The U.S. Department of Energy

FederalTechnology

Alert

A series of energyefficient technologyguides prepared by

the NewTechnology

DemonstrationProgram

Solar Water Heating Well-Proven Technology Pays Off in Several Situations

Solar water heating is a well-provenand readily available technology thatdirectly substitutes renewable energy for conventional water heating. This Federal Technology Alert (FTA) of theFederal Energy Management Program(FEMP), one of a series on new energy-efficient technologies and renewableenergy technologies, describes the various types of solar water heating systems, the situations in which solarwater heating is likely to be cost-effective, considerations in selecting and designing a system, and basic stepsfor installing a system.

There are a variety of different typesof solar water heating systems, but thebasic technology is straightforward. Acollector absorbs heat from the sun andthe system transfers that heat to water.That water is stored for use as needed,with a conventional system providingany necessary additional heating. A typical system will reduce the need forconventional water heating by abouttwo-thirds, eliminating the cost of electricity or fossil fuel and the environ-mental impacts associated with their use.

ApplicationSolar water heating can reduce

conventional energy use at any federalfacility. Savings are likely to cost-effectively pay for system installation in three types of situations. First, anyfacility that pays high utility rates for its conventional water heating is a goodprospect for cost-effective solar waterheating. Many smaller facilities in ruralareas (for example a campground at arecreational area served only by electricpower) are in this situation. Any of several mid-temperature solar waterheating technologies can serve well.Off-the-shelf packages are available andsystems that operate passively withoutpumps or electronic controls are oftenappropriate in warmer climates.

Large facilities such as prisons,hospitals, and military bases with

consistent need for large volumes of hotwater are the second situation wheresolar water heating is apt to be costeffective. Even if conventional waterheating costs are relatively low, econo-mies of scale for large mid- or high-temperature systems can bring costsdown to quite competitive levels.

Swimming pools are the third candidate use for solar water heating.Pool systems will often pay for them-selves in just a few years, particularlyfor pools that are used year round.Relatively inexpensive low-temperaturesystems are quite effective and caneither greatly reduce conventional poolheating bills or extend the season whereheating was considered too expensive.

Software available from FEMP'sFederal Renewables Program at theNational Renewable Energy Laboratory(NREL) (303-384-7509) gives a preliminary analysis of whether solarwater heating would be cost effective foryour situation on the basis of a minimalamount of data. Federal RenewablesProgram staff or this Federal Technol-ogy Alert can help you select an appropriate type and size of system.Reliable off-the-shelf systems can beselected from the Directory of the SolarRating and Certification Corporation(202-383-2570); there are also manyother good systems available. Engineer-ing services will be needed to designlarger systems, but the FEMP Help Line(800-DOE-EREC) can provide manualsand software for detailed economicevaluation and for the Energy SavingsPerformance Contracting Programwhich allows federal facilities to repaycontractors for solar water heating systems through bills for energy savingsinstead of paying for initial construction.

Technology SelectionThe FTA series targets new energy

efficient technologies that appear to havesignificant untapped federal-sectorpotential and for which some federal

SERDPStrategic Environmental Research

and Development Program

Improving Mission Readiness throughEnvironmental Research

DoD

DOE

EPA

installation experience exists. Many ofthe alerts are about new technologiesidentified through advertisements fortechnology suggestions in the Com-merce Business Daily and trade journals,and through direct correspondence inresponse to an open solicitation fortechnology ideas. This FTA describes aclass of renewable energy technologiesof known energy, cost, and environmen-tal benefit, but still with substantialuntapped potential for the federal sector.

Important criteria for selectingamong the various types of solar waterheating systems include temperature ofwater needed, system size, degree offreeze and hard-water scaling hazard,and maintenance need. The table belowsummarizes those considerations.

Case StudyThis alert describes examples in all

three of the likely situations for cost-effective installationshigh conven-tional water heating cost, large consis-tent hot water use, and swimming

poolsand presents a case study fromthe first situation. In lieu of electricwater heating, the National Park Serviceis installing drainback solar water-heating systems on two small and onelarge comfort station at its ChickasawNational Recreation Area in Oklahoma.

At a combined cost of about $22,000,the three systems will provide a total ofabout 136 MBtu (40,000 kWh) ofenergy per year to meet a hot water loadthat averages about 2800 gallons of hotwater per day during the 7 months thatthe area is heavily used. Unlike mostsolar water heating, the Chickasaw systems will operate without conven-tional backup, meeting the full demandmost of the time. The simple paybackperiod for each of the systems is 9 years.

Implementation BarriersThere are no technological barriers to

the use of solar water heating. Its costeffectiveness varies by geographic areaand type of use, but there are suitabletechnologies for all types of use in all

parts of the country. Because it directlyreplaces conventional energy use, solarwater heating will provide energy savingsand environmental benefit to the fullextent of its use. However, it will notalways be cost effective from a straightfinancial perspective. We are not aware ofany likely developments that could lowerthe cost of solar water heating systemssufficiently to consistently compete withthe low cost of natural gas. Solar water-heating is likely to be cost-effective onlyif natural gas is not available, if consistenthigh-volume use provides economies ofscale, and for swimming pool heating.

There are today an adequate numberof good products and skilled systemdesigners and installers. Planned inclusion of solar water heating systemson the GSA purchase schedule should be quite helpful. Most federal facilitymanagers should be aware of solar waterheating, but may not realize its applicabil-ity to their facilities or may have heard of past problems from poor design ormaintenanceunlikely situations today.

Suitable Cost/ft2 for 40 ft2 Freeze Hard water Maintenancesystem size unless noted tolerance tolerance need

Low-Temperature Systems

Unglazed for pools $10-$25 (400 ft2) none good very lowPassive Mid-Temperature Systems

Integrated collector small $50-$75 moderate minimal very lowThermosiphon direct small $40-$75 none minimal low

indirect small $50-$80 moderate good lowIndirect, Active, Mid-Temperature Systems

Flat-plate, antifreeze small $50-$90 excellent good highlarge $30-$50 (30,000 ft2)

Flat-plate, drain back small $50-$90 good good highDirect, Active, Mid-Temperature Systems

Drain down small corrections minimal high

Recirculating small minimal high

High-Temperature Systems

Evacuated tube direct small $75-$150 good minimal highindirect large $75-$150 excellent good high

Parabolic trough large $20-$40 (30,000 ft2) excellent good high

being developed

Solar Water Heating System Characteristics: Factors Useful in Selecting System Type for Particular Situations

AbstractSolar water heating is a renewable

energy technology that is well-provenand readily available and has consider-able potential for application at federalfacilities. Solar water heating systemscan be used effectively throughout thecountry and most facilities will have anappropriate near-south-facing roof ornearby unshaded grounds for installa-tion of a collector. A variety of types ofsystems are available and suitable formany applications. For example, low-temperature unglazed systems can heatswimming pools and associated hot tubs or spas, saving money on conven-tional heating or extending the swim-ming season. In mild climates, passivesystems without pumps or electroniccontrollers can provide low-maintenancehot water for facilities with limited or expensive utility service. High-temperature parabolic trough systemscan economically provide hot water tojails, hospitals, and other facilities inareas with good solar resources thatconsistently use large volumes of hotwater. And active flat-plate systems can service any facility in any area with electric or otherwise expensiveconventional water heating.

This Federal Technology Alert (FTA)of the New Technology DemonstrationProgram, one of a series of guides torenewable energy and new energy-efficient technologies, is designed to

give federal facility managers the infor-mation they need to decide whetherthey should pursue solar water heatingfor their facility and to know how to goabout doing so. Software available from FEMPs Federal Renewables Pro-gram at the National Renewable EnergyLaboratory (303-384-7509) gives a preliminary analysis of whether solarwater heating would be cost effectivefor your situation on the basis of a minimal amount of data.

This FTA describes the main types of solar water-heating systems avail-able and discusses some of the factorsthat make the various types more or less appropriate for particular situ-ations. It also points out the types of situations where solar water heating is most likely to be cost effective andgives examples for each of those situ-ations. In addition this FTA outlines the basics of selecting, evaluating, pro-curing, funding and maintaining a solar water-heating system. Sidebarshighlight indicators that a system willbe effective, tips for ensuring success-ful operation, and pointers for deter-mining system data. A case study for aNational Park Service facility includeseconomic evaluation data and bid specifications. References include solarwater-heating collector manufacturersand system distributors and contacts for federal facilities that are using solarwater heating.

Solar Water HeatingWell-Proven Technology Pays Off in Several Situations

1

Thermosiphon solar water heaters on employee housing at Yosemite National Park

FederalTechnology

Alert

Jim S

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rm/P

IX03

971

ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1About The Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Application DomainBenefitsEnergy-Saving Mechanism

Types of SystemsTypes of Collectors

System DesignInstallation

Federal Sector Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Technology Screening ProcessEstimated Market Potential

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Where to ApplySmall FacilitiesWhere to ApplyLarge SystemsWhere to ApplySwimming PoolsApplication ScreeningSystem Selection and ProcurementEconomic CriteriaFunding Sources

Technology Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18System Maintenance

Case Study Chickasaw National Recreation Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19The Technology In Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20Suppliers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21Who is Using the Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24For Further Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

OrganizationsLiterature

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26Appendix A: Source and Monthly Temperature (F) at the Source for Cold Water Supply in 14 CitiesAppendix B: Example Page from Solar Radiation Data Manual for Flat-Plate and Concentrating CollectorsAppendix C: Federal Life-Cycle Costing Procedures and the BLCC SoftwareAppendix D: Chickasaw Case Study NIST BLCC Comparative Economic Analysis and Cost Estimate DetailAppendix E: Sample Specifications for a Drain Back System from Chickasaw National Recreation Area Case StudyAppendix F: Data Necessary for Evaluating Solar Water Heating SystemsAppendix G: SRCC Rating Page for Flat-Plate Collector Appendix H: SRCC Rating Page for Antifreeze SystemAppendix I: SRCC Rating Page for Drain Back SystemAppendix J: SRCC Rating Page for Thermosiphon System

2

About theTechnology

An estimated one million residentialand 200,000 commercial solar waterheating systems have been installed inthe United States. Although there are alarge number of different types of solarwater heating systems, the basic tech-nology is very simple. Sunlight strikesand heats an absorber surface within a solar collector or an actual storagetank. Either a heat transfer fluid or theactual potable water to be used flowsthrough tubes attached to the absorberand picks up the heat from it. (Systemswith a separate heat transfer fluid loopinclude a heat exchanger that then heats the potable water.) The heatedwater is stored in a separate preheattank or a conventional water heater tank until needed. If additional heat isneeded, it is provided by electricity orfossil-fuel energy by the conventionalwater heating system. By reducing theamount of heat that must be provided by conventional water heating, solarwater heating systems directly substi-tute renewable energy for conventionalenergy, reducing the use of electricity or fossil fuels by as much as 80%.

Todays solar water heating systemsare well-proven and reliable when cor-rectly matched to climate and load. The current market consists of a rela-tively small number of manufacturersand installers that provide reliableequipment and quality system design. A quality assurance and performancerating program for solar water heatingsystems, instituted by a voluntary asso-ciation of the solar industry and vari-ous consumer groups, makes it easier to select reliable equipment with confi-dence. After taking advantage of possi-ble use-reduction measures (see box atright), federal facility managers shouldinvestigate installing solar water heat-ing systems.

Application DomainWater heating accounts for a sub-

stantial portion of energy use at manyfederal facilities. Nationwide, approxi-mately 18% of energy use in residentialbuildings and 4% in commercial build-ings is for water heating. Federal facili-ties with large laundries, kitchens,showers, or swimming pools will likelydevote even more energy to water heat-ing. Solar water heating systems canefficiently provide up to 80% of the hotwater needs of many federal build-ingswithout fuel cost or pollution

and with minimal operation and main-tenance expense.

Solar water heating systems are most likely to be cost effective for facil-ities with water heating systems that areexpensive to operate or with operationssuch as laundries or kitchens that re-quire large quantities of hot water. A need for hot water that is relativelyconstant throughout the week andthroughout the year, or that is higher inthe summer, is also helpful for solarwater heating economics. On the otherhand, hard water is a negative factor,

CD-SS26-B100212

Kilowatt-hoursper square meter

2 to 33 to 44 to 55 to 66 to 7

Map prepared by the NRELResource Assessment Program

3

Fig. 1. Average Daily Global Solar Radiation (on a south-facing flat surface tilted at lati-tude, resource for all but parabolic troughs). Solar water heating can be used effectivelythroughout the country. Available solar radiation is the most important, but not the only factor for cost-effective use.

First Things FirstAs a rule, conservation is the most cost-effective way to reduce water heat-

ing bills. For example, a low-flow shower head costing $9 saves $22 for275 kWh of energy per year for a five month payback. Other examples of hot-water saving measures include faucet aerators, timed or optical-sensor faucets,water-saving clothes washers, dishwashers or other appliances, water heaterinsulation, lower-setting or timed water heaters, and swimming pool covers.These energy efficiency measures are all compatible with solar water heating,and often reduce the size of the systems needed. Reducing hot water use saveson water and sewage as well as energy. For more information, ask the FEMPHelp Line (800-DOE-EREC) about the Water Conservation Program.

particularly for certain types of solarwater heating systems, because it canincrease maintenance costs and causethose systems to wear out prematurely.

Solar water heating can be usedeffectively throughout the country. Thedominant factor in determining effec-tiveness for solar water heating is theavailable solar resource (see Figures 1and 2), but do not dismiss the possibil-ity of using solar water heating because

the facility is in a cloudy area. Otherfactors are also quite important andsolar water heating works better thanmight be expected in areas with lessersolar resources. Cold water supply temperatures (see Figure 3 andAppendix A) increase system efficiencybecause until the fluid being heatedreaches higher temperatures, it losesless heat to the surroundings. Cold airtemperatures hurt solar water-heating

4

Kilowatt-hoursper square meter

0 to 33 to 44 to 55 to 66 to 8

Map prepared by the NRELResource Assessment Program

CD-SS26-B100213

Fig. 2. Average Daily Direct Normal Solar Radiation (on a tracking surface always facingthe sun, resource for parabolic trough). Parabolic trough solar water heating can be veryeffective for large systems, but is best suited to areas with high direct solar radiation.

52 52

52 52

52

55

56

56

56

56

52

484448

4848

4848

48 48

44

5660

60

72

68

68

76

80 7676

76

7272

6864 64

64

606056

56 56

5252

60

CD-SS26-B100203

Fig. 3. Ground Water Temperature in F in Wells Ranging from 50 to 150 Depth. Water supply temperance is also an important factor for solar water heating. Cost effec-tiveness is better if water must be heated from a colder starting temperature.

Recent Track RecordExcellent

The majority of existing solarwater heating systems were in-stalled in the 1980s when privateparties were eligible for a 40% fed-eral residential energy tax credit or a 15% business energy tax creditincentives. (There is currently only a 10% business energy tax credit).Although solar water heating hadcertainly already been around for awhile, there was not yet a matureindustry prepared to handle largevolume sales and installation. In the rush to take advantage of salesspurred by the tax incentives,many systems were poorly de-signed or installed or inadequatelymaintained. This earned solar water heating a bad reputation thatis not deserved by todays indus-try. Solar water heating systems arenow well proven, installers arehighly professional, and the indus-try has demonstrated an excellenttrack record in recent years. (SeeSuppliers on page 21 for list ofmanufacturers of collectors and dis-tributors of systems.) With carefulselection of the right system for aparticular situation, todays solarwater heating installations arelargely free of problems.

Although some solar water heat-ing systems from the 1980s werenot as well designed or installed asthey should have been, the major-ity are still delivering energy withlittle or no maintenance. A 1992 sur-vey of 185 residential systems inColorado, for example, found that65% of the systems were function-ing properly and that half of thosewith problems could be repaired for less than $150. The 1980s wasalso the most active period at fed-eral facilities with 718 systems in-stalled during or shortly after 1981through the Solar in Federal Build-ings Program. If you have an olderexisting systemfunctioning ornon-functioningit would be wellworthwhile to have it examined for possible improvements or reactivation.

performance by increasing loss of heatfrom the collectors to the air. Figure 4shows the performance that can beexpected by average and good solar collectors, respectively, in various partsof the country.

BenefitsBy tapping available renewable en-

ergy, solar water heating reduces con-sumption of conventional energy thatwould otherwise be used. Each unit ofenergy delivered to heat water with asolar heating system yields an evengreater reduction in use of fossil fuels.Water heating by natural gas, propane,or fuel oil is only about 60% efficientand although electric water heating isabout 90% efficient, the production ofelectricity from fossil fuels is generallyonly 30% or 40% efficient. Reducing fos-sil fuel use for water heating not onlysaves stocks of the fossil fuels, but elim-inates the air pollution and climatechange gas emission associated withburning those fuels.

Energy-Saving MechanismAlthough solar water heating sys-

tems all use the same basic method for capturing and transferring solarenergy, they do so with such a widevariety of specific technologies that onealmost needs to learn a whole languageof terms for distinguishing different col-lectors and systems. The distinctions are important though, because variouswater heating needs in various loca-tions are best served by certain types ofcollectors and systems. Systems can beeither active or passive, direct or indi-rect, pressurized or nonpressurized.(Note: the terms open-loop and closed-loop are frequently used to distinguishbetween direct and indirect systems,but technically their meaning is moreequivalent to nonpressurized and pres-surized. To avoid confusion, we will not use them here.) Types of Systems

The most frequently used systems for large facilities, antifreeze systems,are active, indirect systems. Active solarwater heating systems use pumps to circulate a heat-transfer fluid betweenthe collector and the storage tank.Indirect active systems use a heatexchanger to transfer heat from the

circulating fluid to the potable water.Antifreeze systems circulate a non-toxicfluid, usually propylene glycol,through the collector. See Figure 5 orAppendix H.

Even in freezing climates, however,water is often the heat transfer fluid ofchoice. This is because water has excel-lent heat-transfer properties, it is non-corrosive and highly stable, and it isless expensive. The need to prevent thesystem from freezing is, of course, thetradeoff for using water as the heat-transfer fluid. The drain back system

does this by totally draining the heat-transfer fluid out of the collector loopwhenever the pump is off, which iswhenever the water in the collector isnot hot enough to heat the potablewater, and therefore also whenever there is any freeze danger. See Figure 6or Appendix I. In contrast to most indi-rect systems, which are pressurized,many drain back systems use a nonpres-surized heat transfer fluid loop. Non-pressurized systems may use plastic orsite-built tanks that are less expensiveand more durable than pressurized

5

Btu/ft /yr/10002

Average Flat-Plate Collector Performancefor Solar Water Heating

Good Flat-Plate Collector Performancefor Solar Water Heating

240 to 280200 to 240160 to 200120 to 160

123161

188

204

191

173

173

151181197

128

172

226

203

224 262

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238 240257

251 222

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143

147

171

265

225

206Honolulu, HI

134Fairbanks, AK

CD-SS26-B100204

136

145135

134150

CD-SS26-B100205

Btu/ft /yr/10002

320 to 360280 to 320240 to 280200 to 240160 to 200120 to 160

150189

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205

199

177210237154

202

281

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274 300

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307 271

233201

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208204

208213

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250228

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191183

19974

158

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170157 179

151

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196

181

185172

162

191

201

300

275

258Honolulu, HI

163Fairbanks, AK

Maps prepared by the NREL Resource Assessment Program based on data from ASHRAEDesign Manual.Active Solar Heating Systems Design Manual.

Fig. 4. Important factors for solar water heating performance include solar resource, airtemperature, water supply temperature, and collector efficiency.

metal tanks. Evaporated water must bereplaced and being open to the air poses greater corrosion potential, but for a large system there may be signifi-cant savings with a nonpressurized tank.

Direct active systems run the potablewater to be consumed directly throughthe collector. Because they do not re-quire a heat exchanger, they average5%-10% greater efficiency, but theymust, in turn, activate special mecha-nisms to prevent the system from freezing. When control systems sensepotential freeze conditions, valves ondrain down systems shut the servicewater off from the collector loop waterand allow the collector loop water todrain out into a sump or down a drain.Recirculating systems respond to freezedanger by pumping heated waterthrough the collection loop. Althoughfreezing problems have been docu-mented with both of these direct sys-tems in the past, a newly designed valve for the former and careful choiceof the right situations to use the lattermay prevent those problems. Hard water is particularly troublesome fordirect systems, because scale deposits

that form in the collectors can reduceefficiency, increase the likelihood offreeze damage by restricting flow, andeventually shut down a system.

For smaller systems in mild climateswith modest freeze threat, passive sys-tems are also a viable option. Passivesystems do not require pumps or elec-tronic controls, greatly simplifyingoperation and maintenance, making passive systems very attractive for cer-tain situations. These are, in fact, themost commonly used system types inclimates with modest freeze threat.However, because they usually storewater outside at or near the collector,these systems are subject to greater heat loss. In cold climates particularly,this heat loss reduces the efficiency ofthe system in terms of the percentage ofthe solar energy originally absorbed that is eventually used.

Of the two main types of passive sys-tems, integrated collector systems (ICS)store the heated water inside the collec-tor itself. Thermosiphon systems have aseparate storage tank directly above thecollector. In direct thermosiphon sys-tems, the heated water rises from the

collector to the tank and cool waterfrom the tank sinks back into the collec-tor. In indirect thermosiphon systems,heated antifreeze rises from the collec-tor to an outer tank that surrounds thepotable water storage tank and acts as a heat exchanger (be sure it meets anycode stipulations about double-wall heat exchangers for separation frompotable water). See Figure 7 or Appen-dix J. In both ICS and thermosiphonsystems, good insulation of the collec-tor or tank helps prevent freezing andheat loss at night. The critical links,however, are the pipes connecting thecollector or tank to the service waterinside the house. Depending on pipesize and insulation, they can withstandtemperatures that are only so far belowfreezing for only so long, so the geo-graphic areas where these passive systems may be safely used must becarefully calculated. Hard water is again a concern. Also, most roofs willsupport the substantial weight of thewater storage, but this considerationcannot be ignored in adding a system to an existing structure or in designing a new facility.

P

T

TV

T

Sensor wires

Power to pumpsCD-SS26-B100206

Fill Manualair vent

Flat plate collectors

P/Treliefvalve

Temperature gauge

Temperaturegauge

Controller (a)

(b)Heat

transferfluid

pump(c)

Potablewater

systempump

Expansiontank

with air vent

Drain Pressuregauge

Heatexchanger

Solarpreheat

tank(potable)

Auxiliarytank

Boilerdrain

P/Treliefvalve

P/Treliefvalve

P/Treliefvalve

Hotwater

Boilerdrain

Coldwater

Backflowpreventer

6

Fig. 5. Active, Indirect, Two-Tank Antifreeze System

Types of CollectorsThe principal component of a solar

water-heating systemthe collec-torcan be low-temperature, mid-temperature, or high-temperature. Theglazed, flat-plate collectors most com-monly used for commercial or residen-tial domestic hot water are classified asmid-temperature collectors, generallyincreasing water temperature to as much as 160F (71C). As shown in Fig-ure 8, flat-plate collectors consist of aninsulated, weather-tight housing or box, a clear glass or plastic cover glaz-ing, a black absorber plate, and a sys-tem of passages for the heat transferfluid to pass through the collector. Spe-cial coatings on the absorber maximizeabsorption of sunlight and minimize re-radiation of heat. Gaskets and seals at the connections between the pipingand the collector and around the glaz-ing insure a water tight system.

Low-temperature collectors, whichgenerally increase water temperature to as much as 90F (32C), are lessexpensive because they consist simplyof an absorber with flow passages and

have no covering glass (glazing), insula-tion, or expensive materials such as aluminum or copper. These collectorsare less efficient in retaining solarenergy when outdoor temperatures arelow, but are quite efficient when out-side air temperatures are close to thetemperature to which the water is be-ing heated. They are highly suitable forswimming pool heating and other usesthat require only a moderate increase in temperature and are most com-monly used in warmer areas. For thelast several years, they have been themost frequently installed collectors. Inwarm climates, low temperature collec-tors are sometimes used in hybrid sys-tems that heat a pool in the winter andsupplement domestic water heating inthe summer, when pool heating is notneeded.

Large federal facilities or ones withquasi-industrial operations such as laundries may be able to efficiently usemore sophisticated high-temperaturecollectors. Although they are also usedin mid-temperature systems, evacuated-tube collectors can be designed to in-crease water/steam temperatures to asmuch as 350F (177C). They may use a

7

Backflowpreventer

Coldwater

in

Hotwaterout

Tempering valve

Tank-in-tank heatexchanger

Propylene glycol solution CD-SS26-B100208

PT relief valve

Solarcollector

Fig. 7. Passive, Indirect Thermosiphon System

T

T

TV

Flat plate collectors

CD-SS26-B100207

Hotwater

Coldwater

Backflowpreventer

TemperingvalveP/T

reliefvalve

P/Treliefvalve

Boilerdrain

Boilerdrain

Auxiliarytank

Solarpreheat

tank

Immersedheat

exchanger

Sightglass

Vacuumbreaker

Drainback line

Controller(a)

Fill

Sensor wires

Power to pumps

(b)Heat

transferfluid

pump

Temperaturegauge

Temperaturegauge

Fig. 6. Active, Indirect, Two-Tank Drainback System

variety of configurations, but generallyencase both the absorber surface and the tubes of working fluid in a tubularglass vacuum for highly efficient insula-tion. See Figure 9. Evacuated-tube collectors are the most efficient collec-tor type for cold climates with low-level diffuse sunlight. They can bemounted either on a roof or on theground, but they need to be protectedfrom vandalism and can be damaged by hail or hurricanes.

Parabolic-trough collectors use curvedmirrors to focus the sunlight on a re-ceiver tube (sometimes encased in anevacuated tube) running through thefocal point of the mirrors and can heattheir transfer fluid to as much as 570F(299C). See Figure 10. Because theyuse only direct-beam sunlight, para-bolic-trough systems require trackingsystems to keep them focused toward the sun and are best suited toareas with high direct solar radiation.See Figure 2. Because they are particu-larly susceptible to transmitting struc-tural stress from wind loading andrequire large areas for installation,parabolic-trough collectors are usuallyground mounted. For electrical genera-tion or industrial uses that require veryhigh temperatures (greater than 392F[200C]), a heat transfer fluid such as an oil is used, but depending on the de-gree of danger of freezing, antifreeze orwater is used in the heat transfer loopfor domestic water heating systems.Parabolic-trough collectors generallyrequire greater maintenance and

supervision and particularly benefitfrom economies of scale, so are gener-ally used for larger systems.System Design

System design for solar water-heating systems seeks to effectivelycombine solar water heating with con-ventional water heating. Rather than trying to store enough hot water to last through a long period of cloudyweather, solar water heating systemsgenerally have conventional water heat-ing systems as backup. Exceptions,such as the Chickasaw National Recrea-tion Area systems cited later as a case

study, are situations in which a lack ofhot water for a few days is acceptableand the expense of conventional backup is not justified. Typically, a conventional hot water heater drawspreheated water from the solar waterheating system storage tank. If that pre-heated water is not hot enough, the con-ventional water heater operates as itwould if it were starting with cold water and further heats the water until it reaches its set delivery temperature.Occasionally, the solar-heated water (up to 180F [82C]) is too hot for safeuse, so it is mixed with cold water in atempering valve.

As shown in Figure 5, a typicalactive, indirect solar water heating sys-tem consists of one or more parallel-connected glazed flat-plate collectors, astorage tank, a heat exchanger, pipingand valves for the heat-transfer fluid and for the potable water, pumps, andcontrols. Whenever the temperature ofthe water in the collector exceeds that of the stored water by more than a cer-tain amount (usually about 12F [6C]),the controller (a) turns on both pumps (b and c). The heat transfer fluid system pump (b) circulates heatedantifreeze from the collectors to the heat exchanger (where it transfers heatto the potable water) and back to thecollectors. The potable water systempump (c) circulates cool water from

8

Outletmanifold

Inletmanifold

Box

Flow passagesAbsorber

plate BackingTemperature

tolerantinsulation

Temperaturetolerantgasket

Glazing frameSingle ordouble glazing

CD-SS26-B100209

Fig. 8. Flat Plate Collector

Reflector

Supplytube

Return tube

Absorber tube or storagetank with absorber surface

Evacuated space between glassenvelope and absorber surface

Glassenvelope

CD-SS26-B100210

Fig. 9. Evacuated Tube Collector

9the bottom of the storage tank to theheat exchanger for heating and thenback to the top of the storage tank.(Instead of having a separate heatexchanger unit, the heat transfer fluidmay wrap around the potable waterstorage tank either with piping or with a surrounding outer tank.) As water isused from the conventional hot watertank, it is replaced by solar-heated water from the top of the storage tank.Inlet water from the domestic supplysystem flows into the bottom of thestorage tank to keep the system full.

Alternatively, a single storage tankmay be used. A common single-tankdesign disconnects the heating ele-ment(s) from the lower portion of a con-ventional electric water heater. Whenthe solar water heating system is operating, it draws cold water fromthe bottom of the tank and returns theheated water to the top. If the solarheating does not have the water hotenough, the conventional heating ele-ments in the top of the tank bring thewater up to the desired temperature.Although not used much in this coun-try, another single-tank design uses arapid booster or tankless heater in the water line as it leaves the tank toprovide additional heating upondemand, if needed. This option avoidsmaintaining the whole tank at thedesired temperature as most conven-tional water heaters do, minimizingstandby losses. Some two-tank systemsadd a second direct pipe connectionwith appropriate check valves betweenthe two tanks to increase heat flow from the solar storage tank to the con-ventional tank. If the solar storage tankis hotter than the conventional waterservice tank, hot water flows by convec-tion into the service tank, even whenthere is no draw on the system.

The most cost-effective size for asolar water heating system will often be one that is just sufficient to meet thefull summer demand and that meetsapproximately two-thirds of the year-round demand. Including enough capacity to meet more of the winterdemand reduces cost effectiveness bothbecause excess capacity is wasted in thesummer and because it is increasinglydifficult to serve each additional por-tion of the winter demand with the

reduced solar resource. The most cost-effective size can vary widely with spe-cific circumstances, however, and forcommercial building systems espe-cially, it is sometimes best to plan tosupply considerably less than two-thirds of hot water use. The key factorsin determining the most cost-effectivesize for a system are the type and costof conventional fuel and the cost of the solar water heating system to beinstalled.

Good records of past hot-water usehelp greatly to plan an effective solarwater heating system, and it is easy toinstall a water meter on the incomingline to a hot water heater. Water use can vary quite substantially, but for new construction, or if your uses of hotwater are relatively standard, there are rules of thumb to estimate hotwater requirements for various build-ing uses . The handbook guideline forresidential use, for example, is 20 to 30 gallons per person or 65 gallons perhousehold per day. (Note, however,that some more recent studies havefound average use as low as 25-35 gal-lons per household per day.) For officebuildings, you can expect hot water useof 0.5 gallons per person per day. (Thestandard reference for projecting hotwater use is the American Society ofHeating, Refrigeration, and Air

Conditioning Engineers, Inc. [ASHRAE]Applications Handbook, Chapter 44.)

The circumstances for specific largefacilities may vary considerably, but forsmall systems, a general rule of thumbis to have storage roughly equal to onedays hot water use. In a loca-tion withaverage available solar energy, you willneed approximately 0.5 to 1.0 squarefeet of flat-plate collector per gallon of

Fig. 10. Parabolic trough solar water heating system for Adams County, Colorado,Correctional Facility.

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A Few Prescriptions for aSuccessful Solar Water-Heating System Size the system conservatively,

probably to meet at most two-thirds of total hot water use

Pay careful attention to freezeand corrosion protection

Use professional advice and pre-pare the bid package carefully,using an engineering or designfirm or contractor that has expe-rience in designing solar waterheating systems

Ensure that you will have a facil-ity manager committed to renew-able energy and the project

Commit to doing simple systemchecks a couple times per yearand doing all necessary maintenance

storage tank. The daily pattern and con-sistency of hot-water consumption isalso an impor-tant consideration for de-termining the size of collector and stor-age area needed. Uses that demand hot water mostly during the day (laun-dries, lunch service, or car washes, forexample) will require relatively lessstorage than uses such as showers forwhich the heaviest demand occurs atnight or early in the morning.

Installation Solar collectors can be mounted on

the roof of a building or on nearbygrounds. For year-round uses, the mostefficient orientation for the collector isfacing south, tilted at an angle aboutequal to the latitude of the site. (The latitude plus 15 maximizes wintertimeheat collection and latitude minus 15maximizes summertime heat collec-tion.) Collectors can be tilted to theproper orientation with mounting racks. For cost savings and aestheticreasons, however, they are increasinglybeing laid flat against pitched roofs. Ifthe orientation is at all close to optimal,the sacrifice in available energy is usually quite modest. For Denver,Colorado, for example, with a tilt of

latitude minus 15, mounting the collec-tors as much as 45 off of southern orientation loses at most 10% of avail-able solar energy. Similarly, with a truesouthern orientation, you can mountcollectors at up to 25 off latitude tiltwith only 10% loss. Solar resourceinformation for Boulder, Colorado, ispresented in Appendix B as an exam-ple of available data.

Incorporating solar water-heatingsystems in new construction has theadvantages of ensuring that there is anappropriate roof for collector place-ment, allowing for aesthetic design,and reducing installation costs. If thebuilder, architect, or engineer is used to working with solar water heating,it can also save on design cost. But,almost any building can incorporate asolar collector retrofit. It is relativelyeasy to add a solar water heating sys-tem to an existing facility and the economics will be nearly as good.

There are generally relatively fewspecial regulations to consider in install-ing solar water heating systems, butthere are pertinent building, mechani-cal, and plumbing codes. Areas withspecial building regulations because ofearthquake or hurricane danger, mighthave structural requirements limitingthe weight or type of equipment thatcan be placed on a roof. Some localcodes for residential or commercialareas regulate the attachment of collec-tors to roofs or walls. A few jurisdic-tions require rigorous separationbetween the heat transfer fluid and thepotable water in closed-loop systemsthat could rule out single-wall heat ex-changers. Besides regulations such asthese, systems need only comply withstandard plumbing and local buildingcodes.

Numerous manufacturers make quality solar collectors and solar waterheating systems. In addition to check-ing out the various manufacturers, oneway to ensure that your system meetsgenerally applied standards is to installan SRCC-certified system. An inde-pendent, nonprofit organization cre-ated by organizations representing solar equipment manufacturers, stategovernments, and consumers, the SolarRating and Certification Corporation

(SRCC) has instituted a quality assur-ance and performance rating program.As of December 1995, the SRCC hadcertified 3 unglazed collectors and 60glazed flat-plate collectors made by atotal of 12 different manufacturers, plus78 total solar water heating systemsmade by 12 different manufacturers.The SRCC certification process also en-sures that health and safety issues havebeen addressed, that typical code provi-sions are complied with, and that dura-bility and reliability standards havebeen met and are correctly portrayed.There, of course, may be collectors andsystems of acceptable quality that havenot been rated by SRCC.

A complete list of all solar collectorand water heating system manufactur-ers was not available, but Supplierson page 32 lists the manufacturers ofthe SRCC-certified collectors and sys-tems plus manufacturers who belong to the Solar Energy Industries Associa-tion. You can also check the Thomas Reg-ister of American Manufacturers. TheEnergy Information Agencys annualsurvey, reported in the RenewableEnergy Annual, reports 41 active solarcollector manufacturing companiesshipping 7.6 million square feet of col-lectors in 1994. Information on SRCC-certified systems is contained in theDirectory of SRCC Certified Solar Collec-tor and Water Heating System Ratings.Appendices G, H, I, and J illustrateSRCC collector and system rating infor-mation. (The Florida Solar EnergyCenter also rates solar water-heatingsystems.)

Federal SectorPotentialTechnology Screening Process

The FTA series targets technologiesthat appear to have significant untappedfederal-sector potential and for whichsome federal installation experience ex-ists. Many of the alerts are about newtechnologies identified through adver-tisements for technology suggestions in the Commerce Business Daily and tradejournals, and through direct correspon-dence in response to an open technol-ogy solicitation. Those technologies arethen evaluated in terms of potential

10

Factors Contributing to the Cost-Effectiveness of Solar Water Heating

Each factor helpful, but not neces-sary to have all of them. High-cost conventional water-

heating system (more than about$15 to $20 per million Btu)

High daily volume of very-hot-water use (such as for laundries or industrial processes)

Steady demand throughout theweek and year, or highest need inthe summer

Relatively greater hot water useduring the day

Unshaded, south-facing roofspace or sunny, nearby grounds

Good solar resource (see Figures1, 2, 4 and 5)

Cold water supply (see Figure 3and Appendix A)

Soft water

energy, cost, and environmental bene-fits to the federal sector.

Solar water heating is a renewableenergy technology with clearly knownenergy, cost, and environmental bene-fits, and a large number of manufactur-ers of a variety of productsbut stillwith substantial untapped potential forthe federal sector. Solar water heatingwas selected for the New TechnologyDemonstration Program throughresponse to the open technology solicitation.

Estimated Market PotentialThe Office of Technology Assess-

ment reported in 1991 that the U.S.Government owns or leases approxi-mately 500,000 buildings, owns anadditional 422,000 housing units formilitary families, and subsidizes utilitybills for 9 million private households. If the objective were to reduce fossilfuel energy use and associated pollu-tion, regardless of cost effectiveness,the potential application of solar waterheating would clearly be immense.Even limiting application to cost effec-tive situations, opportunities for solarwater heating may still be quite sub-stantial. Combining the large number of military and other housing units with the fact that 18% of residentialenergy use is for water heating and anEnergy Information Administrationstatement that 38% of U.S. residentialwater heating is electric, points to avery large potential application forsmall systems where economics arelikely to be attractive. Federal prisons,hospitals, and barracks are ideal situa-tions for large, high-temperature systems to prove cost effective. An estimate of the number of swimmingpools at federal facilities is not avail-able, but there are certainly a signifi-cant number and the likelihood of solarpool heating being cost effective is quite good.

ApplicationThe cost of operating conventional

or backup water heating systems is thesingle most important factor in deter-mining economic feasibility for solarwater heating systems, but a variety ofother factors are also important. Solarwater heating projects for federal

facilities are most likely to be cost-effective in three situations: Small, residential-size facilities

such as visitor centers, campgroundshowers, or staff housing, whichwould otherwise be dependent upon high-cost energy sources

Large facilities that require large volumes of hot water (more than a thousand gallons per day) or have operations that use high-temperature hot water

Swimming pools.

Where to ApplySmall Facilities

For small federal facility projects, thecost of conventional water heating sys-tems dominates the economic feasibil-ity of solar water heating systems. Ascan be seen from Table 1 below, thecost of conventional energy variesgreatly. Note that these figures arenational averages and utility rates varygreatly by region and individual facil-ity contract. There may be regions inwhich the relative effective energy costof the various energy supplies differsfrom that below. Table 2 shows aver-age utility rates by region. Water heaterefficiencies also vary significantly, par-ticularly for larger heaters, from 77% to97% for electric and from 43% to 86%for gas. You should therefore also inves-tigate the cost-effectiveness of buying amore efficient water heater either on itsown or in conjunction with installationof a solar water heating system.

The cost of solar water heating systems can vary widely dependingupon the circumstances for a specific

installation, region of the country, andother factors and are not generally available as published numbers. To get a ballpark idea, however, we can look at four residential-size systems ap-proved by the Sacramento MunicipalUtility District for its electrical-demand-reduction incentive program. The foursystems are a 42-square-foot indirectthermosiphon system, an evacuated-tube integrated collector system, a 64-square-foot antifreeze system, and a40-square-foot antifreeze system thatuses a wrap around heat exchanger so it needs only one pump instead oftwo. The systems vary in cost from$2,860 to $3,180 and from meeting 61%to 74% of an assumed 57-gallon-per-day demand (averages 8.8 MBtu peryear delivered energy). If we assume20-year continuous operation and 0.5%per year operation and maintenance cost for the two passive systems and 2% per year for the two active systems,the levelized cost for the systems fallsin the $20 to $23 per MBtu range. Look-ing at Tables 1 and 2, we can see thatthis is less than the average cost of elec-tricity for federal facilities, nationallyand for several of the regions, but thereis little chance of competing with othertypes of water heating.

As it happens, many smaller federalfacilities or elements of federal facilitiesare located in relatively remote areaswhere conventional water-heating utility costs are particularly high. Three-quarters of the projects built in the1980s under the Solar in Federal Build-ings Program were small systems (lessthan 100 square feet of collector) for

11

Table 1. Effective Energy Cost for Water Heating Based onNational Average Federal Facility Utility Prices

Average EffectiveFederal Energy Cost Efficiency Energy cost

electricity $21.05/MBtu (7.2/kWh) 91% $23.13/MBtupropane* $5.40/MBtu* (49/gal) 59% $9.14/MBtufuel oil $3.85/MBtu (53/gal) 56%** $6.87/MBtunatural gas $3.65/MBtu (37/therm) 59% $6.19/MBtu(Sources: Energy costs from General Services Administration Energy Analysis and Usage Center forFiscal Year 1995. *Propane is 1994 refiner sales price to end users from the Energy InformationAdministration (Federal facility costs vary widely by individual users and averages are not tracked.)Efficiencies are from Gas Appliance Manufacturers Association April 1995 Consumers' Directory ofCertified Efficiency Ratings for Residential Heating and Water Heating Equipment, pages 155, 193, and195. Data are for 50-gallon first-hour rating, **except for fuel oil, which is for 100-gallon first-hour rating.

facilities in the National Park Sys-tem. Any of the mid-temperaturetechnologies will work well forsmall facilities. Solar water-heatingworks well for general domesticneeds and for isolated facilities such as laundries, show-ers, visitorcenters, ranger stations, and staffhousing.

Off-the-shelf packages areoften quite appropriate for small orremote facilities such as these, and avariety of SRCC-certified systemsare available, so engineering designwork is not necessary. If the poten-tial system involves more than twoor three collectors or will be con-nected to unusual plumbing, electri-cal, or structural systems, a bidpackage will likely be needed for aspecific design. But in most cases,

you will still be able to use off-the-shelfcomponents and the ASHRAE ActiveSolar Heating Systems Design Manual.

In warm climates with limited freeze danger, the low-maintenancenature of passive systems is an attrac-tive feature for isolated locations. Solarelectric cells can provide power to oper-ate solar water heating systems if elec-tric utility connections are unavailable.Even if grid electricity is available,solar cells are an excellent match forsolar water heating pumps and often are used as the main operation controlfor the system. When there is enoughsunlight for the hot-water system to be operating and power is needed to run the pumps, the solar cells are alsoproducing power.

Where to ApplyLarge Systems

Although the cost of conventionalenergy is still the most critical factor forthe economics of solar water heatingsystems, for large federal facilities, it isless likely to be the factor that makes so-lar water heating cost effective. Becauseof their size and because they are lesslikely to be in remote locations, mostlarge facilities will have moderate orlow cost energy available. The cost-effectiveness of solar water heating sys-tems for large facilities may, however,be improved significantly by econo-mies of scale in building a large system.While small systems with less than 100 square feet of collector generallycost between $50 and $90 per squarefoot of collector aperture, that figurecan drop to $40 or $45 per square footfor flat-plate collector systems withmore than 1000 square feet of collector,$30 per square foot for systems withmore than 10,000 square feet of collec-tor, or even $25 per square foot for parabolic trough systems with morethan 20,000 square feet of collector.

As can be seen from Table 3, that re-duction in cost can make all the differ-ence in whether a project will beat outthe conventional energy costs citedabove. The table divides total systemcost (including 2% per year operationand maintenance) by the amount ofenergy the system would produce over a twenty-year lifetime. These costs donot include government acquisitioncosts, which tend to be relatively

12

Small System ExamplesSome examples of recently installed or planned small solar water heating

systems for federal facilities include a system for the Environmental ProtectionAgency (EPA) headquarters offices in Washington, D.C., three systems for anenvironmental education center in the Phoenix, Arizona, area, and three sys-tems for a National Park Service national recreation area in Okla-homa. The480-square-foot active, indirect system recently installed to serve the privatelyowned building housing the EPA headquarters will provide 71% of the de-mand for hot water (approximately 1150 gallons/day), saving $2,656 annuallyin electricity. System costs were shared by EPA and the DOE Solar ProcessHeat Program. With a 10% federal tax credit to the building owner, plus a re-bate from the local electric utility because the system reduces peak demand,the system will pay for itself (simple payback) in 6 to 7 years.

Three small drainback systems will be part of new Bureau of Reclamationfacilities in Lake Pleasant, Arizona. The Bureau is building a classroom build-ing and two dormitories, which it will lease to the Maricopa County OutdoorEducation Center (OEC). The classroom building includes a cafeteria and thedormitories will house 50 students each. The Bureau plans to use solar watereating systems for each of the three buildings at this remote site, not only toreduce the cost of water heating but also to serve as an educational tool for stu-dents. The OEC will be an all-electric facility except for propane for auxiliarywater heating. The system for the classroom building has 70 square feet of collector and 120 gallons of water storage and will meet 64% of the waterheating load. It will save 17,800 kBtu/yr. The systems for each dormitory have145 square feet of collectors and 240 gallons of water storage. The dormitorysystems will meet 45% of the annual load and each save 34,300 kBtu/yr.

A third example is currently being designed by the National Park Servicefor the Chickasaw National Recreation Area in Oklahoma. Three solar water-heating systems are expected to cost $35,000 and have a simple payback of 9 years. The systems will have no backup system, must be designed to shutdown for winter and quickly start up in the spring, and must have very highreliability because of the remote location and the lack of operation and mainte-nance staff. See the case study on page 28 for a complete description andWho is Using the Technology on page 34 for contacts regarding particularprojects.

Table 2. Average RegionalFederal Facility

Utility Prices per MBtuElectricity Oil Gas

Boston 32.31 3.43 6.61New York 33.15 3.80 4.19Philadelphia 22.29 3.51 6.09Atlanta 18.01 4.69 5.32Chicago 20.36 N.A. 3.47Kansas City 15.71 N.A. 3.36Fort Worth 18.64 N.A. 4.29Denver 14.02 4.14 3.83San Francisco 28.67 N.A. 6.51Auburn (Pacific-NW) 13.40 4.63 4.63National CapitalRegion 19.08 3.61 5.04National Average 21.05 3.85 3.65

(To get effective cost as per Table 1, divide electricityprice by .91, fuel oil by .56, and gas by .59)

constant regardless of project size, giv-ing further advantage to larger projects.

As can be seen by comparing Tables 1 and 3, none of our six samplecities can compete with conventionalwater heating paying the effectivenational-average cost for electricity of$23.13/MBtu with small solar waterheating systems costing $75 to $90 persquare foot of collector and only two at$60 per square foot. But with a largersystem costing $40 or $50 per squarefoot, solar water heating is quite com-petitive. These numbers are, of course,ballpark figures that do not take intoaccount the specifics of particular situa-tions, but they do illustrate the impor-tance of either competing againstexpensive conventional water heating or having a large water-heating load that allows building a large enoughsolar water heating system to bringcosts down.

If hot water use is more than 1000 gallons per day or conventionalenergy cost is more than $15 to $20 permillion Btu, prospects are good for alarge solar water heating system toprove cost effective. At more than10,000 gallons per day, parabolic trough systems should be considered.

Nearly all prisons, hospitals, and military bases, and many other federalfacilities with kitchens, laundries, orshowers, use large quantities of hotwater. Many of these facilities also havepopulations that are constant through-out the week and throughout the yearand therefore have consistant water

use. These factors make it worthwhileto consider a solar water-heating sys-temparticularly if conventionalenergy costs are relatively high. As indicated by the case study below, addi-tional savings are often possible duringthe summer by recovering heat fromchiller systems. It is occasionally possible to take further advantage ofeconomies of scale by also providing

hot water for space heating or coolingor other purposes. Current thinking,however, is to look first at providingjust for direct hot water use, becauseadding heating or cooling makes sys-tems more complex and may adverselyaffect economics by increasing the vari-ation in demand throughout the year.

Active indirect systems with flatplate collectors work well for meetinglarge water heating demands, but larger water volumes and need for high-temperature water also make high-temperature parabolic trough orevacuated tube systems attractive, de-pending on the climate. While flat platecollector systems typically provideenough heat to efficiently raise heattransfer fluid temperatures to as muchas 160F (70C), the high-temperaturecollectors operate more efficiently when generating water or steam atmuch higher temperaturesup to 350F (175C) for evacuated-tube collectors and up to 570F (300C) forparabolic trough collectors. So thesesystems are particularly good for facilities with high-temperature waterneeds such as laundries, which

13

Large System ExamplesThe Federal Bureau of Prisons recently awarded a contact to build a solar

thermal system at its correctional institution in Phoenix. Similar to installa-tions at state and local prisons, the system of parabolic trough collectors and athermal energy storage tank will provide hot water for inmates, laundry facili-ties, and kitchens. Another example of a large solar water heating system for afederal facility is a hybrid chiller heat recovery/solar water heating system forthe Prince Kuhio Federal Building in Honolulu, Hawaii. This building has1,083,300 square feet of floor space and houses a number of federal agencies.

The planned hybrid system combines a chiller heat recovery system with adirect solar heating system. It provides 3000 gallons of hot water per day andincludes 1300 gallons of preheat water storage. The chiller heat recovery com-ponent of the system uses a compact brazed heat exchanger with a heat-transfer area of 14 square feet. The optimized solar heating component of thesystem has a solar array with 1361 square feet of collector area on the roof.The hybrid system allows the solar component to be about two-thirds the sizeit would have been without inclusion of the chiller heat recovery.

Because of the Hawaiian climate, freeze protection is not needed and thesolar portion of the system circulates the potable water directly through thesolar collectors without a heat exchanger. The solar component of the systemprovides 55% of the buildings water heating needs, with the total system pro-viding 82% of annual demand. The system meets approximately 75% of thewater heating load in the winter and 90% in the summer. The estimated in-stalled cost for the system is $58,389. The system offsets the need for syntheticnatural gas at a cost of $1.22/therm. The project has a simple payback periodof 9 years and an adjusted internal rate of return of 6.75%.

Table 3. Effective Levelized Cost Per MBtu of Solar WaterHeating at Selected Locations

Installedcost persquarefoot of San Fran- Denver, Chicago, Washing- Orlando, Boston,collector cisco, CA CO IL ton, DC FL MA

$30 $10.15 $9.69 $13.45 $13.99 $13.49 $14.91$40 $13.54 $12.92 $17.94 $18.66 $17.98 $19.88$50 $16.92 $16.15 $22.42 $23.32 $22.48 $24.85$60 $20.31 $19.38 $26.91 $27.78 $26.97 $29.82$75 $25.39 $24.23 $33.63 $34.98 $33.72 $37.29$90 $30.46 $29.07 $40.36 $41.95 $40.46 $44.73Calculations are based on F-chart analysis of energy savings for active flat-plate systems operating con-tinuously for a 20-year life and 2% annual operation and maintenance cost. Operation and maintenancecosts and value of energy savings are escalated at the rate of inflation (0% real) and discounted at 3%.

typically use water as hot as 180F(82C); kitchens, which typically usewater temperatures from 140F to 195F (60C to 91C) for dishwashing;or industrial processes.

Where to Apply SwimmingPools

One of the most consistently cost-effective uses for solar water heatingsystems is for heating swimming pools.Low-temperature collectorsmost ofwhich are for swimming poolshaveaccounted for the majority of solarwater heating systems sold since 1991(more than 85% on a square-foot basisin 1993). Many military bases and otherfederal facilities have swimming pools,so there may be many cost-effectiveopportunities for installation of solarswimming pool heaters. If you have apool and it is now heated, you may reap great savings, because solar pool-heating systems frequently pay forthemselves within two to four yearseven when replacing natural gas heat. If your pool is not now heated, you may be able to extend your season byseveral months. If you are faced withbudget cuts, energy savings may allowyou to keep a pool open.

The pools filter system pumps the water through the collector and the heat storage is in the pool itself.Because only a modest temperatureincrease is needed, most systems useinexpensive, unglazed low-temperaturecollectors, which are often essentiallysystems of water tubes built into darkplastic. Off the shelf packages aregenerally appropriate and maintenanceis minimal. Some smaller systems areoperated manually or with timers, butlarger systems are operated by elec-tronic sensors and controls. When thecollector temperature is sufficientlygreater than the pool temperature, adiverting valvethe only movingpartdiverts water from the filter sys-tem through the collector loop. As withother hot water uses, conservation ofgenerated heat is generally the mostcost-effective investment and swim-ming pool covers should be consideredat the same time as a solar water heat-ing system.

14

Swimming Pool ExamplesSandia National Laboratories has helped Camp Pendleton in Southern

California refurbish an inactive solar pool heating system at one of the Camps recreational swimming pools. The refurbishment was completed in the summer of 1995 for $10,000. The collector array has 2560 square feet ofunglazed collectors using copper pipes. If the pool was used year-round, itwould save $8,000 per year in natural gas. This pool is used only 3-4 monthsper year but was chosen as a pilot project for Camp Pendleton. The MarineCorps has six more pools with non-operating solar water heating systems atthe Camp, and these are each used year round. Now that the pilot has beencompleted, the Marines are looking into refurbishing the other six systems as well.

The Barnes Field House on the Fort Huachuca Army Base in FortHuachuca, Arizona, uses a 2000-square-foot solar system for pool heating, seeFigure 11 above. The system was installed in June 1980, and supplies heat fora 3500-square-foot indoor pool. The system meets 49% of the annual load andoffsets the need for 835 MBtu of natural gas per year.

A noteworthy example of local government use of solar water heating is the city of Santa Clara, California, solar pool heating program. Since 1975, thecitys municipal utility has been providing for the design, installation, and on-going maintenance of solar pool heating systems. The pool owner pays an initial installation fee to cover the value of the labor and permanent materialsrequired to install a solar heating system. The recoverable components, includ-ing the panels and automatic controls, are rented; the monthly fees are set bythe city council as a Solar Utility Rate. Each user and the city enter into acontract that defines the responsibilities of each party and sets the monthlyutility fee proportional to the size of the solar energy system. Fees are de-signed to repay installation costs as though repaying an amortized loan for aterm equal to the expected life of the equipment. To date, more than 300 of the800 pools in the city are heated by the citys solar program.

Fig. 11. Solar water heating system for indoor pool at Barnes Field House, Fort HuachacaArmy Base.

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Brochures on covers and solar waterheating systems for swimming poolsand a software package that can evalu-ate the economic feasibility for yourpool are available from the EnergyEfficiency and Renewable Energy Clearinghouse. Call 1-800-DOE-ERECand ask for the Energy Smart Poolspackage.

Application ScreeningThe first step toward installing a

solar water heating system is to assessyour hot water needs. How much hotwater at what temperature do your various facilities use (or are new facili-ties expected to use), on what kind ofschedule? How much do you pay forthe energy to heat that water? Can yousave money with a more efficient con-ventional water heater? What options do you have for reducing hot water useor lowering the temperature of waterprovided?

The next step is to obtain a prelimi-nary estimate of whether solar waterheating will be cost-effective. TheFEMP Federal Renewables Program atthe National Renewable Energy Labora-tory has developed a computer pro-gram known as FREScA that can makesuch a preliminary as-sessment for you.See the How Do You Figure sidebaron page17 for a list of the necessary in-formation. (For swimming pools, youcan use Energy Smart Pools softwareinstead of FREScA.)

For smaller projects, a clearly posi-tive FREScA calculation will often besufficient to proceed to system pur-chase. For large systems, a positiveFREScA assessment should be fol-lowed up with a formal feasibility study (see Economic Criteria below).Larger projects will likely require a pri-vate engineer at some point, but theFEMP Federal Renewables Programstaff can provide fairly extensive assistance.

A general rule of thumb for federalfacilities is that a renewable energy in-stallation should pay for itself withinabout ten to fifteen years. Because thelifetime of a system can be as much as 30 years, that means you can look for-ward to as much as 20 years of freeenergy.

System Selection and Procurement

As a general rule, the optimal type of solar water heating system dependson the increase in water temperaturethat the system will be used for. Low-temperature systemswith no coverglazing or insulationabsorb a highpercentage of the available solar heatbut also lose sizable amounts ofenergy. They are therefore best for

uses such as swimming pools that onlyrequire a modest increase in water temperature. Adding glazing and insulation cuts down on heat absorptionbut greatly increases heat retention, sothe added cost of mid-temperature sys-tems is cost effective for most applica-tions requiring greater increases inwater temperature. High-temperaturesystems, such as evacuated tubes with their very high insulation and

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The Right Collector for the Right UseSolar collector efficiency is a function of optical gain1 minus heat loss2.

Collectors for low-temperature applications (like swimming pools) have highoptical gains (no cover glass and high surface absorptivity) but they also havehigh heat loss because they are uninsulated. Mid-temperature collectors, fordomestic water heating, have cover glass and insulation to reduce heat loss,but the cover glass results in slightly lower optical gains due to reflection ofsunlight off the glass. High-temperature collectors such as evacuated tubes and focusing parabolic troughs also have optical losses from cover glass andfocusing reflectors, but they retain heat at very high temperatures making them ideal for high-temperature applications like absorption cooling and power generation.

The type of collector best suited to a particular application depends both onthe temperature above ambient to which the water is to be heated and on col-lector cost. The following table of energy generation per area of collector (based on selected collectors from the SRCC Directory) shows that low-temperature collectors are indeed the most effective for low-temperature applications; mid-temperature collectors are the best for medium-temperatureapplications; and high-temperature collectors are the best for high-temperatureapplications. For low-temperature applications the more expensive insulated collectors offer no advantage, but at high temperatures they are essential to collect solar heat.

Unglazed Pool Glazed Collector Evacuated TubeHeater (low) (mid-temp) Collector (high)

Optical Gain .87 .74 .50Coefficient1

Heat Loss 21.3 W/M2C 4.9 W/M2C 21.3 W/M2CCoefficient2 (3.7 Btu/hrft2F) (.9 Btu/hrft2F) (3.7 Btu/hrft2F)Amount Temperatureof Water Entering the Clear Day (6.4 kWh/m2day-2000 Btu/ft2day) Heat DeliveryCollector Exceeds Ambient

5C (9F) 4.1 kWh/m2day 4.0 kWh/m2day 3.0 kWh/m2day(low) (1300 Btu/ft2day) (1250 Btu/ft2day) (1000 Btu/ft2day)20C (36F) 1.5 kWh/m2day 3.2 kWh/m2day 2.8 kWh/m2day(medium) (470 Btu/ft2day) (1000 Btu/ft2day) (900 Btu/ft2day)50C (90F) 0 kWh/m2day 2.0 kWh/m2day 2.4 kWh/m2day(high) (0 Btu/ft2day) (640 Btu/ft2day) (770 Btu/ft2day)1fraction of sunlight captured as heat2mulitiplier for the amount that the temperature of the return water that enters the collector exceeds outside air temperature, to determine heat loss from the collector. For example, the heat collected by a glazed collector heating water from 60C when it is 0C outside and the sun is shining at 1000 W/m2, would be: .74 (1000 W/m2) - 4.9 W/m2C (60C - 0C) = 446 W/m2

parabolic troughs with their concentra-tion of the sunlight, are most effectivewhen used to provide either very largeamounts of hot water or high tempera-ture uses such as kitchens, laundries, orindustrial uses. (See sidebar on page 15for detailed discussion.) Table 4 sum-marizes characteristics that may makecertain system types particularly suit-able or inappropriate for your facility.

Having found that a solar water heating system is likely to be cost effective for your facility, chosen one or two appropriate system types, anddetermined the approximate size of thesystem, you can now probably pick outthe most appropriate products from the SRCC Directory (for smaller sys-tems) and proceed toward purchase inaccordance with Federal AcquisitionRegulations. For most agencies thismeans small purchase agreements based on a request for quotes for proj-ects costing less than $25,000, requestsfor quotes including notice in theCommerce Business Daily for projectscosting from $25,000 to $50,000, and

going out for bids for anything morethan $50,000. (A new electronic mailadvertising system in the works willallow requests for quotes to be used for anything up to $100,000.)

For smaller systems, specifics onyour hot water usage pattern, water supply temperature, and detailed utility rate schedule will probably besufficient additional data for potentialvendors to supply the cost, perform-ance, and other information you need to select a system and to decide whether to proceed. It is not quite likegoing to the discount store for a con-ventional home water heater, but complete off-the-shelf systems are avail-able. FEMP is working on getting solarwater heating systems on the GSA pur-chase schedule (perhaps by 1997, checkwith the FEMP Help Line), which willmake it easier to obtain specific modelsat fixed prices. They are also develop-ing product recommendations for solarwater heating systems. In the mean-time, certified systems from the SRCCDirectory are a place to start, and there

may be many other good systems tochoose from.

For larger systems, you will needengineering help to select an optimumsystem and do a detailed economicassessment for that system (see Eco-nomic Criteria below). You may haveto go out for bids to hire an engineer todesign the system, but can probably doso with a sole source contract for profes-sional services. The designer cannotthen be a vendor for the system but can write the specifications for the bidrequest and either install or supervisethe system_s installation. Appendix E isan example of specifications used forthe Chickasaw National RecreationArea case study. Check with the FEMPFederal Renewables Program (303-384-7509) for other previously pre-pared specifications that may be moresimilar to your planned system.

Economic CriteriaThe policy for evaluating whether

solar water heating or other renewableenergy projects are cost effective andtherefore appropriate for federal facili-ties are contained in 10 CFR Part 436Aof the Code of Federal Regulations. The prin-cipal criterion of these regulations is that the life-cycle cost (value in baseyear dollars of all costs for the fullanalysis period) for the project must beless than any alternatives, including projected utility payments with theexisting water-heating system. (Threesimilar criteria may be used instead for retrofit projects, and projects withinsignificant cost are presumed costeffective.)

Executive Order 12902 goes beyondthe cost-effectiveness regulations tostipulate that if a project will pay foritself (simple payback period-time forsavings to return the cost of the invest-ment) in less than 10 years, it shall bebuilt (Sections 103 and 303). For mostsituations the 10 year payback criterionwill be more rigorous than the life-cycle-cost criterion. Many projects willmeet the life-cycle-cost criterion eventhough their simple payback issomewhat longer than 10 years. Agen-cies must build projects with a simplepayback of less than 10 years, but may

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Table 4. Solar Water Heating System Characteristics: FactorUseful in Selecting System Type for Particular Situation

Suitable Cost/ ft2 Freeze Hard Mainte-system for 40 ft2 tolerance water nancesize for unless noted tolerance need

Low-Temperature SystemsUnglazed pools $10-$25 none good very low

(400 ft2)Passive Mid-Temperature SystemsIntegrated collector small $50-$75 moderate minimal very lowThermo- direct small $40-$75 none minimal lowsiphon indirect small $50-$80 moderate good lowIndirect, Active, Mid-Temperature SystemsFlat-plate, antifreeze small or $50-$90 excellent good high

large $30-$50(30,000 ft2)

Flat-plate, drainback small $50-$90 good good highDirect, Active, Mid-Temperature SystemsDrain down small corrections minimal high

beingRecirculating small developed minimal highHigh-Temperature SystemsEvac- direct small $75-$150 good minimal highuated tube indirect large $75-$150 excellent good highParabolic trough large $20-$40 excellent good high

(30,000 ft2)

also build any project that meets thelife-cycle-cost criterion.

Life-cycle cost analysis calculates thesum during the life of the project of thepresent value of investment costs, op-eration and maintenance, replacementcosts, and energy costs, minus salvagevalue of replaced parts. A manual forlife cycle costing (NIST Handbook 135),an annual set of prescribed energyprices and discount rates (NISTIR 85-3273), and Building Life-Cycle Cost(BLCC) software (NIST 4481) are allavailable by calling the FEMP HelpLine at 800-DOE-EREC. (Some agenciesallow simpler life-cycle calculations,but the BLCC is required if FEMP fund-ing is involved. You may also needMeans Mechanical Cost Data [availablefrom 800-448-8182] for estimating sys-tem component costs.)

In addition to determining whether a project is cost effective, economicanalysis helps to determine the size ofthe solar water heating project that willminimize costs during the life of theproject. The cost of conventional waterheating options will usually be the big-gest factor in determining optimal proj-ect size. The higher the conventionalwater-heating cost, the larger portion of the load you are likely to be able tomeet effectively with a solar water heat-ing system. Calculating the resultingsavings in conventional water heating(subtracting any operation and mainte-nance cost for the system) and using anappropriate discount rate or interest factor to compare present system cost to future savings determines whetherthe system is a worthwhile investment.The prescribed discount rate for evalu-ating renewable energy projects for federal facilities for 1995 is 3%. A lowdiscount rate such as this favors futuresavings over initial investmentandthus encourages renewable energy proj-ects such as solar water-heating systems.

Although standard life cycle costanalysis does not include a way to takecredit for environmental externalitiessuch as benefits of reducing fossil fuelconsumption, these may be an impor-tant consideration if the economic efficiency calculation is close. TheNational Park Service has developedguidelines for calculating and includ-ing avoided air emissions resulting from reduced electrical power

production in their internal economicevaluation of large energy efficiencyand renewable energy projects (DougDeNio, 303-969-2162). Some agencieshave chosen to relax the economic evaluation criteria somewhat for

showcase buildings in new facilities or demonstration projects at existingfacilities. Projects must be basically costeffective, however, or else they do notmake good demonstrations.

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How Do You Figure?To obtain a preliminary analysis of whether solar water heating would be

cost effective for your situation, use the Federal Renewable Energy ScreeningAssistant (FREScA) software package, available from the Federal RenewablesProgram at the National Renewable Energy Laboratory (NREL): 303-384-7531.Federal Renewables Program staff can also do the analysis for you, if you pro-vide the following data: Hot water use in gallons per day Fuel type and cost Zip code Incoming cold water temperature Outgoing hot water supply temperature Area of southern exposure roof or nearby grounds available for system Tilt and direction of roof area.

To obtain comprehensive solar resource data (the FREScA system doesinclude solar resource data based on your zip code) request the NREL SolarRadiation Data Manual for Flat Plate and Concentrating Collectors (see Appendix B)or the CDRom of the National Solar Radiation Data Base.

To estimate hot water use, check your hot water use records; install a meter and track usage; or project demand based on average use for variousfacilities as found in the American Society of Heating, Refrigeration and Air-Conditioning Engineers Handbook of Applications. Typical usage per dayper occupant in gallons is 20-30 for housing, 30 for hospitals, 5 for diningfacilities, and 3 for other uses.

To determine incoming water temperature (may vary considerably with season), call your water utility, check the supply with a thermometer, or refer to Figure 3 and Appendix A. In some instances, the average annual air temp-erature also serves as a rough indication of water supply temperature.

To calculate system output information more rigorously than the prelimi-nary analysis provided by FREScA, use a computer tool such as F-chart, orconsult with Federal Renewables Program staff or a solar water-heating system supplier.

The optimum size for collector and storage will depend upon fuel cost, yourhot water use pattern, and the cost of the system being considered, but expectstorage to roughly match one days use and collector size to be approximately1.0 square feet per gallon of storage. (The resulting system should meet asmuch as about two-thirds of annual demand.) Precise optimization of systemsize will require both a calculation of output and an evaluation of the econom-ics of contemplated systems.

To evaluate the economics of a contemplated system in detail, use the FEMP Life-Cycle Costing Handbook and associated BLCC software (call the FEMPHelp Line at 800-DOE-EREC), or consult with the FEMP Federal RenewablesProgram or a private engineer.

To evaluate the economic feasibility of covers and solar water-heating systems for your swimming pool, use Energy Smart Pools software, also avail-able from the FEMP Help Line.

Funding SourcesThe first place to look for funding is

regular internal agency funding: localpurchasing authority for very small projects; Congressionally-approved line items for very large projects; andregular agency funding. Special agency-specific funds, such as the DefenseDepartments Energy ConservationInvestment Program, may be availablefor energy efficiency and renewableenergy projects. Although there is notexpected to be any funding available for Fiscal 1996, the Federal Energy Effi-ciency Fund of the U.S. Department ofEnergy and other programs have pro-vided funding assistance for renewableenergy projects at federal facilities in the past. Call the FEMP FederalRenewables Program (303-384-7509)for the current status of any availablefunding.

An important new financing optionavailable to federal facilities is energysavings performance contracting(ESPC). A private energy services con-tractor designs and installs the system,paying the full cost of parts and labor,or the project can be financed by a thirdparty. The federal facility pays nothingup front beyond initial feasibility stud-ies. The contractor is responsible foroperating and maintaining the systemand training facility personnel in its use. The facility then pays the contrac-tor for the energy received as a dis-counted percentage (usually about 15%less) of what it would have cost fromthe utility. The facility pays these util-ity savings bills for a specified con-tract period (up to 25 years) from itsutility or operation and maintenancebudget, after which the facility retainsthe savings and equipment. Thus thecontractor and the facility share the savings in utility costs. (There are nowquite a few companies set up to doenergy service contracts; an associationis listed on page 34.) The facility mustannounce intent to consider ESPC pro-posals in the Commerce Business Daily,but may accept unsolicited proposals.The DOE has a list of pre-qualifiedenergy service companies and modelprocurement documents, as well as amanual on the ESPC program (forcopies, call the FEMP Help Line at800-DOE-EREC).

Through 1995, 17 performance con-tracts at a total cost of approximately$30 million have been awarded underthe ESPC program (mostly energy effi-ciency so far, but solar water heating isclearly eligible). Both the contractorsand FEMP are developing a trackrecord and experience base that willhelp make projects go more smoothly.FEMP is currently working on settingup indefinite quantity contracts to allow qualified contractors to serve anyeligible federal facility project within aregion.

The obvious advantages of perfor-mance contracting are limited initialinvestment, no capital investment, nooperation and maintenance responsibil-ity, and no technical or financial risk forthe success of the project. ESPC con-tracting is especially attractive for verylarge projects that require substantialcapital outlay or extensive operation and maintenance. However, if funds can be obtained to build a project,straight agency funding brings the fullcost savings back to the facility for thelife of the project. Also, even with pre-qualified contractors, the paperworknecessary for performance contractingis significant enough to make it unat-tractive for smaller projects for whichconstruction can be more easily funded.

More than half the states and manylocal governments do provide incen-tives for solar thermal collector or solarcell system purchases. These programsare not generally directly applicable tofederal facilities, but may be helpful incertain situations.

Utility company incentives for de-mand reduction and load managementare currently an important non-federalsource of financial assistance for solarwater heating systems. Demand-sidemanagement activities, such as promot-ing solar water heating systems, cansave a utility from investing in systemexpansions or help them comply withair quality programs. Among the utili-ties that have been actively providingrebates or other financial incentives fornew solar water heating systems are the Sacramento Municipal Utility Dis-trict, Florida Power and Light, and theEugene Water and Electric Board.Wisconsin Public Service and the

Hawaiian Electric Company are devel-oping programs.

Although most programs such asthese were designed for residential cus-tomers, they also genera