Thermodynamics

504 | ThermodynamicsWorking fluidEnergyREGENERATOREnergyFIGURE 9–25A regenerator is a device that borrowsenergy from the working fluid duringone part of the cycle and pays it back(without interest) during another part.There are two other cycles that involve an isothermal heat-addition processat T H and an isothermal heat-rejection process at T L : the Stirling cycle andthe Ericsson cycle. They differ from the Carnot cycle in that the two isentropicprocesses are replaced by two constant-volume regeneration processesin the Stirling cycle and by two constant-pressure regeneration processes inthe Ericsson cycle. Both cycles utilize regeneration, a process during whichheat is transferred to a thermal energy storage device (called a regenerator)during one part of the cycle and is transferred back to the working fluid duringanother part of the cycle (Fig. 9–25).Figure 9–26(b) shows the T-s and P-v diagrams of the Stirling cycle,which is made up of four totally reversible processes:1-2 T constant expansion (heat addition from the external source)2-3 v constant regeneration (internal heat transfer from the workingfluid to the regenerator)3-4 T constant compression (heat rejection to the external sink)4-1 v constant regeneration (internal heat transfer from theregenerator back to the working fluid)The execution of the Stirling cycle requires rather innovative hardware.The actual Stirling engines, including the original one patented by RobertStirling, are heavy and complicated. To spare the reader the complexities,the execution of the Stirling cycle in a closed system is explained with thehelp of the hypothetical engine shown in Fig. 9–27.This system consists of a cylinder with two pistons on each side and aregenerator in the middle. The regenerator can be a wire or a ceramic meshTTTT Hq1 in2T Hq in1 2T Hq in1 2s = const.s = const.v = const.Regenerationv = const.P = const.RegenerationP = const.4q out3T L4q out3T L4q out3sssT L1PPP1FIGURE 9–26T-s and P-v diagrams of Carnot,Stirling, and Ericsson cycles.T H = const.q in4 1qq inin2424q outq out3q out 33T L = const.TH = const. T H = const.RegenerationT L = const.T L = const.Regenerationv v v(a) Carnot cycle (b) Stirling cycle (c) Ericsson cycle2