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

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

An Assessment of Unglazed 

Solar Domestic Water Heaters 

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-550-37759 

December 2005 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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AN ASSESSMENT OF UNGLAZED SOLAR DOMESTIC WATER HEATERS 

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

This paper investigates cost-performance tradeoffs in 

replacing glazed collectors with unglazed ones in solar 

domestic water heaters (SDWH). The collector models 

are based upon accepted test standards, with the unglazed 

model explicitly including wind and sky infrared flux. A 

conventional glycol system is modeled, with glazed and 

unglazed collectors. Annual efficiency for a 40-ft2

, 

unglazed SDWH is approximately constant across the 

United States at 21±1%. This efficiency is slightly more 

than 1/2 that of an equal-sized glazed system with 

selective absorber coating, and ~2/3 that of a glazed 

system with nonselective coating. Considering an SDWH 

system, exchanging glazed collectors for unglazed ones of 

the same size generally increases the cost of saved energy 

(Csav, defined as total-cost/total-savings). However, if the 

unglazed collector area Aungl is increased to compensate 

for reduced performance, Csav may significantly decrease, 

as long as Aungl/Aglaz < ~5. General expressions for 

Csav,ungl/Csav,glaz are derived for cases of equal collector 

area and equal savings (where the unglazed area is 

increased until savings are equal). If larger areas are not 

problematic, unglazed SDWH can offer lower Csav at 

lower first cost. 

1. INTRODUCTION

Unglazed polymeric collectors have long been used in 

solar pool heating systems, and they dominate the U.S. 

solar market [Aungl,sold ≈ 20*Aglaz,sold (1)]. With low cost 

and good performance, solar pool heaters can attain 

simple payback < ~5 years, corresponding to ≥ ~20% 

return on investment. In the United States, such favorable 

economics are likely needed to create significant markets 

for SDWH, as in (2,3). External factors such as subsidies 

and/or fossil fuel cost escalations may dominate future 

SDWH market growth (as happened in the early 1980s), 

but the focus here is on potential hardware cost reduction, 

as in (3). Since unglazed collectors can be significantly 

lower in cost than glazed collectors, there is some cost 

reduction potential for unglazed SDWH, and assessment 

of that potential motivated this work. 

 

At 40 ft2

 size in large quantities, the ratio of the unit area 

cost of an unglazed polymer pool collector to that of a 

glazed collector (R$ungl/$glaz) is ~20% (3), a very 

significant cost reduction. However, impact on system 

cost depends also on the other costs, shown in Fig. 1. 

When the other costs are low, as in the do-it-yourself case 

in Fig. 1, unglazed SDWH systems cost significantly less 

than glazed (40% in the Fig. 1 example), and could be 

attractive. For the new-construction glazed case (our 

focus here), the other costs are more significant. System 

cost reduction is fractionally less (16% in the example). 

Other groups have been working on unglazed SDWH, and 

they are being marketed by firms in India (4) and Europe 

(5). Prototypes of unglazed SDWH using conventional 

roofs as absorbers have been tested (6). Larger-scale, 

roof-integrated unglazed systems reducing space 

conditioning and DHW loads are considered in (7,8). 

However, performance and costs for unglazed SDWH are 

generally not grasped as well as for glazed SDWH. A 

cost-performance comparison between glazed and 

unglazed SDWH with consistent energy modeling is the 

objective of this paper. We describe the modeling, 

simulate energy savings, and give general solutions for 

Csav in two cases, where the collector areas are equal (=A 

case) and where the system savings are equal (=Q case). 

1
New Construction and DIY Markets:

Cost Comparisons

$0

$250

$500

$750

$1,000

Collector

BOS

Installation

Marketing

Total /4

NewCon, SelCol

NewCon, Ungl

DIY, SelCol

DIY, Ungl

Fig. 1: First cost components of a glycol system in new 

construction (NewCon) and do-it-yourself (DIY) markets, 

with glazed and unglazed collectors. 

2. PERFORMANCE MODELING

Performance of unglazed collectors is reasonably wellunderstood. Fig. 2 shows efficiency (η) of the glazed and 

unglazed collectors used here vs. the “operating 

parameter” (γ ≡ ΔTin-amb/Isun). The unglazed collector has a 

higher loss coefficient (steeper slope). The performance 

of unglazed collectors in pool heating applications is good 

because γ values are mostly small in that application (e.g., 

0.01 o

C-m2/W @ ΔTinlet-ambient = 5oC, Isun = 500 W/m2). 

Thinking has been that, since the operating parameter is 

much larger for SDWH (e.g., 0.04 o

C-m2/W when ΔTinletambient = 20°C, Isun = 500 W/m2

), an unglazed collector 

would operate too inefficiently. 

Collector Efficiency and 

Distribution Comparison

0

0.2

0.4

0.6

0.8

1

1.2

-0.01 0.01 0.03 0.05 0.07 0.09

(T_in - T_amb)/I_sun [C-m2/W]

Efficiency

-

0.20

0.40

0.60

0.80

Distr. Fraction

Glz/Sel

Unglz-1 m/s

Unglz-5 m/s

Dist-Gl/Sel-Alb

Dist-Unglz/Alb

Fig. 2. Collector efficiencies vs. the operating parameter 

γ, glazed and unglazed (at two vwind), in Phoenix, AZ. 

Also shown are the distribution functions γglaz and γungl. 

Annual distributions of the operating parameter γ are also 

shown in Fig. 2. For the glazed system, the γ distribution 

peaks at about twice that of the glazed, indicating the 

glazed system operates hotter. Although ηungl ≈ 0 where 

the γglaz distribution is largest, the efficiency of the 

unglazed system at lower vwind is reasonably good where 

γungl is large, and the unglazed SDWH can still save 

substantial energy. 

An empirical characterization of the collectors always 

provides the highest confidence. The unglazed collector 

model is based on an international test standard that 

explicitly incorporates wind and sky infrared (9,10). The 

unglazed collector efficiency is expressed as 

ηcol = FrαnK(θ) – FrUl(Tin – Tamb)/Inet. (1) 

Inet includes both short-wave solar and net sky blackbody 

radiation between sky and ambient temperatures, Inet = Isun

– ε/α*IIR. Efficiency is relative to Inet. FrUl and Frαn are 

taken as linear in vwind (10), Frαn = ao – a1vlocal, and FrUl = 

bo + b1vlocal. ai, bi values used here are given in Table 1. 

The largest uncertainty comes from estimating terrain and 

shielding affects on vlocal. It is assumed here that 

vlocal/vTMY = 0.3, as in (2). Uncertainty in this ratio (~0.2 - 

~0.5), causes ~10%-20% variation in performance and 

dominates uncertainty in the analyses here. 

The glycol system simulated here is shown in Fig. 3. Key 

system parameters are given in Table 2, with more details 

in (11). Simulations of performance were done with 

TRNSYS (12). Daily draw volume is kept fixed at an 

invariant 64 gal (242 l), which is the standard “rating 

load” in the U.S. (13). The time-of-use profile assumed is 

a typical “double-humped” profile, taken from (14). 

Mains inlet temperature scales directly with site annual 

average temperature, and varies sinusoidally with time of 

year, as in (14). Storage/area is set at 1.5 gal/ft2

. 

3. PERFORMANCE RESULTS

U.S. maps of the annual savings for a 40 ft2

 glycol SDWH 

with a selective glazed collector and an unglazed collector 

are shown in Figs. 4 and 5, respectively. These maps tend 

to directly mimic maps of the annual incidence of solar 

radiation, because the system annual efficiency (see 

below) is relatively constant, independent of location. 

Efficiency decreases relatively for systems that are 

“oversized” and saturate significantly during the summer 

(system “overheats” and losses increase, implying 

lowered efficiency). This effect shows up in the drop in 

savings in southern Florida, where the loads are lowest 

because mains inlet temperature is highest. A similar 

drop-off in efficiency that occurs for some southwestern 

locations is not evident in the performance maps. 

2
Fig. 3: The Heliodyne indirect glycol system, Heliopak 

model. Taken from (13), where symbols are defined. 

TABLE 1: COLLECTOR PARAMETERS

Collector Fr ταn FrUl [W/m2

] 

Selective glazed1 0.779 4.77 

Nonselec. glazed1 0.768 7.245 

Unglazed2 0.88 - 0.029*vlocal 10.24 + 4.69*vlocal 

1. From the SRCC OG100 directory, posted at (13). 

2. From (10), test results for a pool collector. 

TABLE 2: 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.) 15.24 m (50 ft) 

 U-value 2.27 W/m2-°C 

SDWH performance can be usefully described by an 

annual efficiency ηann ≡ Qsav,ann/Qinc,ann. Fig. 6 shows ηann

vs. the site annual average temperature for a 40 ft2

 SDWH 

with three different collectors: selective-glazed, nonselective-glazed, and unglazed collectors. In each case, 

the efficiency is reasonably constant across the United 

States. That ηann across sites is constant when system size 

and load volume are kept constant is noted in (15) for a 

glazed glycol system. Here, ηann,glaz is ~39% ± 1%, and 

ηann,ungl is ~21 ± 1%, so that Rη = ηann,ungl/ηann,glaz ~0.54; 

i.e., the unglazed system saves slight more than 1/2 as 

much as the glazed selective system, at 40 ft2

 size. Rη is 

~0.64 for a nonselective glazed SDWH (12). Nearly 

identical efficiencies with the same three collectors were 

reported in (12) for a typical drainback system. A 

thermosiphon system showed slightly lower efficiency 

with non-selective and polymer collectors (12). 

Fig. 4: U.S. map of annual savings (GJ) for a 40 ft2

 glycol 

SDWH with a glazed, selectively coated collector. 

Fig. 5: U.S. map of annual savings (GJ) for a 40 ft2

 glycol 

SDWH with an unglazed collector (same scale as Fig. 4.) 

Annual Efficiency vs Site Temperature

0%

10%

20%

30%

40%

0 10 20

Annual Average Temperature (Co)

Efficiency

30

Selective

Nonselective

Unglazed

Fig. 6: ηann vs. Tamb,avg for a 40 ft2

 SDWH, with two 

glazed collectors and an unglazed collector. 

A continental U.S. map of the efficiency ratio Rη for 40 

ft2 systems is given in Fig. 7. Rη is fairly constant at 0.54 

± 0.02. Rη is lower along the coasts and in the Midwest, 

where winds tend to be higher. Rη also tends to be lower 

3
where the load is lower, as in southern Florida and Texas. 

Rη is higher in spots with lower wind velocities and 

higher loads. Because of the low spatial density of TMY 

sites, the contours in Fig. 7 are somewhat dependent on 

the contouring algorithm and should not be taken too 

literally. Although glazed collector performance is also 

affected (slightly) by the wind, this variation is not taken 

into account. Note that the maximum deviation of Rη

(±0.05) is less than 10% of the average ratio, and it is a 

useful approximation that Qsav,ungl ≈ K* Qsav,glaz. K is a 

function of system size and collector types (e.g., 0.54 with 

the selective glazed collector at 40 ft2

 size). 

Fig. 7. U.S. map of the ratio ηann,ungl/ηann,glaz for a 40 ft2

glycol system. The glazed collector is selective. 

Fig. 8 shows the solar fraction of the glazed and unglazed 

SDWH vs. Acol, at three sites. The glazed system saves 

more than the unglazed system at all areas. Starting with 

the smallest area, the glazed system’s performance 

increases with area (which increases system temperature) 

faster than that of the unglazed system. The glazed 

systems in Miami and Albuquerque saturate at ~7 m2

. The 

glazed system did not saturate in Madison, with higher 

loads and lower radiation. None of the unglazed systems 

showed saturation (up to maximum Acol = 20 m2

). At a 

given area, savings are smaller in Madison because the 

incident solar radiation is less. The Miami glazed system 

slightly outperforms the Albuquerque glazed system, even 

though radiation is lower, because the Albuquerque 

system saturates more in summer; this ordering reverses 

for unglazed SDWH, which don’t saturate in the summer. 

4. COST-PERFORMANCE TRADEOFF

Because the unglazed collector changes both first cost and 

savings, it is useful to compare a single normalized metric 

between systems (2,3,16): Csav ≡ $sys,total/Qsav,total. The total 

system cost $sys,total includes the present value of repair 

costs (2,16). Csav normalizes for the simultaneous effects 

of lowered system cost and lowered performance. It also 

compares directly with the cost of energy from 

conventional fuels. It is a commonly-used metric in 

tradeoff analysis, as in (2,3). 

Solar Fraction vs. Collector Area

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10 12 14 16 18 20

Collector Area (m2)

Solar Fraction

Albuquerque- glaz

Madison- glaz

Miami- glaz

Albuquerque- ungl

Madison- ungl

Miami- ungl

Fig. 8: Solar fraction vs. collector area for a glazed and 

unglazed collector at three sites: Albuquerque, NM, 

Madison, WI, and Miami, FL. 

There are two useful cases considered here: equal Acol

(=A) and equal Qsav (=Q). In the =A case, the unglazed 

collector has the same area as the glazed collector. 

Collector cost is reduced by 80%, but the system cost 

reduction depends on the ratio R$col/$sys and savings are 

also reduced [Qsav,ungl/Qsav,glaz (≡RQ,=A) < 1]. RQ,=A is a 

function of the collector area Acol, as shown in Fig. 9. The 

ratio initially decreases as Acol increases, because system 

temperatures are increasing and the unglazed system 

operates less efficiently. When the glazed system starts to 

saturate, the unglazed system is still increasing in savings. 

Thus, RQ,=A starts to increase with area (not true for 

Madison because saturation did not occur in Madison). 

Assuming that only the collector cost differs between the 

glazed and unglazed SDWH, it is easy to show that for 

equal collector areas, the ratio RCsav,=A can be expressed as 

RCsav,=A = [1–R$col/$sys(1 – R$ungl/$glaz)]/RQ,=A. (2) 

RCsav,=A depends on the ratios R$col/$sys, R$ungl/$glaz, and 

RQ,=A. Contours of RCsav,=A in the plane (RQ,=A, R$col/$sys) 

are shown in Fig. 10. Unglazed systems are more costeffective than glazed systems when RCsav < 1, as in the 

upper right corner of Fig. 10. This region corresponds, 

e.g., to pool systems. Unglazed systems are less costeffective when RCsav >1, as in the lower left side of Fig. 

10. This region corresponds to cases where BOS and 

installation costs are significant (i.e., low R$col/$sys), as in 

typical new construction or retrofit cases. 

For the =Q case, the unglazed collector area is increased 

such that Qsav,ungl = Qsav,glaz, with Aungl/Aglaz ≡ RA,=Q. Fig. 

4
11 shows RA,=Q as a function of Acol,glaz. At small sizes, 

RA,=Q is relatively small, because operation is at lower 

temperatures where the unglazed system is relatively 

efficient. As area increases, RA,=Q increases because 

increased system temperatures further favor the glazed 

collector. It is not understood why RA,=Q is significantly 

higher for Miami beyond ~2m2

. For Albuquerque or 

Madison, an unglazed system of ~12 m2

 provides energy 

savings equivalent to a ~3 m2

 glazed system, with ~50% 

larger area needed for =Q in Miami. It is easy to show 

that for the =Q case, the ratio Csav,ungl/Csav,glaz (≡RCsav,=Q) is 

RCsav,=Q = [1–R$col/$sys(1 – R$ungl/$glaz*RA,=Q)] (3) 

Ratio Qsav,ungl/Qsav,glaz vs. Acol

0.4

0.6

0.8

1.0

0 4 8 12 16 20

Collector Area (m2)

RQ,=A

Albuquerque

Madison

Miami

Fig. 9: Plot of the ratio Qsav,ungl/Qsav,glaz (RQ,=A) vs. the 

glazed collector area, for three sites. 

Contours of RCsav,=Q in the plane (RA,=Q, R$col/$sys) are 

shown in Fig. 12. RCsav,=Q is 1 when RA,=Q = 5, because we 

have assumed R$ungl/$glaz = (1/5). This break-even point 

corresponds to Aglaz ~2.8 m2

 in Miami, and ~3.3 m2 at the 

other two sites. For RA,=Q <5, RCsav,=Q < 1 and the 

unglazed is more cost-effective. Note that with R$col/$sys

~0.2, and RA,=Q ~ 4, the advantage is fairly small, ~5%. 

Thus, for the =Q case, the larger-sized unglazed SDWH 

appear cost-effective compared with the smaller-sized 

glazed SDWH, less so as R$col/$s decreases and solar 

fraction increases. For the general case, it would be 

expected that for area ratios somewhat near RA,=Q, the Csav

ratio would be less than one (exact results can be 

estimated for such cases using Fig. 8 data). The key ratios 

RA,=Q and RQ,=A were derived for only three sites. Further 

work is needed to know those ratios for any site, although 

the three sites characterized here give some guidance. 

5. CONCLUSIONS

Unglazed SDWH are less expensive than glazed SDWH, 

but operate less efficiently and yield lower maximum 

temperatures. System efficiencies were computed to be 

~38%, ~33%, and ~21% for a 40 ft2

 glycol system with a 

glazed selective, glazed nonselective, and unglazed 

collector, respectively. At equal areas, Csav,ungl > Csav,glaz, 

and that tradeoff is not cost-effective for cases of interest 

here. However, when the unglazed SDWH area is 

increased so that Qsav,ungl = Qsav,glaz (possible at lower solar 

fractions), then the substitution is cost-effective as long as 

R$ungl/$glaz*RA,=Q < 1. 

Contours of RCsav, =Acol Case 

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.1 0.3 0.5 0.7 0.9

R$col/$sys

RQ,=A

0.4

0.6

0.8

1

1.25

1.5

2

RCsav,with=A = 

Csav,ungl/Csav,glaz

Decreasing R

Increasing R

Fig. 10. Contours of RCsav vs. (RQ,=A, R$col/$sys). The 

collector cost ratio R$ungl/$glaz is kept fixed at 0.2. 

Aungl/Aglaz (for = Q) vs. Aglaz

1

2

3

4

5

6

7

01234

Collector Area (m2)

RA,=Q

Albuquerque

Madison

Miami

Fig. 11. Ratio of collector areas for =Q (RA,=Q) vs. Aglaz, 

for 3 sites. The lines end when the implied Aungl > 20 m2

. 

Contours of RCsav, =Qsav case 

1

2

3

4

5

6

7

0.1 0.3 0.5 0.7 0.9

R$col/$sys

RA,=Q

0.5

0.7

0.9

1

1.1

1.3

RCsav,for=Q = 

Csav,ungl/Csav,glaz

Decreasing R

Increasing R

Fig. 12. Contours of RCsav,=Q vs. (R$col/$sys, RA,=Q). The 

collector cost ratio R$ungl/$glaz is kept fixed at 0.2. 

5
6. ACKNOWLEDGMENTS

The authors acknowledge the U.S. Department of 

Energy’s Solar Heating and Lighting (SH&L) subprogram 

for funding this work. The support of Tex Wilkins and 

Glenn Strahs, SH&L program managers, is gratefully 

acknowledged. 

7. NOMENCLATURE

Symbols:

a,b = Constant coefficients 

A = collector area 

Csav = Cost of saved energy = $sys/Qsav

fsol = solar fraction 

Fr = Heat removal factor 

I = Solar or infrared incidence (solar power/area) 

K = Incidence angle modifier (IAM) or constant 

Qsav = Energy saved over analysis period 

RA,= Q = Ratio of Aungl/Aglaz such that Qsav,ungl = Qsav,ungl

RCsav = Ratio of Csav/Csav 

RQ,=A = Ratio of Qsav,ungl/Qsav,glaz with equal areas 

R$coll/$sys = Ratio of glazed collector to glazed system cost 

R$ungl/$glaz= Ratio of unit area cost, $ungl/$glaz 

T = Temperature 

Ul = Collector loss coefficient 

v = Wind velocity (subscripted local or TMY) 

$ = Cost, item as denoted by subscripts 

αn = Collector absorptivity at normal incidence 

ε = Collector emissivity 

η = Efficiency (collector or system) 

θ = Incidence angle, in IAM function K(θ) 

Subscripts

amb = ambient condition 

col = collector 

glaz = glazed 

in = mains inlet, or inlet to collector 

inc = incident solar on collector 

IR = net infrared radiation between sky and ambient 

local = wind at the collector location 

net = net of short and (ε/α)*(long-wave radiation) 

sun = short-wave radiation 

sys = system 

TMY = Typical Meteorological Year data 

total = total cost or total savings 

ungl = unglazed 

η = efficiency 

8. REFERENCES

(1) Energy Information Agency: Solar Thermal and 

Photovoltaic Collector Manufacturing Activity 2003, 

available at http://www.eia.doe.gov/fuelrenewable.html

(2) Burch, J., Hillman, T., and Salasovich, J., “Cold 

Climate Solar Domestic Water Heating Systems: Life 

Cycle Analyses and Opportunities for Cost Reduction,” 

Proc. ASES 2005, Orlando, FL, August 2005. 

(3) Solar Energy Technology Program: Multi-year 

Technical Plan 2003-2007 and Beyond. DOE/GO102003-1774, August 2003. 

(4) Sintex International Ltd. in India offers an unglazed 

thermosiphon system, described online at 

http://www.sintex-plastics.com/products/building/

solarwaterheater.htm

(5) Energie Solaire in Switzerland offers an unglazed, 

selectively coated steel collector, described online at 

http://www.energie-solaire.com/en/products_roof.htm

(6) Colon, C., and Merrigan, T., “Roof Integrated Solar 

Absorber: The Measured Performance of ‘Invisible’ Solar 

Collectors,” Proc. ASES 2001, ASES, Boulder, CO 

80302. 

(7) Baer, S., “Passive Cooling and Drainback Heating 

with Unglazed Radiators/Absorbers – The Architectural 

Cool Cell(TM),” Proc. ASES 2001, ASES, Boulder, CO 

80302. 

(8) Burch, J., Salasovich, J., and Thornton, J., 

“Geographical Variation in Performance of an Unglazed 

System Meeting Water Heating and Space Conditioning 

Loads,” Proc. ASES 2004, ASES, Boulder, CO.

(9) ISO 9806-3:1995(E), Test methods for solar collectors 

– Part 3: Thermal performance of unglazed liquid heating 

collectors.” Case Postale 56, CH-1211 Geneve 20, 

Switzerland. 

(10) Harrison, S.J., McClenahan, D., and Nielsen, V.H., 

“The Performance of Unglazed Solar Collectors,” Conf. 

Proc., 15th Annual Conference of the Solar Energy 

Society of Canada, Penticton, B.C., June 19-21, 1989. 

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

Heating Systems,” M.S. Thesis, ACEE Dept., Univ. of 

Colorado, Boulder, CO, December 2004. 

(12) TRNSYS description can be found at: 

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

(13) SRCC Standard 100 and Standard 300 can be found 

at http://solar-rating.org/ 

(14) Hendron, B., “Building America Research 

Benchmark Definition,” available online at 

http://www.eere.energy.gov/buildings/building_america/p

a_resources.html

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

for Solar Water Heating,” ASES 99, ASES, Boulder, CO. 

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

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

Water Systems,” ASES 99, ASES, Boulder, CO 

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DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

An Assessment of Unglazed Solar Domestic Water Heaters 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-550-37759 

5e. TASK NUMBER

SH06.5001 

6. AUTHOR(S)

J. Burch, J. Salasovich, and T. Hillman 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-550-37759 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Conference paper investigating cost-performance tradeoffs in replacing glazed collectors with unglazed collectors in 

solar domestic water heating systems. 

15. SUBJECT TERMS

solar domestic water heating; solar water heating; cold-climate solar domestic water heating; cold-climate solar water 

heating 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18