Thermodynamics

hot NH 3 H 2 O solution, which is weak in NH 3 , then passes through aregenerator, where it transfers some heat to the rich solution leaving thepump, and is throttled to the absorber pressure.Compared with vapor-compression systems, absorption refrigeration systemshave one major advantage: A liquid is compressed instead of a vapor.The steady-flow work is proportional to the specific volume, and thus thework input for absorption refrigeration systems is very small (on the orderof one percent of the heat supplied to the generator) and often neglected inthe cycle analysis. The operation of these systems is based on heat transferfrom an external source. Therefore, absorption refrigeration systems areoften classified as heat-driven systems.The absorption refrigeration systems are much more expensive than thevapor-compression refrigeration systems. They are more complex andoccupy more space, they are much less efficient thus requiring much largercooling towers to reject the waste heat, and they are more difficult to servicesince they are less common. Therefore, absorption refrigeration systemsshould be considered only when the unit cost of thermal energy is low andis projected to remain low relative to electricity. Absorption refrigerationsystems are primarily used in large commercial and industrial installations.The COP of absorption refrigeration systems is defined asCOP absorption Desired outputRequired input Q LQ gen W pump,in Q LThe maximum COP of an absorption refrigeration system is determined byassuming that the entire cycle is totally reversible (i.e., the cycle involves noirreversibilities and any heat transfer is through a differential temperature difference).The refrigeration system would be reversible if the heat from thesource (Q gen ) were transferred to a Carnot heat engine, and the work outputof this heat engine (W h th,rev Q gen ) is supplied to a Carnot refrigerator toremove heat from the refrigerated space. Note that Q L W COP R,rev h th,rev Q gen COP R,rev . Then the overall COP of an absorption refrigeration systemunder reversible conditions becomes (Fig. 11–22)COP rev,absorption Q L h th,rev COP R,rev a 1 T 0ba bQ gen T s T 0 T LChapter 11 | 633(11–12)Q gen(11–13)where T L , T 0 , and T s are the thermodynamic temperatures of the refrigeratedspace, the environment, and the heat source, respectively. Any absorptionrefrigeration system that receives heat from a source at T s and removes heatfrom the refrigerated space at T L while operating in an environment at T 0has a lower COP than the one determined from Eq. 11–13. For example,when the source is at 120°C, the refrigerated space is at 10°C, and theenvironment is at 25°C, the maximum COP that an absorption refrigerationsystem can have is 1.8. The COP of actual absorption refrigeration systemsis usually less than 1.Air-conditioning systems based on absorption refrigeration, calledabsorption chillers, perform best when the heat source can supply heat at ahigh temperature with little temperature drop. The absorption chillers aretypically rated at an input temperature of 116°C (240°F). The chillers performat lower temperatures, but their cooling capacity decreases sharply withT LSourceT sReversibleheatengineT 0environmentQ genW = h rev Q genQ L = COP R,rev × WW = h rev Q gen = ( ) 1 – T 0 Q genT sTQ L = COP R,rev W = (LT ) W0 – T LEnvironmentT 0ReversiblerefrigeratorT LRefrigeratedspaceQCOP rev,absorption = L= ( )( )1 – T 0 T LQ genT sT 0 – T LFIGURE 11–22Determining the maximum COP of anabsorption refrigeration system.