Alternative Heating Sources in Vermont Greenhouses

The Green Team: Phil Croker, Kae Fink, Kristin McDonald, Dylan Peters

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Contents

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Heating Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Ground Source Heat Systems . . . . . . . . . . . . . . . . 8 Compost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Solar Thermal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Air Source Heat Systems . . . . . . . . . . . . . . . . . . .32

Comparative Analysis. . . . . . . . . . . . . . . . . . . . . . . . . 36 Incentives and Financing Sources . . . . . . . . . . . . . 40 Energy Service Provider Model . . . . . . . . . . . . . . . .40 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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Introduction The current report examines the economic and technical feasibility of alternative

energy sources to power greenhouses in Vermont. The topic is especially relevant as renewed focus on local foods has peaked interest in season extension in farms across Vermont. In addition, the potential introduction of a carbon pollution tax has increased the economic salience of alternative heating sources. Thus, it is an opportune time to explore options to reduce the cost of fuel and the environmental impact associated with running a greenhouse.

This analysis considers the characteristics of alternative (non-fossil fuel) greenhouse heating technologies to assess economic feasibility in terms of their use in Vermont greenhouses; we will also analyze how the use of these technologies may evolve in the future. Background: Vermont Greenhouse Heating Requirements

Figure 1.

A greenhouse designed for year-round crop production, shown in operation in mid- February (Pete’s Greens, Craftsbury, VT).

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A growing demand for locally grown produce in Vermont, especially outside of the standard growing season, has led to increasing utilization of greenhouses, which provide a sheltered and climate-controlled environment for crop growth. As of 2012, greenhouse growing in Vermont accounted for 2.6 million square feet (60 acres) of Vermont’s land area, and resulted in $24.5 million in crop revenue.1 By design, greenhouses are constructed to passively trap solar radiation, which tends to raise the average temperature inside the greenhouse relative to the external environment. In regions with higher insolation and a milder climate, growers are able to produce crops year-round relying only on this passive solar heating. However, in Vermont, passive solar is frequently not adequate to maintain appropriate growing conditions through the “shoulder seasons” (early spring and late fall), let alone through the winter. In order to prevent crop damage, supplemental heating in colder periods is often required. The specific heating requirement is highly crop-dependent: certain hardier crops that thrive in upper-latitude temperate climates, such as spinach, claytonia, and upland cress, can be produced year-round in Vermont with minimal to no active heating; however, certain other crops, such as tomatoes and cucumbers, require soil temps of 85 °F and an air temp of 55 °F for optimal growth.2 Thus, in Vermont, just under half of all greenhouse space (1.5 million square feet) is estimated to be “heated space,” meaning that active heating systems (i.e., systems to supplement passive solar radiation) have been installed.3 These “heated spaces” may incorporate soil-heating systems, air-heating systems, or a combination of both. Heating the soil is significantly more efficient than heating the air, as the thermal mass of soil results in greater storage of heat, and thus less heat is required to maintain a stable soil temperature than the equivalent air temperature. Additionally, heating the soil leads to increased root activity, which boosts crop growth. However, air heating is essential to maintaining crops throughout colder periods, as damage to the above-ground portions of the crops occurs when the air temperature drops below freezing.

1 Chris Callahan and Vern Grubinger, “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in Vermont” (University of Vermont Extension, September 4, 2015), http://www.uvm.edu/vtvegandberry/Pubs/BiomassHeatingVermontGreenhouses2015Report.pdf. 2 Pete Johnson, March 19, 2016. 3 Callahan and Grubinger, “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in Vermont.”

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a) b)

Figure 2.

a) soil-bed heating, where hot water flows through pipes buried under crop beds; b) air heating, where hot air exits through vents located in the upper regions of the

greenhouse (Pete’s Greens, Craftsbury, VT). An estimated 10-20% of total cost in greenhouse production is attributable to energy usage,4 70-85% of which is attributable to greenhouse heating.5 This represents the second-highest cost to greenhouse owners, behind labor expenses.6 As of 2012, estimated aggregate cost of greenhouse heating in Vermont was $1.8 million annually. As the majority of these heating systems are fossil fuel-based (i.e., run on propane or fuel oil), greenhouse heating has also been estimated to account for 7.58 million pounds/yr of CO2 equivalents (CO2E) in emissions, or roughly equal to the emissions from 8.2 million miles of car travel.7 In the five-year period between 2007 and 2012, as total heated greenhouse space increased by ~30% across the state, total heating costs increased ~90% and total CO2E emissions increased by ~30%.8

4 John W. Bartok, “Increase the Efficiency of Your Greenhouses” (Storrs, CT: University of Connecticut, n.d.). 5 Scott Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating” (Wisconsin Focus on Energy / Rural

Energy Issues, University of Wisconsin, n.d.); Patricia A. Rorabaugh, Merle H. Jensen, and Gene Giacomelli, “Introduction to Controlled Environment Agriculture and Hydroponics” (Tuscon, AZ: Controlled Environment Agriculture Center, Campus Agriculture Center, University of Arizona, Fall 2002).

6 Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating.” 7 Callahan and Grubinger, “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in Vermont.” 8 Ibid.

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Figure 3.

A 5,000 gallon propane tank shown outside the greenhouse it is used to heat. The house uses approximately 300 gallons per week of propane, with the air heating system alone

using 3 gallons per hour (Pete’s Greens, Craftsbury, VT).

Figure 4. Traditional fossil fuel heating systems: a) a larger propane boiler, which connects to an

external exhaust vent; b) a small, on-demand boiler currently used to heat hot water for soil bed heating, and that will soon be adapted for above-ground radiant heating; c) heat pump

for on-demand boiler (Pete’s Greens, Craftsbury, VT).

a) b) c)

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Currently, fossil fuel-based systems dominate the realm of greenhouse heating. In

addition to the fact that they have simply been around for longer than the emerging alternatives, these fossil fuel systems hold several distinct advantages:

1) Flexibility and adaptability. Traditional propane heaters (Figure 4a) can be moved between greenhouses in the course of several hours, allowing for greater flexibility in distribution of heat than would a large, static system. In recent years, smaller portable propane heaters have also been developed (Figures 4b-c), which are even more readily transportable.9 2) Co-benefits of fossil fuel heaters. The use of propane heaters releases CO2 into the greenhouse when the systems are run unvented, which has been observed to boost crop growth.10 3) Technological maturity. Fossil fuel systems are an established technology; growers are comfortable with their operation, and continued use of these systems avoids the management time and cost that would accompany the learning curve of a new technology.11 4) Ease of use. Fossil fuel heaters tend to be straightforward to operate and are minimally labor-intensive. 5) Economics. With the currently low prices of propane, fossil fuel heating makes economic sense. At the current juncture, it is difficult to make a shift to alternative heating sources a priority when fossil fuel systems are not prohibitively expensive.12

The advantages of fossil fuel heating in the greenhouse context are numerous, which may make significant change to the current system challenging. However, the USDA has reported an significant increase in food-related energy use between 1997 and 2007 (from 12.2% to 15.7% of total national energy use),13 which suggests that energy usage in the food system, including in greenhouse heating, is a growing issue on a national scale. Within Vermont, the state’s comprehensive energy plan has established the goal of reducing total energy consumption per capita by 15% by 2025, and by more than a third by 2050. Additionally, the state has pledged to progressively meet up to 90% of its energy needs with renewable sources by 2050; in the context of heating, these sources include wood and biomass, biofuels, and heat pumps powered by electricity from renewable sources.14 The food sector, including greenhouses, has an important role to play in meeting these commitments: The Vermont Farm-to-Plate initiative has established the goals of simultaneously reducing farmers’ production expenses, improving environmental impacts, and minimizing the use of fossil fuels while maximizing renewable energy, energy 9 Johnson, interview. 10 Frederic Jobin-Lawler, March 25, 2016; Johnson, interview. 11 Vern Grubinger, March 22, 2016. 12 Ibid. 13 Ellen Kahler et al., “Farm-to-Plate Strategic Plan” (Vermont Sustainable Jobs Fund, July 2013). 14 “2016 Vermont Comprehensive Energy Plan” (Montpelier, VT: Vermont Department of Public Service, 2016).

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efficiency, and energy conservation.15 In addition to these policy-based drivers, the growing popular concern about the contribution of greenhouse gas emissions to global climate change will likely result in increasing pressure on producers to limit their fossil fuel usage. And finally, from an economic perspective, the volatility of fossil fuel prices in the past decade (Figure 5) suggests that fossil fuels may not always be the most economic choice for greenhouse heating; in fact, one solar and geothermal installer has remarked that the interest of individuals in alternative heating systems spikes during periods of high fossil fuel prices.16 All of these factors indicate that investigating the feasibility of alternative, non-fossil fuel heating sources for greenhouse applications is of interest.

Figure 5.

Residential heating oil and propane costs in Vermont from 2006-2016.17 It has repeatedly been stressed, in both previous literature18 and in personal communications19, that, especially in the greenhouse context, efficiency measures must preclude any consideration of alternative heating sources, as minimizing structural heat loss can drastically limit the necessary capacity of a heating system. These efficiency measures include improving the material covering of the greenhouse (e.g., by utilizing double-layer poly films and/or IR-treated films); insulating side walls; installing energy curtains; reducing leaks through measures such as weather stripping and caulking; building wind breaks; carefully managing humidity; ensuring proper circulation; and using heat-retaining row covers (several of these efficiency measures are shown in Figure 6). Details on the various aspects of greenhouse efficiency have been documented extensively

15 Kahler et al., “Farm-to-Plate Strategic Plan.” 16 Andy Cay, March 17, 2016. 17 Data from USDOE EIA: Vermont Weekly Heating Oil and Propane Prices (October – March). 18 A. J. Both, “How Can You Lower Your Greenhouse Energy Bill?” (New Brunswick, NJ: Rutgers University, August 2008);

Rorabaugh, Jensen, and Giacomelli, “Introduction to Controlled Environment Agriculture and Hydroponics”; T. E. Bond, James F. Thompson, and Ray F. Hasek, “Reducing Energy Costs in California Greenhouses” (Cooperative Extension, University of California, n.d.); Bartok, “Increase the Efficiency of Your Greenhouses”; Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating.”

19 Andy Cay, March 17, 2016; Ben Luce, March 14, 2016.

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elsewhere,20 and will not be examined in detail in this analysis. Instead, this report will analyze various alternatives to traditional fossil fuel heating technologies in the greenhouse context to suggest their strengths and limitations, and thereby suggest their overall viability for greenhouse heating applications.

Figure 6. Several greenhouse efficiency measures in practice: a) circulation fans are optimized for

maximum heating efficiency; b) row covers spread over crop beds at night to reduce heat loss; covers allow 80% light penetration; c) automated air vents used to efficiently cool the greenhouse by

preventing excessive opening and closing (Pete’s Greens, Craftsbury, VT).

20 Both, “How Can You Lower Your Greenhouse Energy Bill?”; Rorabaugh, Jensen, and Giacomelli, “Introduction to Controlled Environment Agriculture and Hydroponics”; Bond, Thompson, and Hasek, “Reducing Energy Costs in California Greenhouses”; Bartok, “Increase the Efficiency of Your Greenhouses”; Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating.”

a) b)

c)

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Alternative Heating Systems Ground Source Heating Technical Overview

Ground source heating systems (GSHS), frequently referred to as geothermal heating systems, fundamentally operate by transferring heat between the ground, which maintains a relatively stable temperature of between 45-60 °F in Vermont21, and an indoor space. This transfer of heat, which is reversible to accommodate both heating and cooling functionalities within an indoor environment, operates as follows: A) Heating mode

1. Heat is transferred from the ground to a “working fluid” (either water or a water-antifreeze mixture, depending on whether the GSHS is an open loop or closed loop system, respectively).

2. Working fluid is pumped from the ground loop to the heat pump circuit. 3. The heat pump circuit, as shown in Figure 7, consists of an evaporator, a

condenser, and a compressor. The low-grade heat from the ground loop working fluid is transferred to a refrigerant fluid in the heat pump circuit, and the cold working fluid is returned to the ground loop to be re-warmed.

4. The refrigerant fluid is passed through the compressor, causing its temperature to increase significantly. This refrigerant fluid then transfers heat to the distribution system.

B) Cooling mode (essentially the reverse of heating mode) 1. Heat is transferred from a hot indoor space to the refrigerant in the heat

pump circuit. 2. The refrigerant passes through the compressor, upgrading the heat, and

then transfers heat to cold working fluid in the ground loop. 3. As the hot working fluid passes through the ground loop, heat is

transferred to the surrounding ground (which serves as a sink as well as a source of heat). Cool water is then pumped back up to complete the ground loop.

It should be noted that GSHS – as with the other alternative heating technologies

examined in this report – require electricity, in this case to operate both the pumps that drive water from the ground loop to the heat pump circuit and the heat pump compressor; notably, this electricity does not necessarily come from renewable sources. However, these alternative technologies are typically classified as “sustainable heating sources,” because they deliver more thermal energy than they consume in electrical energy. In GSHS, for every unit of electrical energy consumed by the pumps and the compressor, 3 to 4 units of

21 “How Geothermal Systems Work,” Green Mountain Geothermal, 2016,

http://www.vermontgeo.com/how_geothermal_systems_work.html; Ben Luce, “Heating Your Home or Business in Vermont with a Geothermal System” (Northeast Vermont Development Association, February 2011), http://www.nvda.net/files/GeothermalHeatPumpsGuide.pdf.

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heat energy are transferred. This corresponds to a typical whole-system Coefficient of Performance (COP; the ratio of rate of heat delivery to the electricity required to run the heat pump, typically measured in BTU/hr) of between 3-4 for a closed loop system, or between 3.5-5 for an open loop system.22

Figure 7.

Schematic representation of generic heat pump. Directionality of arrows shown for heating mode, and water-to-water distribution system is depicted.23

The ground-loop portion of ground source heat systems can be broadly categorized

into open loop and closed loop systems (Figure 8). In an open loop system, the ground loop working fluid is water that is drawn from and returned to a single source, such as a well or a pond; in a closed loop system, the ground-loop working fluid is water or a water-antifreeze mixture that is circulated underground in a closed loop of piping, which can be arranged either horizontally or vertically. Closed loop systems have the advantages of reduced excavation costs and a decreased pumping load relative to open loop systems. However, due to the low thermal conductivity of the plastic piping in which the working fluid is housed, the temperature of the working fluid in a closed loop system tends to be around 32 °F, as opposed to the 50 °F working fluid in an open loop system.24 This leads to a decreased efficiency of closed loop 22 Luce, “Heating Your Home or Business in Vermont with a Geothermal System”; “How Geothermal Systems Work.” 23 http://www.energygroove.net/technologies/heat-pumps/. 24 Andy Cay, March 17, 2016.

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systems, because the circulating working fluid must be kept extremely cold in order to facilitate heat transfer, which increases the load on the heat pump.25 Closed loop systems also, by design, require significantly more piping, which can cut away at the savings achieved through reduced excavation costs; this generally results in greater total costs for closed loop systems than for analogous open loop systems.

Figure 8.

a) Open loop ground source heat system, featuring a standing column well; b)-c) Closed loop ground source heat systems, featuring b) horizontal and c) vertical piping arrangement.26

Although closed loop systems in general tend to be less efficient than open loop systems, one variant of a closed loop system – so-called “direct exchange,” or “DX,” systems – have COPs that are competitive with, or even in some instances exceed, that of open loop systems. In DX systems, a refrigerant fluid is circulated directly in the ground in conducting pipes (typically copper or steel). This enables much higher heat transfer than in standard closed loop systems, but also requires more expensive piping and increased amounts of refrigerant fluid as compared to standard closed loop systems. Thus, DX systems are comparative in terms of efficiency to open loop systems, but tend to cost significantly more than both standard closed loop systems and open loops systems. In Vermont, where well drilling is broadly viable, open loop systems – and specifically standing column well systems – are suggested to be the optimal choice for GSHS design due to their higher efficiency and lower costs.27 A schematic of a standing column well connected to a GSHS is presented in Figure 9.28 25 Ibid.; Luce, “Heating Your Home or Business in Vermont with a Geothermal System.” 26 Luce, “Heating Your Home or Business in Vermont with a Geothermal System.” 27 Andy Cay, interview; Luce, “Heating Your Home or Business in Vermont with a Geothermal System.” 28 Alain Nguyen, Philippe Pasquier, and Denis Marcotte, “Multiphysics Modelling of Standing Column Well and

Implementation of Heat Pumps Off-Loading Sequence,” in Proceedings of the 2012 COMSOL Conference in Boston (COMSOL Conference, Boston, MA, 2012), https://www.comsol.no/paper/download/151259/nguyen_presentation.pdf.

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Figure 9.

Schematic of standing column well for use in ground source heating system.29 John Rhyner has provided an apt comparison of a standing column well to “a long, narrow glass of water with a straw inserted to the bottom,” where the “glass” is a borehole drilled through the upper layers of soil and into the bedrock (supported by steel casing in the shallow soil region) and the “straw” is what’s known as a “porter shroud,” or a plastic pipe inserted in the borehole and extending to the bottom of the hole.30 The bottom portion of the shroud has holes drilled into it, which allows water to flow from the borehole into the shroud. Groundwater from the surrounding region will fill the well; the nature of geologic and hydrologic features in the immediate area of the well will determine the standing water level in the well, as well as the flow rate at any given well depth. At the top of the shroud, just below the standing water level, a submersible pump is installed; this pumps water from the shroud up into the heat pump loop system.31 Standing column wells can either be excavated specifically for a GSHS, or can be adapted from existing wells.32 Where feasible, the utilization of an existing well to serve as the standing column well for an open loop GSHS greatly reduces the cost of installing an open loop system. In order to effectively meet GSHS requirements, the well must have a

29 Alain Nguyen, Philippe Pasquier, and Denis Marcotte, “Multiphysics Modelling of Standing Column Well and

Implementation of Heat Pumps Off-Loading Sequence,” in Proceedings of the 2012 COMSOL Conference in Boston (COMSOL Conference, Boston, MA, 2012), https://www.comsol.no/paper/download/151259/nguyen_presentation.pdf.

30 Jill Ross, “Bringing Geothermal to the City,” Water Well Journal, December 2010, 27–31. 31 Ibid. 32 Luce, “Heating Your Home or Business in Vermont with a Geothermal System.”

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minimum flow of 1.5 gallons per minute (gpm),33 but optimal flow is in the range of 90-150 gpm.34 Distribution Systems The distribution loop portion of the ground source heat system can likewise be broken down into two broad categories: in this case, distribution can take place through water-to-air systems or water-to-water systems. In a water-to-air system, heat is transferred from the refrigerant in the heat pump circuit to an air handler, which produces hot air for distribution in a forced air distribution system. In a water-to-water system, heat is transferred from the refrigerant in the heat pump circuit to a loop of water (the distribution loop), which can then be used either in conjunction with an air handler in another location or to heat the space directly. In the context of greenhouse applications, it has been suggested that water-to-water systems would be greatly favored due to the ability of these systems both to store thermal energy and to target delivery, both spatially and temporally.35 Additionally, water-to-water systems allow for direct soil heating, a practice employed by most major greenhouse growers that both reduces overall heating costs and improves the efficiency of crop growth, as compared to air heating systems.36 Within the scope of water-to-water systems, there are several different specific distribution systems that have specific applicability to greenhouse growing. Burying pipes directly in the soil or in concrete slabs improves heat conductivity, allowing for lower-temperature water to be sufficient for effective heating. Heat can also be transferred via a radiation from pipes placed low to the ground; this provides needed air heating, but targets it where young plants most need it. Costs and Benefits Benefits

From a technical perspective, ground source heating systems have one of the highest efficiencies of any type of heating system: total system efficiency for GSHS has been estimated at around 500%,37 which means that for every unit of energy put in to the system to drive the pumps and the compressor, 5 units of heat are released to the indoor space. Once installed, GSHS have a relatively long lifespan: components in the indoor distribution loop typically last for around 25 years, while components in the outdoor ground loop last for more than 50 years with minimal maintenance demands.38 A comprehensive analysis of GSHS utilized in a residential or small business context has suggested that the use of this system, as opposed to a fossil fuel-based heating system, leads to CO2 emissions reductions of at least 45% if the electricity to run heat pump is

33 Andy Cay, interview. 34 Nguyen, Pasquier, and Marcotte, “Multiphysics Modelling of Standing Column Well and Implementation of Heat Pumps

Off-Loading Sequence.” 35 Ben Luce, March 14, 2016. 36 Pete Johnson, March 19, 2016. 37 Andy Cay, interview. 38 Luce, “Heating Your Home or Business in Vermont with a Geothermal System.”

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fossil-fuel based; these CO2 savings would be even greater if this electricity were to come from renewable sources, such as solar PV or wind. Finally, from a pure cost perspective, it has been suggested that for moderate heat users in a relatively efficient structure, cost savings associated with the installation of a GSHS tend to be in the range of 60-70% of total annual fossil fuel costs.39 These cost savings are derived from replacing purchases of heating oil or propane with purchases of a relatively small amount of electricity to drive the pumps and the compressor. These savings correspond to a 10-15% rate of return on initial investment in the system,40 which, in turn, translates to a simple payback period of around 10-20 years.41 It should be noted that these savings may be reduced in the context of greenhouses, which are, by design, leaky structures compared to residential homes. Conversely, the rate of return is expected to increase and payback period decrease as usage of the system increases, as in the context of year-round greenhouse heating. Costs

Although monetary savings are realized over a moderate payback period for ground source heat systems, these systems tend to have extremely high upfront capital costs of installation. Total costs are highly site-specific, and thus it is difficult to suggest a “typical” or “average” price point for GSHS. However, a recent GSHS system installed in a 3,500 ft2 residential home with a highly efficient building envelope cost around $90,000 for 6 T (72,000 BTU/hr) of heating capacity.42 Costs of GSHS installation are primarily attributable to drilling and excavation costs for either a standing column well (open loop system) or the infrastructure for a closed loop system. Typically (although, as has been noted, costs are heavily dependent on the specific nature of the project and site), drilling and excavation costs are in the range of $10,000-$30,000. Vertical closed loop systems tend to have the highest excavation costs, as a significant number of deep boreholes are required for optimal functionality. Excavation costs for horizontal closed loop systems tend to be roughly half of the excavation costs for vertical closed loop systems, but an adequate amount of horizontal land space is required for installation, and soil in this area must be at least four feet deep. Open loop standing column wells tend to be the least expensive, and if an existing well at the site of interest has suitable dimensions and water flow, excavation costs can be avoided entirely.43 In addition to drilling and excavation costs, the costs of piping must be considered. Closed loop systems require significantly more piping, as a greater surface area is needed for heat collection in the ground loop due to the low conductivity of the plastic piping. So-called direct exchange systems, which utilize copper piping, are drastically more efficient than standard closed loop systems (often achieving COPs of above 5, which places them solidly in the range of the efficiencies of open loop systems), but the cost of copper piping is significantly higher than the plastic piping that is used in standard closed loop systems.44

39 Andy Cay, interview. 40 Ibid. 41 Ibid.; Luce, “Heating Your Home or Business in Vermont with a Geothermal System.” 42 Andy Cay, interview. 43 Luce, “Heating Your Home or Business in Vermont with a Geothermal System.” 44 Ibid.

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Finally, the heat pump itself is not inexpensive: standard pumps that are currently available cost around $2,500 per ton of installed capacity.45 It should also be noted that, given present technological capabilities, a backup furnace would likely be required in conjunction with a GSHS for emergency heating needs on the coldest nights, when GSHS capacity is insufficient to maintain the greenhouse temperature. This furnace would be relatively small, and thus would demand little, although nonzero, capital investment. Improvements in heat pump technology are anticipated to ultimately eliminate the need for this backup furnace.46 Current Use Although all new heating systems – and especially renewable or nontraditional heating systems – are expected to have an initial learning curve, the complexity associated with a ground source heat system is not typically one that can be overcome with repeated use. GSHS are not user-friendly systems; the networks of piping and pumps and the electrical components of the automated control system are usually monitored by technical experts associated with the manufacturer or installer, and users of these systems typically won’t be able to fix system problems on their own.47 In the context of Vermont growers, this could be a significant limitation to GSHS viability: farmers would likely prefer a system that is easy to operate and that they can readily fix without the aid of technical support agents. In the long term, however, GSHS may be a promising option for sustainable greenhouse heating as the regional food system shifts to having a higher percentage of food grown in New England. It is expected that the trend towards greater local food production, already being observed in Vermont, will continue, and even accelerate, in the coming years due to the pressuring influences of climate change, economic conditions, a growing local food movement, and policy measures such as Vermont’s Farm-to-Plate initiative. With a significant increased demand for local food, there will necessarily be more year-round growing and a higher percentage of food grown in greenhouses; in this case, the need for heating could very likely outstrip the potential of alternative heating sources such as biomass and compost. Once installed, GSHS are environmentally benign, in contrast to technologies such as biomass that require ever-increasing amounts of material inputs and thus can lead to environmental degradation. Finally, because the heat pumps and compressors in a GSHS are driven by electricity, there is the potential to use renewable energy for this purpose; it has been suggested that GSHS would be an ideally suited heating technology in a solar PV-based energy landscape.48 Ground source heating systems have, to this point, been primarily limited to residential applications, and specifically, residential applications for affluent homeowners. The only known instance of a greenhouse in the Vermont area heated with GSHS is an installation done by Integrated Solar in Marlboro, NH, which consists of a small greenhouse connected to a residential home that draws from the same system that heats the home.49

45 Ibid. 46 Luce, interview. 47 Andy Cay, interview. 48 Ibid. 49 Ibid.

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Because GSHS are only capable of delivering relatively low-grade heat (water can be heated up to 125 °F, as compared to 180 °F for a fossil fuel furnace), they are ill-suited to under-insulated and leaky structures that would require extremely high rates of heating. By design, greenhouses – and especially hoophouses and smaller greenhouses – tend to have low structural efficiency, and thus might require more heating than a GSHS can deliver during the coldest days of the year. This, combined with the relative nascence of ground source heating technology and a resulting lack of farmer familiarity with the systems, as well as a significant capital cost, has led to minimal interest in geothermal systems for greenhouse heating in Vermont.50 Although greenhouses heated with GSHS are not currently operational in Vermont, there is a significant and thriving example of such a greenhouse in Compton, Quebec. L’Abri Vegetal, an organic farm that produces a variety of produce and herbs year-round, installed a 2,000 m2 (half-acre), 220 kW (~750,000 Btu/hr) capacity DX GSHS in 2008. The choice was made to install a horizontal DX system, rather than a standing column well, due to uncertain bedrock conditions in the area and economic favorability of the DX system (notably in contrast to what is seen in Vermont). Fred Jobin-Lawler, the current owner, purchased the farm after the system had been installed, and was not able to share details about the initial installation, but estimated that the system cost 270,000 CAD. Since the GSHS installation, L’Abri Vegetal has seen a decrease in oil usage of over 60%, even as farm production has gone from a 10-month to a 12-month operation. At the time of the installation, the Canadian government was offering a 30% federal subsidy for alternative heating technologies, which significantly boosted the financial viability of the system; Jobin-Lawler stressed that the subsidy was the primary reason the system has been economically feasible for the farm. L’Abri Vegetal has not yet seen drastic cost savings from the GSHS, and has been disappointed with the system’s technical performance: the observed COP has been only in the range of 2.5-2.7, although the system is rated at a COP of 4-5. In spite of this, Jobin-Lawler views the system as successful, as it is something that can “make [l’Abri Vegetal] shine in a green way” (the farm’s website proudly touts a commitment to sustainable growing, proclaiming, “On voit la vie en VERT à l'Abri Végétal!” – We see life in GREEN at l’Abri Végétal!). Further economic benefits could be found through utilization of Quebec’s carbon markets, but Jobin-Lawler believes that the savings (an estimated few hundred CAD) are not worth the time and cost of establishing the farm in the carbon market. Generally, Jobin-Lawler seems to view the greatest strength of the GSHS as its contribution to the farm’s “green” values, rather than its technological or economic contributions to farm production.51 Conclusion Technologically, ground source heating systems seem to be viable in Vermont. Bedrock that is conducive to well drilling, as well as significant groundwater availability, allows for effective installation of standing column wells, and, although the colder climate contributes to lower average ground temperatures and thus decreased system efficiency, this factor does not significantly diminish the technical suitability of GSHS in the state.52

50 Luce, interview. 51 Frederic Jobin-Lawler, March 25, 2016. 52 Luce, interview.

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Economically, however, GSHS seem unlikely to be a broad-based solution for greenhouse heating in Vermont. Despite significant savings over time, both in economic and CO2 terms, the major upfront cost of these systems is prohibitive in the context of the rural, relatively low-income farming population.53 The payback period for GSHS is at least 10, and up to 20, years for moderate heat users, and this is well beyond the typical payback period that growers tend to look for in their investments. However, it seems likely that GSHS might be viable for large commercial year-round growers, who utilize a significant amount of heat and therefore would realize system savings over a shorter period of time.54 This technology does not seem suited to adoption by small growers, or those who are seeking to utilize greenhouses only for season extension Compost Introduction

In the late 1970s, French permaculturist Jean Pain presented the world with a profound new system for thermal heat exchange. His groundbreaking invention was, somewhat anticlimactically, nothing more than a water hose wrapped around a mound of woody debris and dirt. An unknowing visitor taking a stroll through Pain’s farm could have easily dismissed his brainchild as a pile of waste waiting in idle to be forgotten. In reality, though, his compost heap was a fully throttled engine of microbial decay churning out over 30,000 Btus per hour. That thermal output, transferred into coiled water tubes to heat his home, was enough to spark a revolution in the way the permaculture community began to think about harnessing cheap, naturally abundant thermal energy.55 But Pain’s ingenuity hasn’t remained confined in the niche realm of permaculture. As the market for local food in Vermont grows, many sustainability-minded farmers are beginning to think about compost heating as a productive season extension fuel source in their traditional operations.

While designs for compost heat transfer systems vary widely in complexity and fixed costs, all systems share a common desire to tap into compost’s promise of naturally abundant, low variable cost energy. Depending on the efficiency of a system, active compost heat exchangers can offer anywhere from 1,000-2,000 Btu/hour/compost ton and require only minimal electricity requirements to power the pumps that move hot water through the thermal loop. Meanwhile, Vermont’s 135,000 head dairy industry and 12 existing licensed commercial compost facilities offer a preexisting supply chain for raw compost material. With modest adoption of compost heating, this supply chain has the potential to scale up and offer both buyers and sellers of compost a more efficient marketplace for transactions without the characteristic capital barriers associated with distribution channel scaling.

Still, compost heating has its setbacks, particularly in the context of Vermont’s harsher climate and farmers’ limited access to capital. The most efficient compost thermal recovery designs can cost up to $55,000 dollars per unit and offer varying simple paybacks

53 Andy Cay, interview. 54 Luce, interview; Andy Cay, interview. 55 Poulain, Nicolas (1981). "Jean Pain: France's King of Green Gold." Reader's Digest.

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of anywhere from 5 to 20 years.56 Simultaneously, like many other alternative fuel sources, compost heating often requires a backup thermal generator, such as a propane powered heat pump or wood stove. With a wide range of temperatures in the shoulder season months of February-April, compost alone can be a risky strategy for season extension for farmers growing higher thermal demand crops like tomatoes, peppers, and eggplant. While more sophisticated systems are capable of “banking” thermal output in water storage tanks, these types of setups can take up valuable growing space and can simultaneously prove quite costly.57

As farmers consider whether compost thermal heat adoption makes sense for their operation, two factors emerge as paramount. First, simple paybacks on the more sophisticated thermal heat exchangers are invariably tied to scale. Small season extension operations simply can’t justify the initial capital expenditure associated with the most efficient closed loop setups. Second, there is always a balance between cost and hassle. Unlike a gas-fired water heater or electric powered heat pump, composting necessitates significant variable labor inputs that, if excessive, threaten the attractiveness of the investment. The key barrier compost heating will need to overcome in the years ahead—beyond our current anemic fossil fuel costs—will be standardizing a system of installation and maintenance for smaller scaled systems that minimizes operational risk and increases system predictability.

Technical Overview

Current thermal compost applications in Vermont can generally be divided into two categories—passive radiant heating and active thermal heat transfer. The latter can be divided further on a continuum of technological complexity. Passive Radiant Heating

This is far and away the simplest and the cheapest application used to harness the thermal output of compost’s microbial decay. The heat generated from the compost is simply radiated into the soil and air surrounding the crop, usually from barrels or cartons positioned underneath a raised bed. Thus, this open loop system requires no heat exchange infrastructure nor any added electricity inputs.58

In the late 80s, the New Alchemy Institute experimented with a fairly basic passive thermal setup in Massachusetts. Barrels filled with 4 cubic meters of manure were lined across the perimeter of a 700 square foot growing house. Large fans were positioned against the walls to blow through the barrels, moving energized water vapor from the compost to the soil. The energy transfer was completed when the water vapor condensed on top of the soil, resulting in a “heat blanket” across the bed. Technically, the electricity-consuming fans made the setup an active thermal unit. But at an annual electricity cost of only $40 to power these blowers, the active component of the system proved economically negligible. 59

56 Brown, Gaelan. "Interview With AgriLab." Personal interview. 15 Mar. 2016. 57 Grubinger, Vern. 22 March 2016. 58 Whalen, Michael (2009). "Manure Compost as Passive Greenhouse Heating." Love Apple Farms. 59 Chambers, Donald (2009). “The design and development of heat extraction technologies for the utilization of compost thermal energy”. Galway Mayo Institute of Technology Education and Training Awards Council.

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Understandably, passive compost heating is attractive in its low cost of installation and minimal technical demands. But this simplicity comes at a significant cost. The energy radiating from the compost lacks any thermal concentrating force, resulting in a fairly inefficient heat transfer from bin to the plant target.60 Accordingly, most passive compost employment is confined to commercial operations in more temperate climates and non-commercial gardening.

Figure 10.

Passive compost thermal heating by installing raised beds over heaps (South Pine Street City Farm in Kingston, New York).61

Active Thermal Heat Transfer

Jean Pain’s early woodchip mound serves as inspiration for the modern heat exchange systems we see in use today. In many respects, Pain’s setup, while less complex than contemporary competitors’ designs, is still viable for commercial use in Vermont today. At its foundation, the system acts like any other common heat exchanger, concentrating thermal compost output into a closed water or antifreeze loop that ultimately heats the soil. With larger mounds and higher temperatures, the system can be attached at the end to a water-to-air radiator to allow for radiant air greenhouse heating in addition to ground source heat.

60 Cay, Andy. "Integrated Solar Interview." Personal interview. 17 March 2016. 61 Image source: http://smallfarms.cornell.edu/2014/01/14/local-compost-materials-heat-community-greenhouse/

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Figure 11.

French permaculturalist, Jean Pain, standing next to his compost heat extraction prototype.62

Of advanced applications, the most efficient system to date—designed and retailed

by Vermont based Agriblab Technologies—employs an isobar technology that uses hot and humid compost air as the primary medium of heat transfer (Figure 12). Compared to the Pain’s hydronic medium, the isobar vapor technology can generate up to 40 percent more Btus/hour/ton of compost, resulting in a water capture temperature of around 146 degrees F. Moreover, it has a mechanized churning device that voids the need for significant manual compost turning.63

Diamond Hill Custom Heffers—a 2,000 head dairy farm in Sheldon, VT—was one of the first adopters of Agrilab’s isobar technology. Partly motivated by a desire to manage manure runoff, Diamond Hill owner Terry Magnan saw compost heat recovery as a regulatory compliance investment dovetailed with a nuanced thermal energy strategy. The system, which was installed in 2006, cost around $400,000 and can produce up to 150,000 Btus per hour. Much of this heat is used to warm calf barns and calf milk, but Agrilab’s founder, Joseph Ouellette, notes that the heat could be diverted to nearby commercial greenhouse operations as well. Agrilab envisions these large compost thermal extraction setups, in addition to increasingly prevalent methane digesters, as energy hubs that season extension growers could latch onto for cheap, reliable, and renewable heat.64

62 Image source: http://www.journeytosustainability.com/energyfromcompost/ 63 Brown, Gaelan. "Interview With AgriLab." Personal interview. 15 Mar. 2016. 64 Tucker, Molly (2006). “Extracting Thermal Energy from Composting”. Biocycle, 47.8.

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Figure 12.

A diagram of the Agrilab heat recovery system, which pumps hot vapor through an isobar array to heat water in the bulk tank.65

The question remains, however, if high volume compost thermal heat is truly

market competitive. When it came to financing the innovative installation, Mr. Magnan turned to both state and federal renewable energy funds for support. Ultimately, he procured a $197,000 grant from the USDA’s Natural Resources Conservation Service in addition to a $50,000 grant from the Vermont Agency of Agriculture, Food and Markets.66 The project likely wouldn’t have been pursued without this outside funding. Moreover, even if the Diamond Hill setup is financially viable for replication, Agrilab’s vision of excess thermal generation as a potential marketable revenue source remains unproven. The model hinges on growers intentionally setting up operations in the proximity of large, biowaste intensive dairy farms.

In many ways, though, this hub and spoke arrangement is analogous to existing, market validated alternative fuel sourcing strategies in the combined heat and power market. For example, in 2009 a BMW manufacturing plant in South Carolina constructed an 8-mile pipeline to tap into methane emissions from a nearby landfill. The resulting steam generated through combustion currently satisfies 50 percent of the plant’s thermal energy needs and reduces its annual utility bill by 30 percent.67 Key to the project’s success was recognizing and coordinating compatible energy demand and supply loads within a contained geographic zone.

Vermont diary farmers and season extension growers could apply this fuel sourcing strategy to compost. Recycling manure inputs through two process lines— aerobic

65 Image source: http://agrilabtech.com/products-new 66 Tucker, 2006. 67 Gowrishankar, Vignesh (2013). “Combined Heat and Power Systems: Improving the Energy Efficiency of our

Manufacturing Plants, Buildings, and Other Facilities”. Natural Resources Defense Council.

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composting for thermal heat extraction and anaerobic digestion for methane production—could potentially diversify revenue streams and increase energy recovery efficiency. This simultaneous generation of methane through digestion and hot water through composting would, in an ideal case, attract energy intensive agrarian businesses—greenhouses, meat processing facilities, and perhaps even wood treatment operations—into a consolidated economic hub.

Figure 13.

Hub and Spoke model for manure-centric VT regional energy economy. Costs and Benefits

Fossil fuel prices aside, compost heat adoption hinges on ease of use and integration. Unlike the “flip it on and walk away” mentality associated with traditional fuel use, both the traditional Jean Pain hydronic system and Agrilab’s isobar system require the user to acquire a general understanding of compost microbiology and attend to the pile. Between selecting an appropriate Carbon to Nitrogen ratio, maintaining air flow to reduce anaerobic decay, and monitoring internal mound temperature, the oversight demands can add up.68 At the same time, though, a single compost heap can maintain a relatively consistent output load for several weeks. In some applications, a steady load can be maintained for months.69

While Agrilab’s isobar model maximizes thermal output and minimizes labor input, it’s difficult to see how their current price point is practically compatible with the acute seasonal thermal demands of season extension greenhouses. This is not to suggest, of 68 Gorton, Sam. “Compost Power”. Cornell Small Farms Program. http://smallfarms.cornell.edu/2012/10/01/compost-

power 69 Gorton.

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course, that there isn’t a market for Agrilab’s heat exchanger at higher scales of production. Under the manure based regional energy hub model, high thermal output technology can create additional revenue streams that justify the cost. Increased coordination between Vermont’s dairy farmers and season extension growers on energy planning has the potential to reveal unrecognized market incentives for compost thermal extraction at commercially and industrially viable scales. Beyond distributive generation, simpler, low cost compost heat exchangers are arguably an attractive option for sustainability minded folks looking to avoid the traditionally high fixed cost of carbonless energy produced by, say, a ground-source heat loop. A simple Jean Pain style heat exchanger could be constructed for under $1,000. With an abundance of both compost knowledge and compostable material, Vermont offers a fertile backdrop for compost heat exchange experimentation at smaller scales of production. With proper funding incentives, growers could tinker with Jean Pain applications and create a network of regional compost thermal extraction knowledge. Still, for growers whose primary motivation is maximizing shoulder season productivity, there is a constant opportunity cost of fiddling with unfamiliar technology. An additional obstacle compost will have to overcome of course, beyond standardizing procedures to minimize labor inputs at lower scales of production, is storage and control. Unlike a propane tank for which you can turn up the dial when temperatures take a sudden dive, compost heat exchangers lack traditional variable thermal controls. Agrilab has experimented with the possibility of “banking” Btus for these peak demand moments, a storage mechanism that would allow heat accumulated over many days to be applied for the coldest February chills.70 But such an application would require larger storage tanks that take up a greater share of precious heated soil space. A more likely solution would arguably be to install a small backup thermal system that could cover this peak demand Btu margin, perhaps a small wood fired stove or air source heat pump. Conclusion Compost heat extraction will likely never become a staple energy alternative for Vermont season extension growers. But as a cog in the scaled out manure centric thermal hub model, the technology could prove viable. Moreover, for permaculture enthusiasts interested in tinkering with sustainable energy systems, composting processes surely present exciting integration and biological maintenance puzzles. Solar Thermal Introduction

The cleanliness and abundance of solar energy has made it an attractive option for non-fossil fuel based heating systems. A greenhouse by nature is designed to capture and utilize this energy, so when it comes to finding ways to increase the efficiency of greenhouses, it is natural that finding a way to further utilize the sun’s energy is the first thing that comes to mind. One way of doing this is through solar thermal heating systems, 70 Brown, Gaelan. "Interview With AgriLab." Personal interview. 15 Mar. 2016.

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which involve collecting and storing the sun’s heat through a variety of methods in order to redistribute it as needed (these systems differ from solar electricity, such as photovoltaic cells, which implies the direct conversion of solar energy into electricity). There are passive solar thermal systems, which complete this process without the use of mechanical inputs, and active systems, which increase their heating capabilities through mechanical components such as fans and pumps. Contrary to popular belief, solar thermal systems do not just provide heat – they can provide cooling and ventilation services as well, making them an extremely versatile investment.

Solar thermal energy systems offer enormous potential for the expanding use of greenhouses in Vermont because they are more environmentally efficient than traditional fossil fuel-based heating systems, but lack the complexity and the expense of more sophisticated green energy systems, such as heat exchange pumps or photovoltaic. Solar thermal systems are long-term resources, they do not require additional fuel, have extremely low emissions rates, have less payback time, and are often low maintenance.71 These characteristics make thermal heat, including the utilization of thermal mass as well as active and passive systems, a promising source of heat to support the expansion of greenhouse use in Vermont. Technical Overview Thermal Mass

Passive solar energy is one of the easiest and most efficient forms of greenhouse heating. It simply involves stones, cement, water or water-filled barrels and other passive thermal mass materials that serve as heat sinks, capturing the sun's heat during the day and radiating it back at night. Of these materials, water is the most adept at storing the sun’s energy (Table 1).

Table 1. Thermal mass material and Btu/sq ft.72

There are various ways to increase the capture of solar energy in thermal mass. One of the most important aspects of increasing thermal mass is orientation of materials. While

71 Reda Hassanien Emam Hassanien, 2016 72 Data from: http://www.greenhousegarden.com/thermal-massheat-storage

MATERIAL VALUE (BTU/Sq Ft./degree F)

Brick 24 Concrete 35 Earth 20 Sand 22 Steel 59 Stone 35 Water 63 Wood 10.6

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most greenhouses are constructed with orientation in mind, retro-fitting greenhouses to better incorporate thermal mass is unfortunately quite difficult. As a result, most farmers’ use of thermal mass involves simply placing barrels of water throughout the greenhouse, or using ground material especially adept at capturing the sun’s energy. In this case, the utilization of thermal mass is limited by the amount of space thermal mass objects take up. While this use of thermal mass can incrementally increase a greenhouse’s warming ability and perhaps dampen strong fluctuations in temperature, it does little to provide systematic heating. The best strategy is to encourage new greenhouses to incorporate thermal mass into their construction. While even a fully integrated thermal mass design will not provide enough independent heat to forego an independent heating system, it will significantly reduce the total heating load of the finished greenhouse. This in turn will reduce the energy requirements and give farmers more flexibility in choosing a heating system. Solar Thermal Systems

A similar, albeit more complex, concept to thermal mass is utilized by passive solar thermal systems. In a solar thermal system, heat is absorbed above ground during the day using a heat-transfer medium - often water or an engineered working fluid but air can also be used. The heat transfer medium is then pumped through pipes underground, where the radiant heat warms the soil during the night. At night, fluid is cooled and stored, and can then be released during the day to keep the house cool. One of the most important components of the solar thermal system is the solar collector, a device that absorbs the sun’s radiation and transfers it to a working fluid. From there, the fluid is either pumped into a storage tank where it can be drawn on for colder or cloudy days, or directly into the heating system.

There are three types of stationary solar collectors: conventional flat plate collectors, evacuated tube collectors, and compound parabolic collectors (Table 2).

Table 2. Different types of collectors and their properties.73

73 Image source: Kalogirou, 2004.

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In a flat plate collector, solar radiation passes through a flat glazed surface and is

absorbed by a transport medium (Figures 14, 15).

Figure 14.74 View of a conventional flat plate collector model.

Figure 15.

Conventional flat plate collectors on a residential roof. 75 While flat-plate solar collectors offer the simplest device for solar collection, their

design makes them prone to early deterioration due to weathering, resulting in reduced performance. 76 Flat-plate solar collectors also experience greatly reduced efficiency in cloudy or cold weather. 77 As a result of these drawbacks, the market for flat plate collectors has been slow to develop.78

74 Image source: Kalogirou, 2004. 75 Image source: http://gogreenheatsolutions.co.za/sites/default/files/kollektor_teton.jpg 76Kalogirou, 2004 77 Kalogirou, 2004 78 Yin, 2005

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Evacuated tube collectors present a more complex but more versatile design. They consist of a heat absorbing pipe in a vacuum sealed tube (Figure 16). The vacuum reduces heat loss, allowing the evacuated tube collector to be more efficient at lower angles and to have a better overall performance throughout the day.79

Figure 16. Schematic of an evacuated tube and an evacuated tube collector in use. 80

Compound parabolic collectors utilize a trough of reflective material to focus solar

radiation into a heat absorber (Figure 17). This system has the advantage of being efficient gathering solar energy over a wider range of incidence angles than the other two systems.81

Figure 17.Two different models of compound parabolic collectors. 82

79 Kalogirou, 2004 80 Image source: http://thesolarpanelpeople.com/greentimes/wp-content/uploads/2011/01/IMG_0109.jpg ;

http://www.homepower.com/sites/default/files/articles/ajax/docs/5_Evac-detail-shadows.jpg 81 Soteris, 2004 82 Image source: https://www.e-education.psu.edu/files/egee401/image/lesson07/Parabolic.jpg ; http://andyschroder.com/static/images/CPCEvacuatedTube/IMG_1562.jpg

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There are two types of solar thermal systems: passive and active. A passive system can either be an integral collector or a thermosiphon system. The integral collector systems are generally cheaper but unfortunately are not well suited for climates where temperatures can fall below freezing. Thermosiphon systems are more expensive but also more reliable. In this system, hot water naturally rises above cool water, eventually being pushed into a storage tank, where it then cools and sinks, falling back into the collector to be reheated and displacing the warmer water back into the storage tank (Figure 18).

Figure 18. Thermosiphon circulation in a simple solar water heater. 83

Solar thermal heating systems can and often are active, using electricity or some other

energy source to power a pump to move the fluid throughout the system. If desired, a heat exchanger can be also used to increase efficiency of energy conversion, which in turn increases electricity and capital requirements. Active solar systems are considerably more complex in both design and mechanism than a passive system. However, the increased heat production makes active solar systems an attractive option to many greenhouse growers. Costs and Benefits

In many ways, solar thermal is an extremely advantageous heating system, producing energy that is one-tenth the cost per kilowatt of photovoltaic power. Unfortunately the cost-barriers to active solar thermal systems are extensive, even more so because a back-up heating system would still be required. A medium-scale solar thermal system is estimated to cost around $6,000 to purchase and $3,500 to install, and most have payback periods of around 4 to 8 years depending on the cost of the fuel they are replacing.84,85 These thermal systems are effective at warming soil to preserve roots in cold weather but are not capable of warming an entire greenhouse. So while passive 83 Image source: http://www.treehugger.com/renewable-energy/do-solar-thermal-hot-water-heaters-still-make-

sense.html 84 Öztürk, 2005 85 Soteris, 2004

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solar thermal systems can be used to heat the soil, that will only help to extend the season and it is difficult to justify installing a system that will only be useful for a quarter of the year.86 One way to overcome this is to find alternative uses for the system, and this is one benefit of the air to ground heat exchangers – their ability to provide cooling as well as heating increases their overall viability.

Active solar thermal systems – by far the most common – can vary in price considerably. One study found that a solar thermal system costing a total of $14,675 could provide 83% of the heat load for a 24 foot by 60 foot greenhouse in Vermont for May and June and between 10% and 20% of the heat load in April and March.87 This cost translated to price of about $15.29 per million BTU if the system was assumed to have a 20 year lifespan.88 Whether or not this makes the system economic depends on fuel prices among other factors, but in general, the cost of solar thermal heating must arrive at around $30 to $15 per million Btu’s to make the system financially viable.89 The above system was estimated to result in a reduction of 1.3 tons of CO2 emissions if used for three months.90

Solar thermal systems are also limited by size requirements. Huge storage tanks are often needed to last through inclement weather. When located in the greenhouse heat loss from the tank can still work to warm the greenhouse; energy that would be completely lost if the tank were located outside. However, the tanks can take up tremendous amounts of space and storing them inside is rarely feasible. For example, a tank equipped to store enough heat to warm a 24’W x 60’L x 10’H house for three days of cloudy weather would need to be as large as 6000 gallons. In a crowded greenhouse, this could represent a significant loss of space.

Another limiting size factor is panel space. One farmer estimated that to install an approximately 250,000 BTU system would require 27 panels at around three by five feet each. Many farmers will feel that the reduction in emissions is not worth losing that much space, especially when it comes with such a hefty price tag. This system would probably cost around $16,000, as where a propane heater system of similar power would cost around $2,000.91

One advantage of a solar thermal system is that once it is installed, subsequent energy is either free or extremely cheap (depending on electricity requirements for a heat exchanger). This gives substantially more flexibility and control to farmers who would not have to worry about minimizing fuel costs. One farmer said that once he had installed a solar system, he would extend the season in which he was using heat in order to provide better moisture control.92 In this sense, the increased flexibility granted by solar thermal allows the farmer more control over his system and the ability to diversify his operations. Of course one of the advantages of solar thermal is that there are numerous ways to improve efficiency. Toying with transmission fluids and surfaces can dramatically

86 VERN call 87 Callahan, 2010 88 Callahan, 2010 89 Bezdek et al. 1979 90 Cllahan, 2010 91 Interview with Gildrien 92 Callahan 2010

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affect power output, while improvements in heat exchanger technology can reduce electricity requirements and improve efficiency (Table 3).

Table 3. Effects of various transmission fluids and surface on overall heat transmission coefficient. 93

To date the capital requirements for solar thermal systems mean they have been most prevalent in residential use. The use of thermal systems in residences in Vermont has however been favorable. Vermont thermal system installer Ed Butler estimated that a typical system provides 100 percent of a family’s hot water needs May through September, with only 100 watts of outside electricity to power a pump. The rest of the year, the system needs an alternative heat source, but can still provide around 60 percent of heating needs. He estimated the total payback period as eight to ten years.94

While home solar thermal systems are growing in popularity, residential systems can get extremely complex and are rarely user friendly (Figure 19).95 This complexity combined with the larger cost means that most residential systems are not transferable to greenhouses.

93 Image source: engineeringtoolbox.com 94 Costanzo, 2007 95 Integrated Solar Interview

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Figure 19.

Design of a residential off-grid thermal heating system.96 While the use of small-scale thermal systems has been relatively overlooked in the US, the system’s relative simplicity and cost-effectiveness has made it a popular source of energy in other parts of the world. In China, passive solar systems have sprung up as an efficient way to bring heat and hot water to a population of 1.3 billion and use of solar thermal systems is currently increasing at a rate of approximately 30% annually.97 One study found that most Chinese houses equipped with thermal systems saved around 60-80% of energy that would otherwise have been used with only a 12-40% increase in required initial investment and a payback period of 3 to 10 years.98 There are a handful of farmers using solar thermal systems in the US. Jeremy Gildrien, a farmer in Vermont, previously had purchased a solar thermal system so as to reduce his dependence on fossil fuels. A six panel system rated for eight to ten megajoules a day cost around $10,000 to install. The farmer was able to have the entire installation paid for through a REAP grant. He overall likes the system, especially the sustainability component and the quietness. As he said, there was something culturally significant about growing plants without propane or machinery. Despite this generally positive viewpoint, the grower also cited several complaints common with new energy sources:

“There’s a lot of history and engineering behind making these things [traditional systems] work. With all of this new tech, there’s no history of use. There’s not an industry built up around making these things work well. Plus these energy systems weren’t even designed for greenhouses, they’re designed for insulated buildings with a constant heat load, so there’s lots of things you have to work around. That coupled with the fact that you can’t buy it off the shelf. Propane air furnace you buy off the shelf and you’re good to go. Not

96 Image source: netgreensolar.com 97 Lu and Luo, 2002 98 Yin, 2005

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this stuff. You don’t even know how to install them. Not everyone wants to spend the time and mental energy figuring this stuff out. That coupled with the price. Not everyone has that kind of cash flow.”

These concerns characterize every new energy systems. A lack of easily

accessible technological support for installation and maintenance dramatically reduces the ease of use when compared to traditional systems. The costs further complicate the equation because most farmers will be required to put together a grant application process to receive funding. Overall, there needs to be a very large incentive for a farmer to actively seek out a solar thermal system (if he or she has heard about them at all) and given current fuel prices and the lack of developed industry, that incentive is not adequately large. While solar thermal systems are not currently very popular, both passive and active solar thermal have potential in the long-term for wide spread use in Vermont. Passive solar thermal systems are only capable of delivering relatively low-grade heat and as such will not usually be able to provide enough heat for a greenhouse or hoop house; a back-up heating system will almost always be needed to supplement in case temperatures drop too low or weather is overcast. While active solar thermal systems are capable of providing more heat at more consistent levels, the complexity of the system makes it unattractive to some farmers, and without funding, the costs may be prohibitively expense for the small-scale Vermont farmer. Despite these drawbacks, the relative ease of installation and use make both active and passive solar systems attractive options for varying degrees of season extension. Conclusion While these systems are unlikely to become widespread as a main source of heating, their potential for supplemental heating for season-extension is favorable. They are relatively low technology and low cost, and have the benefit of acting as both a source of heating and cooling. While retrofitting standing greenhouses with thermal heating systems may be difficult, including systems in new greenhouse designs are fairly straight forward and can provide substantial benefit. Encouraging their implementation is a matter of familiarizing farmers with these systems as well as providing technical support and funding.

There are some disadvantages when considering solar thermal systems’ application in Vermont. Similar to photovoltaic, solar thermal relies on consistent energy from the sun to create energy. If weather is cloudy for too long, the system will lose its ability to provide heat and a back-up system will be needed. Additionally, the colder the ambient temperature becomes, the more heat it takes to warm the greenhouse to the necessary temperature, and the quicker the solar thermal system loses power. These attributes make solar thermal systems ill-adapted to Vermont weather, which can experience extended days of both high and low temperatures, especially during the shoulder seasons.99 These technical drawbacks are small compared to the prohibitive costs or technological complexity of other renewable energy sources. Additionally, while this

99 Benoy, 2014

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section has focused mainly on solar thermal energy systems, the incredible natural efficiency of thermal mass should not be overlooked. Farmers should be encouraged to utilize these systems as much as possible. Air Source Heat Systems Introduction

Air source heat systems (ASHS) were first proposed as an alternative heating and cooling method for greenhouses during the oil shock of the early 1970s. However, until recently, ASHS adoption in greenhouses has remained low due to a low coefficient of performance (COP) and a high cost of installation. ASHS tend to be less expensive and easier to install than ground source heat systems (GSHS), which has allowed them to become the more popular of the two for regular heating and cooling demands. However, until recently, ASHS remained too inefficient to be practical in greenhouses. Due to the popularity of ASHS for other applications, competition has recently led to significant improvements in the COP and has reopened the possibility for use in greenhouses.100 However, very few studies exist to determine the actual performance and practicality of the pumps in a greenhouse setting.101 Technical Overview Air source heat pumps can be utilized in two different ways to heat a greenhouse. Air-to-Air Heat Pump Systems Air-to-air heat pump systems use electricity to transfer heat from the outside air to the inside of the greenhouse. The heat pump directly heats the air of a room using a wall-mounted box.102 The reversible refrigeration system consists of a compressor and two coils made of copper tubing (one indoors and one outdoors), which are surrounded by aluminum fins to aid heat transfer. 103 The system uses a fan to pass air through the first set of coils that absorb the large volume of low temperature heat. This heat is then converted to a small volume of high temperature heat by the heat pump’s compressor. The high temperature refrigerant is then passed over the second set of coils, which pick up the heat and transfer it into the greenhouse in the form of warm air. The now cool liquid refrigerant then makes another trip back outside to pick up more heat from the outdoor air, and the cycle continues whenever additional heat is required by the space. The process can also be reversed to cool a greenhouse during warm weather.

100 “Air Source Heat Pumps.” US Department of Energy. available at: http://energy.gov/energysaver/air-source-heat-

pumps 101 Tong, Y., et al. "Greenhouse heating using heat pumps with a high coefficient of performance (COP)." Biosystems

engineering 106.4 (2010): 405-411. 102 I. Staffell, D. Brett, N. Brandon and A. Hawkes, 2012. A review of domestic heat pumps, Energy Environ. Sci., 5, 9291-

9306. http://www.academia.edu/2300785/A_review_of_domestic_heat_pumps 103 “Air Source Heat Pumps.” US Department of Energy. available at: http://energy.gov/energysaver/air-source-heat-

pumps

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Figure 20. Air source heat pump technical diagram.104

Air-to-Water Heat Systems

Air-to-water heat pump systems use the same technology as air-to-air heat pump systems, but utilize water to store the heat collected. The upgraded heat is transferred through refrigerant piping to a refrigerant-to-water brazed plate heat exchanger in the hydrobox. The water is then circulated through floor heating systems, fan coil units, low temperature radiators, or stored in a hot water tank.105

During the shoulder months of early spring and late fall, as well as during sunny winter days, greenhouses require ventilation to release excess solar heat during the daytime hours. Air-to-water heat pumps can also be utilized inside the greenhouse to remove the excess heat during the day, store it in water, and release it back into the air at night. 104 Image source: Nepus.com 105 Image source: Daikin Whitepaper by Daikin AC Americas, Inc. Accessed on July 26, 2014,

http://hydronicheatpump.com/altherma/pdf/Daikin_Altherma_Binder/01_%20Brochures_FAQ/Daikin% 20Altherma%20White%20Paper.pdf

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Figure 21. Infrastructure of air source heat systems.106

The COP of ASHS is affected by outdoor and indoor operation conditions. Heat

pumps are able to more effectively extract energy from a respective source when the source temperature is near that of the temperature of the space to be heated. By the same principle, heat pumps are less effective when they are asked to heat during very cold temperatures. During optimal conditions, ASHS can have a COP of almost 6. However, when the source temperature drops below 10 °F, the COP falls to about 1, and the heat pump has no advantage over electrical heating. A study done in 2010 found that with an average temperature of 42 °F, air source heat pumps average a COP of 4. However, as temperatures become colder the COP drops. In colder climates, the COP falls to an average just above 2.107

ASHS can be utilized in a few different ways to heat a greenhouse. Air-to-air or air-to-water heat pump systems can be used to transfer outdoor heat into the greenhouse in the same manner as for residential homes or commercial office buildings. However, air-to-water heat pumps, due to their ability to store heat, can be placed inside of a greenhouse to extract the excess heat as passive solar heating raises temperatures higher than needed to grow crops.

A recent study done by the University of New Hampshire examined two identical 34 x 60 ft greenhouses, with one greenhouse operating an air-to-water heat pump as a complementary heating method. The system used two heat pumps, which were piped in parallel with the hydrobox, located just above the heat pumps. The hydrobox transferred the heat to and from a roughly 1900 gallon insulated hot water tank.

106 Image source: Daikin.com 107 Luce, Ben. “Reduction in Cold Season Greenhouse Heating Fuel Via Recovery of Excess Solar Heat with Heat Pumps.”

University of New Hampshire. Draft (2014)

Compressor Unit Hydro Box Hot Water Tank

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During February to April of 2014, the conventional greenhouse used 711 gallons of oil while the greenhouse with the heat pump system used only 456 gallons. The heat pump greenhouse used an extra $145 in electricity during the period.108 Similar to the findings of previous studies, the heat pumps were most effective during the shoulder months of late fall and early spring. The UNH study found that the air source required almost no supplementary heat use during this time. Costs and Benefits

Given the very large thermal energy demands of a typical cold season greenhouse, the use of an ASHS to heat a greenhouse by transferring thermal energy directly from the ambient air is not a very attractive option due to the relatively low COPs currently achievable in cold climates. However, using ASHS inside a greenhouse to capture excess solar heat allows for higher COPs. The COP for the transfer of heat from air to water inside greenhouses was found to be a promising 4.8. However, when using the heat pumps in reverse to transfer the heat back into the greenhouse at night, the overall COP drops to 2.15, similar to that of the traditional air-to-air heat pump. Fortunately, the authors of the University of New Hampshire study believe if they had used a bed heating or similar radiant system to redeliver the heat into the greenhouse, rather than transferring the heat back through the heat pump, they would have been able to significantly increase the COP.109

While ASHS will likely require a supplementary heat source during cold winter months, when utilized effectively they are able to offer increased efficiency and reduced emissions compared to traditional fossil fuel heating methods. ASHS may be ideal for farmers looking to utilize greenhouses for season extension rather than year-round growing, as less supplemental heat will be required. Additionally, ASHS offer off-the-shelf availability, simple installation, and relatively low upfront costs. Conclusion

It is important to keep in mind that the use of ASHS has just recently become efficient enough to potentially become feasible. The authors of the University of New Hampshire study utilized an innovative new application for air source heat pumps that showed promising but still ultimately unproven results. Combined with an effective and efficient heat redistribution system, air-to-water heat pump systems during shoulder months have the potential to be as effective as GSHS in greenhouses with a significantly lower upfront cost.

In summary, compared with fossil fuel sourced heating systems, the potential advantages of using air heat pumps for greenhouse heating include:110

108 “’Heat-pump' Up Greenhouse Heating; Cut Heating Oil Use.” Farm Progress.com. (2014) available at:

http://farmprogress.com/story-heat-pump-greenhouse-heating-cut-heating-oil-use-9-121119 109 Luce, Ben. “Reduction in Cold Season Greenhouse Heating Fuel Via Recovery of Excess Solar Heat with Heat Pumps.”

University of New Hampshire. Draft (2014) 110 Tong, Y., et al. "Greenhouse heating using heat pumps with a high coefficient of performance (COP)." Biosystems

engineering 106.4 (2010): 405-411.

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1. Reductions in primary energy consumption and costs for heating. The heat pump operates by moving or transferring and upgrading heat, thus consuming far less electricity than would be required to generate heat. 2. Reductions in the total emission of CO2 and atmospheric pollutants. Also, because the heat pump is driven by an electric motor, no pollutants are produced locally. 3. Achievement of spatially uniform air temperatures and moisture. 4. Enhancement of photosynthesis, transpiration, and uptake of water and nutrients by using the fan installed in the heat pump, due to the decrease in humidity. 5. Quieter operation, even with fans. Comparative Analysis

In order to determine optimal applications of the various alternative heating

technologies that have been explored and to assess their potential for use in Vermont greenhouses, a direct comparison across five key indicators has been conducted. These indicators include:

1) Lifespan: The average expected years of system operation before units must be replaced

2) Coefficient of Performance (COP): A measure of system efficiency; reported as a ratio of heat delivered to electricity required to run the system

3) Capital Cost: The upfront cost of installation of a heating system, inclusive of labor and installation costs

4) Payback Period: The expected time, in years, that savings accrued from a system will outweigh capital costs; this metric depends both on capital costs and usage of the system

5) Ease of Use & Integration: A qualitative metric incorporating both the regular work required by growers to operate and maintain a system and the initial installation of a new system into a greenhouse

In this analysis, assessment of each alternative heating technology according to these indicators has been conducted based on a review of existing literature and conversations with industry experts. Where available, data on biomass systems has been included; a comprehensive review of biomass technologies was not included in this report, as UVM’s Chris Callahan and Vern Grubinger have undertaken a full analysis of biomass heating systems in the context of Vermont greenhouses, which should be referred to for

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specific technical details.111 The results of this comparative analysis are presented in Figure 22. The data presented reflect averages or, where relevant, ranges of values determined in the course of our research.

Figure 22.

Comparative analysis of alternative heating technologies across five key indicators.

111 Callahan, Chris, and Vern Grubinger. “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in

Vermont.” University of Vermont Extension, September 4, 2015. http://www.uvm.edu/vtvegandberry/Pubs/BiomassHeatingVermontGreenhouses2015Report.pdf.

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Based on this comparative analysis, the benefits and drawbacks of each of these systems have been compiled in Table 4.

Table 4. Comparisons of benefits and drawbacks of alternative heating technologies, based on

five key indicators.

Specific Comparisons of GSHS and ASHS Technologies Ground source heat systems (GSHS) and air source heat systems (ASHS) rely on the same heat pump technology, but differ in three fundamental ways: the heat source, the heat collector system, and their efficiency rates. The Heat Source - ASHS collect heat from the outside air, meaning that efficiencies vary depending on the temperature. GSHS collect heat that is just underneath the surface of the earth, which has a consistent temperature regardless of season. The Heat Collector System - GSHS rely on a working fluid that is circulated through pipes buried in the ground to absorb the ground’s heat. ASHS draw their heat from the surrounding air using a fan to pass air through its refrigerant system. Efficiency Rates – GSHS can draw on a consistently warm source of heat in the ground, allowing for a consistent COP of around 4. ASHS have a much less consistent COP depending on the temperature that usually ranges from 2-5 but can drop close to 1 during very cold temperatures.

Currently, GSHS have a stronger market presence than ASHS; this is advantageous in terms of improving customer familiarity with these systems, and competition between multiple suppliers also tends to lead to lower costs and improved functioning. However, GSHS are drastically more expensive than ASHS, due to the need for excavation and the installation of piping networks. ASHS, on the other hand, have a much simpler installation and retrofit process.

Biomass Compost GSHS ASHS Solar Thermal

+

• Payback period • COP • Cost (Jean Pain)

• Lifespan • COP

• Ease of use • Cost

• COP • Ease of use • Cost • Lifespan

- • Difficult to use • Cost (Isobar)

• Difficult to use • Cost • Payback period

• COP • Reliance on weather (sun)

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It is our opinion that GSHS should be explored as an alternative heating source for large commercial year-round growers, but that ASHS may be more viable for smaller growers and those looking to use heating systems only for the shoulder seasons.112 Recommendations Based on the results of Table 4, we have determined that the alternative technologies explored broadly fall into two categories: those that are most appropriate for season extension (moderate usage during the shoulder seasons of early spring and late fall), and those that are most appropriate for year-round use. A summary of these recommendations is presented in Table 5.

Table 5. Recommendations of optimal applications for each alternative heating technology.

Biomass Compost GSHS ASHS Solar Thermal

Shoulder season heating

Jean Pain (small scale) systems for shoulder seasons

Isobar (large scale) systems for year-

round, large commercial operations

Year-round use for large commercial applications; can operate in both

heating and cooling mode

Shoulder season heating

Shoulder season heating; cooling during

summer months

In the specific context of Vermont, which is currently dominated by small-scale farmers who utilize greenhouses primarily for season extension, solar thermal appears to be the most promising alternative heating technology. The only significant drawback to solar thermal that has been identified in this analysis is the necessity of a backup heating system for lengthy periods of minimal sun. As technologies improve, it may be possible for these backup heating systems to utilize renewable energy, but currently, it seems likely that these backup needs will be met with propane heaters. While this is not ideal from a greenhouse gas standpoint, growers may be more willing to experiment with a new, alternative heating technology such as solar thermal if they simultaneously maintain smaller propane systems, which they are familiar with and are known to be adaptable and easy to use. Thus, the combination of solar thermal and a smaller backup propane system is suggested to be the most well suited option for greenhouses in Vermont, of the alternative heating technologies under consideration in this analysis. 112 Ben Luce, “Reduction in Cold Season Greenhouse Heating Fuel Via Recovery of Excess Solar Heat with Heat Pumps

(Draft),” Fall 2014, http://lsc-natural.weebly.com/uploads/4/0/1/3/40134717/technical_report.pdf; Andy Cay, March 17, 2016.

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Incentives and Financing Sources

Vermont has declared a goal of achieving 20% renewable energy by 2017 as part of the state’s Sustainably Priced Energy Enterprise Development (SPEED) program.113 In order to achieve this ambitious goal, the state offers numerous opportunities for financing and tax rebates that could be applied to farmers using alternative heating methods in greenhouses. There are also several federal pathways for achieving funding or rebates. In general, incentives and financing come in the form of tax rebates, low-interest loans, cost rebates, grid-metering (where utilities will buy energy generated from renewables, often at a premium) and grants. While pathways to achieve financing appear to be numerous, there are in fact constraints that limit the financing support infrastructure in Vermont. Thermal technologies are not tied to electricity incentives, and therefore only qualify for incentives specifically intended for thermal systems. These incentives are often considerably less than similar electricity incentives. For example, none of the incentives offered by Efficiency Vermont apply to thermal systems. This lack of funding can dramatically impact the cost-effectiveness of thermal systems and does not take into account the embedded energy and carbon savings provided by thermal energies. Additionally, the majority of incentives for clean energy are based in tax rebates. This means that the amount of the incentive is constrained by the amount of taxes the farmer pays. Given that many of these farms are low-scale, small margin operations and therefore pay lower taxes, tax rebates do not always provide an adequate incentive. The small size of most greenhouse operations in Vermont also disqualify them for project size requirements in many programs, and also make grid-metering an unlikely source of funding. As renewable energy systems grow in popularity, the Vermont and federal governments are doing more to actively incentivize their adoption. In 2015, the USDA encouraged small rural businesses to apply for loans and grants for the installation of alternative energy systems, including newer technologies such as solar thermal, renewable biomass, and geothermal.114 In 2015, the Renewable Energy Resource Center launched a pilot program to incentivize the installation of up to 25 solar heating systems.

As the expansion of greenhouse use and season extension continues in Vermont, it is essential that the government incentivizes the adoption of clean energy systems. Energy Service Provider Model

As growers consider alternative fuel strategies, financial and operational constraints are ubiquitous. How long will a project take to achieve a simple payback? What is the lifespan of a given system? Is there a way to seamlessly integrate the new fuel source into my existing operations? Even if a grower can reconcile these overlapping anxieties, upfront costs can throw a wrench into implementation.

The energy service provider (ESP) model (Figure 23) aims to address the most persistent of these implementation barriers. As one study on the ESP’s effectiveness 113 Clean Energy Authority, 2015 114 http://biomassmagazine.com/articles/12472/usda-seeks-applications-for-reap-loans-grants

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summarized, “energy service contracts allow the client to reduce operating costs, transfer risk and concentrate attention on core activities.”115 While we have not found an energy purchase agreement model catered strictly to thermal needs in the agricultural sector, the concept is a simple extension of thermal purchase agreement (TPA) in other sectors of the economy.

Figure 23.

Proposed Energy Service Provider Model for the funding and development of alternative heating sources in Vermont greenhouses.

One hundred percent of the new fuel system would be funded upfront with ESP

capital. Each month, instead of paying their typical fuel bill, growers would send a service payment to the ESP, which would reflect that month’s marginal fuel cost, amortized fixed cost, and the ESP’s return on equity. As the owner and chief operator of the asset, the ESP would be contractually responsible for system reliability, including maintenance and routine performance monitoring and verification. In this sense, the TPA essentially acts as both a financing vehicle as well as an insurance policy on the fuel system asset. Through supply chain scaling and a consolidation of operational expertise, the ESP can simultaneously drive down project cost.

The TPA model would be applicable to alternative heating methods with large upfront costs and simple paybacks stretching well beyond their fossil fuel competitors. The amortized payment schedule internalized in the TPA structure would allow farmers to afford sustainable heating methods that otherwise would not be feasible. There is also a

115 Sorrell, S. (2007). The economics of energy service contracts. Energy Policy,35(1), 507-521.

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certain degree of operational expertise associated with each of these technologies that make the transfer of project risk a salient selling point for growers. A grower wouldn’t have to worry about adjusting compost heap inputs, or fixing a cracked pipe in a geothermal loop; tasks of this sort would be transferred to the ESP.

Of course, the ESP would be placing a bet on building scale across the region. A few potential projects sprinkled throughout the state wouldn’t satiate entrepreneurial interest. In order for the model to be enticing on the supply side, it would be appropriate to think about total demand from a regional standpoint. An ESP market entrant would likely look to aggregate demand across the Northeast and possibly offer numerous alternative fuel services, reflecting variability in sub-regional fuel demand. Nonetheless, all projects in the service provider’s management would be tied together in a bundled portfolio, with more conservative fuel implementation schemes hedging the risk presented in more ambitious projects.

One potential setback to the greenhouse thermal TPA model has to do with the ESP’s financing scheme. In most TPAs, projects are funded through a blend of service provider equity and third-party debt. Both the debt and the equity require the due diligence of customer credit screening, but often the credit requirements dictated by the issuers of debt are far more stringent. This could make the majority of projects in Vermont unbankable. Moreover, the ESP needs to ensure adequate asset recovery in the event that a grower defaults on his or her service payment. With systems like geothermal and compost heating, asset recovery can present challenges. How can an ESP recover the cost of drilling a geothermal well or constructing an embedded compost system?

Of course, there are likely ways around both the customer credit and asset recovery constraints. On the credit side, there are funding vehicles specifically catered to promoting renewable fuel adoption. Green banks in states like New York and Connecticut are currently at work pumping hundreds of millions of dollars into alternative energy investments. In Vermont, the Vermont Economic Development Association provides financing for energy-related projects throughout the state, but projects typically fall in the $500,000-1 million range, well beyond greenhouse thermal costs. Nonetheless, government sponsored loan programs—both at the federal and state levels—can take on higher customer credit risk for thermal TPAs if they’re positioned to drop into smaller project financing opportunities. Meanwhile, on the asset recovery front, service providers and OEMs have begun to produce modular generating units. Creating flexible applications allow companies to reduce risk and expand their potential customer base.

The adoption of sustainable alternative heating methods in greenhouses will likely require a dedicated effort from farmers, manufacturers, service providers, and government sponsored programs. To ensure that these methods can be feasible, numerous routes will need to be explored. The TPA model represents one option that can help farmers get over financial and operational hurdles to make heating greenhouses with sustainable systems achievable.

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Conclusion As both demand for local food and weather variability in Vermont increase, the increased reliance on greenhouses will magnify the need for more efficient and less environmentally detrimental heating systems. The heating systems we chose to examine – ground source heat systems (GSHS), compost, solar thermal, and air source heat systems (ASHS) – all have the potential to be used in greenhouses across Vermont in various circumstances. While their dependence on optimal weather conditions mean they cannot provide heat year round, ASHS and solar thermal both provide inexpensive low-grade heat that makes them useful for season extension in smaller greenhouses. While Jean Pain compost systems cannot provide as much heat as ASHS and solar thermal systems, their ability to provide consistent heat makes them a valuable source of heat year round. Meanwhile, the more complex Isobar compost systems and GSHS could provide efficient, high-grade heat that would be economic for large-scale operations. All systems have the potential to be successful if effectively applied in optimal conditions. While theoretically these systems have applicability across the industry, real barriers still exist that impede widespread adoption. The high installation cost of these systems relative to traditional heating systems discourages their use by growers chiefly concerned with maintaining their bottom lines. Relatively cheap fuel prices further extend the payback period, making the investment even riskier. There are also significant technological issues. Many of the technologies are originally designed for residential use, where structures are well insulated and generally demand a consistent heat load. As such, there is not a standardized procedure for translating those systems for use in greenhouses, which are usually poorly insulated and demand more irregular heating loads. Additionally, traditional heating systems are usually fairly easy to purchase and install, and if a grower has an issue with the system it is relatively easy to get help fixing it. Unconventional systems, on the other hand, do not have such a built-up institutionalized system of support – a farmer must contact an expert on the system to determine what size equipment is required, how to install it, and how to fix it if it runs into mechanical issues. Finally, as these technologies are not in widespread use many farmers are not even aware they are an option. For the broad adoption of these technologies in Vermont, these barriers must slowly be eroded. This means standardizing the installation process of the systems, perfecting and advancing the technologies, disseminating information on alternative options, insuring financing is available, and providing technological and mechanical support. One way of effectively achieving these results could be the utilization of an Energy Service Provider model where private companies own the thermal assets, which they then effectively lease to farmers. In this way, the lack of institutionalized technological support could be addressed while also reducing costs. In the meantime, organizations focused on increasing agricultural efficiency should emphasize the most viable technologies as alternative options. Based on efficiency, costs, and ease of use we have identified solar thermal (likely

Greenhouses 44

in conjunction with a back-up propane system, depending on growers’ individual needs) as the most viable option. While the previously mentioned barriers mean that these technologies have yet to be widely adopted in Vermont, they all have potential to mitigate the environmental issues associated with emissions from greenhouses. As greenhouse use becomes more widespread and the agricultural industry is faced with a growing need to increase efficiency, this environmental aspect as well as concerns over fuel costs will make these alternative heating systems more promising options. In the meantime, it is necessary to reduce barriers to adoption as much as possible through innovative business models, increased financial incentives for adoption as well as technological research, and increased industry support. Doing so will greatly facilitate the transition to a more sustainable industry.

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References Background: Chris Callahan and Vern Grubinger, “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in Vermont” (University of Vermont Extension, September 4, 2015), http://www.uvm.edu/vtvegandberry/Pubs/BiomassHeatingVermontGreenhouses2015Report.pdf. Johnson, Pete. “Pete Johnson Interview”. March 19, 2016. Callahan and Grubinger, “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in Vermont.” John W. Bartok, “Increase the Efficiency of Your Greenhouses” (Storrs, CT: University of Connecticut, n.d.). Scott Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating” (Wisconsin Focus on Energy / Rural Energy Issues, University of Wisconsin, n.d.); Patricia A. Rorabaugh, Merle H. Jensen, and Gene Giacomelli, “Introduction to Controlled Environment Agriculture and Hydroponics” (Tuscon, AZ: Controlled Environment Agriculture Center, Campus Agriculture Center, University of Arizona, Fall 2002). Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating.” Callahan and Grubinger, “Promoting Adoption of Biomass Fuels for Heating Vegetable Greenhouses in Vermont.” Frederic Jobin-Lawler, March 25, 2016; Johnson, interview. Grubinger, Vern. “Vern Grubinger Interview”, March 22, 2016. Ellen Kahler et al. (2013), “Farm-to-Plate Strategic Plan” (Vermont Sustainable Jobs Fund, July 2013). Vermont Department of Public Service (2016).“2016 Vermont Comprehensive Energy Plan” J. Both, “How Can You Lower Your Greenhouse Energy Bill?” (New Brunswick, NJ: Rutgers University, August 2008); Rorabaugh, Jensen, and Giacomelli, “Introduction to Controlled Environment Agriculture and Hydroponics”; T. E. Bond, James F. Thompson, and Ray F. Hasek, “Reducing Energy Costs in California Greenhouses” (Cooperative Extension, University of California, n.d.); Bartok, “Increase the Efficiency of Your Greenhouses”; Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating.”

Andy Cay, March 17, 2016; Ben Luce, March 14, 2016.

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Both, “How Can You Lower Your Greenhouse Energy Bill?”; Rorabaugh, Jensen, and Giacomelli, “Introduction to Controlled Environment Agriculture and Hydroponics”; Bond, Thompson, and Hasek, “Reducing Energy Costs in California Greenhouses”; Bartok, “Increase the Efficiency of Your Greenhouses”; Sanford, “Reducing Natural Gas / Propane Use for Greenhouse Space Heating.” Ground Source Heating Systems: Luce, Ben (2016). “How Geothermal Systems Work,” Green Mountain Geothermal, http://www.vermontgeo.com/how_geothermal_systems_work.html Luce, Ben (2001). “Heating Your Home or Business in Vermont with a Geothermal System” http://www.nvda.net/files/GeothermalHeatPumpsGuide.pdf. “Heat Pumps,” EnergyGroove.net http://www.energygroove.net/technologies/heat-pumps/. Alain Nguyen, Philippe Pasquier, and Denis Marcotte (2012) “Multiphysics Modelling of Standing Column Well and Implementation of Heat Pumps Off-Loading Sequence,” in Proceedings of the 2012 COMSOL Conference in Boston. https://www.comsol.no/paper/download/151259/nguyen_presentation.pdf. Ross, Jill (2010), “Bringing Geothermal to the City,” Water Well Journal, 27–31. Nguyen, Pasquier, and Marcotte, “Multiphysics Modelling of Standing Column Well and Implementation of Heat Pumps Off-Loading Sequence.” Johnson, Pete. “Pete Johnson Interview”. March 19, 2016. Compost: Brown, Gaelan. "Interview With AgriLab." Personal interview. 15 Mar. 2016. Cay, Andy. "Integrated Solar Interview." Personal interview. 17 Mar. 2016. Chambers, Donald (2009). “The design and development of heat extraction technologies for the utilization of compost thermal energy”. Galway Mayo Institute of Technology Education and Training Awards Council. Poulain, Nicolas (1981). "Jean Pain: France's King of Green Gold." Reader's Digest. http://journeytoforever.org/biofuel_library/methane_pain.html Sylla, Youssouf Boundou, et al. (2006) "Feasibility study of a passive aeration reactor equipped with vertical pipes for compost stabilization of cow manure." Waste Management and Research 24.5.

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Gorton, Sam. Compost Power. Cornell Small Farms Program. http://smallfarms.cornell.edu/2012/10/01/compost-power Tucker, Molly (2006). “Extracting Thermal Energy from Composting”. Biocycle, 47.8. Whalen, Michael (2009). "Manure Compost as Passive Greenhouse Heating." Love Apple Farms. http://www.growbetterveggies.com/growbetterveggies/2009/03/manure-compost-as-passive-greenhouse-heating.htm Solar Thermal: Benoy, Russel (2016). "Storing Excess Daytime Solar Energy to Heat Greenhouse at Night!" Storing Excess Daytime Solar Energy to Heat Greenhouse at Night. http://www.builditsolar.com/Projects/Sunspace/GreenhouseHX/HeatPumpEnergyStorage.pdf Bezdek, Roger H., Alan S. Hirshberg, and William H. Babcock (1979). “Economic Feasibility of Solar Water and Space Heating”. Science 203 (4386). American Association for the Advancement of Science, 1214–20. http://www.jstor.org/stable/1747235. Callahan, Christopher (2010). “Feasibility and Cost/Benefit Study of Solar Hot Water Greenhouse Heating System” University of Vermont. https://www.uvm.edu/vtvegandberry/Pubs/Solar_Hot_Water_for_Greenhouse_Heat.pdf. Clean Energy Authority (2015). "Vermont Solar Rebates and Incentives." http://www.cleanenergyauthority.com/solar-rebates-and-incentives/vermont/. Kalogirou, Soteris A (2004). "Solar Thermal Collectors and Applications." Progress in Energy and Combustion Science 30.3, 231-95. https://eva.fing.edu.uy/pluginfile.php/55828/mod_resource/content/1/kalogiru.pdf. NetGreen Developments LDA (2012). "NetGreen HEAT." NetGreen Heat. 2012. Web. 24 Mar. 2016. http://www.netgreensolar.com/netgreen_heat_promo/body/netgreen_system.html. NC Clean Energy Technology Center (2016). "Programs." DSIRE. US Department of Energy. http://programs.dsireusa.org/system/program?state=VT "Overall Heat Transfer Coefficients for Some Fluids and Heat Exchanger Surfaces." The Engineering Toolbox http://www.engineeringtoolbox.com/overall-heat-transfer-coefficients-d_284.html.

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Öztürk, H. Hüseyin (2005). "Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating." Energy Conversion and Management 46, no. 9: 1523-1542. Reda Hassanien Emam Hassanien, Ming Li, Wei Dong Lin (2016) “Advanced applications of solar energy in agricultural greenhouses” Renewable and Sustainable Energy Reviews, Volume 54, Pages 989-1001, http://www.sciencedirect.com/science/article/pii/S1364032115011740 Yin, Zhiqiang (2005) Development of solar thermal systems in China, Solar Energy Materials and Solar Cells, Volume 86, Issue 3, 31 March Pages 427-442, ISSN 0927-0248, http://dx.doi.org/10.1016/j.solmat.2004.07.012. Air Source Heat Pumps: “Air Source Heat Pumps.” US Department of Energy. http://energy.gov/energysaver/air-source-heat-pumps Luce, Ben and Krug, Brian (2014). “Reduction in Cold Season Greenhouse Heating Fuel Via Recovery of Excess Solar Heat with Heat Pumps.” University of New Hampshire. Tong, Y., et al. (2010) "Greenhouse heating using heat pumps with a high coefficient of performance (COP)." Biosystems engineering 106.4: 405-411. “‘Heat-pump' Up Greenhouse Heating; Cut Heating Oil Use.” FarmProgress.com. http://farmprogress.com/story-heat-pump-greenhouse-heating-cut-heating-oil-use-9-121119 Comparative Analysis: Cay, Andy. "Andy Cay Interview", March 17, 2016. Luce, Ben 2014. “Reduction in Cold Season Greenhouse Heating Fuel Via Recovery of Excess Solar Heat with Heat Pumps (Draft),” http://lscnatural.weebly.com/uploads/4/0/1/3/40134717/technical_report.pdf.