are affected by government protocols shaping facility siting. Environmental permitting and approvals by state and federal entities will shape how CAES technologies are developed and deployed. For more information, see Appendix 1‐A on siting responsibilities. As CAES plants store compressed air in subsurface rock formations, the geologic characteristics of the site are paramount for


facility operation. Candidate sites are currently being investigated by the Geology Team (see Section 4), and the Economic and Policy team is working closely to better understand how geologic data will affect plant design and potential operation. Site selection will affect economic characteristics that will affect facility costs and the cost of electricity. Infrastructure costs, such as access to the high‐voltage electric transmission network and easy delivery of natural gas, will affect the financial viability of CAES.

Finally, the ability of CAES projects to be constructed on the Mesabi and Cuyuna Ranges remains dependent on access to land and mineral rights. Earlier work on the viability of the PHES project (Fosnacht et al., 2011) on the Mesabi Range addressed this issue, “The largest mineral rights owner and a major landowner is the State of the Minnesota. Purchasing the land to build a PHES facility is complicated by the fact that property rights may be severed, which means that landowner may not hold the mineral rights.” These issues remain important for any CAES development.

Task 1 is complete, but we will continue to track legislation and bills that could affect CAES and energy storage at the federal, state, and RTO levels.

1.3.2. Task 2: Develop Preliminary Regulatory and Socio‐Political Criteria for Assessment Guidelines

Regulatory and socio‐screening criteria for CAES site assessment include the following factors and depend on CAES site selection. Once potential sites are selected, we will examine the following: current land use, patterns of property ownership (surface and mineral), permitting considerations.

Current land use and ownership – For permitting process and community acceptance, we will review and survey residential population density, current industry activities, ecological features and recreational uses of the potential site.

Permitting – The Minnesota Public Utilities Commission (PUC) has permitting authority for any future CAES plant that seeks rate recovery in Minnesota. Any large energy facility1‐1 proposed to be built by a utility and that seeks rate recovery in Minnesota must obtain a Certificate of Need from the PUC before construction begins. Certificate‐of‐need proposals must demonstrate a need for the facility. Under the Power Plant Siting Act (Minnesota Statute 216E) in the review process, an environmental impact statement (EIS) on the project and a contested case hearing are conducted (Minnesota Rules Chapter 7850).

Work on Task 2 is complete, as we have identified stakeholder groups and assessed their interests. We are now coordinating with the environmental assessment team to complete the description of permitting criteria.

1.3.3. Task 3: Assess Socio‐Political Factors Affecting CAES Implementation

CAES stakeholders include both direct beneficiaries (facility owners, electric utilities, system operators, owners of variable electricity sources) and indirect beneficiaries (residential, commercial, and industrial

1‐1 A large electric facility is classified as: any electric power generating plant or combination of plants at a single site with a combined capacity of 50,000 kilowatts or more and transmission lines directly associated with the plant that are necessary to interconnect the plant to the transmission system (Minn. Stat. § 216B.2421, subd. 2)

https://www.revisor.mn.gov/rules/?id=7850

https://www.revisor.mn.gov/rules/?id=7850

https://www.revisor.mn.gov/statutes/?id=216b.2421


customers). Additionally, gatekeeper organizations include regulators (Minnesota Pollution Control Agency (MPCA), Minnesota Department of Natural Resources (MNDNR), Public Utilities Commission (PUC), Federal Energy Regulatory Commission (FERC)), economic development agencies, e.g., Iron Range Resources and Rehabilitation Board (IRRRB), as well as nearby residents of communities in the vicinity of potential CAES sites. Other stakeholders include private landowners, recreational users, mine owners and mineral rights holders and environmental non‐profit organizations.

Drawing from the literature, our research has identified several socio‐political factors with the potential to affect CAES implementation, including: increasing prices for iron ore and non‐ferrous minerals; shifting natural gas prices; Minnesota state renewable portfolio standards; and decreased support for national greenhouse gas (GHG) emission limits or national renewable energy standards.

Work on Task 3 is linked with the work on Task 2 and will be completed in conjunction with that task.

1.3.4. Task 4: Identify Life Cycle Parameters

Task 4 focuses on identifying parameters for life cycle analysis of CAES projects. However, as no CAES life‐cycle analysis is scheduled for this project, the resources allocated for this task are being redeployed to better develop the economic modeling effort in Tasks 5 and 6. Assess policy factors affecting implementation is presented in conjunction with Tasks 1‐3 and Task 6. This work will be further developed as a synthesis of all policy team research.

1.3.5. Task 5: Analyze Economic Factors Affecting CAES Implementation

We have concentrated project resources to better understand the economics of CAES. The following section provides a literature review plus a description of the model we are developing. In conjunction with the policy work, this will allow us to better understand the implications and value of CAES development.

1.3.5.1. Literature Review on Compressed Air Energy Storage

The economic and policy literature on CAES can be divided into two classes. The first class of articles focuses on the value of independent CAES. The second class deals with the value of CAES with wind integration.

• The value of independent CAES.

Several studies estimated the value of electricity storage. First, Sioshansi and Denholm (2009) analyzed the arbitrage value of pricing‐taking storage devices in PJM, an RTO which part of the Eastern Interconnection grid operated an electric transmission system from 2002 to 2007 with welfare effects. They pointed out that difference between on‐peak price and off‐peak price and the volatility of price for natural gas and electricity have raised interest in the potential economic value for electricity storage. In addition, the impact of load‐shifting for larger amounts of storage and reductions in arbitrage can threaten the economic value for plants. However, it will be offset by increases in social welfare due to increase in consumer surplus. Yucekaya (2013) also emphasized that the fluctuation of prices for electricity and natural gas impacts the revenue


of CAES directly. Based on 100 simulations in the Turkish market, the author showed that the investment to CAES would be economically feasible for the given market prices and could be implemented. Second, Sioshansi et al. (2009) and Jenkin and Weiss (2005) also noted that depending on its location, storage can have some transmission‐related benefits. Third, regarding ownership, Sioshansi et al. (2011) suggested that a private‐sector investor could not have incentive to invest CAES plants in a restructured market such as PJM, and treating storage as a regulated asset like transmission or distribution infrastructures would be better considering potential benefit from storage such as congestion relief, deferred transmission, and better grid and asset utilization. Fourth, the authors also showed that compared with pure storage devices, the CAES device purchases 44% less energy, choosing from lower‐cost hours. Lastly, Drury et al. (2011) estimated the value of CAES considering both operating reserves revenue and arbitrage revenue in several U.S. markets. They found that conventional CAES systems could earn an additional $23±10/Kw‐yr by providing operating reserves, and adiabatic CAES systems could earn an additional $28 ±13/Kw‐yr. They also found that arbitrage‐only revenues are unlikely to support a CAES investment in most market locations, but the addition of reserve revenues could support a conventional CAES investment in several markets. Adiabatic CAES revenues are not likely to support an investment in most regions studied.

• The economics of CAES with wind integration.

Most studies on CAES with wind integration suggested that the CAES plant is likely to be unprofitable. Denholm and Sioshansi (2009) suggested that the advantage of co‐location of wind and storage is a decrease in transmission requirements, but the disadvantage of it is a decrease in the economic value of energy storage compared to locating energy storage at the load. Fertig and Apt (2011) showed that for various price scenarios, most CAES plants are unprofitable. Considering revenue from regulation markets raises the value of CAES slightly. Even though social benefits of CAES with wind integration include the avoidance of the construction of new generation capacity, improved air quality during peak times, and increased economic surplus and subsidy from government is considered, the private cost of the CAES system will not be covered. Mauch et al. (2012) also tested whether a wind farm with CAES can survive in the day‐ahead market. They found that annual income for the wind‐CAES plants would not offset annualized capital costs, even considering the market prices with a carbon price. Madlener and Latz (2013) also analyzed the economic feasibility of CAES with various capacity scenarios. The feasibility of CAES plants depends on entering both the spot market and the reserves market. Without the revenue from reserves market, building CAES plants is not viable. An independent CAES plant is found to be more profitable than a CAES with integration. In the Madlener and Latz (2013) study, diabatic CAES is more profitable than adiabatic CAES.

1.3.5.2. Key economic issues affecting CAES implementation

Based on previous studies related with CAES, the operating cost, including the price of natural gas and electricity, potential project profitability in the electricity market and the optimal duration of the operating cycle, impacts the value of CAES plants. In particular, considering CAES has a relatively high operation cost per kW installed and the major revenue of CAES are from arbitrage and ancillary service revenues, major key issues affecting the value of CAES plants are the following: 1) natural gas price; 2) the type of plant – Independent CAES plant or Coupling CAES plant with wind farm; and 3) uncertainties


in the price of electricity. Technical factors affecting the value of CAES are heat rate, energy ratio (energy efficiency factor), power ratio (power efficiency factor), ramp rate, response time, and storage duration period. Financing factors affecting the value of CAES are capital costs, real estate and taxes, construction, and permitting period.

According to the financial revenue from the arbitrage model by buying the electricity with off‐peak price and selling it with on‐peak price, it suggests that a CAES plant could be likely to operate profitably (see Appendix 1‐B); however, additional analysis would be needed to address uncertainty in construction costs, land acquisition costs, and the evolution of electricity price in the market.

Additional revenues for a CAES plant to generate revenue need to be investigated, including the value that such a plant could offer in: 1) countering the system balancing and system reliability costs; 2) avoiding the need to build additional transmission; and 3) providing ‘regulation reserves’ in operating reserves market.

Work remaining on Task 5: • Optimal operation hours – annual operating hours, with daily and monthly optimization, expected

net revenue, seasonal optimization; • Exploration of options to monetize system balancing, system reliability of a CAES plant and revenue

of the CAES plant in operating reserves market; • Analyzing how the price of electricity (LMP) evolves by using time‐series econometrics technique for

each node in potential site; • Analyzing how the value of CAES plants affected by the change of natural gas price; and • Finding the optimal size of CAES plants with load curve.

1.3.6. Task 6: Compare First Order Economic Estimates of a CAES Project with the Previously Developed PHES Analysis.

Work remaining on Task 6 is to compare CAES with PHES investigation in May and June 2014.

1.4. REFERENCES

Denholm, P., and Sioshansi, R., 2009, The value of compressed air energy storage with wind in transmission‐constrained electric power systems: Energy Policy, v. 37, p. 3149‐3158.

Drury, E., Denholm, P., and Sioshansi, R., 2011, The value of compressed air energy storage in energy and reserve markets: Energy, v. 36, no. 8, p. 4959‐4973.

Fertig, E., and Apt., J., 2011, Economics of compressed air energy storage to integrate wind power: a case study in ERCOT: Energy Policy, v. 39, p. 2330‐2342.

Jenkin, T., and Weiss, J., 2005, Estimating the Value of Electricity Storage: Some Size, Location and Market Structure Issues: October Electricity Energy Storage Applications and Technologies Conference, San Francisco, CA.

Madlener, R., Latz, J., 2013, Economics of centralized and decentralized compressed air energy storage for enhanced grid integration of wind power: Applied Energy, v. 101, p. 299‐309.


Mauch, B., Carvalho, P., and Apt, J., 2012, Can a wind farm with CAES survive in the day‐ahead market?: Energy Policy, v. 48, p. 584‐593.

Sioshansi, R., Denholm, P., Jenkin, T., and Weiss, J., 2009, Estimating the value of electricity storage in PJM: arbitrage and some welfare effects: Energy Economics, v. 31, p. 269‐277.

Sioshansi, R., Denholm, P., and Jenkin, T., 2011, A comparative analysis of the value of pure and hybrid electricity storage: Energy Economics, v. 33, p. 56‐66.

CAES Progress Report – Economic and Policy Team Appendix 1‐A 12

APPENDIX 1‐A – ENVIRONMENTAL PERMITS AND APPROVALS MATRIX

CAES Progress Report – Economic and Policy Team Appendix 1‐B 21

APPENDIX 1‐B – ECONOMIC MODELING BASED ON REVENUE FROM ARBITRAGE

In Day‐Ahead market, we estimate the expected revenue from arbitrage. In other words, CAES plant is charged when the price is off‐peak and it generates electricity by discharging compressed air when the price is on‐peak. To provide a basic financial performance for the arbitrage‐only scenario, we modeled stochastic dynamic optimization based on discrete state, discrete choices and discrete time (hours). Especially, since the price of electricity is volatile and is not easy to predict, we suppose that the price of electricity is stochastic variable.

Assumptions 1. Prices are based on day‐ahead market price data from January 9, 2009 to June 30, 2013 for the

MP.MP BOS4 node, which is the closest generation node for potential sites. 2. The Power capacity of storage device (k) is 100MW and the Energy capacity of storage device (K*h)

is 100*Operating hour (10h) =1000MWh. 3. The Roundtrip efficiency of storage device (η) is 0.8 that means that it discharges 80MW when it

charges 100MW. 4. According to the Henry Hub Natural Gas Futures Contract data, the price of natural gas (Png) is

5$/GJ. 5. Heat rate of CAES expander (γ) is 2 GJ/MWh. 6. Variable O&M cost (Ve) is 3$/MWh. 7. Discount factor (δ) is 1/1.08. Dynamic Optimization Model

• Stochastic Bellman Equation

• Decision Variables are

• State Variables are the price of electricity and the storage level. The price of electricity ( ) is a

stochastic variable and torage level , which is a total energy in storage at the beginning of hour t (MWh) is a deterministic variable.

• State transition function is the followings.

• means quantities of electricity charged at time t and means quantities of electricity discharged at time t.

• Reward function F(S ( is consist of the followings.

F(S ( = [F1 F2 F3] where

F1 = 0 if X= idle, 0

F2 = * – (Png * γ + Ve)* if X=discharge,

F3= * – Ve* if X=charge,

CAES Progress Report – Economic and Policy Team Appendix 1‐B 22

A model is designed to find the optimum operating strategy such that the power request of the site is met and the total expected revenue is maximized. Expected results indicate that for this simplified model, electricity can be produced profitably as long as the price variation is large. After completing this model, we will investigate how frequently the CAES plant is in generating mode during week 1 and how much the plant sells energy when the price is at certain level.

Except for revenue from arbitrage, CAES plant is expected to get more revenue from reserves market, according to the incentives for stored energy resources (SER) in MISO. To estimate revenue from reserves market, we will modify above model and figure out the expected revenue from CAES plant.

CAES Progress Report – Facilities Team 23

2. CAES PROGRESS REPORT – FACILITIES TEAM

Jeff Marr

2.1. REVIEW OF THE FACILITIES TEAM SCOPE

The mission of the Facility Assessment Team is to assess technical systems that will be required in order to implement the CAES system at the chosen location or locations on a preliminary or first pass basis. The assessment includes the following tasks:

Task 1: Technology assessment and review of published literature and evaluation of existing/planned facilities;

Task 2: Evaluation of conventional and advanced compressed air energy storage technologies; Task 3: Assessment of surface and subsurface facilities configurations; Task 4: Assess plant operational characteristics (daily, annual, design life timescales); and Task 5: Development of Deliverables: “CAES Assessment guidelines,” “CAES Site Screening

methodology,” and “Screening Assessment of CAES Potential.”

Through these five tasks the Facilities Team is contributing to development of project deliverables and milestones as listed elsewhere in this report.

The Facilities Assessment Team is led by Jeff Marr of the St. Anthony Falls Laboratory, University of Minnesota and includes key personnel from University of Minnesota, Duluth ‐ Department of Civil Engineering (Associate Professor Carlos Carranza‐Torres) and University of Minnesota, Twin Cities ‐ Department of Mechanical Engineering (Professor Perry Li). The team will also include engineers from Great River Energy (GRE), Minnesota Power (MP), and Duluth Metals (DM). Consulting Services to the team will be provided by Barr Engineering.

2.2. ANNUAL PROGRESS SUMMARY: FACILITY ASSESSMENT TEAM

2.2.1. Task 1: Technology assessment and review of published literature and evaluation of existing/planned facilities

This task focuses on a comprehensive review of available literature on CAES, including peer reviewed journals, white papers, government and state reports, and websites. A portion of the literature review focuses on the theory and physics forming the basis of CAES. The team conducted a detailed investigation into the fundamental physics and theories behind CAES and studied the primary approaches to CAES. The team also evaluated several CAES facilities that were either planned or constructed. The outcomes of this task include: 1) internal report, “Summary of the Physics of CAES” (Appendix 2‐A), and 2) internal report, “Annotated bibliography of relevant papers on conventional and advanced CAES” (Appendix 2‐B). This task is 95% complete.

2.2.2. Task 2: Evaluation of conventional and advanced compressed air energy storage technologies

This task is on identifying prototyped and/or constructed CAES facilities focusing on location, size, components, layout, and other elements that provide physical characterization of the facility. Once


identified, the team conducted a study of each site using any available published information and conducted interviews with original equipment manufacturers (OEM), vendors and facility operators to gain deeper understanding for facility design, operation, and costs. The outcomes of this task will include an internal report on critical CAES design specification of existing and prototype CAES technologies. This internal report is 90% complete.

2.2.3. Task 3: CAES feasibility assessment and preliminary schematic of surface and subsurface facilities configurations

This task focuses on identifying critical design specification for surface and subsurface CAES facility components. Utilizing the outcomes of the Task 1 and Task 2, the Facility Assessment team, supported by the other three teams, will develop a list of criteria required for viable CAES facilities.

The outcomes of this task include: 1) internal report, “CAES feasibility assessment and preliminary schematic design parameters” (Appendix 2‐C). The report will address both conventional and advanced CAES. This report is 75% complete and will be 100% complete by December 2013.

2.2.4. Task 4: Assess plant operational characteristics (daily, annual, design life timescales)

This task focuses on developing preliminary operational characteristics of the facility such as time required to discharge and recharge storage chambers, estimates of round‐trip efficiencies, and ramp up and ramp down times. The assessment relies heavily on published literature and existing facility characteristics and attempts to address both conventional and advanced CAES. The outcomes of this task will include an internal report summarizing the likely operational characteristics of CAES facilities ranging is technology, storage capacity and power. This task is 5% complete.

2.2.5. Task 5: Development of Deliverables: “CAES Assessment guidelines,” “CAES Site Screening methodology,” and “Screening Assessment of CAES Potential”

In this task, information gathering Facility Assessment Team Tasks 1‐4 is combined with information from other groups to develop project deliverables critical for the second phase of the project in which the screening assessment is applied to potential sites in northern Minnesota’s Iron Range. The outcomes are project deliverables: CAES Assessment Guidelines, CAES Site Screening Methodology, and Screening Assessment of CAES Potential. The first two deliverables, CAES Assessment Guidelines and CAES Site Screening Methodology, will draw heavily on Task 2 and Task 3 internal reports. The deliverable Screening Assessment of CAES Potential will apply the screening methodology developed by the team to identified sites within the Minnesota’s iron ranges.

2.3. ADVANCED COMPRESSED AIR STORAGE TECHNOLOGIES

• During the initial stages of the project, it became clear that the emergence of advanced CAES technologies would need to be considered in this project. These emerging approaches also do not add to the carbon emission, as no additional hydrocarbon fuel is needed. The facilities team expanded the scope of the literature and technology review to include this technology in our


feasibility analysis. In general, advanced CAES offers greater round trip efficiencies to energy storage and also may reduce the capital equipment costs and allow a decrease in the size of underground cavern through use of higher pressure storage. In Tasks 2‐4 of the project, advanced CAES is also integrated into the study through consideration of commercial technologies including General Compression, Lightsail Energy, SustainX, and the NSF‐supported CAES for wind turbines project at the University of Minnesota.

2.4. CALENDAR YEAR 2014, QUARTER 1 ACTIVITIES

In the first three months of 2014, the Facilities Team will finalize site selection criteria, which will include consideration of both conventional and advanced CAES. We will support the rest of the team in applying criteria to potential sites and identify 2‐3 sites for full feasibility analysis. The facilities team will work closely with the Economics Team in further analysis of how the facility design and operation is integrated into the MISO energy market. We will work to identify strategies and barriers for energy storage participation.

CAES Progress Report – Facilities Team Appendix 2‐A 26

APPENDIX 2‐A – DRAFT SUMMARY OF THE PHYSICS OF CAES

Internal Project Report (Draft)

_____________________________________________________________________________________

Report Title: Summary of the Physics of Compressed Air Energy Storage

Project: Compressed Air Energy Storage (CAES) in Northern Minnesota Using Underground Mine Workings

Date: November 2013

By: Caroline Hughes, Jeff Marr, Perry Li

_____________________________________________________________________________________


1. Summary This report provides a general overview of the physical principles behind compressed air storage. This document is written for the internal team such that all team members have a basic understanding for the technology.

2. Gas turbines and the Brayton Cycle Compressed air energy storage was developed out of gas turbine technology in the early 1970s. Gas turbines are most often used to produce thrust, as in a jet engine, but are also used to run generators in power plants. Gas turbines use air, water vapor, or another gas as their working fluid. The gas turbine cycle compresses, heats, and then expands its working fluid to generate electricity or forward motion (as in a jet turbine). The electric generator, compressor, and turbine are all on the same shaft. The input to a traditional gas turbine is a combustion fuel, usually natural gas. An input of heat from combustion occurs between compression and expansion to ensure the system can maintain its own motion in addition to doing some work. The outputs of the gas turbine are: (1) an exhaust stream with sufficient energy to go back into the compressor and keep it running, and (2) shaft work (spinning) of an electrical generator. The ratio of the energy input to the energy output is:

The thermodynamic cycle of a conventional gas turbine is called the Brayton Cycle or the Joule cycle. The steps of the cycle are shown in Figure 1.1:

Figure 1.1. Brayton cycle. From www.mae.wvu.edu

1 – 2: Constant‐entropy compression process

2 – 3: Constant‐pressure heating (usually by combustion of fossil fuel)

3 – 4: Constant‐entropy expansion process

4 – 1: Constant‐pressure cooling

CAES Progress Report – Facilities Team Appendix 2‐A 28 In an open cycle, the working fluid may be discarded at step 4 and fresh air can be taken in at state 1 so that cooling the fluid is not required.

Conventional compressed air energy storage is thermodynamically very similar to a gas turbine that follows the Brayton cycle. The main difference is that CAES separates compression and expansion in time with a storage phase in between. The use of energy is slightly different: a conventional CAES plant uses electric energy from the grid to compress air and inject it into a storage cavern, then mix it with natural gas for combustion at a later time in order to run a turbine and generator. In contrast with the gas turbine described above, the input‐to‐output ratio of a conventional CAES plant is:

Ideally, the compressor would be able to use inexpensive electricity during off‐peak hours to compress the air. At a later time when the demand for electricity and its retail value increase, the plant can discharge the stored air and generate electricity. In that sense, the efficiency may not be as important as the price of fuel and the difference in price of electricity between charging and discharging. CAES plants can be used independently to level loads on the grid at large. CAES plants are also potentially useful for improving the capacity factor (“firming”) of intermittent renewable energy such as wind.

3. Fluid Energy The total energy per unit mass in a body of fluid is the sum of internal energy, kinetic energy, potential energy, and “flow work,” which refers to the movement of mass across the boundary of a control volume. The internal energy includes a thermal component, which for an ideal gas refers to the kinetic energy of molecules, measurable as temperature. There is also a potential energy component of internal energy, but it is not important in the context of compressed air storage.

Flow work is defined as the work required to move a fluid through the cross‐section of a control volume, as if by a piston. The piston exerts a force on the fluid over a certain distance. Consider the following:

F = Force exerted by piston in order to move the fluid P = Fluid pressure A = Cross‐sectional area of control volume

W = Work done by piston L = Distance through which the piston moves the fluid

The total energy per unit mass of a flowing fluid can then be written as shown, where lowercase letters indicate values per unit mass:

Energy = kinetic + potential + internal + flow

Note that if the fluid is not moving through a control volume, PV is zero—the V represents the volume the fluid moves through, not the volume the fluid occupies when it is stationary.

CAES Progress Report – Facilities Team Appendix 2‐A 29 Also, potential and kinetic energy can be ignored in this context, as they refer to macroscale properties like elevation and velocity in a single direction. These do not change from one end to the other of a compressed air storage plant. Internal energy (particularly the thermal component of it) and the flow work are much more important.

4. Conventional CAES Plants A simplified schematic of a CAES plant is shown in Figure 4.1.

Figure 4.1. Simplified CAES plant schematic.

Figure 4.2. Two‐stage compression train.

More detail of the compression phase is shown in Figure 4.2. The compression is broken up into several stages to reduce the pressure drop across each individual machine. By pausing the compression and stripping some heat with an intercooler, the required compressor work is decreased, and the machines are saved from operating at extremely high temperatures. The cost requirements of an intercooler are usually recovered in the form of avoided mechanical failure. The temperature of the air leaving the compressor may be around (VALUE), but it is “aftercooled” with a heat exchanger to about 50° before injection in order to avoid thermal stresses on the cavern walls.


Figure 4.3.Two‐stage expansion train.

Figure 4.3 shows the expansion process that generates electricity. The actual generator, which is connected to the turbines by a shaft, is not shown. The air comes out of storage and is mixed with the exhaust of the turbines, which pre‐heats it so that less energy is consumed in the combustion chamber. The fuel heat rate, which is a measure of the energy consumed per unit of generated electricity, improves by about 25% as a result of this process, which is called regeneration or recuperation. Also, the expansion process is split into two stages with a second heating step between them, to counteract the cooling effects of expanding the air. Reheating serves the similar purpose in expansion as intercooling does in compression: by breaking up the expansion process, the pressure difference from inlet to outlet of each one is smaller, and the addition of heat increases the work output of the air on the turbine.

Conventional compressed air storage plants are inherently inefficient, and it is counterintuitive that they should require an input of fossil fuel in addition to the electricity they use to compress. The main input of energy is in the form of the electricity that is used to compress the air. However, some of this energy is lost afterwards, either by intentional cooling or by dissipating into the walls of the storage cavern. Pressure is maintained throughout the storage process, but combustion is required to restore the lost thermal energy.

5. Emerging technologies Unlike power plants, which must convert stored energy into useful work, storage plants do not need to generate a net output of work. Fuel is only required because such a large portion of the thermal energy imparted to the air is dissipated during storage. If thermal energy were stored or recaptured, fuel consumption would be unnecessary.

Engineering an effective thermal store is the primary technical challenge involved with the “Advanced Adiabatic” compressed air energy storage plant under development in Germany, nicknamed ADELE. Instead of discarding heat during intercooling and aftercooling, the heat would be stored in a material such as concrete or ceramic. After storage, air exiting the cavern would undergo heat exchange with the thermal energy store instead of going through combustion. Because of the extremely high temperature difference between the thermal energy store and the surrounding environment, preventing heat dissipation is a significant challenge.

CAES Progress Report – Facilities Team Appendix 2‐A 31 Under development by several small companies, including Berkeley‐based LightSail, Isothermal CAES does away with the need for combustion by preventing the compressed air from gaining more thermal energy than can be easily stored. Instead, the heat is stripped from the air gradually during the compression process, stored, and returned to the air in a similar manner during expansion. Because the air is prevented from increasing to more than a few degrees above ambient temperature, the primary challenge of this process is matching the heat exchange rate with the rate of compression or expansion. Heat transfer rate is proportional to the difference in temperature between the two materials and the contact surface area. By using extremely small droplets of water, it is possible to achieve the necessary heat transfer rate. The thermal energy may be stored at approximately 20 degrees above ambient temperature.

6. Quantifying Stored Energy The work per unit mass for compressing or expanding an ideal gas is given by:

W = work done by or on the gas (depending on whether it is compression or expansion) Vatm = initial volume of gas Vcomp = compressed volume of gas P = pressure

6.1 Adiabatic Processes Succar (2008) takes the approach of solving the work integral on the basis of specific volume (lowercase v) and specific pressure (lowercase p) for a two‐stage adiabatic expansion process that goes from a high pressure down to atmospheric pressure. With adiabatic processes, as found in conventional CAES

systems, then PVk is constant throughout the process, where k (often also written as ) is the ratio between the specific heat capacities at constant volume (cv) and constant pressure (cp):

The end result is the work per unit mass, w:

Ti = temperature pi = specific pressure Subscript 1 is associated with the input to the first (high‐pressure) turbine Subscript 2 is associated with the input to the second (low‐pressure) turbine Subscript “atm” is associated with exhaust and atmospheric pressure.

The total amount of stored energy, E, that will be output during expansion can be found by integrating the work per unit mass with the mass flow rate over time:

mT = mass flow rate of air and fuel through the system t = time to discharge the storage cavern

CAES Progress Report – Facilities Team Appendix 2‐A 32 Depending on how the plant is operated, the mass flow rate through the system and the work per unit mass may be constant or they may vary as the storage is depleted. A realistic operating scenario is that the cavern pressure is allowed to change as the air is depleted, whereas the inlet pressure to the turbine is throttled so it is constant. Throttling the air flow at the turbine inlet means that the mechanical work is constant over time, but because the cavern pressure is decreasing, the mass flow rate varies. Taking these assumptions into consideration, Succar’s solution for energy output can be completed:

mT = total mass flow rate mF = fuel mass flow rate mA = air mass flow rate Vs = storage volume ps = storage pressure. ps1 is the minimum storage pressure that the cavern is allowed to be depleted to, and is equal to the inlet pressure at the first (HP) turbine. ps2 is the maximum pressure that the cavern reaches, and is determined by the outlet pressure that the compressors are capable of achieving.

The total energy generation for the adiabatic case was adapted to a MATLAB code that computes generated power for a series of possible cavern volumes and discharge times. The results are shown in Table 5.1.1.

The code requires several inputs. This example case uses operating parameters from Dresser‐Rand’s line of products.

Operating Parameters

Maximum operating storage pressure (ps2) Minimum operating storage pressure (ps1) Inlet temperature to the HP turbine (T1) Inlet pressure to the HP turbine (p1) Inlet temperature to the LP turbine (T2) Inlet pressure to the LP turbine (p2) Atmospheric pressure (Patm) Storage temperature (Ts) Heat rate (H)

Physical Constants

Ratio of specific heat capacities (k1) at T1 Ratio of specific heat capacities (k2) at T2 Molar weight of air (Mw) Ideal gas constant (R) Lower Heating Value of natural gas (LHV)

6.2 Isothermal processes The case of isothermal expansion or compression is somewhat more straightforward because the temperature of the gas remains constant throughout the process, and the use of specific temperature and pressure is not necessary. An ideal gas law relationship applies:


Accounting for the fact that the minimum pressure is atmospheric (i.e., not zero), the work done during compression or expansion is expressed as:

The ratio (Pcomp/Patm) is substituted for (Vatm/Vcomp) is substituted as because the uncompressed volume of air is unknown, and the end result, as expressed in Li (1975) is:

7. The Storage Cavern An important characteristic of a potential rock mass is permeability. Permeability should be on the order of 10‐9 mm2 (which corresponds to a hydraulic conductivity of 10‐6 cm/s at 20° C) (Giramonti et al., 1978). Granite and limestone generally satisfy this. Even with a relatively impermeable rock, however, pressure losses of 1‐2% can be expected while the air is in storage.

To further protect against pressure loses, the storage cavern can be hydraulically compensated using a “water curtain.” Air loss can be minimized provided that the water pressure surrounding the cavern is greater than the air pressure inside the cavern (Zhongkui et al., 2009).

If the cavern is operated without hydraulic compensation, the pressure will decrease as air is extracted. However, the entire cavern does not need to be depleted with every charge‐discharge cycle. An example from the proposed plant shows how the cavern may be discharged and re‐charged partially over the course of about 4 days. The cavern is charged back up to full capacity to for about 3 days to complete a week‐long cycle. Note that the pressure values in this example are significantly higher than the maximums used at both the McIntosh and Huntorf plants, indicating the improvements in compression machinery over the past 30 years.

Thermal stresses


8. References Giramonti, A.J., Lessard, R.D., Blecher, W.A., and Smith, E.B., 1978, Conceptual design of compressed air

energy storage electric power systems: Applied Energy, v. 4, p. 231‐249.

Li, K.W., 1975, Compressed air storage in gas turbine systems: Journal of Engineering for Power, v. 62, no. 4, p. 640‐644.

Zhongkui, L., Wang, K., Wang, A., and Liu, H., 2009, Experimental study of water curtain performance for gas storage in an underground cavern: Journal of Rock Mechanics and Geotechnical Engineering, v. 1, no. 1, p. 89‐96.

CAES Progress Report – Facilities Team Appendix 2‐B 35

APPENDIX 2‐B – DRAFT ANNOTATED BIBLIOGRAPHY

Internal Project Report (Draft)

_____________________________________________________________________________________

Report Title: Annotated Bibliography of Relevant Papers on Conventional and Advanced CAES


Date: November 2013

By: Jeff Marr, Caroline Hughes, and Perry Li

_____________________________________________________________________________________

CAES Progress Report – Facilities Team Appendix 2‐B 36 1. Summary This report contains findings from a literature and project review of utility scale compressed air energy storage (CAES) technologies. The report is organized into sections focused on technology overview, thermodynamics, machinery, rock mechanics, economics, wind integration, and case studies. A brief summary of each paper is provided. The papers are presented in each section in alphabetical order by lead author’s last name.

2. Technology Overview

Giramonti, A.J. and Lessard, R.D. (1974) Exploratory Evaluation of Compressed Air Storage Peak‐Power Systems. This paper provides and analysis of compressed air energy storage integration into peaking system for transmission‐scale power distribution. The paper suggests that pumped hydro is an economic and flexible storage method however references challenge in acquiring land, land use issues and transmission line construction. Conventional CAES is proposed as an alternative peak‐power application. The technology is referred to as Compressed Air Power or CAP. The primary concept of CAP is to decouple the compressor and turbine so they operate at different time intervals and the use of a separate storage facility for the compressed air. The paper provides an effective summary of the temperature‐entropy changes for a multi‐stage compression and expansion conventions CAES facility.

The above‐ground facilities will include standard or “familiar” equipment including: compressors, burners, turbines, intercoolers, clutches, regenerators, and motor/generators. Subsurface facilities represent the greatest unknowns or “principal uncertainties.” The technical and economic feasibility of storing compressed gas underground is viewed as the greatest challenge.

The paper provides a preliminary analysis of CAP performance under certain assumptions of the system. One outcome is demonstration that the energy requirements of the compressor can be reduced by using multiple stages of compression rather than a single compressor.

The paper provides a brief economic discussion, which is based on 1970s values and therefore difficult to apply for the current project. Capital equipment costs for surface facilities are estimated at $65/kW and underground at $10‐30/kW.

The paper provides a brief review of environmental impacts. The paper suggests that CAP has less environmental impact than other storage technologies. CAP has lower fossil fuel consumption compared to standard natural gas peaking plants, lower sulfur and particulate emissions as well.

Giramonti, A.J., Lessard, R.D., Blecher, W.A., and Smith, E.B. (1978) Conceptual design of compressed air energy storage electric power systems. The paper gives an overview of conceptual CAES with sections on operation, compressed air storage facilities, CAES plant design considerations, environmental aspects, performance, comparative economics, and prospective improvements. While the paper is older, the information is relevant and useful for this IREE study. Overview information is provided on the general concepts of CAES. Summary information is provided on underground cavern types and considerations include air leakage rates and permeability (for water) of 10‐6 cm/s. With the section on plant design, the paper discusses major systems components such as compressors, intercoolers, aftercooling, recuperators, and plant layout. The paper concludes with a comparative economics section that highlights the importance of

CAES Progress Report – Facilities Team Appendix 2‐B 37 considering the utility system and includes consideration of the magnitude and duration of peak loads plus off peak loads available for compression.

McBride et. al. (2012) Mechanical energy storage. This paper provides an overview of CAES and advanced adiabatic and isothermal CAES. The paper begins with an overview of the compressed air and the thermodynamic implication of compression. Work must be performed to compress a gas. During compression, input work is converted into elastic potential energy and heat. In adiabatic compression the heat remains in the gas resulting in the temperature of the gas to rise. Adiabatic expansion coverts the pressure potential and thermal potential back into work. Isothermal compression works to remove all the heat generated by compression. So the work input into compression resides in the pressure potential energy within the gas and thermal potential energy that is outside the gas. In theory, both adiabatic and isothermal compression could have efficiencies of 100%; however, this is not possible because of mechanical and parasitic energy loss.

The paper goes further to describe the operation of large scale, classical CAES using natural gas combustion turbines. In Classical CAES, part of the heat generated from combustion is used to add heat into the expanding gas prior to entering combustion chamber. The paper describes a simple calculation of round trip energy efficiency for classical and advanced CAES. In Classical CAES, the denominator contains a term that is the energy released by combusting fuel. The Adiabatic and Isothermal systems discussed propose systems that do not use natural gas combustion. This supposes that a purpose of the combustion is to re‐heat the gas. Finally, the paper gives a summary of existing CAES facilities and names Huntorf, Germany, and the McIntosh plant in Alabama. Specification for each of these facilities is provided.

The paper provides a current status report on research for Advanced CAES including isothermal and adiabatic. For adiabatic the challenge is developing high‐temperature compressors and expanders that can handle these large temperature and pressure ranges. For isothermal focus is on developing rapid and continuous heat exchange.

Pfenninger, H. and Baden (1975) Hydroelectric and compressed‐air pumped‐storage schemes. The paper is from 1975, but it provides an overview of energy production, load and balancing. The first part of the paper focuses on describing how base load and peaking plants may be used in combination along with storage to provide the energy needs of a variable daily load profile. The paper states that base load facilities need to run at a flat rate in order to protect systems and realize mechanical design life. It also states that it is economically ideal to run base load plants (high‐cost thermal plants) at full load during low load demand periods. Because base load may run in excess of load, it is ideal to be able to store excess energy and suggests that hydraulic pump and compressed air storage plants are means for storing energy. The paper described the pumped hydro energy storage (PHES) approach. The focus is on Europe. The paper describes CAES and suggests it will be used in locations where PHES is not feasible. The paper gives a very brief summary of the types of caverns – man made and natural. The paper discusses the need for water availability to moderate the temperatures of the compressed gas. In general, this paper is a bit old and is a very brief summary of energy and storage technologies.

Pockley, S. (2008) Compressed air energy storage. This paper provides an overview/summary of compressed air energy storage as a viable energy storage technology for Australia’s energy portfolio. The paper provides an overview of CAES, the physics of compressed air, a summary of isothermal and adiabatic compression, as well as polytrophic compression. The paper provides brief summaries of various compressor types and a discussion on the underground storage systems. The review is very light and only discusses the positive aspect of the

CAES Progress Report – Facilities Team Appendix 2‐B 38 caverns. It does not discuss the challenges of finding viable underground storage. The paper provides a summary of CAES economics suggesting that CAES facilities can produce 2 or 3 times that of conventional gas turbine facilities. The paper concludes with summaries of two operational facilities – Huntorf, Germany, and McIntosh, Alabama. The author provides summaries of design, operational experience, thermodynamics, and maintenance for the Huntorf facility.

Succar, S. and Williams, R.H. (2009) Compressed air energy storage: Theory, resources, and applications for wind power. This paper is a highly comprehensive and relevant paper for this project and serves as a primary reference for the facilities team. The paper is lengthy and therefore only a high‐level summary is provided here. The paper has the following sections: 1) Background, 2) CAES operation and performance, 3) Aquifer CAES geology and operation, 4) Wind/CAES system in baseload power markets, and 5) Advanced technology options. In each section, detailed information is provided for each sector of CAES.

Ter‐Gazarian, A.G. (2011) Energy storage for power systems. This text is a chapter (Chapter 7) from the book Energy Storage from Power Systems (see full citation in reference section). The paper begins with a general physical derivation for compressed air storage for isobaric and isothermal compression. The next section focuses on the basic concept of a CAES facility and differences between conventions gas‐turbine and CAES. In conventional gas‐turbine, the author summarizes the process in which a compressor compresses air that is then combined with fuel and combusted in a combustion chamber. The combusted, high temperature combusted gas drives a turbine connected to a generator. Approximately two‐thirds of the energy produced by the turbine is used to drive the compressor and one‐third generates electric energy. CAES uses the same topology except clutches and storage systems are added, which act to separate the compressor from the turbine.

The volume of air reservoir is the determined by the amount of energy to be stored based on the requirements of the power system. The compressor on the other hand, because the systems are decoupled, is sized based on the length of time available to recharge the reservoir (size of cavern, pressure, volume, and characteristics of diurnal pricing). The paper makes the point that the key advantage of CAES in the fact that the nameplate electrical energy output of the facility is 3x higher than conventional systems since two‐thirds of output energy from a conventional system flows back to the compression stage. In CAES, the electric generator can be one‐third the size of a conventional plant, which is a significant costs advantage.

The paper discusses the subsurface facilities and provides summary of various underground mediums including salt and hardrock caverns. Constant‐pressure caverns are discussed.

The paper provides a summary of the power extraction system or turbine‐generators. It includes a summary of compressor arrangements and the advantages of inter‐cooling. A relationship for compressor work is provided that includes effects of temperature increases across the compressor. Cooling water requirements for large facilities can be significant and need to be considered.

The paper describes two technological advances that help CAES. The first advance is increasing the inlet temperature of the air into the combustion chamber, which increases the Charge energy factor or electric energy output. The second advance is utilizing re‐heat mechanism to use exhaust heat from the LP turbine to pre‐heat expanded air coming into the HP turbine. This reduces the fuel heat rate of the facility.

CAES Progress Report – Facilities Team Appendix 2‐B 39 The paper goes on to discuss adiabatic compression and then provides detail review of the Huntorf and McIntosh facilities. The paper concludes with a short section on dispatch and economic limitations. Discussion of ramp up time is provided for Huntorf, as well as a discussion on alternative fossil fuel types.

Zaug, P. (1975) Air‐storage power generating plants. This paper was published at the time the Huntorf facility was contracted but not yet built. The paper provides an excellent overview of conventional CAES theory and used Huntorf characteristics to demonstrate design and operational parameters. The topology of a conventional CAES system is described including multistage compression and expansion/turbines; inlet temperature considerations; reservoir characteristics and sizing; and a brief discussion on reservoir temperature considerations. A major contribution of this paper is Figure 5, which is also referenced by Succar and William (2008), that provides the relationship for minimum size of subsurface reservoir based on one of three pressure/volume scenarios and desired energy production. This table is very useful in feasibility design of conventional CAES.

3. Thermodynamics

Li, K.W. (1975) Compressed air storage in gas turbine systems. This is a short paper written in the 1970s on the idealized thermodynamic operation of conventional CAES facilities. The paper describes physics‐based relationships for the thermodynamics of these systems. There are sections on thermodynamic analysis, fuel costs estimations, optimizations, and power plant sizing. This paper is very much focused on theoretical thermodynamic analysis and is perhaps more detailed than required for the IREE project.

Rabbani, M., Dincer, I., and Naterer, G.F. (2012) Thermodynamic assessment of a wind turbine based combined cycle. The paper provides a simulation‐based evaluation of a coupled wind turbine and combined cycle power plant. The paper is informative but has little relevance to the IREE CAES study.

Lienhard IV, J.H., and Lienhard V, J.H. (2012) A heat transfer textbook. This is a general textbook on heat transfer.

4. Machinery

Berman, P.A. (1979) Turbo‐machinery for CAS. This paper from the late 1970s provides a summary of three configurations of a conventional CAES facility and presents operating characteristics for the three approaches. The paper suggests that CAES facilities can be constructed using off‐the‐shelf components.

Langston, L.S. and Opdyke, Jr., G. (1997) Introduction to gas turbines for non‐engineers. This paper provides a very simple summary of gas turbines including aircraft engines and land‐based gas turbines. The paper covers gas turbine usage, gas turbine cycles – focusing on the Brayton cycle, and gas turbine components.

CAES Progress Report – Facilities Team Appendix 2‐B 40 [Online] http://www.cast‐safety.org/pdf/3_engine_fundamentals.pdf, Fundamentals of Gas Turbine Engines. This paper provides a summary of gas turbines considering an aircraft jet. The paper covers the following topics – the gas turbine cycle, basic principles, performance and efficiency, engineer sections (details), effects of turbine temperature, effects of atmospheric conditions, and compressor stall/surge. The paper provides a useful entry into understanding the design and operation of gas turbines.

5. Rock Mechanics

Kim, H.M., Lettry, Y., Park, D., Ryu, D.W., Choi, B.H., and Song, W.K. (2012) Potential and evolution of compressed air energy storage: Energy and exergy analysis. This paper provides a summary of an in situ permeability measurement system that is demonstrated at a pilot CAES facility. The device is demonstrated in a low permeability concrete matrix, high permeability construction joints and in the surrounding rock mass.

Kim, H.M., Rutqvist, J., Ryu, D.W., Choi, B.H., Sunwoo, C., and Song, W.K. (2012) Exploring the concept of compressed air energy storage (CAES) in lined rock caverns at shallow depth: A modeling study of air tightness and energy balance. The paper describes a numerical modeling analysis of a shallow CAES application in concrete line caverns. While the focus of this paper is much more refined than the scope of the present study, the paper highlights the importance of cavern leakage rates, construction methods, cavern/air temperature. The paper focuses on shallow construction and postulates that shallow construction of new caverns can, in some cases, be more economical since costs of construction and location can be less relative to deeper and geological‐driven locations. The paper conducts a numerical study that couples thermodynamics, multiphase fluid flow and heat transport with underground CAES in lined rock caverns and, using this model, the authors explore variables that include permeability, initial liquid saturation, and capillary pressure of the concrete lining and surrounding rock, the lining thickness, and cavern depth.

Kushnir, R., Dayan, A., and Ullmann, A. (2012) Temperature and pressure variations within compressed air energy storage caverns. This paper provides insight from numerical and analytical modeling on the temperature and pressure variation within a subsurface cavern due to thermal exchange with the cavern walls. The authors suggest that thermal effusivity is an important design/siting parameter that should be considered during design. For the purposes of the present study, this paper provides important insights into the details of the cavern; however, it is largely focused for the preliminary feasibility study.

Rutqvist, J., Kim, H.M., Ryu, D.W., Synn, J.H., and Song, W.K. (2012). This paper provides a summary of modeling of temperature and geomechanics and an underground, lined CAES facility. The project is focused on geotechnical issues and not relevant to the facilities team and is therefore not summarized here other than including the abstract from the paper.

Abstract: Coupled nonisothermal, multiphase fluid flow and geomechanical numerical modeling is conducted with TOUGH‐FLAC, a simulator based on the multiphase flow and heat transport simulator TOUGH2 and the geomechanical simulator FLAC3D, to study the complex thermodynamic and geomechanical performance of underground compressed air energy storage (CAES) in concrete‐lined rock caverns. The analysis focuses on CAES in lined caverns at relatively shallow depth (e.g., 100 m

CAES Progress Report – Facilities Team Appendix 2‐B 41 depth) in which a typical operational pressure of 5 to 8 MPa is significantly higher than both ambient fluid pressure and in situ stress. Two different lining options are analyzed, both with a 50 cm thick low permeability concrete lining, but in one case with an internal synthetic seal such as steel or rubber. Thermodynamic analysis showed that 96.7% of the energy injected during compression could be recovered during subsequent decompression, while 3.3% of the energy was lost by heat conduction to the surrounding media. Geomechanical analysis showed that tensile effective stresses as high as 8 MPa could develop in the lining as a result of the air pressure exerted on the inner surface of the lining, whereas thermal stresses were relatively smaller and compressive. With the option of an internal synthetic seal, the maximum effective tensile stress was reduced from 8 to 5 MPa, but was still in substantial tension. One simulation in which the tensile tangential stresses resulted in radial cracks and air leakage though the lining was performed. This air leakage, however, was minor (about 0.16% of the air mass loss from one daily compression) in terms of operational efficiency, and did not significantly impact the overall energy balance of the system.

Shidahara, T., Oyama, T., and Nakagawa, K. (1993). This paper is heavily focused on the hydrogeological evaluation of a CAES site in Japan and was therefore not read thoroughly by the facilities team. The abstract is included here for reference.

Abstract: The compressed air energy storage(CAES) is a much‐awaited new system for load leveling power supply. An economical system must be developed, preventing leakage of stored air (with pressures of more than 20 atm) using groundwater pressure surrounding an unlined cavern in hard rock. The air tightness of the rock around the cavern must be confirmed. In this study, the hydrogeology of the test site was examined prior to field air tightness tests in the borehole. The results indicate that, when evaluating the hydrogeology of the test site related to the air tightness of rocks, it is necessary to understand the geological structure and fracture characteristics of the site. This is done by means of a field survey, investigations and tests in and between the boreholes, and the examination of the distribution of permeability and pore water pressures.

Shidahara, T., Oyama, T., Nakagawa, K., Kaneko, K., and Nozaki, A. (2000). This paper is heavily focused on the geotechnical evaluation of a CAES site in Japan and was therefore not read thoroughly by the facilities team. The abstract is included here for reference.

Abstract: It is necessary for the rock mass that surrounds the excavated storage cavern to maintain its mechanical and hydrological properties in order to keep the stability and air tightness of the storage caverns of the operating compressed air energy storage (CAES) system. A case study for the CAES, which included drilling a borehole of 600 m in depth, was carried out on the Paleogene sedimentary rock that consisted mainly of conglomerate in northeast Kyushu, Japan. Elastic wave velocity corresponds to the sedimentary facies and the mineralogy in the rock matrix that are controlled by the sedimentary cycle. The mechanical properties can be inferred from physical properties such as elastic wave velocity. Hydrological properties such as permeability coefficient and pore water pressure are affected by the grain size distribution and the filling minerals in the rock matrix that are controlled by the sedimentary cycle. Carbonaceous shale and sandstone with abundant filling mineral (calcite), which are distributed at the top of the sedimentary cycle, are inferred to be impervious. It is assumed that the groundwater flow is divided into two zones based on the vertical change of pore water pressure and one of these zones overlies the impervious layers and the other underlies the impervious layers. Finally we concluded that sedimentological and mineralogical studies were required to evaluate the mechanical and hydrological properties of stratified sedimentary rocks. We propose a procedure for the geotechnical evaluation of sedimentary rocks that surround the CAES cavern.

CAES Progress Report – Facilities Team Appendix 2‐B 42 6. Economics

Fertig, E., and Apt, J. (2011) Economics of compressed air energy storage to integrate wind power: A case study in ERCOT. This paper is focused on an economic model developed for wind‐CAES integration and not particularly relevant for the facilities team. The abstract is included here for reference.

Abstract: Compressed air energy storage (CAES) could be paired with a wind farm to provide firm, dispatchable baseload power, or serve as a peaking plant and capture upswings in electricity prices. We present a firm‐level engineering‐economic analysis of a wind/CAES system with a wind farm in central Texas, load in either Dallas or Houston, and a CAES plant whose location is profit‐optimized. With 2008 hourly prices and load in Houston, the economically optimal CAES expander capacity is unrealistically large– 24 GW–and dispatches for only a few hours per week when prices are highest; a price cap and capacity payment likewise results in a large (17GW) profit‐maximizing CAES expander. Under all other scenarios considered the CAES plant is unprofitable. Using 2008 data, a baseload wind/CAES system is less profitable than a natural gas combined cycle (NGCC) plant at carbon prices less than $56/tCO2 ($15/MMBTU gas) to $230/tCO2 ($5/MMBTU gas). Entering regulation markets raises profit only slightly. Social benefits of CAES paired with wind include avoided construction of new generation capacity, improved air quality during peak times, and increased economic surplus, but may not outweigh the private cost of the CAES system nor justify a subsidy.

Greenblatt, J.B., Succar, S., Denkenberger, D.C., Williams, R.H., and Socolow, R.H. (2007) Baseload wind energy: Modeling the competition between gas turbines and compressed air energy storage for supplemental generation. This paper is focused on an economic model developed for wind energy baseload systems (wind and gas as well as wind and CAES). This paper is focused on an economic model and not particularly relevant for the facilities team. The abstract is included here for reference.

Abstract: The economic viability of producing baseload wind energy was explored using a cost‐optimization model to simulate two competing systems: wind energy supplemented by simple‐ and combined cycle natural gas turbines (‘‘wind+gas’’), and wind energy supplemented by compressed air energy storage (‘‘wind+CAES’’). Pure combined cycle natural gas turbines (‘‘gas’’) were used as a proxy for conventional baseload generation. Long‐distance electric transmission was integral to the analysis. Given the future uncertainty in both natural gas price and greenhouse gas (GHG) emissions price, we introduced an effective fuel price, pNGeff, being the sum of the real natural gas price and the GHG price. Under the assumption of pNGeff ¼ $5/GJ (lower heating value), 650W/m2 wind resource, 750km transmission line, and a fixed 90% capacity factor, wind+CAES was the most expensive system at b6.0/kWh, and did not break even with the next most expensive wind+gas system until pNGeff ¼ $9.0/GJ. However, under real market conditions, the system with the least dispatch cost (short‐run marginal cost) is dispatched first, attaining the highest capacity factor and diminishing the capacity factors of competitors, raising their total cost. We estimate that the wind+CAES system, with a greenhouse gas (GHG) emission rate that is one fourth of that for natural gas combined cycle plants and about one‐tenth of that for pulverized coal plants, has the lowest dispatch cost of the alternatives considered (lower even than for coal power plants) above a GHG emissions price of $35/tCequiv., with good prospects for realizing a higher capacity factor and a lower total cost of energy than all the competing technologies over a wide range of effective fuel costs. This ability to compete in economic dispatch greatly boosts the market penetration potential of wind energy and suggests a substantial growth opportunity for natural gas in providing baseload power via wind+CAES, even at high natural gas prices.

CAES Progress Report – Facilities Team Appendix 2‐B 43 7. Advanced CAES

Grazzini, G. and Milazzo, A. (2011) A thermodynamic analysis of multistage adiabatic CAES. The paper is focused on numerical simulation of adiabatic CAES. The work examines design of heat exchanger optimization. The work is quite detailed into the thermodynamics of the heat exchange relationships and equations. The work is more detailed than necessary for the IREE project but provides useful overview information on the potential of adiabatic advanced CAES.

Kim, Y.M., and Favrat, D. (2008) Energy and exergy analysis of a micro compressed air energy storage and air heating and cooling system. The work focused on analysis of small, micro‐CAES systems. The authors perform energy and exergy analysis of eight different configurations of CAES including adiabatic, quasi‐isothermal and various stages of each. The work is only slightly relevant to the IREE CAES project.

Kim, Y.M., Lee, J.H., Kim, S.J., and Favrat, D. (2012) Potential and evolution of compressed air energy storage: Energy and exergy analysis. This is a very comprehensive paper that provides a unique comparison evaluation of various advanced CAES approaches and compares with conventional, existing CAES designs. Evaluations are based on an exergy analysis. Explanation of the exergy analysis is provided and then the various scenarios are studied. This paper provides an encouraging perspective on the potential for advanced CAES as a viable and efficient storage technology for both large and small (distributed power) system.

Mohsen, S. and Li, P. (2012) Modeling and control of a novel compressed air energy storage system for offshore wind turbine. This paper describes an advanced approach that couples the wind turbine drivetrain with a micro CAES system. The approach has advantage both for the wind turbine itself and also for energy and power production. The paper describes enabling technologies including a liquid‐piston air compressor/ expander, and a hydrostatic drivetrain (for the turbine). The authors consider advanced control systems that would accompany the new topology and through the control the generation system greater efficiency, control of transient power, and pushes wind energy toward a baseload character.

Samaniego, F. (2010) Modeling of an Advanced Adiabatic Compressed Air Energy Storage (AA‐CAES) Unit and an Optimal Model‐based Operation Strategy for its Integration into Power Markets. This is a comprehensive graduate thesis focused on and advance adiabatic compressed air energy storage system.

8. Case Studies

Schulte, R.H., Critelli, Jr., N., Holst, K., and Huff, G. Sandia National Laboratories. (2012) Lessons from Iowa: Development of a 270 megawatt compressed air energy storage project in Midwest Independent System Operator. The paper provides a forensic study of the terminated CAES project located near Des Moines, Iowa. The Iowa Stored Energy Park was to be a 270 MW CAES facility and sought to use subsurface rock formation to store compressed air. The project was terminated because of unfavorable geologic conditions. The study was commissioned by the DOE Energy Storage Systems Program and contains valuable and insightful information on the project, project design, and lessons learned. The report is lengthy and comprehensive. The project never moved past the geological investigations; however, many insights go

CAES Progress Report – Facilities Team Appendix 2‐B 44 beyond this part of the design. The main sections of the report are: introduction, project history, economics (costs and economics studies), transmission, markets and tariffs, renewables policy and legislation, siting, project management, geology, and recommendations for follow‐on work.

Sirius Minerals, Dakota Salts. (2011) Compressed air energy storage feasibility in north Dakota. This comprehensive report is developed by Dakota Salt with collaborators Electric Power Research Institute (EPRI) and Schlumberger Water Services (SWS). The report is made of up separate reports from SWS, who conducted research on siting in North Dakota, and EPRI, who developed a North Dakota specific economic dispatch model. The report provides important insights into the importance of the underground works – in this case, the underground cavern was viewed as the most important aspect of the project proceeding surface facility design. The EPRI study suggests that they economics of the project are also key. The study examines MISO specific rules and policy as they pertain to the proposal for CAES in North Dakota. Appendix 2 of the report contains a MS PowerPoint presentation developed by EPRI that provides a wealth of information on how CAES is valued in MISO. This information is very relevant to the IREE study.

Weber, O. (1975) The air‐storage gas turbine power station at Huntorf. The paper provides an overview of the to‐be‐constructed Huntorf conventional CAES facility in Germany. The facility was commissioned in 1977 and so this paper was published during design. The authors provide a background on the design approach and then provide details on the important components of the Huntorf facility including the turbine, control, compressors, generators, fuel, cooling water, operational information, and the underground storage facility.

9. Reference Berman, P.A., 1979, Turbo‐machinery for CAS: International Power Generation, v. 2, no. 9, p. 45‐51.

Bullough, C., Gatzen, C., Jakiel, C., Koller, M., Nowi, A., and Zunft, S., 2004, Advanced adiabatic compressed air energy storage for the integration of wind energy: Paper from European wind energy conference, London, England.

Electric transmission lines and substations [Map], 2007, Minnesota Geospatial Information Office. Retrieved from http://www.mngeo.state.mn.us/maps/ElecTran07.pdf.

Fertig, E., and Apt, J. 2011, Economics of compressed air energy storage to integrate wind power: A case study in ERCOT: Energy Policy, v. 9, no. 5, p. 230‐2342. Retrieved from 10.1016/ j.enpol.2011.01.049

Fundamentals of Gas Turbine Engines. Retrieved from http://www.cast‐safety.org/pdf/ 3_engine_fundamentals.pdf.

Giramonti, A.J., and Lessard, R.D., 1974, Exploratory evaluation of compressed air storage peak‐power systems: Energy Sources, v. 1, no. 3, p. 283‐294.

Giramonti, A.J., Lessard, R.D., Blecher, W.A., and Smith, E.B., 1978, Conceptual design of compressed air energy storage electric power systems: Applied Energy, v. 4, p. 231‐249.

Grazzini, G., and Milazzo, A., 2011, A thermodynamic analysis of multistage adiabatic CAES: Proceedings of the IEEE, v. 100, no. 2, p. 461‐472.

CAES Progress Report – Facilities Team Appendix 2‐B 45 Greenblatt, J.B., Succar, S., Denkenberger, D.C., Williams, R.H., and Socolow, R.H., 2007, Baseload wind

energy: Modeling the competition between gas turbines and compressed air energy storage for supplemental generation: Energy Policy, v. 35, no. 1, p. 1474‐1492.

Kim, H.M., Lettry, Y., Park, D., Ryu, D.W., Choi, B.H., and Song, W.K., 2012, Field evaluation of permeability of concrete linings and rock masses around underground lined rock caverns by a novel in‐situ measurement system: Engineering Geology, v. 137‐138, p. 97‐106.

Kim, H.M., Rutqvist, J., Ryu, D.W., Choi, B.H., Sunwoo, C., and Song, W.K., 2012, Exploring the concept of compressed air energy storage (CAES) in lined rock caverns at shallow depth: A modeling study of air tightness and energy balance: Applied Energy, v. 92, p. 653‐667.

Kim, Y.M., and Favrat, D., 2008, Energy and exergy analysis of a micro compressed air energy storage and air heating and cooling systemP Paper from International refrigeration and air conditioning conference. Retrieved from http://docs.lib.purdue.edu/iracc/950.

Kim, Y.M., Lee, J.H., Kim, S.J., and Favrat, D., 2012, Potential and evolution of compressed air energy storage: Energy and exergy analysis: Entropy, v. 14, p. 1501‐1521.

Kushnir, R., Dayan, A., and Ullmann, A., 2012, Temperature and pressure variations within compressed air energy storage caverns: International Journal of Heat and Mass Transfer, v. 55, no. 21, p. 5616‐5630.

Langston, L.S. and Opdyke, Jr., G., 1997, Introduction to gas turbines for non‐engineers: Global Gas Turbine Newsletter, v. 37, no. 2, Retrieved from http://files.asme.org/IGTI/ Knowledge/Articles/13051.pdf.

Li, K.W., 1975, Compressed air storage in gas turbine systems: Journal of Engineering for Power, v. 62, no. 4, p. 640‐644.

Lienhard IV, J.H., and Lienhard V, J.H., 2012, A heat transfer textbook (4th ed.): Phlogiston Press, Cambridge, MA, 755 p.

Lindblom, U., 1989, The performance of water curtains surrounding rock caverns used for gas storage: International Journal of Rock Mechanics, Mineral Science, and Geomechanics, v. 26, no. 1, p. 85‐97.

McBride, Bollinger, and Kepshire, 2012, Mechanical energy storage: In Ginley, D.S., and Kahen, D. (eds.), Fundamentals of materials for energy and environmental sustainability: Cambridge University Press, New York, p. 628‐631.

Pfenninger, H. and Baden, 1975, Hydroelectric and compressed‐air pumped‐storage schemes: Brown Boveri Review, v. 62, no. 7/8.

Pockley, S., 2008, Compressed air energy storage. Retrieved from http://www.duckdigital.net/ Research/CAES.doc.

Rabbani, M., Dincer, I., and Naterer, G.F., 2012, Thermodynamic assessment of a wind turbine based combined cycle: Energy, v. 44, no. 1, p. 321‐328.

CAES Progress Report – Facilities Team Appendix 2‐B 46 Rutqvist, J., Kim, H.M., Ryu, D.W., Synn, J.H., and Song, W.K., 2012, Modeling of coupled thermodynamic

and geomechanical performance of undergound compressed air energy storage in lined rock caverns: International Journal of Rock Mechanics and Mining Science, v. 52, p. 71‐81.

Schulte, R.H., Critelli, Jr., N., Holst, K., and Huff, G., 2012, Lessons from Iowa: Development of a 270 megawatt compressed air energy storage project in Midwest Indpendent System Operator (SAND2012‐0388), Sandia National Laboratories, p. 96.

Sirius Minerals, Dakota Salts, 2011, Compressed air energy storage feasibility in North Dakota, 142 p.

Shidahara, T., Oyama, T., and Nakagawa, K., 1993, The hydrogeology of granitic rocks in deep boreholes used for compressed air storage: Engineering Geology, v. 35, no. 3‐4, p. 207‐213.

Shidahara, T., Oyama, T., Nakagawa, K., Kaneko, K., and Nozaki, A., 2000, Geotechnical evaluation of a conglomerate for compressed air energy storage: The influence of the sedimentary cycle and filling minerals in the rock matrix: Engineering Geology, v. 56, no. 1‐2, p. 125‐135.

Succar, S., Denkenberger, D.C., and Williams, R.H., 2012, Optimization of specific rating for wind turbine arrays coupled to compressed air energy storage: Applied Energy, v. 96, no. 1, p. 222‐234.

Succar, S., and Williams, R.H., 2009, Compressed air energy storage: Theory, resources, and applications for wind power: Informally published manuscript, Princeton Environmental Institute, Princeton University, Princeton, NJ.

Ter‐Gazarian, A.G., 2011, Energy storage for power systems (2nd ed.): The Institution of Engineering and Technology, London, England, p. 99‐120.

Weber, O., 1975, The air‐storage gas turbine power station at Huntorf: Brown Boveri Review, v. 62, no. 7/8, p. 332‐337.

Zaugg, P., 1975, Air‐storage power generating plants: Brown Boveri Review, v. 62, no. 7/8, p. 338‐347.

Zhongkui, L., Wang, K., Wang, A., and Liu, H., 2009, Experimental study of water curtain performance for gas storage in an underground cavern: Journal of Rock Mechanics and Geotechnical Engineering, v. 1, no. 1, p. 89‐96.

Zunft, S., Jakiel, C., Koller, M., and Bullough, C., 2006, Adiabatic compressed air energy storage for the grid integration of wind power: Paper from Sixth international workshop on large‐scale integration of wind power and transmission networks for offshore windfarms, Delft, The Netherlands.

CAES Progress Report – Facilities Team Appendix 2‐C 47

APPENDIX 2‐C – DRAFT FACILITY DESIGN SUMMARY

Internal Project Report

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Report Title: Design Specification of Existing and Prototype CAES Facilities.


Date: November 2013

By: Jeff Marr, Julie Cornell, and Perry Li

_____________________________________________________________________________________

CAES Progress Report – Facilities Team Appendix 2‐C 48 1. Summary This internal report provides a summary of the facility and technology characteristics for existing or prototype CAES facilities. The source of this information is from the published literature, website, and other communications. This report represents work completed as of November 2013, but we acknowledge that more information continues to be gathered on these systems.

2. Results

Facility Name: Huntorf CAES plant Location: Bremen, Germany Completion date: 1978 Power: 290 MW Duration at full power: 3 hours Depth of cavern: 650‐800 m Caverns: Two underground salt caverns with total of 310,000 m3 Operation pressures: 43 ‐ 70 bar Expanders Stage 1: 46 to 11 bar Stage 2: 11 bar to 1 bar

Facility Name: McIntosh Location: southwestern Alabama Completion Date: 1991 Power: 110 MW Duration at full power: 26 hours Depth of cavern: 1000 ft Cavern: single salt cavern total volume of 540,000 m3 Operating pressure: 45 ‐ 75 bar

Facility Name: Norton (proposed) Location: Norton, Ohio Power: 800 MW (2700 MW) Cavern: 9,600,000 m3 Operating pressure: 55‐110 bar

Facility Name: Iowa Storage Energy Park (proposed) Location: Dallas Center, Iowa Completion Date: Never completed, funding cut. Power: 270 MW

Facility Name: Dakota Salts (proposed) Location: Williston Basin, southeast of Beulah, North Dakota Power: 390 MW Duration at full power: ~50 hours

CAES Progress Report – Facilities Team Appendix 2‐C 49 Facility Name: ADELE (under construction) Location: Sachsen‐Anhalt, Germany Storage Capacity: 360 MW (~1000MWh) Discharge Capacity: 90 MW (~200 MWh) Discharge Duration: 5 hours

Facility Name: Apex CAES (beginning construction in 2014) Location: Tennessee Colony, Anderson County, Texas Output Capacity: 317 MW Depth: 3750 feet

Facility Name: Sustainx Location: Seabrook, New Hampshire Completion Date: September 11, 2013 Power: 1.5 MW

Facility Name: Lightsail Energy (funded startup, research phase) Location: Berkeley, CA Proposed Storage Capacity: under controlled conditions 100 kW Operating Pressure: 206 bar

Facility Name: General Compression R&D Location: Gaines, TX Implemented Location: Watertown, MA Completion Date: December 2012 Power: 100kW (multi‐stage); scaled from 2.4‐1000MW Discharge Duration: 8‐300 hours

9. Reference Fertig, E., and Apt, J., 2011, Economics of compressed air energy storage to integrate wind power: A

case study in ERCOT: Energy Policy, v. 9, no. 5, p. 230‐2342.

Giramonti, A.J., and Lessard, R.D., 1974, Exploratory evaluation of compressed air storage peak‐power systems: Energy Sources, v. 1, no. 3, p. 283‐294.


Kim, H.M., Lettry, Y., Park, D., Ryu, D.W., Choi, B.H., and Song, W.K., 2012, Field evaluation of permeability of concrete linings and rock masses around underground lined rock caverns by a novel in‐situ measurement system: Engineering Geology, v. 137‐138, p. 97‐106.

CAES Progress Report – Facilities Team Appendix 2‐C 50 McBride, Bollinger, and Kepshire, 2012, Mechanical energy storage: In Ginley, D.S., and Kahen, D. (eds.),

Fundamentals of materials for energy and environmental sustainability: Cambridge University Press, New York.

Pfenninger, H. and Baden, 1975, Hydroelectric and compressed‐air pumped‐storage schemes: Brown Boveri Review, v. 62, no. 7/8.

Pockley, S., 2008, Compressed air energy storage. Retrieved from http://www.duckdigital.net/ Research/CAES.doc.

Schulte, R.H., Critelli, Jr., N., Holst, K., and Huff, G., 2012, Lessons from Iowa: Development of a 270 megawatt compressed air energy storage project in Midwest Indpendent System Operator (SAND2012‐0388): Sandia National Laboratories, p. 96.

Sirius Minerals, Dakota Salts, 2011, Compressed air energy storage feasibility in North Dakota, 142 p.

Succar, S., and Williams, R.H., 2009, Compressed air energy storage: Theory, resources, and applications for wind power: Informally published manuscript, Princeton Environmental Institute, Princeton University, Princeton, N.J., p. 81.

Ter‐Gazarian, A.G., 2011, Energy storage for power systems (2nd ed.): The Institution of Engineering and Technology, London, England, p. 99‐120.

Weber, O., 1975, The air‐storage gas turbine power station at Huntorf: Brown Boveri Review, v. 62, no. 7/8, p. 332‐337.

Zaugg, P., 1975, Air‐storage power generating plants: Brown Boveri Review, v. 62, no. 7/8, p. 338‐347.

CAES Progress Report – Geotechnical Engineering Team 51

3. CAES PROGRESS REPORT – GEOTECHNICAL ENGINEERING TEAM

Carlos Carranza‐Torres

3.1. INTRODUCTION

This report summarizes the work done in the first year of the project ‘Compressed Air Energy Storage (CAES) in Northern Minnesota Using Underground Mine Working’ (Fosnacht, 2012) and addresses some first key questions related to geotechnical aspects of the system, in particular, size and shape of required underground cavities and re‐use of existing mining workings in Northern Minnesota. The report is structured in five sections. After this introduction, Section 3.2 presents a literature review of CAES systems (with focus on geotechnical engineering aspects of the system), and also includes a brief technical description of existing and planned CAES plants as found in published literature. Section 3.3 outlines the main considerations to take into account for designing underground excavations for CAES plants. Section 3.4 presents the references cited in this report. Finally, Section 3.5 includes a set of three PowerPoint presentations on various topics (mostly with a geotechnical engineering focus) that have been developed as part of the project, to present advances of the research and promoting discussions within the team during the scheduled meetings for the project.

3.2. LITERATURE REVIEW OF CAES SYSTEMS WITH FOCUS ON GEOTECHNICAL ENGINEERING ASPECTS

A CAES plant, which is generally associated to wind turbines, compresses air when there is an excess of electric energy production in the grid and generates electric energy using a turbine when the demand exceeds the production. The storage of compressed air to produce energy in this way is typically done in underground chambers, which could be existing cavities, e.g., mining rooms, drifts, and shafts, or cavities created with this specific purpose, e.g., in salt formations, new cavities can be relatively easily created by dissolution, although the dissolution process can last for more than a year.

According to Cavallo (2007), the air storage needs not only depend on the pressure of the air to be stored, but also on the power of the plant and the amount of time the compressed air will be expanded, e.g., circulating through turbines, and generating electricity. Cavallo (2007) mentions 200 MW of power for a CAES plant as a reasonable target to achieve, and although he does not mention the lapse of time this power is to be maintained, e.g., the energy to be produced, nor the pressure at which the air will stored, he indicates a rough value of air storage need as being 1,000,000 m3 for a 200 MW target plant.

In terms of the pressure at which the air is to be stored, Li et al. (2011) mentions the value of 7 MPa (or 70 bar) as a value to consider.

According to Denholm and Sioshansi (2009), who have studied the CAES systems from an economical point of view mainly, air storage capacity should be such to be able to maintain discharge for 20 hours at nominal power, e.g., basically the longest possible throughout a day). Also, considering that less time is needed to charge than to discharge the air (the authors mention a typical charge‐discharge ratio of 0.72), Denholm and Sioshansi (2009) recommend using a series of small air compressors to get more flexibility in the use of excess of energy that drives the compressors and so to optimize this excess energy and have less idle lapses in the production of energy by CAES.


Succar and Williams (2008) present a comprehensive treatment of the CAES system. The report by Succar and Williams not only addresses critically important factors to account in the design of CAES system but also presents a review of existing and planned CAES plants. In what follows, the description of actual and planned CAES plants, as outlined in their publication, will be summarized first, followed by the technical considerations presented in their report.

Succar and Williams (2008) describe first the two existing (functional) CAES plants: 1) the Huntorf power plant, near Bremen, Germany; and 2) the McIntosh power plant in McIntosh, Alabama.

The Huntorf plant has been operating since 1978. Originally designed to generate 290 MW, the capacity was increased in 2006 to generate 321 MW of electricity. The plant is capable of sustaining that power for three hours by storing air in two cavities with a total storage volume of 310,000 m3. The cavities were excavated by dissolution method in a salt formation. The cavities do not have lining (‘rock salt,’ or halite, is a rock that creeps in time and is an ideal self‐sealing, basically impermeable rock to host compressed air without the need of lining). The working pressure of the air stored in the chambers varies from 66 to 48 bar (6.6 to 4.8 MPa). The turbine that generates electricity works at 46 bars, so the stored air pressure has to be throttled to this pressure (2 bars of air pressure is mentioned to be lost in throttle and pipes). The report by Succar and Williams (2008) also mentions that in the first years of operation of the plant, the air reacted with the salt in the cavity and oxidized the piping; this resulted in the need of replacing the original metal pipes in the plant (those in contact with the compressed air) by fiber‐glass reinforced plastic pipes.

The McIntosh power plant in McIntosh, Alabama, has been operating since 1991 and is reported to produce 110 MW of electricity. The plant is capable of sustaining that power for 26 hours by storing air in a single chamber of 560,000 m3 (as in the case of the Huntorf plant, the chamber was excavated in a salt formation by the dissolution method, and it does not have lining). The working pressure of the air stored in the chamber varies from 74 to 45 bar (7.4 to 4.5 MPa).

Succar and Williams (2008) also describe two planned CAES plants that have not been built: 1) the Norton CAES plant, in Norton, Ohio; and 2) the Iowa Stored Energy Park, northwest of Des Moines, Iowa.

The Norton CAES plant in Ohio is planned to produce 2,700 MW of electricity, after final expansion. In contrast with the Huntorf and McIntosh plants, an abandoned limestone mine (that was excavated using room and pillar method) will be used to store 9,600,000 m3 of compressed air. The working air pressure ranges between 110 and 55 bars (11 and 5.5 MPa). The construction of the project (owned by First Energy since 2009) is being delayed due to economic reasons. Current low power prices and insufficient demand are mentioned as the causes of the delay in construction.

The Iowa Stored Energy Park was planned to produce 268 MW of electricity. The compressed air was to be stored in an aquifer in porous rock. It was originally planned to be finished and to start operation in 2011. The project was halted during the phase of design. A publication by Schulte et al. (2012) summarizes the eight‐year development of the Iowa Stored Energy Park project and discusses in detail the reasons of the failure of the project. They mention that after many field surveys and after spending considerable amount of money in air injection in‐situ testing, it was determined that the porous rock did not have the required permeability to properly transmit the compressed air within the aquifer. Due to this, and due to the lack of an alternative suitable location for the air storage, the project was terminated.


A report by Rettberg and Holridge (2012) describes another case of a failed CAES project. This project, known as Seneca Lake CAES project near Watkins Glen, New York, was planned to produce between 130 and 210 MW of electricity using a cavern in a salt formation (mined by dissolution) with a storage capacity of 150,000 m3. The range of air pressure was initially considered to vary from 10 to 5 MPa for a period of discharge time of 10 to 12 hours (the charging period was designed to be 8 hours). The system was designed to complete 260 (charge/discharge) cycles per year. Preliminary thermo‐dynamic and geo‐mechanical studies showed that the difference in pressure would produce a change of temperature during charge/discharge cycles of 45oC, with temperatures on the walls of the cavity ranging between 60oC (or 140oF) and 15oC (or 60oF). This high change in temperature was expected to produce spalling of the walls of the cavern (deterioration of the rock due to repeated change from compression to tension), and therefore significant modifications of the original design had to be implemented. Among them, the limiting operating pressures were changed to 10 to 8 MPa, maintaining the original planned size of the cavern (for the rock conditions, the volume of 150,000 m3 was established to be the maximum volume possible). The reduction in differences in pressure and the maximum size of a cavern dictated an increase in the total storage of air needed to produce the target energy for the plant. This increase resulted in the need to construct now three caverns, each of them with the originally planned total volume of 150,000 m3 e.g., air storage requirements tripled in the revised design. The radical changes in the design resulted in a significant increase in the cost for building the plant, making the CAES plant economically unfeasible.

The comprehensive report by Succar and Williams (2008) discusses in depth the technical requirements of air storage for a CAES plant. In particular, the report presents equations and diagrams from where volumes of air storage can be computed as a function of the energy to generate, and as a function of the working pressure of the air. The capacity of air storage depends on the mode in which the air will be stored and retrieved, and the pressure at which the air will enter the turbine. In terms of the pressure at which air will be stored, two possibilities exist: 1) constant pressure storage; and 2) constant volume storage.

In the constant pressure storage (or constant cavern pressure) option, the air in the chamber is compressed by hydrostatic pressure from a water reservoir located on the surface. To get the target working pressure, the depth of the chamber is chosen so as to get the correct hydrostatic pressure to compress the air. Figure 3‐1 illustrates the concept. Succar and Williams (2008) mention that one of the problems to address in the constant pressure scheme is the avoidance of the ‘champagne effect’ by which water could raise through the reservoir shaft and lead to unstable loss of head and blowout of the cavern (Giramonti et al., 1978). The case of constant pressure storage described above is also referred to as Case 1 later in this report.


Figure 3‐1. Constant pressure CAES storage with surface reservoir and compensating water column

(after Succar and Williams, 2008).

In the constant volume storage (or variable cavern pressure) option, the pressure of the air falls during extraction from the chamber. This pressure‐decreasing air can be input into the turbine directly, e.g., using a variable‐pressure turbine or, otherwise, throttled to the working pressure of a (constant‐pressure) turbine. These two possibilities are referred to as Cases 2 and 3, respectively, in the text below.

Of the two options described above, the first one (constant cavern pressure) requires less volume of air storage than the first one (variable cavern pressure). Nevertheless, the second option is ‘more flexible’ than the first, in that it does not require a reservoir on the surface nor a specific depth of the chamber in relation to the reservoir. This may be the reason why existing and planned CAES systems described earlier on favored the use of the second option, e.g., these plants do not use a water reservoir to maintain a constant pressure of the air within the caverns.

Succar and Williams (2008) present equations to compute the storage volume requirements. They distinguish three cases, namely: Case 1, corresponding to constant pressure storage option; Case 2, corresponding to the constant volume storage option and variable turbine inlet pressure; and Case 3, also corresponding to constant volume storage, but considering a constant turbine inlet pressure. The results from these equations are summarized in the diagram of Figure 2.

In the diagram of Figure 3‐2, the horizontal axis represents the maximum pressure of the air (in bars) at which the storage chamber will operate, while the vertical axis represents the so‐called density of energy, which is the amount of stored energy (in kWh) per unit of storage volume (in m3). As discussed earlier, there are three cases of operation of the plant, e.g., Cases 1, 2 and 3, and these are represented by curves with different lines in the diagram, namely, dash‐dot‐dash line for Case 1 (upper most line in the diagram), continuous line for Case 2, and dashed line for Case 3. The curves for Cases 2 and 3 are grouped into pairs corresponding to the ratio of initial‐to‐final pressure during operation, e.g., the ratio ps2 and ps1. It is seen from the diagram that the amount of density of energy provided by Case 2 is


slightly larger than that for Case 3. The inset diagram on top of the main diagram in Figure 3‐2 represents the loss of energy (in percent) for Case 3 as compared with Case 2. From this diagram it is seen, in particular, that the throttling losses become relatively small, e.g., below 10%, for large values of initial pressures, e.g., ps2 > 60 bar. Because the penalty for not throttling the pressure in Case 2 is offset by the benefits of higher turbine efficiency and simplified system operation in Case 3, it is more beneficial to operate a CAES plant as in Case 3, e.g., using constant turbine inlet pressure), as exemplified by the Huntorf and McIntosh plants, both of which use this mode of operation.

To illustrate the use of the diagram in Figure 3‐2, an example is presented in the lower right corner of the figure. The objective of the example is to show how the capacity of storage can be simply obtained from the diagram for a proposed CAES plant when the power of the plant, the time the power will be sustained, the working pressures, and whether a variable turbine inlet pressure (Case 2) or a constant turbine inlet pressure (Case 3), are known.

Also, to illustrate the use of the diagram in Figure 3‐2 in the context of the existing and planned CAES projects described previously, Figure 3‐3 is the same diagram but where the existing plants (Huntorf and McIntosh) and another planned plant plots (the text added at the bottom of the diagram in Figure 3‐3 summarizes the computations needed to find the ordinates of the points representing these cases, namely A, B, and C, respectively). This exercise shows that the points that represent the CAES plants align satisfactorily well within the theoretical expected positions, and therefore illustrates a first estimate of the storage volume required to develop a CAES system at least roughly, by application of equations and/or diagram presented in Succar and Williams (2008). The exercise also suggests that the energy generated by existing and planned plants is significantly below the ‘ideal,’ e.g., maximum, energy that a constant pressure storage option (Case 1) could provide. Considering that in the mining district of northern Minnesota there exists a significant number of lakes formed in abandoned mine pits, a question that arises from the exercise in Figure 3‐3 (in particular the relative position of points A, B and C with respect to the curve for Case 1) is whether the advantage of having artificial lakes above underground mining works could be used to target the development of a constant pressure storage option to boost the production of the envisioned CAES system in northern Minnesota.


Figure 3‐2. Diagram summarizing the relationship between generated energy, storage volume, upper and lower storage pressure and operation case (Cases 1, 2 or 3) described in the main text. The insert (point P and associated text) represents an example of use of the diagram as described also in the main text. After Succar and Williams (2008).


Figure 3‐3. Same diagram as in Figure 2, including the position of the two existing CAES plants (Huntorf and McIntosh) and one of the planned plants (Seneca), described in the main text.


3.3. GEOTECHNICAL CONSIDERATIONS FOR THE DESIGN OF UNDERGROUND EXCAVATIONS FOR A CAES PLANT

When defining the potential location for a CAES plant, the type of air storage method is probably the most important factor to consider. There are two types of storage possibilities, namely: 1) surface storage; and 2) underground storage. A surface storage can be implemented by using large steel or concrete tanks (which tend to have a negative visual impact and tend to be expensive in terms of land use and construction). Alternatively, it can be implemented using large submerged ‘air bags,’ as proposed by Ter‐Gazarian (1994); this is perhaps an option that could be further explored for Northern Minnesota, considering the large number of existing lakes, some of them in the form of abandoned open pit mines, and also the existence of Lake Superior. Underground air storage can be implemented excavating in salt deposits or depleted gas or oil fields (both of which, unfortunately, do not exist in Minnesota) or aquifers, which require very specific geologic requirements, as demonstrated by the failed Iowa Stored Energy Park project described in the previous section. Finally, an underground storage option can be implemented by the use of existing or abandoned mines or by excavating a new cavern for that purpose. This underground storage option will be considered in the remainder of this section.

The volume of air storage is another important factor to consider in the conception of a CAES system. As seen in the previous section, this volume is related to other critically important variables such as the nominal power of the plant and the discharging time (the multiplication of these two variables defining the amount of energy produced by the plant), and the range of operating pressures and the type of system used, e.g., Cases 1, 2 or 3 described in Section 3‐2 and Figure 3‐2.

During discussions with team members of this project, a target power of 100 MW to be sustained for at least 10 hours was envisioned for a CAES system in northern Minnesota. Considering: 1) a typical average working pressure of 70 bars (or 7 MPa); 2) a ratio of initial‐to‐final pressure equal to 1.4, e.g., choosing the more ‘flexible’ constant volume storage option and without distinguishing at this stage on whether Cases 2 or 3 would be chosen; and 3) taking as a basis the very same example worked out in Figure 3‐2 (considering 10 hours instead of 5 hours), the equations/diagram in Succar and Williams (2008) suggest the need of approximately 300,000 m3 of air storage capacity (the actual volume would be 333,333 m3 but, for simplicity, the figure will be rounded off to 300,000 m3).

To put this amount of volume (300,000 m3) into perspective, Figure 3‐4 shows a cross‐section of an existing shaft of an abandoned iron mine on the Cuyuna Range in central Minnesota (cross‐sections of mining drifts, particularly those that were used for access and ore transportation could be expected to have similar cross‐section areas). Without subtracting the space occupied by the support, the area of the cross‐section in Figure 3‐4 is 13.54 m2. Therefore, for getting 300,000 m3 of air storage using these shafts or tunnels, a total length of shaft (or tunnel) of 22,160 meters (or 72,703 feet) will be required (see computations on the right side of Fig. 3‐4). Although this length of tunneling is not uncommon in large mines (and probably could be found in some abandoned mines of central and northern Minnesota, after dewatering them), and given the amount of volume involved, it seems worth exploring the possibility of using not only existing mining excavations but new openings that could be created with the objective of air storage.

With this purpose, in what follows, a general discussion about technical considerations for storage options, applicable to both existing and new excavations, is provided.


Figure 3‐4. Cross sectional area of a mining shaft of the abandoned Cuyuna mine in central Minnesota. Drawing provided by J. Oreskovich.

Kovari (1993) discussed some basic considerations for gas storage in rock cavities. Although the considerations presented in the article by Kovari are oriented towards the storage of natural gas (for both compressed and liquefied cases) in new cavities that will be excavated for this particular purpose, many of the considerations are still valid for the case of a CAES system and so they will be reviewed below.

According to Kovari (1993), cross‐sectional shapes of cavities should be preferably circular, although if the long‐axis of the cavern is horizontal, some practical disadvantages arise for circular shapes, e.g., excavation equipment operates efficiently on planar surfaces, and typically a compromise is reached by using a horizontal floor and curved walls and roof. A horse‐shoe or truncated elliptical cross‐section, as shown by the two sketches below in Figure 3‐5, are possible options.


Figure 3‐5. Possible shapes of cross sections for CGES (Compressed Gas Energy Storage) openings – adapted from Kovari (1993).

Also in regard to the shape of the opening, according to Kovari (1993), the volume‐to‐surface ratio for the opening is another important factor to consider. The larger the volume‐to‐surface ratio, the smaller the amount of reinforcement and/or support needed for the same storage volume. The best option would be a spherical cavity (the geometric construction with largest volume‐to‐surface ratio), but for construction reasons, e.g., need to have planar floors for equipment to work on efficiently, this shape of cavity is difficult to implement. The next option includes a toroid shape, but this shape can lead to hoop tensile stresses on the walls closest to the center of the toroid, e.g., the inner core with negative curvature, and so a toroid shape has to be discarded as well. From a construction point of view, a cylindrical opening, e.g., with truncated or slightly curved floor as needed, is the next ideal shape. Then, with reference to targeting to have a large volume‐to‐surface ratio for the cavity, a cylindrical (or truncated cylindrical shape) with a ratio of diameter‐to‐length (of cylinder) closest to one, seems to be the best alternative.

The next consideration is whether the openings have to be chambers (or tunnels) or shafts (the difference between chambers‐or‐tunnels and shafts is that the former have axes that are horizontal, or near horizontal, while the latter have vertical, or near vertical). According to Kovari (1993), chambers or tunnels tend to have a large floor surface area; therefore, accumulation of sediment and/or bacterial growth is a possibility. In this regard, shafts seem to be a better alternative. Also chambers/tunnels have a large roof area, which will be a matter of concern in that reinforcement or support will be necessary in cases of rock masses with average‐to‐poor quality (although in the case of rock masses with very good quality, chambers or tunnels without support nor reinforcement would be possible as well; in these cases, using chambers/tunnels or shafts would not make a difference in terms of required support). Shafts do have the advantage that they can be constructed with circular shape (maintaining a flat floor for excavation equipment to work on), although shafts have the disadvantage that they are more time consuming and more expensive to build than tunnels of similar shape and volume. In view of what is mentioned above, for rocks of very good quality, as could be expected in the iron‐formations or in the Duluth Complex in northern Minnesota, there seems not be a marked difference in using chambers (or tunnels) or shafts as storage openings.


According to Kovari (1993), another consideration is whether to use a single cavity or a group of cavities. In hard rock, even under extreme favorable conditions, the cross‐sectional area of cavities seldom exceeds 800 to 1,000 m2 – this would correspond to diameters of 30 to 35 m if circular cross‐sections are considered. For example, for the volume of 300,000 m3 mentioned previously, which was envisioned for a CAES plant in northern Minnesota, a cavern with a typical section of 600 m2 yields a length of 500 m (or 1640 ft.), which, although still far from having a diameter‐to‐length (of cylinder) ratio close to one, as suggested by Kovari (1993), would at least diminish the amount of length needed in comparison with the shaft section discussed in Figure 3‐4. With regard to the required length of cavern of 500 m computed above, having 3 caverns of length 167 m each or, alternatively, 4 caverns of length 125 m each would be a better option than having a single longer cavern (of 500 m). This is because with various caverns used to store air, there exists the opportunity of performing maintenance operations in a chamber without having to stop the entire plant. Also, a group of cavities would also allow to increase progressively the capacity of storage and, therefore, the capacity of the plant.

Another fundamental aspect to consider in the conception of a CAES system is whether the caverns will be lined or unlined. This particular topic is still being studied at the time of writing this progress report, and for this reason the issue will be treated only partially.

In the context of storing compressed natural gas, Sofregaz and LRC (1999) states that the term “lining” refers to the need to install a barrier to stop leakage of gas (or air) into the rock mass. The term lining does not refer to support, e.g., concrete rings or steel sets applied on the walls of the opening, or reinforcement, e.g., cables or rockbolts, that are typically installed in underground openings such as tunnels, caverns and shafts with the purpose of maintaining structural stability of the opening. In this regard, it could be mentioned that (at least at first sight) underground openings in typically hard rocks of Minnesota’s three iron‐formations or the Duluth Complex would not impose the demand of significant (if any) support or reinforcement to guarantee the structural stability of the openings.

With the understanding that lining refers to that layer of material, e.g., steel, concrete or plastic, installed on the periphery of the cavern to avoid leakage of air/gas, Sofregaz US Inc. (1999) mention that the liner should be designed mainly to withstand the internal pressure of the gas stored, and also to withstand straining that will be expected to occur when the surrounding rock mass undergoes minor movements, e.g., due to slipping along jointing or similar, due to the repeated cycles of loading and unloading that the cavern will be subject to during its service life (see also Rutqvist et al., 2012).

According to Sofregaz US Inc. (1999), lining for underground gas storage usually consists of thin steel plates inside the cavern. This steel lining is welded in place and the void between the rock and the steel plates filled with concrete. Because the interface between the concrete and steel is supposed to accommodate movement, a viscous layer or film derived from petroleum is typically installed behind the steel plates before backfilling the void between steel and rock with concrete.

Sofregaz US Inc. (1999) also mention that a synthetic membrane could be used instead of the steel lining. In such case, attention must be given to the fact that some of the condensates contained in natural gas may soften plastic materials, and therefore long‐term imperviousness could not be guaranteed. This problem may not be such in the case of compressed air storage for a CAES plant. Considering that using a synthetic membrane in place of a steel layer could bring down the costs for lining the opening, this option should be investigated for the case of an air storage for a CAES plant, if the option of using lining will be considered to prevent leakages.


Another important factor to consider in the design of the cavity is its depth relative to the ground surface. The selection of the depth also depends on whether the cavity will be lined or unlined, as briefly explained below.

In general, lined caverns can be emplaced at shallow depths provided the balance between the uplift force generated by the internal pressure and downward force associated with the weight of the rock above the cavern is such that uplift is prevented (considering an appropriate factor of safety, as normally done in geotechnical design). Figure 3‐6 illustrates the concept of determining the depth of lined caverns using this approach. When considering the weight of the material above the cavern, a conical ‘wedge’ with lateral sides inclined 30 to 45o with respect to the vertical are typically considered (Sofregaz US Inc., 1999 and Rutqvist et al., 2012). It should be emphasized that the approach represented in Figure 3‐6 is a simplistic approach that among others does not consider the initial stresses existing in the ground. For example, from laboratory and numerical experiments, Tunsakul et al. (2013) found that the angle formed by the sides of the conical wedges and the horizontal at the time of failure can actually vary between 0 and 90o. In view of this, more detailed analyses, e.g., using finite element models which account for all mechanical variables involved in the problem, should be preferred.

Figure 3‐6. Determination of depth of emplacement of a shallow lined cavern for gas/air storage.

Caverns can also be constructed without liner (see Blindheim et al., 2004; and Kovari, 1993).

In the case of unlined caverns, leakage can be prevented by choosing a depth such that the hydrostatic pressure of the water surrounding the cavern is equal or greater than the pressure at which the gas is to be stored (see Fig. 3‐7). In this concept, when the hydrostatic pressure of the water exceeds the pressure of the gas (a case that will indeed occur during operation), water will enter the cavity and will have to be collected at the bottom of the cavern and periodically pumped out of the cavern. Also, due to


the seepage of water into the cavern, attention will have to be placed on the groundwater table, with particular regard to the depression of the water table that will tend to develop and which will influence the initially predicted values of hydrostatic pressure at the level of the cavern.

An alternative way of preventing leakage, e.g., when no phreatic surface exists or when smaller depths than dictated by a phreatic surface under hydrostatic conditions is desired for the caverns, is to use water curtains (see Section 5, Groundwater/Environmental Team report). In this concept, represented in Figure 3‐8, water is injected under pressure (equal or greater than the pressure at which the gas is to be stored) in water galleries which surround the cavity. This system requires constant pumping of water during operation and is more expensive than the one discussed earlier on (see Fig. 3‐7).

Figure 3‐7. Determination of depth of emplacement of an unlined cavern for gas/air storage.


Figure 3‐8. a) Cross sections of unlined caverns with two water curtains configurations: umbrella (left) and circumventing (right) configurations – from Kovari (1993); and b) plan view of air cushion surge unlined chamber with water infiltration – from Blindheim et al. (2004).

3.4. REFERENCES

Blindheim, O.T., Broch, E., and Grov, E., 2004, Gas Storage in unlined caverns ‐ Norwegian experience over 25 years: Tunnelling Underground Space Technol., v. 19, no. 4‐5, p. 367. Available online: http://www.ctta.org/FileUpload/ita/2004/data/abs_c25.pdf.

Cavallo, A., 2007, Controllable and affordable utility‐scale electricity from intermittent wind resources and compressed air energy storage (CAES): Energy, v. 32, no. 2, p. 120‐127.

Denholm, P., and Sioshansi, R., 2009, The value of compressed air energy storage with wind in transmission‐constrained electric power systems: Energy Policy, v. 37, no. 8, p. 3149‐3158.

http://www.ctta.org/FileUpload/ita/2004/data/abs_c25.pdf


Fosnacht, D.R., 2012, Compressed Air Energy Storage (CAES) in Northern Minnesota Using Underground Mine Working. Project proposal submitted to the Initiative for Renewable Energy and the Environment, Institute on the Environment, University of Minnesota.


Kóvari, K., 1993, Basic consideration on storage of compressed natural gas in rock chambers: Rock Mechanics and Rock Engineering, v. 26, no. 1, p. 1‐27.

Li, P. Y., Loth, E., Simon, T.W., Van de Ven, J.D., and Crane, S.E., 2011, Compressed air energy storage for offshore wind turbines: Proceedings of the 2011 International Fluid Power Exhibition (IFPE), Las Vegas, NV, March, 2011. Available online in the NRRI website: http://www.nrri.umn.edu/ egg/REPORTS/CAES/References/Lietal.pdf.

Rettberg, J.W., and Holdridge, M., 2012, Seneca compressed air energy storage (CAES) project: Final phase 1 technical report, National Energy Technology Laboratory.

Rutqvist, J., Kim, H‐M., Ryu, D‐W., Synn, J‐H., and Song, W‐K., 2012, Modelling of coupled thermodynamic and geomechanical performance of underground compressed air energy storage in lined rock caverns: International Journal of Rock Mechanics & Mining Sciences, v. 52, p. 71‐81.

Schulte, R.H., Critelli, N., Holst, K., and Huff, G., 2012, Lessons from Iowa: Development of a 270 MW compressed air energy storage project in Midwest independent system operator: A study for the DOE energy storage systems program, Sandia report SAND2012‐0388.

Sofregaz US Inc., 1999, Commercial potential of natural gas storage in lined rock caverns (LRC): Topical report prepared for the U.S. Department of Energy, 101 p.

Succar, S., and Williams, R.H., 2008, Compressed air energy storage: Theory, resources and application for wind power: Report for the Energy Systems Analysis Group.

Ter‐Gazarian, A., 1994, Energy storage for power systems: In Peter Peregrinus Ltd. (ed.), Chapter 7: Compressed air energy storage: Hert, United Kingdom, p. 99‐120.

Tunsakul, J., Jongpradist, P., Kongkitkul, W., Wonglert, A., and Youwai, S., 2013, Investigation of failure behavior of continuous rock mass around cavern under high internal pressure: Tunnelling and Underground Space Technology, v. 34, p. 110‐123.

3.5. POWERPOINT PRESENTATIONS (DRAFT VERSIONS) DEVELOPED FOR DISCUSSION OF GEOTECHNICAL ENGINEERING ASPECTS OF UNDERGROUND EXCAVATIONS FOR CAES PLANTS

Appendix 3‐A presents a set of three PowerPoint presentations that have been developed as part of the project, to present advances of the research and promoting discussions within the team during the scheduled meetings for the project. The titles of the presentations are as follows:

http://www.nrri.umn.edu/egg/REPORTS/CAES/References/Lietal.pdf

http://www.nrri.umn.edu/egg/REPORTS/CAES/References/Lietal.pdf


- Presentation 1 (of 3): Initial literature review about general geotechnical aspects of CAES plants.

- Presentation 2 (of 3): Geotechnical considerations dictating the design of underground excavations for CAES plants.

- Presentation 3 (of 3): Compiled bibliography (geotechnical engineering focus).

CAES Progress Report – Geotechnical Engineering Team Appendix 3‐A 67

APPENDIX 3‐A – POWERPOINT PRESENTATIONS

CAES Progress Report – Geology Team 76

4. CAES PROGRESS REPORT – GEOLOGY TEAM

Julie Oreskovich

4.1. OVERVIEW

Work of the Geology Team has been focused on locating documentation of underground mining activity on the Cuyuna, Mesabi, and Vermilion Iron Ranges of east central and northeastern Minnesota. Existing underground workings hold potential as repositories for compressed air energy storage (CAES). These workings are likely flooded with water, which could prove beneficial in terms of providing a water curtain for compressed air containment.

Literature searches were conducted for geologic and mine‐specific references. Map collections were searched for evidence of underground mining features. Mine maps were scanned and geo‐referenced, and underground mining features were digitized. Since much of this work had been done previously through the Minnesota Department of Natural Resources (MNDNR) for the Mesabi Range, particular attention was paid to the Cuyuna and Vermilion Ranges.

In addition, initial investigations were made relative to creating an underground cavern for CAES (Geology Team Objective 4). This was primarily a literature search.

4.2. INTRODUCTION

Underground workings for the central and eastern portions of the Mesabi Range, as well as a small portion of the western Mesabi Range, were mapped through the Minnesota Department of Natural Resources (MNDNR) Underground Mine Mapping Project (2006‐2011). These can be viewed and are available in digital format on the MNDNR web site (http://www.dnr.state.mn.us/lands_minerals/ underground/index.html) or can be obtained from the MNDNR Division of Lands and Minerals Office in Hibbing, MN (contact Dale Cartwright: [email protected]).

This investigation was extended beyond the Mesabi Range to include mines of the Cuyuna and Vermilion Ranges (Fig. 4‐1). Important to this determination was proximity to major electric transmission lines. Figure 4‐2 illustrates the location of Great River Energy and Minnesota Power transmission lines and electrical substations relative to the Cuyuna Range. It also shows the two districts of the Cuyuna Iron Range (North Range and South Range) as well as the Emily District.

Mapped underground workings for the remainder of the western Mesabi Range and the Cuyuna and Vermilion Ranges, with the exception of the Soudan Mine, have not been previously compiled and digitized. Investigation of CAES potential for these areas began with a search of public and private collections for maps and documents related to underground mining. Maps and features were and are being made digital and worked with within a Geographic Information System (GIS) framework. The mapping platform used for this project is ESRI® ArcGIS ArcMap™ 10.1. Progress to date is reported below.

http://www.dnr.state.mn.us/lands_minerals/underground/index.html

http://www.dnr.state.mn.us/lands_minerals/underground/index.html

mailto:[email protected]


Figure 4‐1. CAES location map.


Figure 4‐2. Cuyuna Iron Range.

4.3. OBJECTIVE 1: Locate underground workings, including maps, on the Mesabi, Cuyuna/Emily, and Vermilion Iron Ranges for potential use as CAES storage caverns.

4.3.1. Progress/Findings to Date

4.3.1.1. Mesabi Range

A GIS shape file of the underground workings on the Mesabi Range has been compiled from previous work done at MNDNR Lands and Minerals in Hibbing, MN. Additional maps for a potential site, the Utica Extension Mine, were obtained from Great Northern Iron Ore Properties in Hibbing. This particular property sits within designated mineland west of Hibbing and south of Hibbing Taconite’s pit. It is surrounded by extensively mined (underground) land that may preclude future mining activity by


Hibbing Taconite (Fig. 4‐3). The shaft area is visible from aerial photos. This site consists of upper and lower main drifts that extend west and then north from the shaft. These drifts could potentially be isolated from a second set of main drifts on the same levels that extend north from the shaft and connect with workings to the north and east. There has been no apparent working from the drifts extending to the west.

The Mesabi Range poses a problem in siting a potential CAES in existing underground workings as the shafts were typically put down off the natural ore body but still within the iron‐formation. Technological advances are extending the southern limits for mining taconite by open pit methods. Most of the existing underground workings will likely be engulfed as existing taconite operations continue to mine southward. Possible exceptions to this condition are several underground mines that lie to the south of the Minorca taconite pit in the Virginia Horn area. Among these are the Onondaga and Lincoln mines.

4.3.1.2. Cuyuna Range/Emily District

The Cuyuna Range and Emily District offer potentially better siting opportunities for CAES in existing underground workings. Steeply folded rocks here, as opposed to relatively flat‐lying strata on the Mesabi Range, led to linear ore bodies of restricted width. Several main hoisting shafts, such as those at the Armour No. 1 and Armour No. 2 mines, were emplaced not only off the ore body but entirely outside of the iron‐formation.

Much of the project work to date has focused on this region as follows:

1. A search was conducted for documents related to underground mining on the Cuyuna Range and Emily Ranges, particularly those with information specific to individual mines. A database compiled from multiple source documents gives specifics for each mine, including locations, time and kind of mining operation, shaft types and depth, overburden depth, ore types, geology, and more.

2. MNDNR, MN Dept. of Revenue, and the Office of Surface Mining Reclamation and Enforcement’s (OSMRE) National Mine Map Repository collections and files were searched for maps indicating underground mine workings on the Cuyuna Range and the western Mesabi Range. A request for maps was submitted to OSMRE on January 11th and the documents were received February 21st on 2 flash drives containing a total of 1,464 digital files (over 84 gigabytes of data). Since the files were simply assigned an image number, they were reviewed, organized and re‐named by properties. The documents include surface and underground plan view and cross‐section maps. Paper and Mylar maps found at MNDNR and Dept. of Revenue were scanned at MNDNR via a large‐format scanner to make them digital. Digital maps were geo‐referenced, and mine features including shafts, drifts, mined extents and caved areas were digitized. Geo‐referencing of mine maps from the Cuyuna Range proved to be a difficult undertaking, as they were often referenced to forty acre section lines that differ from today’s survey standards; landmarks, such as roads and buildings, have been altered or removed. An on‐line search yielded a 1913 Standard Atlas of Crow Wing County that proved very useful in geo‐referencing the early mines of the Cuyuna.


Figure 4‐3. U

tica Exten

sion M

ine.


3. Collections housed at the MNDNR Cuyuna Recreation Area office in Ironton, MN and at the Croft

Mine Historical Park in Crosby, MN have been briefly looked at and permission is pending to remove desired maps from the premises for the purpose of scanning. A listing of known mines on the Cuyuna Range has also been provided to the archivist at the Iron Range Research Center (IRRC) in Chisholm, MN and awaits his attention.

4. A series of mine shafts located north of the cities of Ironton and Crosby present potential for CAES. The Armour No.1 mine shaft is the deepest at 800 feet. It is located outside of the iron‐ formation in potentially competent rock. Haulage drifts extend over 400 feet from the shaft before reaching the ore body. The Armour No. 2, #2 Shaft is the next deepest at 525 feet. Workings of these mines, as well as the Bonnie Belle and Ironton mines, are shown in Figure 4‐4. Each color represents a level or sublevel in the mines. The Armour No. 1 and Armour No. 2 mines have not been completely mapped. There are at least 59 levels and sub‐levels in the Armour No. 1 mine. The Armour No. 2 mine has at least 28 levels and sub‐levels. A 3‐D rendering of part of the Armour No. 1 mine is provided in Figure 4‐5. It shows the shaft and main haulage drifts extending to the ore body.

5. To date, maps have been obtained showing evidence for underground workings at 28 mines.

Geo‐referencing of maps and digitizing of mine features is ongoing. Figure 4‐6 illustrates many of the mines on the Cuyuna North Range. Underground mine shafts are indicated in red. Much of this area comprises the Cuyuna Recreation Area that is managed by the MNDNR Parks and Trails Division. It hosts a 25‐mile‐long mountain bike trail system over and among legacy mining features (stockpiles, pits) that has gained a national reputation. While the land was given for surface use, the intent was that it would revert to its primary purpose as mineland as opportunity arose (James Sellner, MNDNR, pers. comm., October 10, 2013). This situation may prove an impediment to CAES as there is no current mining activity on the Cuyuna Range.

4.3.1.3. Vermilion Range

The Vermilion Range was briefly investigated. The Soudan Mine has competent rock and large rooms that could conceivably host CAES. However, the Soudan Mine is a State Park that provides public tours of the underground mine. It also contains a large underground physics laboratory. These factors likely preclude its use for CAES.

At least twenty underground mines were started on the Vermilion Range. The largest of these, the Chandler, Pioneer, Savoy, Sibley, Soudan, and Zenith mines, shipped from 1.8 M to > 41 M long tons of natural ore (Skillings, 2005), some from both underground and open pits. Stenlund (1988) documents 12 mines that were started on the Vermilion Range, most of which amounted to little more than a shaft and exploratory drifts.

Since distance from electric transmission lines must be taken into consideration in siting a CAES facility, the Vermilion Range has been assigned low priority status at this time. There are maps available from the U.S. Steel aperture card collection housed at the IRRC in Chisholm and from a collection stored in Ely. These have not been collected to date.


Figure 4‐4. A

rmour No. 1, A

rmour No. 2, B

onnie Bell, and Ironton M

ines.


Figure 4‐5. P

artial ren

dering of the Arm

our No. 1

Mine.


Figure 4‐6. M

ap of Cuyuna cities and m

ining features.


4.4. OBJECTIVE 4:

Investigate excavating/rehabilitating potential underground CAES storage cavities in different rock types (Group), e.g.,

a) Mesabi, Cuyuna, Vermilion, and Emily iron ranges; b) Duluth Complex, including working with Duluth Metals; c) Investigate other Minnesota River Valley granites, Mentor Mafic Complex, etc. for CAES

storage potential; and d) Other geologically favorable rock types in other MN locations in different rock types

near GRE, MP, transmission lines.

4.4.1. Progress/findings to Date

1. An on‐line search led to discovery of a document detailing the geological examinations, physical tests, and design plans for a proposed underground liquid propane storage cavern in Minnesota (Fenix and Scisson, Inc., 1960). Such a cavern was constructed near Erskine, Pope County, MN. It has been in operation for decades. This document has application for the geotechnical team in terms of evaluating sites for a potential newly constructed CAES cavern.

2. Creating underground caverns on the Mesabi Range for pumped hydro energy storage (PHES) was investigated as part of NRRI’s earlier investigation of PHES potential on the Mesabi Range. Specific underground ore bodies were looked at and reported on in Fosnacht et al. (2011). These same ore bodies pose potential for CAES.

3. The Emily District north of the Cuyuna Range has a significant manganese deposit. Cooperative

Mineral Resources (CMR), a subsidiary of Crow Wing Power, is working to develop the best project plan for extracting manganese from an 80‐acre parcel north of the City of Emily (http://www.cwpower.com/manganese.shtml). One method that has been tested on site is extraction by solution mining via a borehole. This method produces a cavern that could potentially be used for CAES. Contact has been made with a representative of Crow Wing Power.

4.5. ADDITIONAL PROJECT WORK

4.5.1. Presentations (Appendix 4‐A)

January 9, 2013, Hinckley: “Sites of Underground Mining Activity in Minnesota – Potential CAES Study Areas”

May 30, 2013, Duluth: “CAES Project Update: Identifying Existing Underground Mine Features”

August 12, 2013, Hinckley: “CAES Potential in Existing Underground Mines in Minnesota – Scenarios for Discussion”


4.5.2. Bibliographies

Preliminary Cuyuna Range Geologic Bibliography

Preliminary Cuyuna Range Mining Bibliography

4.6. REFERENCES

Fenix & Scisson, Inc., 1960, Feasibility Report Mined LP‐GAS Storage Cavern Near Erskine, Minnesota,

Study prepared for Solar Gas (Minnesota), Inc.

Fosnacht, D.R., and PHES Study Team, 2011, Pumped Hydro Energy Storage (PHES) Using Abandoned Mine Pits on the Mesabi Iron Range of Minnesota – Final Report: University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR‐2011/50, 599 p.

Stenlund, M., 1988, Ghost Mines of the Ely Area (1882‐1925).

CAES Progress Report – Geology Team Appendix 4‐A 87

APPENDIX 4‐A – GEOLOGY TEAM PRESENTATION

CAES Progress Report – Groundwater/Environmental Team 99

5. CAES PROGRESS REPORT – GROUNDWATER/ENVIRONMENTAL TEAM

Rebecca Teasley

5.1. GROUNDWATER MODELING

The Environmental Team is determining how groundwater movement may be affected by construction and operation of a Compressed Air Energy Storage facility. Models have been constructed in MODFLOW for various conditions for the sites of interest. The project team is examining the use of existing underground caverns as a site for Compressed Air Energy Storage. The Mesabi Iron Range of Minnesota has multiple abandoned underground mines that can be considered for storage sites. Current open pit mines on the Mesabi Iron Range may make it difficult to use nearby existing mines because of blasting concerns. Abandoned underground mines on the Cuyuna Range may be a more realistic option, although many were converted to open pit mines or are now flooded. Future precious metal mining operations in the Duluth Complex may be another option to consider converting into CAES storage facilities after the mining operation is complete. Selection of a site will greatly impact the results of the model, but the Environmental Team has been examining the most important factors to consider for a CAES storage facility.

A number of variables for site selection are being examined by other project partners and will be integrated into the modeling at a later date. At this time, the major design questions related to impacts on groundwater are dewatering of the abandoned mines, leakage of air from the cavern, the potential use of a water curtain for unlined caverns and reservoirs to maintain constant pressure in the cavern. Changes in pressure in the cavern from dewatering or air leakage could alter the surrounding water table.

A design decision that has to be made in a CAES operation is how to confine the caverns. Theoretically, if the cavern is deep enough, water pressure could be high enough to keep air contained. Gas escape may not occur as long as the water pressure along all possible escape paths increases for some small distance in the direction of potential gas escape (Goodall et. al, 1988). Alternatively, a water curtain could be built to contain air with increased water pressure or the cavern could be lined with an airtight material. Lining materials need to be able to withstand fatigue from the cyclic compression and decompression of CAES operation. Thermal stresses in the lining would also be an issue with adiabatic compression.

5.1.1. Model Configuration

To set up a MODFLOW model for regional groundwater flow, a series of variables were selected based on local geology. Although the final site has not been selected, a sensitivity analysis of the variables was conducted to determine which had the largest impact on the final results and should be carefully considered in the final model. An example of the model geometry is displayed in Table 5‐1. This layer configuration represents an assumed cavern in the Cuyuna Range with a water curtain. Water curtains are discussed in the following section on Boundary Layers. Grid spacing was set at 100 ft. with a further refinement near the cavern. Cavern spacing was set at 25 ft. by 50 ft. to capture the effects of the CAES operation.


5.1.2. Model Parameters

Assuming the geology of the Cuyuna Range, values for Storage, Specific Yield, and Hydraulic Conductivity were selected. Storage coefficients ranging from ‐0.00005 to 0.005 were selected for confined aquifers. Specific Yield is a function of Porosity, so Porosity values were selected as less than 0.1 and decreased with depth. These values were selected to represent a reasonable physical range found in the region (Todd and Mays, 2005). A sensitivity analysis demonstrated that the changing the values for Storage, Specific Yield and Porosity had negligible impact on the final heads in the model. This result demonstrates that the final model does not need to include precise values for these parameters.

While the model demonstrates little sensitivity to some parameters, it was found that the model results are sensitive to values for Hydraulic Conductivity. To model hydraulic conductivity, an anisotropy ratio of Kz/Kh was set at 0.1. The values of Hydraulic Conductivity set in the model are shown in Table 5‐2. Table 5‐1. MODFLOW Layer Properties.

Layer Description Top Elevation

(ft.) Thickness (ft.)

1 Sand and gravel 500 100

2 Bedrock 400 100

3 Water curtain in bedrock 300 1

4 49 ft. between cavern and water curtain 299 49

5 Mine drift in bedrock 250 50

6 Bedrock 200 200

Bottom of model 0

Table 5‐2. Typical Value for Hydraulic Conductivity used for Modeling.

Zone Description Kh (ft./day) Kz (ft./day)

1 Sand and gravel in layer 1 300 30

2 Bedrock in layers 2‐4 3x10‐3 3x10‐4

3 Bedrock in layers 5‐6 10‐6 10‐7

4 Water curtain variable variable

5 Cavern variable variable


5.1.3. Importance of Hydraulic Conductivity

The results were found to be sensitive to the values used for Hydraulic Conductivity. If the ratio of Hydraulic Conductivity values is increased between the layers, the value of head in the model decreases. Additionally, the individual layer values of hydraulic conductivity do not impact heads as much as the rate of change of hydraulic conductivity with depth (Fig. 5‐1). Hydraulic conductivity for the selected site must be carefully classified as is shown by the model results.

Figure 5‐1. Maximum Model Head with Ratio of Hydraulic Conductivity to Depth.

5.1.4. Boundary Conditions

For effective air storage, water at the cavern boundary should flow toward the cavern or have no flow. Outwards movement of water is possible at the bottom of the cavern, so to prevent air from leaking from the cavern bottom should be saturated with water or lined (Liang and Lindblom, 1994). To model groundwater flow, there must be a boundary condition to represent the air water interface at the edge of the cavern. This boundary needs a better technical understanding for modeling and will be affected by design decisions such as physical geology of selected location, cavern lining, cavern dimensions and operating air pressure. Two simplified boundary conditions were selected for modeling; a constant head boundary and a water curtain. If a constant head boundary is assumed, it implies an infinite supply of water in the nearby area and no changes in head will occur at the boundary. The second boundary condition is a water curtain that is used for providing a pressurized boundary condition in unlined caverns.

For the model, a water curtain is installed 40 to 50 ft. above the top of a cavern to provide a pressure that is greater than the pressure in the cavern. Boreholes are spaced at about 10 ft. along the length to


allow the water curtain to at least cover the roof of the cavern (Fig. 5‐2). Flow for a 855 psig curtain is about 13.5 gmp (Bauer et. al, 2012).

The water curtain was modeled in MODFLOW as a series of analytic element (AE) wells (Fig. 5‐3) and as a series of injection wells. The modeling showed that for operating pressures ranging from 900‐2000 psi, the current model is requiring injection rates of 45‐120 ft.3/day to achieve pressures greater than cavern operating pressures. The project team must determine if this is a reasonable setup for the CAES project.

Figure 5‐2. CAES Cross‐Section with Water Curtain (Bauer et al., 2012).


Figure 5‐3. Cavern Model with Multiple AE Wells for a Water Curtain.

Boundary condition modeling considerations include changes in system flow and distance of boundary conditions from flow system. If the stress on a flow system would cause changes in a natural boundary, e.g., head drawdown from a new well, the boundary would no longer be physically reasonable. Distance to lateral boundaries should be at least three times the depth of the flow system, after that increasing the distance has only slight influence on the model (Lehn Franke et. al, 1987). For example, if a cavern has a depth of 300 ft., the model should extend 200 ft. below it for a total model depth of 500 ft. The distance to lateral boundaries should be about 1500 ft.

5.1.5. Remaining Questions

To complete the model of regional groundwater flow, a few questions must be answered. A site must be selected so that the geology is well known. The modeling has shown that the model is sensitive to values of hydraulic conductivity. Additionally, boundary conditions must be characterized including the local geology, the surrounding water table depth, the operating pressures, and how the air will be contained (lining or a water curtain). The geometry and layout of the cavern are also important parameters for the model and finally, existing groundwater data must be used to calibrate the model.

5.2. ENVIRONMENTAL PERMITTING ISSUES

A number of environmental and permitting issues will be answered by selection of the site. The following provides a summary of the issues currently examined.

5.2.1. Geology and Soil

5.2.1.1. Soil Erosion

Soil erosion is assumed to likely only occur during construction. Soil erosion and sedimentation control plans should be included in project development.


5.2.1.2. Seismicity

Could affect project safety and should be evaluated when/if project site is identified (Table 5‐3). Table 5‐3. Recorded historical earthquakes affecting Minnesota (adapted from USGS, 2009).

Date Severity of the Shock Quake Center Area affected in

Minnesota

1860 Fairly strong Central Minnesota, US Entire state

Sept. 3, 1917 Intensity VI Central Minnesota, US Entire state

Nov. 15, 1877 Strong shock Eastern Nebraska, US SW Minnesota

May 26, 1909 Intensity VII Illinois, US SE Minnesota

Feb. 28, 1925 ‐‐ Quebec, Canada Slightly felt in Minneapolis, MN

Nov. 1, 1935 Strong Timiskaming, Canada Slightly felt in Minneapolis

Nov. 9, 1968 Intensity I‐IV South‐central Illinois, US

Austin, Glencoe, Mankato, Minneapolis, Rochester, MN

5.2.2. Water Quality

There are minimal concern‐sources comparing natural gas storage to air storage. Air storage is easier and safer than natural gas storage since CAES requires lower pressures, eliminates fire and explosion potential of natural gas storage, and leaks or venting to the atmosphere is not an environmental consideration with compressed air (Allen, 1985).

5.2.3. Groundwater

Groundwater concerns that could result from CAES in an underground facility depend on the location and depth of the cavern. If an existing cavern is used, it is likely that it would need to be dewatered prior to conversion to a storage facility. The impact of dewatering a cavern is dependent on the properties of the nearby rock and soil. Another concern is the potential for air escape into groundwater. The implementation of a water curtain or the use of a reservoir to keep the cavern at a constant pressure would also have a notable impact on groundwater.

5.2.4. Surface Water

• Exchanges between groundwater and surface water depend on the site selected; • Increased stormwater could occur from impervious surfaces; and • Use proper runoff, soil erosion, and sedimentation control practices during construction.


5.2.5. Biological Resources

• Biological resources include plant communities, wildlife communities, fishery resources, and sensitive species and sensitive habitats;

• Rare plant species unlikely to be present due to heavily disturbed mined land in area (depends on location);

• Regional forest‐sensitive wildlife/plant species; and • Endangered, threatened or special concern (ETSC) plants and wildlife.

5.2.6. Impacts Common to Large Facility Construction

5.2.6.1. Agricultural Resources

• Northern MN ‐ primarily pasture and hay crop farming; and • Identify if resources at risk in immediate vicinity of project.

5.2.6.2. Cultural Resources

• Potential impact of construction of power line corridors, new roads, and any structures, buildings, or other features that could overlie or constitute historic or prehistoric resources; and

• Federal, state and local agencies, tribal governments, the Minnesota State Historic Preservation Office (SHPO), and other stakeholders should be consulted.

5.2.6.3. Aesthetic Resources

• Main impact on aesthetic and visual resources would be from construction of power lines. Severity is dependent on visual character and scenic quality surrounding selected site.

5.2.6.4. Recreational Resources

• Consult regulatory and economic development agencies, and NGOs associated with natural resource conservation and recreation early in siting process.

5.2.6.5. Population and Housing

• Not likely to significantly increase or decrease population in area.

5.2.6.6. Air Quality and Noise

• Use dust control efforts during construction process. Potential for noise from compressors/generators?

5.2.6.7. Greenhouse Gas Emissions

• Associated with life cycles of materials used for facility, magnitude depends on scale of project.


5.2.6.8. Hazardous materials

• Primarily released during construction and maintenance phases, depends on scale of project.

5.2.6.9. Environmental Permitting

• FERC does not have authority over the construction or maintenance of power generating plants, and has limited jurisdiction over transmission line siting. (www.ferc.gov/industries/electric);

• Federal agencies that may become involved in licensing a CAES project, especially for environmental issues, include the U.S. Army Corps of Engineers (USACOE), the Environmental Protection Agency (EPA), and the U.S. Fish and Wildlife Service (USFWS); and

• State agencies include MNDNR and Department of Health (if wells become impacted).

5.3. TIMELINE

• Groundwater modeling will be completed 2 months after completion of site selection. • Environmental Permitting will be completed 2 months after completion of site selection. • Final Report.

5.4. REFERENCES

Allen, K., 1985, CAES: The Underground Portion: IEEE Transactions on Power Apparatus and Systems, v. PAS‐104, no. 4, p. 809‐812.

Bauer, S.J., Gaither, K.N, Webb, S.W., and Nelson, C., 2012, Compressed Air Energy Storage in Hard Rock Feasibility Study, Sandia Report, SAND2012‐0540.

Goodall, D.C., Aberg, B., and Brekke, T.L., 1988, Fundamentals of Gas Containment in Unlined Rock Caverns: Rock Mechanics and Rock Engineering, v. 21, p. 235‐258.

Lehn Franke, O., Reilly, T.E., and Bennett, G.D. , 2005, Definition of Boundary and Initial Conditions in the analysis of Saturated Ground‐water Flow Systems – An Introduction: U.S. Geological Survey, Techniques of Water‐Resources Investigation, Book 3, Chapter B5. Available online: http://pubs.usgs.gov/twri/twri3‐b5/.

Liang, J., and Lindblom, U., 1994, Analyses of Gas Storage Capacity in Unlined Rock Caverns: Rock Mechanics and Rock Engineering, v. 27, no. 3, p. 115‐134.

Todd, D.K. and Mays, L., 2005, Groundwater Hydrology: New Jersey, John Wiley & Sons, 3rd edition, 636 p.

USGS, 2009, Minnesota Earthquake History: Webpage, Retrieved on October 10, 2013 from http://earthquake.usgs.gov/earthquakes/states/minnesota/history.php.

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