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Advancement of Distributed Solar Thermal Technologies in the U.S.

Prof. Jane DavidsonMechanical EngineeringUniversity of Minnesotajhd@me.umn.edu

Contributors: Susan Mantell & Lorraine Francis, UMNJay Burch, Gary Jorgensen & Tim Merrigan, NRELMike Rubio, FAFCOEric Lee, DEG & Harpiris Energy

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18%11%

48%

22%

0%

20%

40%

60%

80%

100%ResidentialCommercial

H2O Heating

Water and space heating represent 10% of total U.S. energy consumption.

75% of US households and commercial buildings are appropriate candidates for solar based on population density and climate.

Cost is perceived to be a major barrier to greater implementation

Market Potential

Industry∼32%

Buildings ∼40%

Transportation ∼29%

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Cost Targets

Solar America InitiativeCourtesy of Tim Merrigan, NREL

Goal: Cost-neutral zero-energy homes (ZEH) in all U.S. climate zones by 2020.

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5

U.S. R&D 1999-2009

Low-Cost Solar Hot Water– Paradigm shift from metal/glass

components to integrated systems manufactured with polymeric materials

Building integrated systems

PV/Thermal Systems

Space Conditioning– High density storage, IEA 4224

FAFCO polymer collector

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Low-Cost Polymer Based R&D Characterization of Glazings

– Optical properties– Mechanical integrity– Durability (optical and mechanical)

Absorbers– Materials & Design

Heat Exchangers– Thermal and mechanical design– Effect of hot/chlorinated water on antioxidant diffusion

in polyolefins – Growth and removal of CaCO3 “scale”

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Glazing Materials

Optical performance is stable for UV exposure equivalent to ~6 years in Fl. Acrylic films are too brittle to withstand mechanical stress. Transmittance of ethylene tetrafluoroethylene (ETFE) fluoropolymer films

remained stable in outdoor tests at 3 sites for 6 years and in the Ci5000 WOM for a UV dose equivalent to 39 years.

Equivalent Exposure in FL (y)

70

75

80

85

90

0 500 1000 1500 2000 2500 3000

Total UV Dose (MJ/m2)

Sola

r-W

eigh

ted

Tran

smitt

ance

(%)

0 2 4 6 8 10

Impact Modified Acrylic

Company B; Material 1

Company A; Material 2

Fiber Reinforced Polyester

UV Dose (MJ/m2)Exposure (years)

Impact Modified Acrylic

Fiber Reinforced Polyester

0 2 4 6 8 100 500 1000 1500 2000 2500 3000

Tran

smitt

ance

(%)

70

75

80

85

90

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Heat Exchanger DevelopmentMaterial selection for

strength and minimumconductive resistance

D

tP

0

0.1

0.2

0.3

0.4

0.5

0.6F

EP

NP

(2

5%

Gla

ss)

NP

(u

nre

inf.

)

PE

EK

PE

X

PF

A

PL

S

PP

A

PP

O

PP

S

PT

FE

PV

DF

Th

erm

al C

on

du

ctiv

ity

(W/m

-K)

Reinforced

Unreinforced

Fully wetted, thin-walled structures to help compensate for low thermal conductivity

Copper - 380 W/m-K

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Thermal Design of Immersed HXDeveloped Design Tools for

Polymer Based ICS

1

10

100

1E+02 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08RaD

Nu D

NuD=0.728RaD0.25

R2=0.942 (Eight-tube, P/D = 2.4)

NuD=0.675RaD0.25

R2=0.923 (Single tube)

NuD=2.45RaD0.188

R2=0.566 (240-tube, P/D=1.5 to 3.3)

(Morgan, 1975)

Strategies to Improve Heat Transfer in Storage Tanks

Simple cylindrical baffle increases discharge rates ∼ 30%

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Polymer Durability in Potable Water

PSU: Shows good stability in an oxidative environmentPB/PP/PE (polyolefins): Rate of antioxidant loss will affect life, thickness importantNylon: Not suitable

Established a relationship between mechanical performance and polymer degradation

Creep compliance (stiffness) Chemical Degradation (Oxidation induction time) Tensile Strength

Polybutylene Nylon 6,60

2

4

6

8

10

12

14

Polysulfone

cree

p c

om

plia

nce

(G

Pa

-1) Air 25°C

550 mV, 60°C825 mV, 60°C550 mV, 80°C825 mV, 80°C

Stiffness change after 1500 hours in hot potable water

0%

20%

40%

60%

80%

100%

0 50 100 150 200

Exposure time (hours)

Antio

xida

nt

Plaques exposed to Cl water

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Comparative Study of Scaling

50 µm 1 µm

0102030405060708090

100

0.16 0.22 0.31 0.38

Shear Stress (Pa)

Mas

s R

emov

ed (%

)

PP, PP-r, PEX, PP, PB, Nylon 6,6, HTN and Copper Relative scaling rates depend on water composition Scale forms more rapidly in distilled water than in tap water for identical

temperatures and supersaturation Generally, scaling rates are comparable for polymers and Cu Scale is much easier to remove from PP than from Cu

Polymorphs of CaCO3

Distilled WaterTap Water

Cu

PP

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SunCache ICS

Developed by Davis Energy Group; Commercialized by Harpiris Energy Impact-modified acrylic glazing; PE rotomolded tank with CU HX SRCC certified drain-back system 75 units in field with oldest 6+ years old; 3% call back 5-year warranty 8 person-hour installation http://www.harpiris.com

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Parallel, serpentine Cu HX

Rotationally molded superlinear PE ICS tank

Thermoformed impact-modified acrylic glazing

EPDM seal http://www.harpiris.com

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FAFCO, Inc. Revolution and Hot2O Unglazed active drain back

system Introduced at National Association

of Homebuilders (NAHB) International Builders Show, 2007

SRCC OG-300 Everything is provided in one box

Polymer collectors (48 ft2) Digital controller with animated display 80 Feet of UV resistant polymer tubing and

quick connect fittings Polymer drain back tank Circulation module containing two pumps

and a heat exchanger Roof jacks and mounting hardware Easy to install tank adapter and all plumbing

hardware

www.fafco.com www.hot2o.com

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Solar Electric / Solar ThermalSunEarth / PVT Solar Combined PV/Thermal Array

Ambient air passes under the PV panel and a glazed, selective-surface collector. The heated air is used for space heating and hot water. http://www.pvtsolar.com/

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New Initiatives

Building Technologies

Hydrogen &Fuel Cells

Fuel Cell Technology & Hydrogen Infrastructure

Zero Energy Buildings Goal

Combined Heat and Power Technologies

Efficiency Technologies& Building Integration

Solar

PV and Solar Hot Water Technologies

Distributed Energy

Building Technologies

Hydrogen &Fuel Cells

Fuel Cell Technology & Hydrogen Infrastructure

Zero Energy Buildings Goal

Combined Heat and Power Technologies

Efficiency Technologies& Building Integration

Solar

PV and Solar Hot Water Technologies

Distributed Energy

High Density Storage Triple Play Systems to provide hot water, space heating and

space cooling

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Burch, J. “Solar Thermal for Zero Energy Homes”, NREL 2007 Technology Status, National Renewable Energy Laboratory (NREL), jay_burch@nrel.govDavidson, J.H., Mantell, S.C., and Jorgensen, G., “Status of the Development of Polymeric Solar Water Heating Systems,” in Advances in Solar Energy, D.Y.

Goswami, ed., American Solar Energy Society, vol. 15, 149-186, 2002.Davidson, J.H., Mantell, S.C., and Francis, L.F., “Thermal and Material Characterization of Immersed Heat Exchangers for Solar Domestic Hot Water”, in

Advances in Solar Energy, D.Y. Goswami, ed., American Solar Energy Society, vol. 17, 99-129, 2007.Denholm, P., ”The Technical Potential of Solar Water Heating to Reduce Fossil Fuel Use and Greenhouse Gas Emissions in the United States”, March 2007

NREL/TP 640-41157.Liu, W., Davidson, J.H., and Kulacki, F.A., “Thermal Characterization of Prototypical ICS Systems with Immersed Heat Exchangers,” ASME J. of Solar Energy

Engineering, 127, 1, 21-28, 2005. Kearney, M., Davidson, J.H., and Mantell, S., “Polymeric Absorbers for Flat Plate Collectors: Can Venting Provide Adequate Overheat Protection?,” ASME J. of

Solar Energy Engineering, 127, 3, 421-424, 2005.Davidson, J. H., “Low-Temperature Solar Thermal Systems: An Untapped Energy Resource in the United States,” ASME J. of Solar Energy Engineering, 127,

3, 305-306, 2005.Freeman, A., Mantell, S.C., and Davidson, J.H., “Mechanical Performance of Polymer Tubes Intended for Use in Solar Heat Exchangers,” Solar Energy, 79,

624-637, 2005. Sanft, P., Francis, L., and Davidson, J.H., “Calcium Carbonate Formation on Cross-linked Polyethylene (PEX) and Polypropylene Random Copolymer (PP-r),”

ASME J. of Solar Energy Engineering, 128, 2, 251-254, 2006.Kulacki, F.A., Davidson, J.H., and Hebert, M., “On the Effectiveness of Baffles in Indirect Solar Storage Systems,” ASME J. of Solar Energy Engineering, 129,

494-498, 2007.Haltiwanger, J., and Davidson J.H., “Discharge of a Thermal Storage Tank using an Immersed Heat Exchanger with an Annular Baffle, Solar Energy, 83(2),

193-201, 2009. On line August 2008 http://dx.doi.org/10.1016/j.solener.2008.07.017Merrigan, Tim. Solar Heating Strategic Plan, June 22, 2007. NREL, Golden, CO 80401.. Merrigan, Tim. Report to Congress 2007, Solar Water Heating: Potential Energy Savings, Market Barriers, and Strategies for Wider Deployment. U.S. DOE,

Forestall Bldg, Washington, D.C.Mittelman, G., Alshare, A., and Davidson, J.H., in press, “A Model and Heat Transfer Correlation for Rooftop Integrated Photovoltaics with a Passive Air

Cooling Channel”, Solar Energy.Su, Yan and Davidson, J.H., “Transient Natural Convection Heat Transfer Correlations for Tube Bundles Immersed in a Thermal Storage,” ASME J. of Solar

Energy Engineering, 129, 210-214, 2007.Su, Y., and Davidson, J.H., “Multi-Zone Porous Medium Model of the Thermal and Fluid Processes during Discharge of an Inclined Rectangular Storage

Vessel via an Immersed Tube Bundle,” ASME J. of Solar Energy Engineering, 129, 449-457, 2007.Su, Y., and Davidson, J.H., “Discharge of Thermal Storage Tanks via Immersed Baffled Heat Exchangers: Numerical Model of Flow and Temperature Fields,”

ASME J. of Solar Energy Engineering, 130, 021016-1-7, 2008.Mittelman, G., Davidson, J.H., Mantell, S.C, and Su, Y., “Prediction of Polymer Tube Life for Solar Hot Water Systems: A Model of Antioxidant Loss,” Solar

Energy, 82(5), 452-461, 2008.Wade, A., Davidson, J.H., and Haltiwanger, J, in press, “What is the Best Solution to Improve Thermal Performance of Storage Tanks with Immersed Heat

Exchangers - Baffles or a Partitioned tank?” ASME J. of Solar Energy Engineering. Wang, Y., Davidson, J.H., and Francis, L., “Scaling in Polymer Tubes and Interpretation for Their Use in Solar Water Heating Systems,” ASME J. of Solar

Energy Engineering, 127, 1, 3-14, 2005. Wu, Z., Davidson, J.H., and Francis, L.F., in review, “Effect of Water Chemistry on Calcium Carbonate Deposition on Metal and Polymer Surfaces,” submitted

to Journal of Colloid and Interface Science.Lee, Eric. Harpiris Energy, 25205 Baronet Road, Corral de Tierra, CA 93908 eric@harpiris.com, www.harpiris.comRubio, Mike. FAFCO, mrubio@fafco.comWu, C., Mantell, S.C., and Davidson, J.H., “Polymers for Domestic Solar Hot Water: Long-term Performance of PB and Nylon 6,6 Tubing in Hot Water,” ASME

J. of Solar Energy Engineering, 126, 1, 581-586, 2004.Wu, Z., Francis, L.F., and Davidson, J.H., in press (online Nov. 2008), “Scale Formation on Polypropylene and Copper Tubes in Mildly Supersaturated Potable

Water, Solar Energy. http://dx.doi.org/10.1016/j.solener 2008.10.012

References