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C H I L LE RThe New Era ofAbsorption ChillersTechnological advances are moving absorption chillers forward.By Rajesh DixitAEfficiencyThe main misconception aboutabsorption chillers is that electriccentrifugal chillers are much moreefficient. Consequently, many poten46 www.DistributedEnergy.comJohnson Controlslthough absorptionchillers were introducedover a century ago, itis worth examiningwhat’s new today andwhat’s next, while dispelling someold misconceptions. That’s becausein recent years, absorption chillertechnology has evolved dramatically.Absorption chillers are being usedaround the globe as an efficient solution wherever waste heat recoveryor a low-cost thermal driving heatsource are available, especially as partof a natural-gas or a combined heatand power (CHP) system. The ability to use these sources of thermalenergy, along with using water as therefrigerant, make absorption chillersa viable option in commercial andindustrial—and even food-processingand marine—applications. In manysituations, the business case forabsorption chillers is surprisinglystrong due to the stable/low cost ofnatural gas, rising cost of electricity or grid congestion, and focus onnatural refrigerants.To understand why, this discussion focuses on common questionsabout the efficiency, flexibility, andmaintainability of lithium bromide/water commercial absorption chillersin the 30- to 2,000-ton range.Single-effect double-lift hot water absorption chiller

An average US city with averagechiller performance was considered.The chillers in the comparison wereType of ChillerDesign COPIPLV (COP)Electrical Consumption kW/Tonan industry-average, 1,000-ton electricElectric Centrifugal6.510.70.542centrifugal chiller with variable speeddrive, and absorption chillers of threeSingle-Effect Steam0.770.810.01Absorptiondifferent types: single-stage steam,Double-Effect Steam1.371.560.01two-stage steam, and direct fired, withAbsorptiona minimum entering condenser waterDirect Fired Absorption1.371.560.01temperature of 68 F.The model shows that the higherefficiency of an electric chiller doestial customers assume that absorption chillers do not makenot result in a lower operating cost compared to an absorptioneconomic sense because their energy efficiency appears tochiller. In other words, absorption chillers are not necessarilybe lower. This mistaken idea is surprisingly common evenless cost-efficient to operate. That’s because absorption chillersamong chiller experts.are driven by low-cost heat that can lower the cost of operaThe reason behind this misconception can be seen in Figtion and, therefore, provide a faster payback on the capitalure 1, which uses a common measurement of efficiency—theinvestment. This is not to say that absorption chillers are idealcoefficient of performance (COP)—to compare different 1,000- for every application. Each situation must be carefully evaluton systems at Air-Conditioning, Heating, and Refrigerationated before making the right chiller choice. One thing is clear,Institute (AHRI) conditions.In this comparison, it is importantto note that COPs between centrifugalFigure 2: Correlating COP, design point, and energy costs for different 1,000-ton systemsand absorption chillers are calculated inType of ChillerDesign COPElectricalSteam flow rateNatural gas flowConsumptionratea different manner. It is also important toremember that it is the cost of operation, Electric Centrifugal6.50.542 kW/tonNot applicableNot applicablenot academic comparison of COP values,Single-Effect Steam0.770.01 kW/ton14.8 lb/hr/ton @ 15Not applicablethat makes the business case.AbsorptionpsigTo further investigate the cost-effecDouble-Effect Steam1.370.01 kW/ton8.5 lb/hr/ton @ 115Not applicablepsigtiveness of an absorption chiller, consider AbsorptionDirect Fired Absorption1.20.01 kW/tonNot applicable10 ft /hr/ton*how COPs correlate with actual energycosts as shown in Figure 2.* Assuming a higher heating value of 1,000 BTU/ftThis comparison assumes that botha centrifugal and an absorption chillerare working at design conditions with their respective COPs.however—that low-cost thermal driving heat source must beAlso, it assumes the natural gas cost is fixed at 4/MMBTUavailable. Otherwise, it doesn’t pay to create the driving heatand electric utility costs are 0.1/kWh and 0.2 kWh, respecsource to drive an absorption chiller because it would not betively, with no demand charge (Figure 3).operationally economical.Here’s the conclusion: In both cases, the operating costBased on a first-cost comparison, absorption chillers areof the absorption chiller is significantly less in spite of havindeed more expensive than electric centrifugal chillers. Howing a much lower COP. The point of this simple analysisever, the difference in capital cost is quickly recovered throughis to show that although the numerical COP values of anannual operating savings, as mentioned above. It is importantabsorption chiller are lower than that of electric chillers, itto base the chiller choice not solely on the initial cost, butdoes not necessarily mean that absorption chillers are more rather the life cycle cost that comprises of first cost, mainteexpensive to operate. In fact, the opposite is true, especiallynance cost, and the annual energy cost.when there is low-cost natural gas or waste heat available.Furthermore, in many US citFlexibilityies, the cost of electricity canApplication versatilitybe much higher than the values Figure 3: Simple analysis of ton-hour costs of absorptionAbsorption cooling technolchiller fired by natural gas vs. an electric centrifugal chillerused in the analysis.ogy has evolved over the years,As additional evidence ofresulting in much greaterNatural Gas FiredElectric Centirfugal Chiller Difference %Absorption Chiller cents/ton-hourcost-effectiveness, an analysisflexibility for deployment incents/ton-hourof annual operational cost wasvast numbers of comfort and5.41 at 0.1 kWh electricity cost26%performed using a computerized 4.0industrial process cooling4.010.82 at 0.2 kWh electricity cost63%modeling tool (Figure 4).installations—and even aboardFigure 1: Comparing the coefficient of performance (COP) for different 1,000-ton systems33Distributed Energy June 2018 47

Figure 4: Operating cost comparison between centrifugal and absorption chillers attwo different electric energy cost levels, assuming cost of natural gas is 4/MMBTUand steam is waste ( 2/1,000 lb)Level 1energy costsElectricity: 0.1/kWhSteam (waste steam): 2/1,000 lbNatural gas: 4/MMBTUChiller typeElectric CentrifugalSingle-Stage SteamTwo-Stage SteamDirect FiredChiller cost ofoperation 155,382 117,178 67,272 149,775Plant cost ofoperation 249,106 216,823 176,408 258,847Level 2energy costsElectricity: 0.2/kWhSteam (waste steam): 2/1,000 lbNatural gas 4/MMBTUChiller typeElectric CentrifugalSingle-Stage SteamTwo-Stage SteamDirect FiredChiller cost ofoperation 310,765 117,178 67,272 149,775Plant cost ofoperation 498,212 316,468 285,544 367,919passenger cruise vessels. Absorptionchillers have also been successfullyapplied for gas turbine inlet air coolingapplications or hazardous area outdoorapplications, such as in a refinery or apetrochemical plant.Absorption chillers are generallylonger and heavier than electric centrifugal chillers. Nevertheless, for tightspaces or other dimensional constraintsat the job site, it is possible to reducelength and increase width for the samecooling capacity.Operating rangeIn the real world, however, there ismore flexibility in the operationalrange than suggested by the typicalnominal values shown in Figure 5. Asan example, if the unit is working at50% cooling load, the chilled water andthe cooling (condenser) water flow ratemay also be at 50% of the design value.The acceptable rate of change of flow isapproximately 5% per minute.Effect of tube metallurgy and coolingwater inlet temperatureCooling capacity can vary dependingon the tube material, such as copper,CuNi 90:10, SS316, and titanium. However, there will be a deration (7 18%)in cooling capacity if the tube materialis not copper. This deration also occurswhen the cooling (condenser) waterinlet temperature to the absorber section increases above the design value.For an industry-average, two-stage1,000-ton steam-driven absorptionchiller, if the cooling (condenser) watertemperature increases from 85 F to, say,93 F, the cooling capacity may drop toalmost 750 tons (a 25% deration) whilethe COP stays relatively flat.Absorption chillers are being used around theglobe as an efficient solution wherever wasteheat recovery or a low-cost thermal drivingheat source are available.Johnson ControlsDouble-effect steam-firedabsorption chiller48 www.DistributedEnergy.com