1

TechnologyInstallation

Review

WhiteCapTM Roof Spray Cooling SystemCooling Technology for Warm, Dry Climates

IntroductionWhiteCap is an integrated roof surface

and spray cooling system that is suitablefor warm, dry climates. WhiteCapprovides a means to evaporatively andradiatively cool water by spraying it overa building roof at night. The cooledwater is collected from the roof surface,filtered, and stored for use the followingday, where it is used to cool the buildingeither in conventional HVAC systemsor by passive cooling of the roof deckor building floor.

This case study outlines the theorybehind the WhiteCap system and itsdesign, construction, and operation inthree specific design configurations. Itthen describes the construction, design,and performance of a WhiteCap systemrecently installed on a Federal buildingin Nogales, Arizona. Finally, the casestudy outlines specific considerations fordetermining whether WhiteCap systemsare an appropriate energy-saving technol-ogy for existing or proposed buildingapplications at other sites.

Theory andOperation

The WhiteCap system (Marketedby Roof Science Corporation [RSC] ofCalifornia) comes in three basic designconfigurations. In all configurations, a

water spray system consisting of a pipinggrid and conventional water spraynozzles is installed on the roof surfaceand connected to a water pumping andfiltration system. During the coolingseason, water is sprayed over the roofsurface at night. This water is firstcooled by evaporation during the sprayprocess and then further cooled byradiation to the night sky. The watercycles through this spray-coolingprocess until it is cooled to approximately45-50 ºF, and then it is stored for later usein providing cooling to the building.Different methods of cooled waterstorage and ways that water is used tocool the building result in a number ofdifferent configurations of the WhiteCapsystem. The three principal designconfigurations are discussed below.

WhiteCapR SystemIn the original WhiteCapR system

(see Figure 1a), the roof is constructed toallow a 3-inch-thick layer of water to stayon the roof surface at all times. Inter-locking, 4-inch-thick, polystyrene panelsfloat on the surface of the water. Thesepanels are coated with a white, fire-resistant, protective coating and insulatethe water layer and roof from heat gainduring the day. Spray nozzles aremounted flush with the upper surface ofthe panels and the spray-system pipingruns on the roof surface below. Water ispumped to the nozzles from the waterlayer on the roof. The spray-cooled

The U.S. Department of Energyrequests that no alterations bemade without permission in anyreproduction of this document.

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water landing on the top of the insulationpanels leaks down through joints betweenthe insulation panels and returns to theroof surface, where it is stored for useduring the day. The insulating panelsreduce any heat gain to the water due tohigh outside air temperatures or solarradiation during the day. Water cooled atnight during the spray process providespassive building cooling via conductionthrough the roof of the building and toeither building space directly below theroof or to the air in a return-air plenum.

Because the roof is used for waterstorage in the WhiteCapR design,building roofs must be level and strongenough to support the weight of waterand panels (typically 16-18 lb/ft2) on aregular basis. This is not as significantan obstacle as it sounds because theweight of a conventional gravel roofingcovering may be 50% of the weight ofwater in the WhiteCapR system. Inaddition, live roofing loads on thefloating panels (such as snow loads orpersons walking on the roof) become lesssignificant with WhiteCapR because anyweight on the floating panels displaces anequivalent weight of water underneaththe panels. This displaced water loadfirst spreads the weight over the entirewetted roof surface and then eventuallydrains off the roof, removing the live loadcompletely from the roof.

WhiteCapT SystemIn a WhiteCapT system (Figure 1b),

spray-cooled water drains from the roofat night and is stored in a storage tank.When the building requires coolingduring the day, cold water can be pumpedfrom the storage tank to cooling coils ina forced-air cooling system. Advantagesof the WhiteCapT system over theoriginal WhiteCapR system include easeof retrofit to existing buildings and theability to use roofs that are not com-pletely level. Because water is not storedon the roof, building construction loadsare not increased by the WhiteCapTsystem, and the roof can be convention-ally insulated. However, the WhiteCapTdesign may increase building fan powerrequirements because of the additionof new water cooling coils to theHVAC system.

WhiteCapF SystemA third design, called WhiteCapF

(Figure 1c), calls for the spray-cooledwater to be channeled through coilsembedded in the slab floor of thebuilding, and the cooling energy is

Figure 1a. WhiteCapR System

SG97070007.2

Pump Filter

Conditioned Space

Roof Spray

Cool Air

Fan CoilCoolingDelivery

Night Spray

WaterStorage

Tank

SG97070007.3

Radiant Cooling

Roof Spray

Conditioned Space

Pump

FilterCoil inSlab

Roof Spray

Pump Filter

Water Storage

Conditioned Space

Radiant Cooling

SG97070007.1

Floating Panels

Figure 1b. WhiteCapT System

Figure 1c. WhiteCapF System

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stored in the massive building slab,reducing or eliminating the need forwater storage. In this configuration,most of the cooling is achieved throughradiative and convective interchangebetween the building floor and theoccupied space, as shown in Figure 1c.Because water coils are embedded in theslab, builders must plan for WhiteCapFvery early in the building construction.

As of the date of this publication, nopure WhiteCapF designs have beenconstructed, although slab cooling hasbeen implemented in conjunction withfan-coil cooling in a WhiteCapF/T designfor a large California office building.Because of their similarities, theWhiteCapT design can be readilycombined with the WhiteCapF designin new construction to form this type ofhybrid system. In addition, the storagetank in the WhiteCapT system can beconnected to a vapor-compressioncooling system, allowing the WhiteCapsystem to pre-cool water in the storagetank with the vapor compression systemproviding even further cooling of thestored water.

Design DifferencesWhiteCap differs from other roof-

spray systems in that WhiteCap systemsare designed to use night spray systemsto cool water, which is later used fordaytime cooling in the building. Otherroof-spray technologies rely on thedaytime spray to reduce peak rooftemperatures and thus building coolingload. The principal advantage ofWhiteCap over these other systems isthat the WhiteCap spray cycle is doneat night, resulting in the lowest possiblewater temperature for use in buildingcooling. WhiteCap systems use a controlalgorithm that relies on daytime peaktemperature and evening water tempera-ture measurements to estimate thenumber of spray hours needed to achievea target cool water storage temperature.Spray cycle start time is then establishedby counting backwards from a 6 a.m.target completion time. This algorithmensures operation of the spray systemduring the most beneficial cool morninghours. Actual spray operation continuesat night until the target tank temperatureis reached or the building begins regularoperation.

All WhiteCap configurations haveas their primary benefit a reduction inbuilding cooling energy use. Otherpotential WhiteCap benefits can includeextended roof life (with the WhiteCapR

configuration) and enhanced fire protec-tion as well as the reduction in peakbuilding cooling load and possibledownsizing of mechanical coolingequipment. Potential disadvantages ofincorporating WhiteCap systems intoconventional HVAC cooling systemsinclude increased first cost of construc-tion, increased risk of water damage,increased water usage and increasedbuilding maintenance requirements.

Performance IssuesWhiteCap performance is primarily

dependent on outdoor wet-bulb tempera-ture, and secondarily on nighttime skytemperature. Water cooling is accom-plished first during the spray process, asdirect evaporation of some of the waterspray into the air reduces the temperatureof the remaining water spray. In theory,the lowest possible temperature thatcould be reached using spray coolingalone is the outdoor wet-bulb tempera-ture. Practical considerations limit theminimum water temperature that can beobtained using spray cooling alone to afew degrees above the wet-bulb tempera-ture. However, WhiteCap can reducethe temperature of the water to belowthe wet-bulb temperature by radiatingheat to the night sky.

Cooling via night sky radiation relieson the fact that the effective temperatureof the night sky can be significantlycooler than the ambient air temperature.Thus, an object set outside at night willradiate more heat to the night sky than itwill absorb from the night sky and the netloss of heat will cause the object to coolbelow the surrounding air temperature.In the WhiteCap system, most of thecooling via night sky radiation occurs asthe water film left on the roof from thespray process exchanges radiant heatwith the night sky. Although evaporative/radiative cooling ponds have been usedin the past for building cooling, they aretypically much smaller than the buildingsthat they serve. The large roof areaavailable for the WhiteCap system allowsfor substantial radiative cooling atrelatively little additional building cost.

The effective nighttime sky tempera-ture is a function of both the dry-bulb airtemperature and the humidity contentof the air, with higher humidity reducingthe differential between the effective skytemperature and dry-bulb air temperature.Although not an easily measured param-eter, sky temperature data are available

in Typical Meteorological Year (TMY)weather data files or can be estimatedfrom other weather parameters usingestablished algorithms (M. Martin andP. Berdahl 1984).

In previous experimental work withWhiteCapR technology in California(California Energy Commission 1992),the Davis Energy Group (DEG), RSC’sparent company, developed an algorithmfor estimating hourly cooling energysupplied by the WhiteCap system duringoperation. The algorithm describes thecooling energy provided during the sprayprocess as:

Q (Btu/ft2-hr) = 1.16*(Tcsr

-Tdb

) + 1.68*(T

db -T

wb)

+ 0.125*(Ωa-Ω

b) (1)

WhereΩ

a= (0.01*(T

csr+460°F))4

Ωb

= (0.01*(Tsky

+460°F))4

Tcsr

= Temperature of the waterbefore spray (°F)

Tdb

= Dry-bulb ambient airtemperature (°F)

Twb

= Dry-bulb ambient airtemperature (°F)

Tsky

= Sky temperature (°F)

The same studies have shown thatthe minimum water temperature reachedduring operation was typically 5-10°Fless than the minimum dry-bulb airtemperature reached at night. Thealgorithm has been implemented in aresearch version of the MICROPASbuilding energy simulation program usedby the state of California for estimatingbuilding energy use. RSC has used thisprogram to predict the benefits ofWhiteCap at various other building sites.Insofar as cooling the water for storage,all WhiteCap systems appear to havesimilar performance.

Manufacturer’sClaims and PotentialSavings

Data from past installations of theWhiteCapR system have been analyzedby RSC and have shown these systemsto have effective Seasonal EnergyEfficiency Ratios (SEERs) of 50-100[California Energy Commission, 1992].This compares with SEERs of 8-15 withconventional-packaged HVAC equip-ment. In studies of existing WhiteCapinstallations in commercial buildings,

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WhiteCap systems have providedbetween 30% and 60% of the buildings’annual cooling loads. As an addedbenefit, RSC also claims reduction inrequired capacity of conventional coolingsystems. In dry climates, where summerdry-bulb air temperatures fall below65°F at night, typical WhiteCap systemcapacities are 25 ton-hours per 1,000 ft2

of roof spray area. Performance of theWhiteCap technology is strongly depen-dent on climatic conditions, however,and prior to WhiteCap implementation,performance should be estimated usingEquation (1), shown previously.

Aside from energy savings, otherbenefits claimed for WhiteCapR systemsinclude an extended roof life. This issuggested because most of the degrada-tion to a building roof surface occursbecause of weathering caused by largedaily temperature variations and byultraviolet radiation from the sun.However, WhiteCapR roof systemsprotect the roof with a water layer andinsulation panels and little degradation tothe actual roof surface is expected overtime. Long term panel life has not beenestablished; however, no problems withshort panel life have been reported fromany of the previous WhiteCapR installa-tions.

RSC also suggests that the use ofWhiteCap technology can offset the peakload enough in most buildings to allowfor downsizing the mechanical coolingsystem. The ability to do this will bestrongly dependent on weather conditionsduring the peak cooling days. Climatesthat are relatively dry year round mayderive substantial peak cooling reduc-tions year round. Climates that haverelatively humid peak cooling seasonswill see more limited reductions in thepeak mechanical cooling capacityrequired for the building.

Typical installed costs for theWhiteCap system can vary significantlydepending on specific system designs andbuilding requirements. Installed compo-nent costs are estimated by RSC to bebetween $2.50/ft2 and $4.50/ft2 of roofspray cooling for buildings with 20,000ft2 to 50,000 ft2 roof area. In newbuilding construction, however, it is theincremental cost of the WhiteCap roofingsystem and HVAC components over thecost of a more conventional roof con-struction and HVAC system that is theactual cost to the site. This is particularlytrue of the original WhiteCapR technol-ogy, where the roof design will change to

accommodate the technology. In manynew buildings however, the opportunityto downsize the building chiller cangreatly reduce the incremental cost forthe WhiteCap system.

Development andPast Installations

DEG first began work on theWhiteCap technology in 1979. The firstWhiteCapR prototype was installed on aresidence garage in 1982 and monitoredfor 10 years. During the early phases ofthis testing, the group’s experiments withthe technology suggested it was practicaland showed significant energy savingspotential. In 1987, DEG won a grantfrom the California Energy CommissionEnergy Technologies AdvancementProgram (ETAP) to further develop theWhiteCap technology. In 1991, a6,500 ft2 WhiteCapR demonstration wasput together on the California Office ofState Printing facility in Sacramento,California, and evaluated extensively byDEG and the California Energy Commis-sion. Further interest in the technologyspread in the energy community. Afterextensive evaluation, the NationalInstitute of Science and Technology(NIST) recommended WhiteCap forfunding under the DOE Energy-RelatedInventions Program (ERIP). The ERIPgrant, awarded in 1994, supports newdemonstrations of the technology as wellas implementation issues such as buildingcode certifications. NIST researchestimated annual WhiteCap energysavings potential of 585 million kWhin the low rise commercial buildingmarket (assuming 5% penetration overa 10-year period for new construction).

In 1994, RSC emerged as a spin-off from DEG with a goal to marketWhiteCap technology. Although origi-nally spurred on by a strong belief in theenergy and building life benefits of theWhiteCapR design, RSC has found itdifficult to convince building owners touse the water-ballasted roofing systemor to convince code officials about thesafety of these systems. For this reason,since 1995 RSC has focused on promot-ing the WhiteCapT and WhitecapFdesigns. Net energy benefits are similarwith all three configurations, althoughthe designs using cooling coil deliverysystems may have additional fan energypenalties and system design costs.Existing WhiteCap installations areshown in Table 1.

Maintenance/ServiceRequirements

WhiteCap systems are simple indesign and control, and properly designedsystems should have relatively lowmaintenance requirements. However,the limited number of installations haveresulted in each installation being a testcase and learning platform for futureinstallations.

With WhiteCap systems, the mainrequirement is to keep the water spraysystem operating. To keep the systemfree from particulates, WhiteCap filtersall water passing through the system witha large sand filtration system. The filteris set on an automatic backwash cycle sothat it regularly cleans itself. Reportsfrom past installations have not demon-strated a need for water treatment toprevent bacteria or algal growth as wouldbe required by most open cooling towersystems (California Energy Commission,1992). This is most likely because thewater is not exposed to light for pro-longed periods and plant growth cannotbe sustained. Water treatment to preventscale developing inside the spray pipesand coils has also not proved necessaryin any of the existing installations, andordinary tap water is used for makeupto the system. Installations in sites withmore dissolved minerals in the watersupply may want to consider suchtreatment for WhiteCapT andWhiteCapF installations.

Spray tubes are installed on a slightslope with a small hole drilled into thelowest end of the tube to allow all waterto leak out at the end of a spray cycle.This prevents ice developing inside anyof the tubes during cool weather (in dryclimates, night sky radiation can causeice to start forming even on relativelymild nights).

WhiteCap control algorithms areencoded in a microprocessor controllerand can be field adjusted for specificinstallations. The controllers have a10-year battery backup, and controlsettings are unaffected by power loss.

Other ConsiderationsStorage space is a significant consider-

ation for WhiteCapT designs. To storesignificant amounts of thermal cooling

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energy requires large volumes of the coolwater produced through the WhiteCaproof spray system. Providing for thenecessary storage may be a significantexpense for some sites.

Water cost is not expected to be asignificant expense for WhiteCap systemdesigns. Past evaluations have suggestedtypical water usage to be from 0.15 to0.20 gallons/kBtu cooling provided bythe WhiteCap system (R. Bourne andC. Carew 1996). However, it should bepointed out that water costs associatedwith WhiteCap cooling systems arecomparable to water costs associatedwith mechanical cooling systems relyingon evaporative heat rejection systems(such as cooling towers).

Case StudyDescription

This document describes the perfor-mance of a WhiteCap roofing systeminstalled on a Port of Entry stationbuilding in Nogales, Arizona. Nogalesis located in the high desert country ofsouthern Arizona at an altitude ofapproximately 4,000 feet, and wasthought to represent an excellentclimate for a WhiteCap system.

The building used in the WhiteCapdemonstration is single-story, 8,340 ft2 infloor area. The building is owned by theGeneral Services Administration (GSA)and is used to control and processcommercial transport between the UnitedStates and Mexico. It is occupied byapproximately 30 persons during theday from both U.S. Immigration andU.S. Customs’ offices. Built in 1974,it has concrete walls and built-up roofconstruction. Adjacent to the building isan approximately 7,500-ft2-canopy whichcovers a secondary inspection area for

private vehicles. A single 8,500-cfm airhandler serves eight zones in the build-ing. Although originally constructed as adual-deck, air-handling system, the airhandler presently is not used as such.Heating and cooling equipment schedulesdo not allow the building boiler andchilled water system to operate simulta-neously.

The chilled water system uses a two-compressor, air-cooled chiller, with theair passing over evaporative cooling padsbefore passing over the chiller’s con-denser. Both compressors are equallysized at 21.5 tons, yielding a total chillercapacity of 43 tons. The building’soriginal chiller is still used. Accordingto site personnel, only one of the twochiller compressors ever operates at atime, the other compressor being lockedout from use.

Originally, a WhiteCap demonstrationwas to be added to the roof of a newbuilding constructed at the site, butfunding limitations stalled constructionof the new building. Because of GSA’sinterest in the technology, a scaled-down

WhiteCapT system(see Figure 1b) wasinstalled as a retrofitto the parking canopy,with the energybenefit appliedto the adjacent Portof Entry building.An array of 44 sprayheads and pipingwere installed on theparking canopy toprovide 5,600 ft2 ofspray coverage for theWhiteCap system.

Figure 2 shows a photo of the existingsystem with theroof spray in operation. Previouslyexisting roof drainage on the canopywas channeled to 4-inch piping runningdown the two longest sides of the canopy.These roof drainage lines were thenchanneled into a single 5-inch pipe thatempties into the top of a 10,500-gallon,site-constructed, cold-water storage tank.During operation, the entire roof sprin-kler system is fed by a single 3-inchwater line from the base of the coldwater storage tank.

The WhiteCap pump system usesa single 3/4-hp pool filtration pump toprovide the night water spray, thefiltration of tank water, and to sendwater through a 4-row cooling coilinstalled in front of the existingHVAC coils in the system.

The major construction activity forthis project was fabrication of the waterstorage tank. The tank is of wood framewall construction, with 2 x 10 studsspaced 12 inches on center. R-30fiberglass batt insulation was used toinsulate the sides of the tank. The

Figure 2. Nogales WhiteCap System

Table 1. Existing WhiteCap Installations

Installation SizeSite Date (ft2) Configuration

Davis, CA, Garage Prototype 1982 250 WhiteCapRUniversity of Nebraska Energy Research Center 1989 1,000 WhiteCapRETAP demonstration site Sacramento, CA, Office of State Printing 1991 6,500 WhiteCapR/FBourne home, Davis, CA 1993 1,900 WhiteCapR/FAuburn, CA, Office Bldg. Retrofit 1994 7,500 WhiteCapTLos Angeles, CA, Economic Development Dept. Bldg. 1996 27,000 WhiteCapF/TNogales, AZ, Border Station 1996 5,600 WhiteCapTDavis, CA, Solar Cooperative Housing 1996 9,500 WhiteCapR/F

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top and bottom are insulated to R-25 andR10.8, respectively, using rigid foaminsulation. Plywood faces the interiorand exterior faces of each panel. Exteriordimensions of the tank are approximately9 ft high, 27 ft long, by 8 ft wide. Acustom-made liner hangs inside the tankto hold water. Figure 3 shows a pictureof the tank taken prior to the applicationof a stucco finish over the plywoodsurface.

Installation of the WhiteCap systemwas begun on September 1996 andcompleted over a 10-day period. Totalcost for design and construction of thesystem was approximately $28,500.Table 2 shows approximate installedcosts for this type and size of systemwhen retrofit to an existing building.These unit costs for the Nogales installa-tion are based on a 6,000-ft2 spray area, a10,500-gallon storage tank, and approxi-mately 250-foot separation between thecooling coil and the farthest roof drainlocation. The Nogales installation costsare relatively high on a per unit basisbecause of the small size of the project.The largest single component cost, that ofthe storage tank, can be expected to dropto approximately $1.00/gallon or less fortanks 30,000 gallons or larger.

MonitoringA “bare bones” monitoring system

was installed at the WhiteCap demonstra-tion in Nogales. This system monitoredthe following points:

• outdoor air temperature• upper storage tank temperature

• lower storage tank temperature• return air temperature• pump operation (on or off).

Data were originally collected at10-minute intervals using a Datatakerprogrammable data logger and down-loaded to RSC. The data collectionsystem was installed primarily as a tool totroubleshoot and notify RSC of anypotential problems with the WhiteCapoperation. Although not designed as atool for detailed energy savings calcula-tions, the change in average tank tem-perature during the spray cycle canprovide a rough estimate of the thermalcooling energy provided by the WhiteCapinstallation.

Unfortunately, communicationdifficulties were experienced early in1996, and only the data from the firstmonth after installation were collectedin 1996 (September 25, 1996, to Octo-ber 24, 1996). A site visit in January1997 corrected the problem, and datacollection resumed on January 26, 1997.On a later site visit (March 10, 1997), the

data logger was reprogrammed to takedata at 15-minute intervals.

The system was operated until March26, 1997. On that date, a rupture in thetank wall occurred, draining the systemand forestalling any operation until itsrepair on April 12, 1997, by RSC. Sincethat time, the system has been in more orless continuous operation. The systemhas been shut down for brief periods (1 to3 days) either to accommodate repair tounrelated building systems or to preventovercooling of the building.

Measured PerformanceMeasured performance for the

Nogales WhiteCapT system is based onthe daily reduction in tank temperatureachieved during the spray cycle. Twotemperature sensors were originallyinstalled in the tank system; however,only one remained functioning afterrepair of the tank in April, and as such,can only provide a rough indication oftank water temperature. In addition, notall of the cooling going to the tank iseventually used in the building. Sensibleheat gains through the tank walls, as wellas losses in the distribution system, willreduce the actual cooling energy suppliedto the building.

An hourly estimate of tank storagelosses was made by multiplying thecalculated tank wall conductance (UA~ 41 Btu/hr-F) by the temperaturedifference between stored water andambient air. This calculation was doneonly for the hours the building is occu-pied, since nighttime storage losses areincluded in the measurement of nighttimetank temperature change. This analysisshowed that typical tank losses averagedapproximately 2.0% of cooling energyprovided to the tank. The net coolingenergy supplied to the building iscalculated as the cooling energy suppliedto the tank minus the storage losses. Netcooling energy supplied to the building isshown in Figure 4 for the period fromApril 15, 1997, to June 23, 1997. The

Figure 3. Nogales Water Storage Tank (under construction)

Table 2. Installed Costs for WhiteCapT Retrofit(estimated from Nogales demonstration)

Item Estimated Cost

Roof Spray Piping Array $400 per 1000 ft2 of roof coverageAbove-Grade Storage Tank $1500 per 1000 gallonsCooling Coil $700 per 1000 ft2 of roofPump/Filter Hardware & Controls $4500 per installationConnecting Piping $16 per lineal foot drain to coil

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reduction in supplied cooling apparent inthe month of July is believed to be due tothe onset of the relatively humid “mon-soon” season for this part of Arizona.Better performance is anticipated towardthe later months of the cooling season(late August and September).

The system has pumping powerrequirements estimated at 0.47 kW(3/4 hp motor at 2/3 load) when operatingin either the spray or cooling mode. Nopre-retrofit or post-retrofit electricalpower measurements were taken of theair-handling unit fan. Although theinstallation of the WhiteCapT water coilincreased the pressure drop in the fansystem, the total post-retrofit air flow wasnot reset to the pre-retrofit air flow, andthe actual fan power requirements mayhave been reduced by the new coilinstallation. No such energy credit isgiven to WhiteCap here because this ismore an artifact of the installation than aWhiteCap benefit.

Had the fan airflow been reset to theoriginal fan volume, the additional fanpower requirements would have beenapproximately 0.41 kW for additional fanpower (based on a design pressure dropof 0.15 in. w.g. for the WhiteCap coolingcoil, an airflow rate of 9,400 cfm, and acombined fan/motor efficiency estimatedat 54%).

The pump electrical load is multipliedby the pump operating time to provideestimated electrical energy requirementsfor the WhiteCap system, also shown inFigure 4.

Over the period from April 15, 1997,to August 3, 1997, the WhiteCap systemprovided an average daily cooling energyof 1.39 million Btu/day while using anestimated 9.37 kWh/day of pumpingenergy. To provide a comparison withother cooling systems, the WhiteCapoperated with an average Energy Effi-ciency Ratio (EER) of 149 over this timeperiod. Assuming a COP of 2.8 for theexisting chiller system and using anavoided electrical energy cost of $0.055/kWh (Citizens Utility LGSRate Schedule 1997), theWhiteCap installation hassaved an average of 1,020kWh/week in coolingenergy—worth $56/week incooling energy savings.

The existing installationis not controlled fordemand reduction, insteadallowing for coolingoperation during alloccupied hours. Analysisof the measured data

indicates that WhiteCap cooling rates ofapproximately 93,000, 103,000, 85,000,and 78,000 Btu/hr were experiencedduring peak temperature periods in April,May, June and July, respectively. Oncepumping energy is subtracted, thesesuggest that a typical demand reductionof approximately 9 kW/month will berealized even with the present controlstrategy—worth an additional $85/monthto the site for each month of operation.

Site personnel report that the existingchiller is enabled for seven months of theyear, allowing for an estimated $2,290/yrelectrical cost savings.

Because the existing WhiteCapsystem provides some level of coolingduring all operating hours, modificationof the control strategy to further reducedemand will reduce energy savings. RSCis working on control algorithms that willoptimize demand reduction and energysavings to reduce total electrical cost inother installations. In the long run,however, greater energy cost savingsmay be realized by plumbing the main

Figure 4. WhiteCap Energy Savings for Nogales Installation

chilled water loop directly to theWhiteCap storage tank. WhiteCap spraycooling would be used to cool the storagetank at night, and the chiller used tomaintain the tank water temperature lowthroughout the morning. Both coolingcoils would use cold water from thestorage tank during the cooling process,and the chiller could be shut down duringpeak demand periods.

Maintenance issuesPeriodic maintenance requirements

for the WhiteCap system are shown inTable 3 and are relatively minor. Mostof the maintenance was already beingdone through the site’s scheduledpreventative-maintenance routine. Theonly new maintenance requirement is forcleaning of the WhiteCap HVAC coil.Costs for the five-year cleaning areestimated at $230 for labor andchemicals.

In addition, site personnel report thatthey regularly check the controller aswell as examine the tank water level.

Table 3. Nogales WhiteCapT Maintenance Requirements

Maintenance Frequency

Monthly Annually Every 5 years

Chilled Water Coil Inspect filter and roll to Clean WhiteCapnew section and replace if HVAC coil, removingnecessary accumulated dust and

debris

Roof Drains Check/clean roof drains Clear debris from rooftopand inspect roof surface

0

500

1,000

1,500

2,000

2,500

4/20

/97

4/27

/97

5/4/

97

5/11

/97

5/18

/97

5/25

/97

6/1/

97

6/8/

97

6/15

/97

6/22

/97

6/29

/97

7/6/

97

7/13

/97

7/20

/97

7/27

/97

8/3/

97

Week (ending date)

Coo

ling

Pro

vide

d by

Whi

teC

ap (

kBtu

/day

)0

2

4

6

8

10

12

Ele

ctric

al E

nerg

y U

se (

kWh/

day)

Cooling Provided (kBtu/day Total Electrical Usage (kWh/day)

8

Normally, however, the system isrelatively maintenance free. Automaticcontrols backflush the filter on a regularbasis and automatic fill valves replacetank water loss.

Although scheduled maintenance isminimal, the Nogales WhiteCap demon-stration has experienced some difficultiesin operation, most of which can be tracedto installation and control issues. Theseinclude minor freeze damage and generaldegradation to the original PVC roofspray piping, leaking filters, and theruptured storage tank discussed previ-ously. The original PVC piping for thespray grid was completely replaced withcopper by RSC, and small holes weredrilled in the end of each new spraysection to completely drain water andprevent any further freeze damage to thespray grid. Local contractors for RSCrepaired the tank and leaking filters. TheGSA contract with RSC provides for a10-year service plan during which RSC isresponsible for any repairs to the newlyinstalled system components. RegionalGSA administration reports that RSC hasbeen extremely responsive to the prob-lems encountered.

It should be noted that WhiteCap isnot tied to any one storage tank design. Anumber of conventional, insulated storagetank options are available for WhiteCapFsystems; however, given site constraints,the rectangular wood-frame storage tankwas chosen as the most suitable optionfor the Nogales installation.

In addition to the above, the WhiteCapcooling mode has been manually shut offin several instances where the WhiteCapsystem was overcooling the space. Aswith any new system, more familiaritywith the controls and capability of thesystem should, over time, result in lessneed for manual intervention.

EconomicsA comparison of the life-cycle cost for

the WhiteCap system as opposed to theexisting chiller system was made usingFEMP’s Building Life Cycle Cost(BLCC) computer program and the firstcosts, energy and demand cost savings,and additional maintenance costs outlinedpreviously. System life was estimated at25 years with the existing chiller used asthe basis for comparison. Economiccriteria were the default criteria assumedfor Federal energy conservation projects.The resulting BLCC output showed theNogales WhiteCap installation has adiscounted payback of 16 years (simple

payback in 12 years), with a savings-to-investment ratio of 1.43 and an adjustedinternal rate of return of 5.59%.

Applicability of theTechnology

To decide if WhiteCap systemsrepresent an appropriate cooling systemalternative for a particular site, considerthe following:

• Climate: Although WhiteCapsystems will provide cooling inmost U.S. climates, performanceis enhanced in climates with a drycooling season. As mentionedpreviously, dry climates offer moreopportunity for capacity reductionof the mechanical cooling system.Typical WhiteCap capacity in dryclimates is approximately 25 ton-hours/day per 1,000 ft2 of roofspray area.

• Roof construction: For WhiteCapRdesigns, roofs must be dead leveland capable of supporting the waterand component weight of theWhiteCapR system. In addition,the existence of roof insulation orroof plenum will impact the energysavings with WhiteCapR technol-ogy. Also, determine whetherbuilding codes require that roofsmeet Class A Underwriter’sLaboratories (U.L.) fire protectioncriteria. Roof construction withWhiteCapR systems has not beencertified as meeting these require-ments because the U.L. testrequires that the roof be dry duringthe test (such as might be the caseduring winter weather). Any fire-protection advantages gained byhaving the water pond and roofspray system operating during thecooling season has not beenrecognized by U.L. tests.

• New or retrofit construction:WhiteCapR and WhiteCapFdesigns are much better suited tonew construction than to retrofitapplication. WhiteCapT is wellsuited to building retrofits.

• Water Storage: If WhiteCapTtechnology is being considered, thestorage tank placement must be

examined up-front. As a rule ofthumb, allow about 2 gallons ofstorage for each square foot ofspray area with this system. For an8-foot water height, this results in33.4 ft2 of tank area for each1,000 ft2 of roof spray area.

• Demand Control: Considerwhether demand reduction is adesired feature for the site.WhiteCapR and WhiteCapFsystems have limited control of thetiming of the WhiteCap coolingbenefits. WhiteCapT systems canbe designed to provide precisetiming of the cooling benefits;however, this may be at theexpense of the energy savings.Sites interested in demand controlare encouraged to consider plumb-ing mechanically chilled waterto the WhiteCap storage tank,allowing for maximum energy anddemand savings. This design willalso allow for greater reduction inchiller capacity, with significantfirst cost savings for new buildings.

• Bear in mind that although thetechnology is simple and energysavings are proven, experience withreal installations is still limited.Consider this carefully whendeciding whether to reducemechanical cooling capacity inexchange for WhiteCap cooling.Also consider that althoughWhiteCap maintenance is relativelyminor, any new technology requiresthat the operators understand howit works, what maintenance isrequired, and what to do in caseof failure.

• Cost-effectiveness will be basedon electrical cost and rate structure,mechanical cooling efficiency,WhiteCap installation costs, andbuilding cooling loads. Mechanicalcooling efficiency estimates can bemade from equipment performancetables or through consultationwith the equipment manufacturer.Installation costs will vary consid-erably in the case of buildingretrofits; however, for a first cut,the installation costs for smallsystems can be estimated fromthe data provided for the Nogalesdemonstration. Total costs/ft2 ofspray area can be expected to drop

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for larger installations. A reason-able first estimate for WhiteCapinstallation costs for new buildings30,000 ft2 or larger would be $2.50/ft2. In certain instances, WhiteCapcosts may be offset considerably bydownsizing conventional HVACcomponents. Daily buildingcooling loads can be estimatedusing building simulation models,or, in the case of retrofit construc-tion, analysis of previous buildingenergy use.

• Consider any utility incentivesfor reduction of building electricalenergy use or demand. Because ofthe uniqueness of the technologyand the potential for large savings,utilities may be interested inproviding special incentives. Aswith any cool-storage technology,consider the use of time-of-useelectrical rate schedules.

Because of the various designconfigurations and climatic performancevariation, WhiteCap energy savings arebest estimated using a building energysimulation. Interested parties shouldconsult RSC for such an analysis.

Technology inPerspective

WhiteCap systems represent aneffective energy-saving technology that issuitable for application in many Federalbuildings. In particular, the WhiteCapTsystem design, as used in the Nogalesdemonstration, represents a simple andpractical energy technology to be used inconjunction with conventional mechani-cal cooling systems. Installed correctly,the system should have little in the wayof additional maintenance or operationalrequirements. It is recommended thatenergy managers in warm and relativelydry climates consider WhiteCap as areadily available technology that deliverson its performance claims. Cost-effectiveness of the technology, however,will be strongly dependent on climate,

WhiteCap integration with mechanicalcooling systems, and utility rate structure.Potential users of the technology shouldconsider several WhiteCap systemdesigns to determine the most cost-effective design for their site.

For FurtherInformation

Contacts

ManufacturerRoof Science CorporationJerry Best, President123 C StreetDavis, CA 95616Tel: (916) 757-4844Fax: (916) 753-4125

Nogales DemonstrationCecilia Serrano/Ron SandlinUSGSA Tucson Field Office/ 9PMM-27300 West Congress StreetTucson, AZ 85701Tel: (520) 670-4738

Dennis DeConcini Poe9N Grand AvenueNogales, AZ 85621Tel: (520) 287-4275

Other WhiteCap InstallationsJay HearnlyCalifornia Office of State PrintingTel: (916) 323-0311

Lance ElberlingPacific Gas and Electric(Verifone Installation)Tel: (510) 866-5519

ReferencesBourne, R., C. Carew. 1996. Design andImplementation of a Night Roof-SprayStorage Cooling System. Proceedingsof the 1996 ACEEE Summer Study onEnergy Efficiency in Buildings,Washington, D.C.

California Energy Commission, 1992.Cool Storage Roofs ETAP Project,Consultant Report. Prepared by the DavisEnergy Group. CEC P500-92-014, Davis,California.

Citizens Utility Company Large GeneralService Rate Schedule (effective 1/1/97).

Martin, M., P. Berdahl. 1984. Character-istics of Infrared Sky Radiation in theUnited States, Solar Energy, Vol. 33 pp.321-326.

FEMP Help Desk(800) 363-3732International callers please use (703) 287-8391Web site: http://www.eren.doe.gov/femp/

General ContactBob McLaren, NTDP Program ManagerFederal Energy Management ProgramU.S. Department of Energy1000 Independence Avenue SW, EE-92Washington, DC 20585(202) 586-0572

Technical ContactDavid WiniarskiPacific Northwest National Laboratory2400 Stevens, K5-08Richland, Washington 99352(509) 372-4461e-mail: david.winiarski@pnl.gov

Produced for the U.S. Departmentof Energy by the Pacific NorthwestNational Laboratory

December 1997