National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

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NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Cold-Climate Solar Domestic 

Water Heating Systems: 

Life-Cycle Analyses and 

Opportunities for Cost Reduction 

J. Burch and J. Salasovich 

National Renewable Energy Laboratory 

T. Hillman 

University of Colorado

Presented at the ISES Solar World Congress 

Orlando, Florida 

August 6–12, 2005 

Conference Paper 

NREL/CP-550-37748 

December 2005 
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1

COLD-CLIMATE SOLAR DOMESTIC WATER HEATING SYSTEMS: 

LIFE-CYCLE ANALYSES AND OPPORTUNITIES FOR COST REDUCTION 

Jay Burch 

Jim Salasovich 

National Renewable Energy Laboratory 

1617 Cole Blvd.; Golden, CO 80401-3393 

e-mail: jay_burch@nrel.gov 

Tim Hillman 

University of Colorado 

Architectural, Civil, and Environmental Engineering 

Boulder, CO 80302 

ABSTRACT

To determine if the goal of 50% reduction in the cost of 

saved energy (Csav) is attainable and prioritize research and 

development (R&D) for cold-climate solar domestic water 

heaters (SDWH), life-cycle analyses were done with 

hypothesized lower-cost components in glycol, drainback, 

and thermosiphon systems. Balance-of-system (BOS) 

measures include replacing conventional metal components 

with polymeric versions, and system simplification. With all 

BOS measures in place, Csav could be reduced just over 50% 

with a low-cost, selectively-coated, glazed polymeric 

collector, and slightly under 50% with either a conventional 

selective metal-glass or a non-selective glazed polymer 

collector. The largest percent reduction in Csav comes from 

replacing conventional pressurized solar storage tanks and 

metal heat exchangers with un-pressurized polymer tanks 

with immersed polymer heat exchangers, which could be 

developed with relatively low-risk R&D. 

1. INTRODUCTION

Reducing costs of SDWH is necessary for a substantial

SDWH market to exist in the U.S. (1), assuming no radical 

increases in the cost of fossil fuels. This position is 

supported by numerous market studies. Studies at (2) 

indicate that for substantial interest in SDWH, the installed 

cost should be less than $1K-$1.5K, depending on the 

responders (homeowners, builders, and architects) and 

definition of “substantial interest.” However, the cost of 

systems installed today ranges from roughly $2K to $7K, 

depending on many factors such as market context (e.g., 

retrofit or new construction), local solar installation volume, 

and system type/size (3). Determining possible lower 

bounds for SDWH costs and identifying the best focus for 

cost-reduction R&D motivates the work reported here. 

Because cost-reduction measures may also change 

performance and operation and maintenance (O&M) costs, 

it is necessary to frame cost-reduction analyses in terms of a 

normalized metric such as the cost of saved energy (Csav): 

Csav = Ctotal/Qsaved, 

where Ctotal is the total cost (first cost + present value of 

O&M costs) and Qsaved is the direct auxiliary energy 

savings, discounted over the analysis period (1). This 

definition purposely excludes regulatory-related costs and 

reductions such as demand savings, rebates, tax credits, 

and/or renewable energy credits. These are policy issues, 

and the focus here is on “hard costs” that can be influenced 

by R&D. 

Since 1998, the primary goal of the Solar Heating and 

Lighting (SH&L) subprogram has been to reduce Csav by at 

least 50% (1). The technical strategy is to use polymer 

technology and concomitant system re-design to lower 

costs. Even though performance may be negatively 

impacted by lower-cost polymeric materials, cost reductions 

can be proportionally much larger. Focus was initially 

placed on reducing Csav for SDWH systems appropriate for 

mild climates, where ambient temperatures rarely dip below 

0 o

C. Systems projecting reductions of 30% to 60% in Csav

are nearing market (1). Because of pipe freezing, mild 

climate systems are restricted mostly to parts of Florida, 

Arizona, and California (4). In this study, we consider the 

potential of technology R&D to achieve the goal of 50% 

reduction in Csav specifically for systems appropriate for 

cold climates. 
2. COST MODELING: GENERAL

Ctotal can be broken down into hardware, installation, 

marketing, and O&M. The first three costs compose the first 

cost to the homeowner. Costs are illustrated in Fig. 1 for 

two market scenarios: (i) retrofit at low volume (today’s 

market, mostly) and (ii) new construction at high volume 

(hypothetical). Note from Fig. 1 that BOS costs exceed 

collector costs significantly. The BOS is relatively complex 

for cold-climate systems, and is clearly a candidate for costreduction efforts. Marketing and installation costs for the 

retrofit market are inherently higher than for the new 

construction market. By assuming a healthy, new 

construction market scenario, the analysis avoids the highlyvariable, market-related cost inefficiencies of the presentday retrofit market that distort analyses for technology 

R&D. To keep focus on the “hard” costs, we use a mature 

new-construction context and assume high volumes of 

production. As a result, the “new construction” system costs 

here are significantly lower than costs actually seen in 

today’s retrofit market. 

Retrofit vs New Construction Costs

$0

$1,000

$2,000

$3,000

$4,000

$5,000

Retrofit New construction

First Cost

Market

Install

BOS

Collector

Fig. 1: First-cost comparison in retrofit and new 

construction markets, for a typical glycol system. 

Marketing cost in new construction might include sales 

commission, market materials, and builder mark-up. Sales 

commission and market materials costs would presumably 

be zero when SDWH is a standard feature on all houses, and 

that is assumed here. Builder mark-ups vary (2) and are 

taken here as 25% of builder “hard cost” (hardware + 

installation). If hard costs are reduced by X%, any cost 

proportional to hard cost (such as marketing here) will also 

be reduced by X%. Thus, including this “soft” mark-up cost 

does not distort the conclusions about fractional reduction of 

costs, and it is a real, significant cost that should be 

accounted for. 

Component costs are estimated by several different 

methods. For established components, costs were generally 

based on the lowest of large-volume quotes from supplier 

catalogues or conversations, or Internet listings (5). For 

hypothetical components, costs were estimated from a 

detailed accounting and/or by analogy to similar products. 

Nonetheless, for “hypothesized” components, the costs 

assumed here may also be considered as “reasonable 

technical targets.” Installation costs were estimated from a 

task-time layout as in (6). This is an intermediate level of 

detail, with ~20 separate tasks defined so that each costreduction measure (such as eliminating the load-side pump) 

affects a unique time allocation. O&M costs are computed 

statistically. Each failure mode is postulated (e.g., storage 

tank leak), and parameters are then posited for a 

corresponding three-part failure distribution, as in (3). The 

expectation values of future O&M costs are calculated and 

transformed to present value costs. 

3. SYSTEMS AND COST-REDUCTION MEASURES 

Three system types are considered here: glycol, drainback, 

and indirect thermosiphon. System diagrams from (7) are 

shown in Figs. 2-4. For cold climates, glycol and drainback 

are common, whereas the indirect thermosiphon is relatively 

novel. Although thermosiphons are probably the most 

popular system worldwide, thermosiphons for cold climates 

are problematic because supply and return piping can freeze 

and catastrophically burst. The base-case parameters 

common to the systems are given in Table 1. 

Component variations considered here are shown in Table 2. 

BOS variations include an un-pressurized polymer tank and 

heat exchanger, polymer piping, integrated valve package, 

and solar-side pump removal. Cost reductions assumed for 

the BOS variations are given in Table 4. Details of the BOS 

cost model changes are given in (5). Substituting a photovoltaic-driven (PV) pump for the standard differential 

controller was considered in (5). However, reliability cost 

reductions were far outweighed by additional first cost, so 

the PV-pump option is not considered here. Five different 

collectors were considered, and the assumed collector cost 

and performance parameters are given in Table 3. Three 

polymer collectors were considered: glazed selective, glazed 

non-selective, and un-glazed. The glazed selective polymer 

collector is hypothetical, although it could be developed. 

The other two polymer collector types are on the market. 

For simplicity, it was assumed that for all BOS changes the 

performance impacts were negligible. Only costs (hardware, 

installation, and O&M) change in the BOS substitutions. 

Performance changes from the base case only for collector 

substitutions with a change in the collector parameters. A 

good example of zero impact on performance impact is 

swapping out sweated copper piping for polymer piping 

with the same insulation value. For the tank-heat exchanger 

substitution, the heat exchangers are assumed large enough 

that performance is not significantly altered. Future work 

will relax this assumption. 

2
Fig. 2: The Heliodyne indirect glycol system, Heliopak 

model. Taken from (7), where symbols are defined also. 

Fig. 3: The Radco drainback, model R-DBHX-8-65-D-40P. 

Taken from (7), where symbols are defined also. 

Fig. 4: The SunSiphon indirect thermosiphon from 

SunEarth. Taken from (7), where symbols are defined also. 

TABLE 1: BASE-CASE SYSTEM PARAMETERS

Collector (metal-glass selective) 

 Area 3.72 m2 (40 ft2) 

 Slope 33.7° 

Solar Tank (pressurized)

 Volume 0.227 m3 (60 gal) 

 U-value 0.556 W/m2-°C 

Auxiliary Tank (pressurized)

 Volume 0.15 m3 (40 gal) 

 U-value 0.981 W/m2-°C 

 Setpoint Temp. 51.7 °C (125 °F) 

 Envir. Temp. 20 °C (68 °F) 

Piping (hard copper) 

 Length (sup. + ret.) 15.24m (50 ft) 

 U-value 2.27 W/m2-° 

The base-case solar storage tank is a conventional, 

pressurized, electric-auxiliary storage tank, with glasscoated steel pressure vessel. A side-arm heat exchanger with 

storage-side pump is the glycol base case. For drainback, an 

external heat exchanger in the drainback tank is used. For 

thermosiphon, a pressurized tank with solar fluid in an 

outside annulus (i.e., mantle tanks) is most common and 

was assumed here. A potentially low-cost variation of these 

pressurized tanks is an un-pressurized tank with load-side 

heat exchanger. An additional solar-side heat exchanger is 

required for glycol and thermosiphon (unless all storage 

water is freeze-tolerant, such as with lower-cost brine fluids 

contacting only all-polymer materials). The water storage 

vessel is assumed to be made from thin-wall polymers (thin 

film, membrane or blow-molded). A schematic drawing of a 

thin-film membrane tank is shown in Fig. 5, from (9). A 

similar 100-gal (380-l) commercial tank sold for ~$200 and 

was used in 1990s utility load-shifting programs (10). For 

the drainback system, the un-pressurized tank replaces the 

drainback tank and heat exchanger, storage side pump and 

piping, and the solar storage tank. The heat exchangers in 

the unpressurized tank are presumed to be made of polymer 

materials, as in Fig. 6. These low-cost heat exchangers are 

under development with a DOE partner, as in (1). 

TABLE 2: COMPONENT VARIATIONS

Component Baseline Variation(s) 

All types: 

 Collector Selective Non-selectve; unglzd 

 Storage Pressurized Un-pressurized 

 Heat exch. Metal/copper Polymer tube bundle 

 Piping Hard copper Polymer tubing 

 Valves Piece-by-piece Integrated package 

Glycol/dback: 

 Storage-side 

 pump 

9-10W pump Remove pump (use 

tsiphon or imm. coil) 

3
TABLE 3. COLLECTOR PARAMETERS AND COST

Collector1

 Fr(τα)n FrUl

2

 Cost 

Selective metal-glass3 .779 4.77 $500 

Nonselectve metal-glass3 .768 7.25 $450 

Polymer- selective4 .779 4.77 $250 

Polymer- non-selective3 .739 8.16 $200 

Polymer- unglazed5 0.88-

.029*vw

10.24 + 

4.69*vw

$100 

1. Collectors are all 40 ft2. 

2. Units of FrUl are [W/m2

]. 

3. From SRCC directory (7). 

4. Same parameters as base case metal-glass collector. 

5. From (8), test results for a pool collector; vw = wind speed at 

collector. 

TABLE 4: BOS COST REDUCTION MEASURES

BOS Measure ∆$1

Hardware 

∆$ 

Install2

∆$ 

O&M 

∆$ 

Total3

Glycol only:

Remove pump $82 $22 $73 $220 

Poly. tank/HX $280 $74 $256 $761 

Drainback only:

Remove pump, 

poly tank/HX 

$562 $192 $358 $1390 

Thermosiphon only:

Poly. tank-HX 

& piping 

$400 $30 $542 $1215 

Glycol/DrainBack: 

Poly. piping $70 $284 $148 $553 

All Systems:

Valve package -$25 $130 $0 $131 1

 Savings (+) of the measure over the base case. 

2

 Includes direct labor and consumables, and overhead/profit on 

installation of 100%/50%. 3

 Sum of savings from previous three categories, +25% markup. 

Heat exchanger

Retainer ring

Submersible pump

Polymer film liner

Insulation

Sheet metal cylinder

Rigid foam base

Heat exchanger

Retainer ring

Submersible pump

Polymer film liner

Insulation

Sheet metal cylinder

Rigid foam base

Fig. 5: Low-cost thin-film membrane tank with immersed 

load-side heat exchanger and solar-loop pump, for a 

drainback system [taken from (9)]. 

Cost ~$12

Fig. 6:. Prototype low-cost polymer heat exchanger (10). 

Tubes are 1/8-inch (3 mm) diameter. Tube weaving and 

tube-to-header welding are automated processes, 

contributing to a low cost projection. 

4. RESULTS: PERFORMANCE, COST AND Csav 

The well-known simulation tool TRNSYS (11) was used for 

performance prediction at all U.S. TMY2 sites (12). 

Detailed assumptions and results are given in (5). The 

unglazed results are based on unglazed collector test 

procedures that incorporate wind and sky-infrared affects, as 

in (13). 

The annual efficiency is defined as annual saved energy 

divided by annual solar incidence on the collector. Fig. 7 

shows the efficiency versus TMY2 site annual average 

temperature for a glycol system using the four different

collectors (the two selective collectors have the same 

performance parameters). It can be seen that for each 

collector the annual efficiency is approximately constant, 

falling slightly with increasing site temperature, as in (14). 

Efficiency for the unglazed collector is about 1/2 of the 

glazed/selective efficiency, with the other two collectors 

between these two extremes. 

The system first cost and Csav are shown in Figs. 8-10, one 

figure for each system type. The highest cost and Csav are 

for the baseline system, at far left of each graph. Going left 

to right, the BOS variations are shown first, followed by the 

collector variations. The BOS changes are cumulative, and 

the collector variations are done with all the BOS changes 

in place. Collector substitutions proceed from more 

expensive to less expensive. The non-selective metal-glass 

collector was not done for the drainback and thermosiphon 

systems, because results with the glycol data show that it 

leads to increased Csav relative to other collectors. The 

unglazed collector was not done with the thermosiphon due 

to convergence problems that were not yet resolved. 

The base-case first costs of the glycol, drainback, and 

thermosiphon systems were $3059, $3336, and $2377, 

4
respectively. With all BOS variations in place, first cost was 

reduced by $1203, $1509, and $671 for the glycol, 

drainback, and thermosiphon systems, respectively. The Csav

reductions mirror these BOS cost reductions. With the 

collector variations, the first cost continues to decrease as 

collector costs decrease. However, Csav is reduced from the 

base case only with the selective polymer collector (which 

by definition is equal performance-wise to the selective 

metal-glass collector at half the cost). This is because the 

other three collectors lead to a significantly lower system 

efficiency (see Fig. 7), which is proportionally larger than 

the lowered cost. 

Glycol Solar System

15%

25%

35%

45%

0 10 20

Annual Average Temperature (Co

)

Efficiency

30

Selective

Nonselective

Polymer

Unglazed

Fig. 7: Annual efficiency (savings/incidence) vs. site annual 

average temperature for the glycol system with four 

different collectors. The base case is the selective collector. 

With all cost-reducing BOS variations in place, the lowest 

Csav is obtained using the hypothetical selective polymer 

collector, with > 50% reduction. The 2nd-lowest Csav occurs 

with the base-case metal-glass selective collector, somewhat 

under 50%. The result for the non-selective polymer 

collector is about equal to the selective base-case. Table 5 

summarizes the base case and the best two cases for the 

three system types. First cost is under $1500 only with 

polymer collectors. The cold-climate thermosiphon is the 

lowest-cost system at all stages, with lowest Csav and best 

chance to meet the 50% reduction goal; however, it has the 

highest barriers to overcome (freezing and piping/tank/heat 

exchangers in attic). The non-selective metal-glass collector 

and the unglazed polymer collector are not effective 

measures because performance is lowered proportionally 

more than first costs, compared to the base collector. 

Table 6 gives the % reduction in Csav due to key measures, 

averaged over the system types to which the measure is 

applicable. The polymer tank substitution made the largest 

impact, ~17%, with polymer heat exchangers accounting for 

an additional 9%. The R&D cost and risk are also given in 

Table 6. It would appear that cost reduction efforts focused 

on the tank-heat exchanger combination are a good focus for 

technology R&D, since the reduction is the largest and 

R&D costs would be moderate. 

Cost & Cost-of-Savings/ Glycol 

$1,000

$1,500

$2,000

$2,500

$3,000

$3,500

Base case

One pump

Polymer tank + hx

Integrated piping

Valve package

Non-selective mtl-gls

Polymer selective

Polymer non-selective

Polymer unglazed

First Cost

4

6

8

10

12

COSE [c/kWh]

1st Cost

COSE

BOS Variations Collector Variations

Fig. 8: First cost and Csav for the glycol systems. 

Cost & Cost-of-Savings/ Drainback 

$500

$1,000

$1,500

$2,000

$2,500

$3,000

$3,500

Base case

Polymer tank + hx

Integrated piping

Valve package

Polymer selective

Polymer non-selective

Polymer unglazed

First Cost

2

4

6

8

10

12

COSE [c/kWh]

1st Cost

COSE

BOS Variations Collector Variations

Fig. 9: First cost and Csav for the drainback systems. 

Cost & Cost-of-Savings/ Thermosiphon 

$500

$1,000

$1,500

$2,000

$2,500

Base case

Polymer tank + hx

Valve package

Polymer selective

Polymer non-selective

First Cost

2

4

6

8

10

COSE [c/kWh]

1st Cost

COSE

BOS Variations Collector Variations

Fig. 10: First cost and Csav for the thermosiphon systems. 

5
6

TABLE 5: 1ST COST & Csav FOR BASE & BEST CASES

System: Glycol Drainback Thermosiphon

 Base Best Base Best Base Best 

First cost $3,059 $1,363 $3,336 $1,396 $2,377 $1,213 

Csav 1,2 11.2 5.5/6.3 11.8 5.2/6.3 9.3 4.8/6.6

% reduction2 50%//41% 53%//44% 57%//46% 1 Csav units are (¢/kWh). 2

 For “Csav/Best” and “% reduction” cells, two results are given: 

(polymer selective result)//(metal-glass selective result). 

TABLE 6: MEASURE EFFECT, R&D RISK AND COST

Variation Reduction 

in Csav

R&D 

Risk 

R&D 

Cost 

Remove pump 4.8% None None1 

Polymer storage 17% Low Med 

Polymer heat exch. 9% High Med 

Polymer piping 9% Low2/

Med3 

Low2/ 

Med3

Valve package 7% Low Low 

Non-sel. poly. coll. 0% Med Med 

Selec. poly. coll. 10% High High 

1. A comprehensive study is available (15). 

2. For glycol/drainback, where freeze protection is not needed. 

3. For thermosiphon, where freeze protection is needed. 

5. CONCLUSIONS

Cost and performance were estimated for glycol, drainback, 

and thermosiphon system types over a set of lower-cost 

BOS components and collectors. All BOS measures here 

reduced Csav and were assumed in place when analyzing the 

collector variations. Reductions in Csav of somewhat over 

50% are attained when using a hypothetical polymer 

collector with selective absorber. The 2nd-largest reductions 

in Csav, somewhat under 50%, are attained when using the 

conventional metal-glass selective collector. Csav for this 

latter case was about equal to Csav for the glazed selective 

collector, for all three system types. About 26% reduction in 

Csav came from replacing the pressurized solar storage tank 

with un-pressurized tanks with polymeric heat exchangers, 

which may be a good focus for R&D efforts. 

6. ACKNOWLEDGMENTS

The authors acknowledge the funding from the U.S. 

Department of Energy’s Solar Heating and Lighting 

(SH&L) subprogram and support from Tex Wilkins and 

Glenn Strahs, SH&L program managers. 

7. REFERENCES

(1) Solar Energy Technology Program: Multi-year 

Technical Plan 2003-2007 and beyond. Available at 

DOE/GO-102003-1774 

(2) SDWH market studies are currently available at: 

ftp://ftp.nrel.gov/pub/solar_waterheat-out/market_studies

(3) Burch, J., Salasovich, J., Christensen, C., Lorand, B., 

and Scholten, B., “Cost-Benefit Modeling of Solar 

Domestic Hot Water Systems,” Proc. ASES 99 Solar 

Conference, American Solar Energy Society, Boulder, CO; 

June 1999. 

(4) Salasovich, J., Burch, J., and Barker, G., “Pipe Freeze 

Probability for Passive-solar Water Heating Systems in the 

United States”, Proc. ASES 2004, ASES, Boulder, CO. 

(5) Hillman, T., “Life Cycle Analysis of Solar Water 

Heating Systems”, 12/04, M.S. Thesis, ACEE Department, 

University of Colorado, Boulder, CO. 

(6) Kutscher, C., et al., Low-Cost Collectors/ Systems 

Development Progress Report, SERI/RR-253-1750, 

DE84013022, July 1984. National Renewable Energy 

Laboratory, Golden, CO. 

(7) SRCC Standard 100: Operating Guidelines and 

Minimum Standards for Certifying Solar Collectors, Feb. 

2004. Available at http://solar-rating.org/

(8) Harrison, S.J., McClenahan, D., Nielsen, V.H., “The 

Performance of Unglazed Solar Collectors,” Renewables - 

A Clean Energy Solution, Conference Proceedings, 15th 

Annual Conference of the Solar Energy Society of Canada, 

Penticton, B.C., June, 1989, pgs. 235-239. 

 (9) Wilhelm, W. and Ripel, B., “Low-cost Thin-Material 

Solar Technology, The Key to a Viable Energy Alternative” 

U.S. India Workshop on Solar Energy R&D, CONF850858-4/BNL36815/DE86 000025, 8/85 

(10) Gozce, T., Access Technology, Inc., Bangor, MN, 

private communication. 

(11) TRNSYS description can be found at the University of 

Wisconsin Solar Energy Laboratory Web site: 

http://sel.me.wisc.edu/trnsys/default.htm

(12) Marion, W. and Urban, K., “User’s Manual for 

TMY2,” available at http://rredc.nrel.gov/solar/pubs/tmy2/

(13) Burch, J., Christensen, C., Salasovich, J., and Thornton, 

J., “Simulation of an Unglazed Collector System for 

Domestic Hot Water and Space Heating and Cooling,” Solar 

Energy, 77 (2004), pp. 399–406. 

(14) Christensen, C., and Barker, G., “Annual Efficiencies 

for Solar Water Heating”, Proc. ASES 1999, ASES, 

Boulder, CO 

(15) Dahl, S., “Analysis of Thermosiphon Heat Exchangers 

for use in Solar Domestic Hot Water Heating Systems,” 

6/98, PhD thesis, University of Minnesota, Minneapolis, 

MN. 
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Cold-Climate Solar Domestic Water Heating Systems: Life-Cycle 

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Conference paper regarding research in potential cost-savings measures for cold-climate solar domestic water 

hearing systems. 

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