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BUILDING FOR THE FUTURE

Selecting Right System, Configuration

ogeneration systems generate power and capture heat for

local uses. These systems can reduce operating costs, re-

duce the need for new electric generation, and perhaps, more

importantly, reduce the load on electric transmission systems.

After last summer’s East Coast grid failure, the interest in cogen-

eration systems is higher than ever.

In the long run, developing countries(where electric demand is growing andelectric distribution is strained) can ben-efit most from cogeneration. Every co-generation system built reduces the needfor central generation and transmissionsystem construction, and decentralizespower production, potentially increasingthe security of the electric system.

Aggressive year-round heat recoveryis important in economically justifying

cogeneration systems. Engine generatorsare the most commonly used drives forcogeneration systems in commercialbuildings and campuses. For most prac-tical application sizes, this means that aportion of the summer cooling load mustbe met by an absorption chiller operat-ing on waste heat from an industrial en-gine. In the developing world, whichtends to be more tropical, the need forcogeneration systems to supply cooling

is more acute as there often are few otherpractical applications for waste heat.

Unfortunately, a large body of litera-ture does not exist on the best way to linkengine generators and absorption chill-ers. Surprisingly, even absorption manu-facturers offer no specific guidance,although manufacturers’ sizing programscan be of some help. This is a crosscut-ting question between HVAC and engine-generator manufacturers—two groupswho have had little contact in the past.

Absorber TypesThe first issue is the selection of the

best type of absorption chiller to applyto engine heat rejection. Engine genera-tors reject heat in the exhaust, the jacketwater, the oil cooler, one or more turbo-charger intercoolers, and directly to theengine room. The last three often are toolow in temperature to be practically used.

Temperature limits govern how muchcan be recovered. Engine jacket outlet

By William Ryan, Ph.D., P.E., Member ASHRAE

CCCCC

The following article was published in ASHRAE Journal, September 2004. © Copyright 2004 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 inpaper form without permission of ASHRAE.

ABSORPTION CHILLERS

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Surprisingly, even absorption manufacturers offer no

specific guidance, although manufacturers’ sizing pro-

grams can be of some help.

‘

’temperatures are limited to the 240°F to 250°F (116°C to121°C) range. Heat recovery mufflers are less limited, butthe amount of heat that can be recovered declines with in-creasing inlet water temperature. Overall, if water at or be-low 250°F (121°C) can satisfy the load, the jacket water heatas well as a sizable portion of the exhaust heat can be recov-ered, and heat recovery between 3,800 and 5,000 Btu/kWh(4009 and 5275 kJ/kWh) of electric generation is practical. Ifhigh pressure (>15 psig [>103 kPa]) steam is needed,the jacket heat cannot be used and more exhaust heat iswasted, lowering heat recovery to as low as 1,500 Btu/kWh(1583 kJ/kWh).

Given that high-pressuresteam is not needed in mostcommercial buildings, thelower temperature hot waterapproach can recapture asmuch as 300% more heat.This means that running aless expensive, single-effectabsorber on low-temperatureheat is more desirable thanusing a more efficient high-temperature, double-effectsystem.

Table 1 shows how muchcooling is available from suchsystems. Note that a low-tem-perature, single-effect absorberproduces more cooling per kW of engine generator at a lowerfirst cost than a high-temperature, double-effect absorber.

In addition, single-effect absorbers have somewhat lowermaintenance costs than double-effect systems, and do not re-quire steam, eliminating steam system maintenance issues.Lastly, single-effect absorption chillers operate further fromthe crystallization region than double-effect systems. Figure1 shows a simple, idealized low-temperature system. Figures2 and 3 show examples of these components.

Engine Absorber IntegrationThe way the absorber is connected to the engine is critical

for proper operation. Although absorption chillers can be run

at water temperatures as low as 180°F (82°C), operating at suchlow temperatures may involve a capacity derating. This willrequire oversizing the absorber, effectively increasing the costof the absorption chiller in dollars per useable ton. Clearly,operating the energy transfer between the engine and the ab-sorber at the highest temperature practical is desirable. How-ever, the ultimate limitation comes from a source most designersdo not initially expect: the maximum temperature of returnwater to the engine jacket allowed by the engine manufacturer.

Industrial engine manufacturers contacted thus farrequire jacket return water temperature at 207°F (97°C) orbelow. The return temperature to the exhaust gas heat ex-

changer or water-cooled si-lencer is not as limited. Sowhat are the effects on prac-tical supply temperature tothe absorber?

Figure 5 shows a deratingchart for two domestic manu-facturers of single effect hotwater-driven absorption sys-tems. (Figures 6 and 7 are thesame chart with specificsample temperatures.) Themultipass line uses data fromboth manufacturers for chill-ers with the greatest numberof passes available. The chartis somewhat simplistic in that

a customer can, by working directly with the manufacturer,order specific changes that can improve capacity somewhat.Therefore, Figure 5 should be used to give a good first esti-mate of absorber derating.

The charts are plotted with inlet temperature on the verti-cal axis and outlet temperature on the horizontal axis. Thecapacity factor scale on each line shows the percentage of theoriginal rating these machines produce at any particular inletand outlet water temperatures. Where the user’s system fallson these charts can make a dramatic difference in actual ab-sorber capacity.

The chart shows lines for both a single-pass and multipassflow arrangement. In a single-pass arrangement, the hot wa-

One of two MW generators.

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Percent CostAbove

Electric(At 500 tons)

25%

100%

Low Temp.System

High Temp.System

Min.

Max.

Min.

Max.

HeatProduction,

Btu/kWh

3,800

6,000

1,500

2,000

AbsorberCOP

Single Effect0.7

Single Effect0.7

DoubleEffect

1.2

DoubleEffect

1.2

CoolingAvailable,

tons/kWGen.

0.22

0.35

0.15

0.2

ter flows through the absorber generator once before exitingthe absorption chiller. In a multipass arrangement, the waterflows back and forth through the generator from two to fourtimes before exiting. The longer flow lengths of multipassarrangements remove more heat from each gallon of hot wa-ter, resulting in a greater temperature drop through the ab-sorber. The other alternative, running the hot water throughthe generator tubes more slowly, generally is not practical asthe water flow may become laminar and the heat transfer ratemay deteriorate.

Using these charts for our simplified engine-absorber sys-tem, it will be seen that the critical limitation is the returnwater temperature to the engine jacket.

If the maximum return water temperature to the engine jacketis 207°F (97°C), the maximum temperature of water leavingthe absorber is 207°F (97°C). This gives the situation shown inFigure 7. Using a multipass arrangement, the absorber couldtake in water at 230°F (110°C)with a capacity factor of ~84%and still produce the desired 207°F (97°C) outlet water tem-perature. Feeding water to the absorber at any higher tempera-ture other than 230°F (110°C) would raise the leaving watertemperature above 207°F (97°C). This excess heat would have

to be thrown away before the water reenters the engine jacket,thereby lowering the overall efficiency of the system. With asingle-pass machine, the maximum hot water inlet tempera-ture would be 220°F (104°C) and the capacity factor would be~74%. A multipass arrangement produces significantly less ab-sorber derating than a single pass.

In addition, the multipass allows a 23°F (13°C) hot waterrange (difference between absorber input and output water

Hot Water

WaterCooledSilencer

Exhaust

Absorber

ChilledWater toCooling

Load

Hot Water

WaterCooledSilencer

Exhaust

Absorber

ChilledWater toCooling

Load

207°FMax.

Table 1 (left): Comparison of low and high temperature approaches. Figure 1 (right): Ideal engine absorber interconnection.

Figure 2 (left): Absorption chiller at GTI cogeneration facility. Figure 3 : Exhaust heat recovery heat exchanger at GTI facility.

Figure 4: Engine absorber interconnection.

ABSORPTION CHILLERS

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temperatures), whereas the single pass allows only a13°F (7°C) range. The larger range of the multipassmeans that less water has to be pumped to and through theabsorber to supply a givenheat input. This helps tocompensate for the higherpressure drop of a multipassarrangement.

If the maximum return wa-ter temperature to the enginejacket was 190°F (88°C),as quoted by some enginemanufacturers, the situationbecomes appreciably worse,as shown in Figure 8.

With a 190°F (88°C) maxi-mum engine jacket enteringtemperature (and therefore a190°F [88°C] absorber leav-ing water temperature) themaximum entering water temperature for a multipass absorberis 212°F (100°C), with a capacity factor of ~65%.

Table 2 illustrates how sensitive the absorber capacity fac-

tor is to engine jacket entering water temperature, and alsothe value of using multipass machines. Both absorber derat-ing, requiring installation of a larger absorber, and the larger

hot water flows, requiringlarger piping and pumps,can make a significant dif-ference in first cost.

There are situationswhere single-pass arrange-ments will make sense,specifically where tempera-ture ranges must be kept lowor the water flow rate mustbe high for some other sys-tem-related reason. How-ever, with engine coolant,the limited return tempera-ture to the engine and theability for an engine togenerate high (250°F or

more [121°C or more]) leaving water temperatures suggestthat engine heat recovery is a problem best solved with amultipass chiller.

280

260

240

220

200

180

160140 150 160 170 180 190 200 210 220 230 240

Inle

t Te

mp

. (°F

)

Outlet Temp. (°F)

60%Capacity

Factor

100%

90%

70%

100%

90%

80%

70%

60%

50%

40%

80%230°F

220°F

207°F

280

260

240

220

200

180

160140 150 160 170 180 190 200 210 220 230 240

Inle

t Te

mp

. (°F

)

Outlet Temp. (°F)

60%Capacity

Factor

80%

100%

90%

70%

100%

90%

80%

70%

60%

50%

40%

MultipassSingle-Pass

MultipassSingle-Pass

Figure 5 (left): Derating charts for two American manufacturers. Figure 6 (right): Operating points to achieve 207°F (97°C)return to jacket temperature.

Engine Water FlowRequired

For 100 Tons, gpm

571

428

264

214

171

156

137

90

Number ofPasses

1

1

1

1

Multipass

Multipass

Multipass

Multipass

Maximum EngineJacket Inlet, °F

180

190

207

220

180

190

207

220

Maximum AbsorberInlet Temp., °F

186

198

220

236

200

212

232

258

Resulting CapacityDerating Factor

37%

50%

73%

86%

50%

65%

87%

101%

Chiller SizeRequired to Deliver

100 tons, Tons

270

200

137

116

200

143

115

99

Table 2: Resulting derating situation and water flows for 100 tons (350 kW) of heat recovery cooling.

Figure 7: Operating points to achieve 190°F (88°C) return tojacket temperature.

280

260

240

220

200

180

160140 150 160 170 180 190 200 210 220 230 240

Inle

t Te

mp

. (°F

)

Outlet Temp. (°F)

60%Capacity

Factor

100%

90%

70%

100%

90%

80%

70%

60%

50%

40%

80%

212°F

198°F

190°FMultipass

Single-Pass

HEAT RECOVERY

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How Heat Exchangers Make the Situation WorseDesigners often place a heat exchanger between the ab-

sorber and the engine jacket. Engine manufacturers may ac-tually recommend this as it relieves them of any concernsabout the capability of the engine water pump to handle thepressure drop through the absorber. Unfortunately, this is notdesirable from an overall sys-tem standpoint.

If the heat exchanger has a10°F (5.5°C) drop, as is typi-cal of shell and tube arrange-ments, and the maximumreturn temperature of the en-gine is 190°F (88°C), themaximum output temperaturefrom the absorber becomes180°F (82°C). As shown inTable 2, this results in a fur-ther derating of the absorber,increasing the design size ofthe absorber from 143 to 200 tons(503 to 703 kW) just to effectivelyproduce 100 tons (352 kW).

Some designers also voice con-cerns about any leakage in the ab-sorber generator heat exchangerpotentially contaminating the enginecoolant system. However, even whenthe absorber is running, the genera-tor operates below atmospheric pres-sure, whereas the jacket coolantsystem is at or above atmospheric.When the absorber is shut down, thegenerator is far below atmosphericpressure. Any leakage in generatortubing would admit jacket water tothe absorber, rather than leak bro-mide solution to the jacket water.

Finally, although a heat exchanger dividing the two waterflows allows the absorber’s hot water flow pump and engine

pump to be in separate circuits, other ways exist of handlingthis, as described in the next section.

Pumping IssuesMoving jacket water through the absorption chiller directly

involves overcoming the pressure drop within the hot waterpiping that runs through thegenerator. The longer the flowpath, the greater the pressureloss. Therefore the pressuredrop increases with the num-ber of passes used.

As shown in Table 3, mov-ing from single to multipassmachines both raises the pres-sure drop and lowers the waterflow rate, resulting in similarpower consumption. Pumpingpower does rise with lowerwater temperatures in either

pass arrangement. However, it remainsa small quantity compared to the cool-ing derived. The values in Table 3 donot include pumping needed to sendthe hot water from the engine to theabsorber. A system with considerabledistance between the engine and theabsorber will consume more power.

Some engine generators may beequipped with a pump on the engine.However, this pump will have beensized to move jacket water througha radiator and back to and throughthe engine. It may not be sufficientto handle pumping through the ab-sorber. An additional pump mayneed to be added to circulate cool-ant through the absorber. Also, the

pressure drops shown in Figure 9 are for water and will behigher than for the ethylene glycol water mixtures generally

Pre

ssu

re D

rop

, ft o

f wat

er

90 100 200 300 400 500Gallons Per Minute

20

10

5

2

1

3 Pa

ss2

Pass

1 Pa

ss

Figure 9: Pressure drop in a 160 ton (563 kW)(nominal rating) hot water absorption chiller.

WaterCooledSilencer

180°FExhaust

190°F190°F

Absorber

Figure 8: Commonly used heat exchanger worsens situation.

Engine Water Flow RequiredFor 100 tons, gpm

571

428

264

214

171

156

137

90

Number ofPasses

1

1

1

1

Multipass

Multipass

Multipass

Multipass

MaximumEngine Jacket

Inlet, °F

180

190

207

220

180

190

207

220

MaximumAbsorber Inlet

Temp., °F

186

198

220

236

200

212

232

258

ResultingCapacity

Factor

37%

50%

73%

86%

50%

70%

87%

101%

Chiller SizeRequired to Deliver

100 tons, tons

270

200

137

116

200

143

115

99

Table 3: Pump power across differing flow conditions.

PressureDrop, ft

10

6

2

2

22

15

12

8

Pump Powerat 60%

Efficiency, hp

2.41

1.08

0.24

0.14

0.00

1.59

0.98

0.69

0.30

ABSORPTION CHILLERS

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used in engine jackets to prevent radia-tor freeze-up in the winter.

Sizing and ControlIn most commercial building appli-

cations, cooling, heating, power (CHP)or cogeneration systems make the mosteconomic sense when sized to coverabout 50% of a facility’s electric load.1,2

The generator operates nearly continu-ously to cover 40% to 60% of the fullelectric load. The infrequent peak elec-tric loads are covered by utility power.Properly connected, engine generatorsin the 32% efficiency range can provideenough heat to power, at most, 250 to300 tons (880 to 1055 kW) of single-effect absorption chiller for each MWof generator installed.

Commercial buildings require between 5 and 15 W/ft2 (54and 161 W/m2). This includes all electric loads including elec-tric cooling systems. Conversely, each 400 ft2 (37 m2) of build-ing requires roughly 1 ton (3.5 kW) of cooling.

CHP systems in commercial buildings, as suggested in Table4, feature the following:

• CHP systems tend to have more favorable economics inlarger installations. In general, for a given climate and utilityrate structure, the amount saved by the systems is relativelyconstant per square foot of building floor space. However,the first cost of the system declines precipitously with build-ing size, making paybacks shorter for larger systems.1,2

• Engine generators are the most practical way of generat-ing electricity in systems in the 0.5 MW to 2 to 3 MW range.This would cover buildings up to ~500,000 ft2 (~46 450 m2),which includes the vast majority of commercial buildings.Systems serving larger floor space loads, where turbines maybe more practical, would include only very large hospitalsand collections of buildings on central heating and coolingsystems. The emerging technology of microturbines maychange this situation soon.

• The cooling provided by an absorption chiller oper-ating on generator waste heat tends to cover typicallyabout one-third of the peak cooling load. The remainderof the cooling needs must be provided by either con-ventional chillers and/or by supplementary firing of theabsorber.

Given that the absorption chiller is operated on reclaimedheat from a generator, the absorber should be the first chilleroperated (the “lead” chiller) whenever a cooling load ispresent. Other chiller capacity should then be brought on onlyas needed.

Controlling the SystemDiagrams presented thus far have shown very simple sys-

tems. The next issue to handle is controlling the output of theabsorber to match the needs of the cooling system. The stan-dard method is to control the volume of hot water flow throughthe generator with a two-way or three-way control valve.Given that the overall water flow through the engine jacketmust remain constant, and that some water flow should bemaintained through the water-cooled silencer, the three-wayarrangement lends itself to this system.

In addition, during periods when the absorber is producingless than full cooling or is shut down, the unused heat fromthe engine jacket must be rejected to the environment. At thispoint, the dump heat radiator is introduced into the system,as shown in Figure 10. A cooling tower could also be used.Given that this cooling tower is also needed for the absorp-tion chiller, this can be a practical arrangement for CHP sys-tems. However, in colder climates, engine cooling to a coolingtower will require cooling tower operation in freezing weather,generally requiring a dry sump.

Notice that an expansion and pressurization system also hasbeen added. The water system must be kept at a pressure above

Cost of InstalledSystem

$1,400/kW

$1,200/kW

$800/kW

Building Size, ft2

100,000

500,000

1,000,000

Peak ElectricLoad

700 kW

3,500 kW

7,000 kW

Cogen Sizeat 50%

350 kW

1,750 kW

3,500 kW

Absorber Size, tons

87

438

875

Peak CoolingLoad, tons

250

1250

2500

Table 4: Approximate sizing of CHP systems in commercial buildings.

Cost of SystemPer ft2 of Building

$5.50

$3.33

$2.80

Exhaust

207°FMax

Dump Radiator

Expansion andPressurization

Tank

Absorber

Absorber Control Valve

WaterCooledSilencer

T

Figure 10: Layout showing control system and heat rejection.

ChilledWater toCooling

Load

HEAT RECOVERY

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the boiling point of the hottest water inthe system.

A water-cooled silencer (for exhaustgas heat recovery) equipment manufac-turers’ diagram is shown in Figure 10.An exhaust gas diverter valve controlsthe water-cooled silencer. When the en-gine operates and some or all of the wasteheat is not needed, the valve opens andsends exhaust gas around the heat ex-changer. Even when this is done, someflow of water should continue to passthrough the silencer to prevent boilingdue to any possible diverter valve leak-age. In Figure 10, the silencer flow by-passes the radiator, but does mix with thecooling flow. Therefore, the return to thesilencer should not significantly exceedthe temperature out of the engine jacket,even when heat usage is zero.

Space and Water HeatingHaving handled the cooling issues, the more straightforward

recovery for space and water heating should be incorporated.As previously mentioned, water jackets typically can pro-

duce hot water in the 240°F to 250°F (116°C to 121°C) range.The critical temperature is the allowable entering temperatureto the water jacket, and engine manufacturers may limit this tobelow 207°F (97°C). If there is no productive heat load thatwill lower the return water to 207°F (97°C), much of the en-gine jacket heat will be wasted. This has a negative impact oncogeneration economics.

Figure 11 details an entire interconnection arrangement be-tween an engine generator, an absorption chiller, and a heatingdelivery system. The absorber uses engine coolant directly withno intermediate heat exchanger to minimize derating.

Using a heat exchanger for a building space heating sys-tem almost always is required. Building heating systems in-volve extensive piping, and, running engine coolantthroughout the building generally is not practical. In addi-tion, the highest temperature used for most heating applica-tion is 180°F (82°C), well below the maximum engine returntemperature. When the space heating load is sufficient, all ofthe recovered heat can be used and the waste heat radiatorremains inoperative. In addition, in larger commercial build-ings, domestic hot water often is generated as part of the over-all heating system. This single heat exchanger would thenpick up the domestic hot water load, as well.

Low-Temperature Absorption ChillersSingle-effect absorption chillers also can be built that op-

erate, with little derating, on lower temperature hot water.These chillers use a different structure to the generator,allowing lower temperature hot water to be used moreeffectively. To date, there has been no American productionof such equipment. Given a strong enough market in cogen-eration systems, these chillers may become available inthe future. Some foreign equipment of this type hasbeen brought into the market, but this is beyond the scope ofthis article.3

ConclusionApplying absorption chillers in conjunction with engine-

driven cogeneration systems requires some thought aboutoverall operation. Otherwise, the cost of the absorbers maybecome too high or the system may produce less cooling thanexpected. In cogeneration economics, recovered heat offsetssome of the generator’s fuel consumption by reducing boilerfuel consumption. With the rising cost of fossil fuels, maxi-mizing the useful recovery of heat from a cogeneration sys-tem is more important than ever.

References1. Ryan, W., C. Haefke and M. Czachorski. 2002. Evalua-

tion of Commercial Markets for BCHP Applications, Gas Tech-nology Institute.

2. Ryan, W. 2002. “Economics of Cogeneration.” ASHRAEJournal 45(10):34–40.

3. 2004. Century Single Stage Absorption Chiller, ProductSpecification Sheet.

William Ryan, Ph.D., P.E., is a research engineer at the Uni-versity of Illinois Chicago.

T Radiator Operation Triggered byLeaving Temp. Exceeding 207°F

Water Pump

<207°F

Water CooledSilencer

Exhaust

Single EffectAbsorption

Chiller

CWR

CWS

HWS180°F Max.

HWR160°F Max.

HeatExchanger

ToHeatingLoad

FromHeatingLoad

FromCoolingLoad

ToCoolingLoad

Figure 11: Possible connection arrangement between engine generator and heatrecovery system.