D e c e m b e r 1 9 9 9 A S H R A E J o u r n a l 3 1

A SHRAE JOURNAL

Evaporative Cooling and TESAfter Deregulation

By Mike Scofield, P.E.Member ASHRAEandKeith Dunnavant, P.E.Member ASHRAE

Because 29% to 43% of commercialbuildings peak electrical demand can beattributed to air-conditioning refrigera-tion systems,2 owner interest in thermalenergy storage (TES) designs will in-crease as an alternative to more conven-tional central cooling plant designs. Theability of TES systems to mitigate peakrefrigeration energy consumption duringhot weather will provide building own-ers with a trump card in their energy ratenegotiations with the utility of theirchoice.

This article explores the energy sav-ings and first cost advantages of combin-ing an indirect/direct evaporative cool-ing air-handling unit design with that of aTES central plant in the arid western cli-mate. The physical size, and conse-quently the installed cost, of TES systemsmay be substantially reduced whenevaporative cooling augments mechani-cal cooling. By collecting and routing thebuilding return air through the exhaustside of an air-to-air heat exchanger, muchof the energy contained in this exhaustairstream can be recycled—serving tocool (or heat) the entering outdoor air to

he first summer in California after deregulation illustrateshow expensive electrical energy will become in the afternoonhours between 12:00 p.m. and 6:00 p.m. On July 29, because

of the national heat wave, wholesale electrical energy costs rose froma pre-dawn off-peak rate of $24 per mega-watt up to $151 per mega-watt consumed between 4:00 p.m. and 5:00 p.m.—an increase of 629%.1

be delivered to the space. In the coolingcycle, the exhaust side of the air-to-airheat exchanger may be wetted, allowingthe dry building exhaust air to evaporatesome of this water and thereby act to en-hance the heat exchanger’s ability to ex-tract heat from the hot entering outdoorairstream.

This is the essence of indirect evapo-rative cooling (IEC), and because thebuilding (return) air is controlled, a con-sistent, and significant, cooling effect isachieved, which is shown in Figure 1.Note that the outdoor airstream after IECvaries by less than 8°F (4.4°C) from theextreme dry bulb (DB) to the 0.4% dewpoint (DP) design condition. Building in-door air quality (IAQ) is improved, sinceevaporative cooling designs introduce100% outdoor air (O/A) to the building.When rigid, corrugated media type directevaporative cooling (DEC) is used as asecond cooling stage, the cooling mediawashes out much of the microscopic pol-len and dust that would otherwise be cir-culated with the supply air. Measurementsof indoor air contaminant levels, takenduring the summer months in a large of-

fice building where such DEC was used,were found to be significantly lower thanthe norm for commercial office buildings. 3

In designing a TES system, the con-sulting engineer should always try tominimize the building air-conditioningcooling load to reduce the size and firstcost of both refrigeration plant and TESsystem. The evaporative cooling conceptdescribed in this article is one of severalload reduction techniques that will makea TES installation a better investment forthe owner.

Evaporative Cooling SystemsFigure 1 shows that, even during in-

frequent high ambient humidity excur-sions, in Sacramento, Calif. an IEC sys-tem with a 70% wet bulb depression ef-fectiveness (WBDE) would reduce sys-tem cooling loads when compared to aconventional system with 25% minimumO/A and full economizer capability. The1997 ASHRAE Handbook—Fundamen-tals now includes design values for WBand DP design conditions in Chapter 26,“Climate Design Information.” 4

In the dry western U.S., there are usu-ally many hours where the ambient DBtemperature is greater than 55°F (13°C),but the coincidental WB temperature issufficiently low such that 55°F (13°C) DBsupply air may be achieved with DEConly. In Sacramento, using a DEC ele-ment with a WBDE equal to 90%, thereare 1,344 such hours (neglecting supply

About the Authors

Mike Scofield, P.E., is president of Conservation Mechani-cal Systems in Sebastopol, Calif. He received the 1987ASHRAE Journal best paper award for “Building Venti-lation: A Heat Pipe Economy Cycle.”

Keith Dunnavant, P.E., is the national sales manager ofDes Champs Laboratories Commercial Division.

The following article was published in ASHRAE Journal, December 1999. © Copyright 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paperform without permission of ASHRAE.

TTTTT

3 2 A S H R A E J o u r n a l D e c e m b e r 1 9 9 9

fan heat) during a typical year as definedby typical meteorological year (TMY2)weather data. This represents 27.5% ofthe annual hours where the ambient DBis above 55°F (13°C)—hours where con-ventional systems, without evaporativecooling, would be required to use refrig-eration to provide 55°F (13°C) DB sup-ply air to the building.

When a 70% WBDE IEC element isadded in combination to the 90% WBDEDEC element, there are 1,448 annual hours(neglecting supply fan heat) where 55°F(13°C) supply air can be achieved with-out refrigeration, but where conventionalsystems would require refrigeration to pro-duce that same 55°F (13°C) supply air.Although the cooling tonnage is usuallylow during these hours compared to de-sign day requirements, because there aresuch a significant number of central Cali-fornia hours typically residing in this en-velope, the annual ton-hours avoided byevaporative cooling here is substantial.

Indirect and Direct EvaporativeCooling

IEC WBDE quantifies the ability of anair-to-air heat exchanger to cool the O/ADB temperature to approach the build-ing exhaust air WB temperature (refer toChapter 19 of the 1996 ASHRAE Hand-book—HVAC Systems and Equipment fordefinitions of both IEC and DECWBDE). With IEC, no moisture is addedto the O/A. During the normal IEC pro-cess, the O/A is sensibly cooled, result-ing in a reduction not only in the DB tem-perature, but also in the WB temperature.Consequently, because of the reductionin WB temperature during IEC, whenDEC is added downstream of IEC, thecooling effect is greatly enhanced result-ing in a significant reduction in annualton-hours compared with IEC savingsonly. In Sacramento, the annual coolington-hours using two-stage evaporativecooling (70% WBDE IEC) combinedwith 90% WBDE DEC) are reduced by40.9% compared to that of IEC (70%WBDE) alone.

The DEC component provides anadded cooling benefit only when the am-bient DP temperature is less than the de-sired supply air temperature. This is be-cause DEC, a constant WB process, addsmoisture to the supply air as it cools. Simi-

Figure 1B: 0.4% WB design condition.

Figure 1: This figure shows the flow schematics for a conventional (25% mini-mum outside air) system with full economizer capability and a 100% outdoor airsystem with 70% WBDE indirect and 90% WBDE direct evaporative cooling com-ponents. For both systems, the return air scfm is assumed to be 90% of thesupply air scfm. The numbered points on these schematics correspond to theprocess points shown on the three psychrometric charts. Cooling load reduc-tions of 36.2% for the extreme dry-bulb design condition (Figure 1A); 9.5% forthe 0.4% wet-bulb design condition (Figure 1B); and 6.1% reduction for the

Figure 1A: Extreme DB design condition.

D e c e m b e r 1 9 9 9 A S H R A E J o u r n a l 3 3

E V A P O R A T I V E C O O L I N G

larly, IEC provides a cooling benefit onlywhen the return air WB temperature isless than the DB temperature of the O/Aentering the indirect evaporative cooler.Thus, the system controls typically turnoff DEC when the ambient DP is greaterthan 55°F (13°C), and IEC is normallydisabled when the ambient DB is less than62°F to 64°F (17°C to 18°C) (typical re-turn air WB temperatures).

Face and bypass dampers on the air-to-air heat exchanger are essential forbest control of any two-stage evapora-tive cooling system, as they allow foreconomizer cooling and full modulationof IEC, while similarly allowing for op-timal DEC control by regulating theamount of heat recovery as needed toachieve tighter supply air temperaturecontrol. The DEC system may be fur-nished in two sections (4 in. (102 mm)media located downstream of 8 in. [203mm] media) to allow for two stages ofDEC control, further enhancing overallunit performance.

Free HumidificationThe DEC component may be used dur-

ing the winter months to add moisture tothe dry supply air and prevent space RHfrom falling below 30%, as recommendedby ASHRAE Standard 62-1999, Ventila-tion for Acceptable Indoor Air Quality.The cold, dry outdoor air is preheated asit flows through the air-to-air heat ex-changer, thereby allowing the DEC ele-ment to humidify this air without expend-ing energy for preheating that normallywould be required.

Because central California has a mildwinter climate, the heat exchanger oftenis able to heat the O/A well above 55°F(13°C), allowing the air to be humidifiedwithout cooling the air below the normaldesired supply air temperature of 55°F(13°C), thereby eliminating the need forreheat. 5

Consequently, during all except thecoldest periods, the supply air may behumidified for “free.” The only addedcosts are the water, the electricity to runa fractional horsepower DEC pump, andthe minimal added fan energy to movethe S/A through the DEC media. To pre-vent excess humidification, it is suggestedthat a 2 in. (51 mm) rigid, corrugated me-dia type DEC element be added for winter

Figure 1C: 0.4% DP design condition.

0.4% dew-point design condition (Figure 1C) are indicated for the 100% O/Atwo-stage evaporative cooling design relative to the conventional system with25% minimum outside air operating in Sacramento, Calif. Shaded areas on thepsychrometric charts indicate ambient conditions where refrigeration may beeliminated with the evaporative cooling design but would be required to achieve55°F (13°C) supply air with the conventional system.

Two-StageEvaporative Cooling System

Conventional Cooling System with Full Economizer Capability

3 4 A S H R A E J o u r n a l D e c e m b e r 1 9 9 9

humidification, located downstream of the 12 in. (305 mm) me-dia used for summer cooling.

System ComparisonFigure 2 compares the Sacramento month-by-month refrig-

eration ton-hour per 1,000 scfm (472 L/s) requirements of threesystems: 70% WBDE IEC; 70% WBDE IEC combined with90% WBDE DEC; and conventional system with full econo-mizer capability, introducing a minimum of 25% O/A. All sys-tems assume building return air at 75°F (24°C) DB and 64°F(18°C) WB, which is at 55% RH. The TMY2 weather data forSacramento was used in this analysis that shows the potentialfor annual cooling energy savings using evaporative cooling.

Based on a 24-hour per day, seven day per week duty cycle,the ton-hours per 1,000 scfm (472 L/s) required to develop a55°F (13°C) supply air condition would be reduced in each ofthe 12 months of the year by using evaporative cooling Sys-tems 1 or 2, compared to the conventional System 3.

While the results are dramatic in the month of April, withthe two-stage evaporative (IEC/DEC) cooling system chalkingup an 87.7% reduction in ton-hours compared with the conven-tional system, the results are nonetheless impressive during themonth of July, where the same ton-hour comparison results in astill mighty 38.6% reduction—and this is the least percentagereduction for all 12 months.

On an annual basis, using the same 24-hour per day, seven-day per week duty cycle, the ton-hours per 1,000 scfm (472L/s) would be reduced from 5,876 for System 3 to 2,588 forSystem 2, a 56% reduction (Figure 5, Sacramento). A 70%WBDE IEC component without DEC (System 1) would re-duce annual ton-hours per 1,000 scfm (472 L/s) by 25.5%, from

5,876 down to 4,377, when compared to the conventional Sys-tem 3 (Figure 5, Sacramento). The biggest impact that two-stage evaporative cooling systems have on building refrigera-tion requirements occurs during the spring and fall months insemi-arid climates such as California’s (Figure 2). During win-ter months, evaporative cooling can replace refrigeration dur-ing all except the most extreme weather conditions.

Again, using Sacramento TMY2 weather data, Figures 3 and4 compare the tons per 1,000 scfm (472 L/s) required by thethree supply air designs to develop 55°F (13°C) supply air onan hour-by-hour basis—for both a typical high DB day (Figure3) and a typical high DP day (Figure 4). During the more nor-mal arid conditions illustrated by the August 20 TMY2 data(Figure 3), it is evident that evaporative cooling componentsproduce their biggest impact on refrigeration cooling require-ments in the afternoon hours between 12:00 p.m. and 6:00 p.m.,the same time that electric rates are highest.6

The two-stage evaporative cooling design reduces the after-noon (12:00 p.m. to 6:00 p.m.) ton-hours on this typical highdry bulb day by 52%, while indirect evaporative cooling only(no direct evaporative cooling) reduces ton-hours by 30%.

High humidity conditions in Sacramento are illustrated bythe TMY2 sample high dew point day Figure 4. The peak dewpoint on this day (August 22) of 62.1°F (17°C) occurs over atwo-hour period between 1:00 p.m. and 3:00 p.m. Peak tonsper 1,000 scfm (472 L/s) of supply air are typically higher onsuch high humidity days than they are on high dry bulb days forall three systems (Figure 1).

During high humidity, the direct evaporative cooling com-ponent does not contribute to system tonnage reduction as itdoes during dry ambient conditions. Still, Figure 4 reveals a13.4% reduction in the afternoon (12:00 p.m. to 6:00 p.m.) ton-hours for the 100% outdoor air indirect evaporative coolingsystem compared to the 25% outdoor air economizer cycle.

Other California Cities

Figure 2: This figure shows the month-by-month refrigera-tion cooling load requirements per 1,000 scfm (472 L/s) ofsupply air for an IEC, IEC plus DEC, and a 25% minimumoutdoor air conventional system with full air side econo-mizer. The results were derived for a 24-hour per day, 365days per year duty cycle using TMY2 weather data for Sac-ramento, Calif. All systems supply air at 55°F DB (13°C) withbuilding return air at 75°F DB (24°C) and 64°F WB (18°C)(55% RH). The IEC wet-bulb depression effectiveness wasselected at 70% and the DEC saturation effectiveness at90%. Fan heat was not included.

Figure 3: This figure compares the daytime hour-by-hour ton-nage per 1,000 scfm (472 L/s) of supply air required for eachof the three air system designs on a typical hot, dry summerday in Sacramento, Calif. The ambient conditions for thisday were taken from the TMY2 data set for Sacramento andthe same system parameters apply as for Figure 2.

D e c e m b e r 1 9 9 9 A S H R A E J o u r n a l 3 5

E V A P O R A T I V E C O O L I N G

Figure 5 shows that the Central Valley of California (Bakers-field and Sacramento), and surprisingly San Francisco, offerthe greatest potential for cooling ton-hour reduction. Southerncoastal cities like Los Angeles and San Diego have higher cool-ing requirements but seem to be less influenced by the dry east-erly (Santa Ana) winds in a typical year than is San Francisco.

Building Load Profile for Extreme Summer DesignFor thermal storage design, an accurate building load pro-

file calculation is essential. Whether the refrigeration systemis sized for full storage, partial storage for load leveling, orpartial storage for demand limiting, the most extreme summerambient design condition must be analyzed. The TMY2 dataused in Figures 2 through 5 are acceptable in analyzing cool-ing requirements during typical climate conditions. The de-signer sizing a TES central plant, where day-to-day coolingstrategies are used, must size his refrigeration machine and stor-age capacity to meet load requirements of the most extremeday of the year for the maximum benefit to be realized.

The highest DP temperatures typically do not occur coinci-dentally with the highest DB values. In most dry climates likeSacramento, the extreme summer DB temperatures occur whenair moves into that area from a hot dry location.

For Sacramento and all of the West Coast, these extremeconditions occur when a easterly wind brings hot dry air fromthe inland mountains towards the coast. Figure 20 (Page 11-3)of the Ecodyne Weather Data Handbook 7 shows that a Sacra-mento extreme high DB temperature of 108°F (42°F) is coin-cident with a mean WB reading of 72°F (22°C). This repre-sents a wet bulb depression of 36°F (2.2°C) that would makean evaporative cooling design highly effective in reducing ton-nage requirements for a TES system (Figure 1).

During periods of high ambient DP, because the coinciden-tal DB temperature and solar gain are typically off peak, thetotal building cooling requirements are reduced relative to the

hot arid periods. Consequently, on VAV systems, the airflowdelivered to the space is reduced during this time—indicatingthat even though the required refrigeration in tons per 1,000scfm is highest at this condition, the peak building refrigera-tion requirements may not occur during such high ambient hu-midity conditions. A detailed hour-by-hour building load analy-sis must be conducted to establish actual peak cooling require-ments.

Installed Cost ReductionTES designs have been used effectively to shave peak after-

noon electrical demand costs throughout the U.S. In most ofthe West Coast, as well as in other dry climates, evaporativecooling systems effectively eliminate the need for refrigerationduring many, if not most hours, and reduce the required refrig-eration capacity at the design condition. Consequently, TESdesigns are complemented by the inclusion of IEC and DEC inthe air-handling unit designs, as they provide a significant re-duction in chiller plant and thermal storage capacity, and asso-ciated first costs, while greatly reducing day-to-day off-peakhours of operation of the refrigeration system.

We can estimate the chiller plant and thermal storage systemfirst cost reductions that would be achieved by using the two-stage evaporative cooling system relative to the conventionalcooling system with full economizer capability as shown inFigure 1. For a 1,000 ton (3517 kW) variable air volume (VAV)application, at 400 scfm (189 L/s) per ton, the total supplyairflow would be 400,000 scfm (189 000 L/s) at the extremeDB condition of 108°F (42°C). Using a building load profiletypical for Sacramento on a design DB day, we estimate thevolume of 55°F (13°C) S/A to be delivered at each of the hoursbetween 8:00 a.m. and 6:00 p.m., which is a 10-hour period.

Although Figure 3 does not depict the extreme DB day inSacramento, it does illustrate the relative performance of a two-stage evaporative cooling system compared with a conventionalsystem on a very hot, dry day—one that is typical of the ex-treme DB day. From Figure 3, and the volume of 55°F (13°C)S/A delivered at each hour, we calculate that the two-stage

Figure 5: This figure uses TMY2 weather data to comparethe annual ton-hours per 1,000 scfm (472 L/s) of supply airfor the three systems operating in five California cities. Thesame system parameters apply as for Figure 2.

Figure 4: This figure compares the daytime hour-by-hourtonnage per 1,000 scfm (472 L/s) of supply air required foreach of the three air system designs on a less typical highdew point day in Sacramento, Calif. The ambient condi-tions for this day were taken from the TMY2 data set (high-est dew point day) for Sacramento and the same systemparameters apply as for Figure 2.

3 6 A S H R A E J o u r n a l D e c e m b e r 1 9 9 9

evaporative cooling system uses 4,409 ton-hours less than theconventional system over the ten-hour period between 8:00 a.m.and 6:00 p.m.

For the purpose of our analysis, we assume that this same4,409 ton-hour reduction would be achieved on the extremeDB day. Using a cool storage installed cost of $100 per ton-hour, typical for TES systems, the savings would amount to$440,900, not including any allowance for structural or rent-able floor space savings. The chiller, operating over the 14 off-peak hours between 6:00 p.m. and 8:00 a.m., would be reducedin capacity by 315 nominal tons (1108 kW), or 485 actual tons(1706 kW) when producing the 22°F (–5.6°C) chilled water/glycol required to make ice.

With installed chiller costs estimated at $1,200 per ton, thesavings here would be $582,000. Thus, the total installed costsavings to the TES system resulting from using two-stage evapo-rative cooling would be $1,022,900 for the 1,000 ton (3517kW) Sacramento example

The two-stage evaporative cooling components would add$1.75 to $2.00 per cfm of supply air to the installed cost of atypical large custom air-handling unit, or $800,000 using thehigher value. This cost estimate is based on a 70% WBDE di-rect spray heat pipe air-to-air heat exchanger IEC componentwith modulating face and bypass dampers coupled with a 12in. (305 mm), 90% WBDE rigid corrugated media DEC com-ponent, divided into 4 in. (102 mm) and 8 in. (203 mm) sec-tions for two stages of DEC control.

Subtracting the installed cost addition of $800,000 from theinstalled cost reduction of $1,022,900, we net a $222,900 sav-ings in the overall installation. This cost savings assumes thatthe peak TES system cooling day for the 100% O/A two-stageevaporative cooling system is the same peak cooling day thatthe conventional 25% minimum O/A system is most likely tooccur on—the extreme DB day.

The proof of this assumption requires a detailed hour by hourbuilding load analysis using a program such as DOE2. However, itis certain that a two-stage evaporative cooling system, supplying100% O/A, would always require less TES and chiller capacitythan a conventional system providing a minimum of 25% O/A.

While the potential installation cost savings are impressive,two-stage evaporative cooling systems provide other benefitsequal in magnitude. Figure 5 shows an annual reduction from5,876 ton-hours per 1,000 scfm (472 L/s) for the conventionalsystem, compared with 2,588 ton-hours per 1,000 scfm (472 L/s) for the two-stage evaporative cooling design introducing100% outdoor air in Sacramento. For the 1,000 ton (3517 kW)VAV example, assuming an average cooling mode S/A volumeequal to 75% of the maximum, or 300,000 scfm (141 600 L/s),this calculates out to 986,400 ton-hours per year of additionalchiller down time.

Even after deducting for the added costs connected with IECand DEC, such as the added fan energy to move the supply andexhaust airstreams through the evaporative cooling compo-

nents, IEC and DEC pump energy, and the water costs, annualoperating cost savings approaching $100,000 would be ex-pected for this 1,000 ton (3517 kW) VAV example in the centralvalley of California.

Parasitic losses for the evaporative cooling components onboth supply and exhaust sides should be selected at less than1 in. w.g. (249 Pa) to minimize fan energy costs. For VAV appli-cations, secondary oversized bypass dampers may be used toshunt the static penalty of the evaporative cooling componentsduring mild economizer cooling hours. These dampers may beincluded by the air-handling unit manufacturer at a minimaladditional cost and are easily implemented into the DDC con-trol algorithm.

ConclusionThe marriage of evaporative cooling and TES systems in

dry western climates has many benefits beyond installation andoperating cost savings. High occupancy applications such asschools, prisons, nursing homes, and hospitals benefit from theintroduction of all outdoor air. Improved air quality with win-ter humidification and reduced risks of airborne infection prom-ise to increase productivity and reduce absenteeism for crowdedindoor environments.

AcknowledgmentsThe authors wish to thank Dave Vasey of FAFCO for pro-

viding valuable TES system operating and installed cost data,and Ned Brush and Doug Des Champs for their assistance increating the figures used in this article.

References1. White, M. 1998. Associated Press, Los Angeles, Calif., July 29.

2. Silvetti, B. and M. MacCracken. 1998. “Thermal storage and de-regulation.” ASHRAE Journal, 40(4):55–59.

3. Colvin, T.D. 1995. “Office tower reduces operating costs with two-stage evaporative cooling system.” ASHRAE Journal 37(2):23.

4. 1997 ASHRAE Handbook—Fundamentals, Chapter 26, Tables 1Aand 1B.

5. Scofield and Bergman. 1997. “ASHRAE Standard 62R: a simplemethod of compliance.” HPAC Magazine (10):74.

6. Schiess, K. 1998. “RTP + TES = ?” Engineered Systems (10):102.

7. Ecodyne. Weather Data Handbook. McGraw-Hill p. 11-3.

Please circle the appropriate number on the Reader Service Card at the back ofPlease circle the appropriate number on the Reader Service Card at the back ofPlease circle the appropriate number on the Reader Service Card at the back ofPlease circle the appropriate number on the Reader Service Card at the back ofPlease circle the appropriate number on the Reader Service Card at the back ofthe publication.the publication.the publication.the publication.the publication.

Extremely Helpful ........................................................................................................ 450

Helpful ........................................................................................................................ 451

Somewhat Helpful ...................................................................................................... 452

Not Helpful ................................................................................................................. 453