Thermodynamics

or any kind of porous plug with a high thermal mass (mass times specificheat). It is used for the temporary storage of thermal energy. The mass ofthe working fluid contained within the regenerator at any instant is considerednegligible.Initially, the left chamber houses the entire working fluid (a gas), which isat a high temperature and pressure. During process 1-2, heat is transferredto the gas at T H from a source at T H . As the gas expands isothermally, theleft piston moves outward, doing work, and the gas pressure drops. Duringprocess 2-3, both pistons are moved to the right at the same rate (to keep thevolume constant) until the entire gas is forced into the right chamber. As thegas passes through the regenerator, heat is transferred to the regenerator andthe gas temperature drops from T H to T L . For this heat transfer process to bereversible, the temperature difference between the gas and the regeneratorshould not exceed a differential amount dT at any point. Thus, the temperatureof the regenerator will be T H at the left end and T L at the right end ofthe regenerator when state 3 is reached. During process 3-4, the right pistonis moved inward, compressing the gas. Heat is transferred from the gas to asink at temperature T L so that the gas temperature remains constant at T Lwhile the pressure rises. Finally, during process 4-1, both pistons are movedto the left at the same rate (to keep the volume constant), forcing the entiregas into the left chamber. The gas temperature rises from T L to T H as itpasses through the regenerator and picks up the thermal energy stored thereduring process 2-3. This completes the cycle.Notice that the second constant-volume process takes place at a smallervolume than the first one, and the net heat transfer to the regenerator duringa cycle is zero. That is, the amount of energy stored in the regenerator duringprocess 2-3 is equal to the amount picked up by the gas during process 4-1.The T-s and P-v diagrams of the Ericsson cycle are shown in Fig. 9–26c.The Ericsson cycle is very much like the Stirling cycle, except that the twoconstant-volume processes are replaced by two constant-pressure processes.A steady-flow system operating on an Ericsson cycle is shown in Fig. 9–28.Here the isothermal expansion and compression processes are executed in acompressor and a turbine, respectively, and a counter-flow heat exchangerserves as a regenerator. Hot and cold fluid streams enter the heat exchangerfrom opposite ends, and heat transfer takes place between the two streams. Inthe ideal case, the temperature difference between the two fluid streams doesnot exceed a differential amount at any point, and the cold fluid stream leavesthe heat exchanger at the inlet temperature of the hot stream.T HT HChapter 9 | 505q inRegeneratorT LT Lq outFIGURE 9–27The execution of the Stirling cycle.State1State2State3State4RegeneratorHeatT L = const.CompressorT H = const.Turbinew netq inq outFIGURE 9–28A steady-flow Ericsson engine.