Maximum Obtainable Efficiency For Engines And Refrigerators Based On The Stirling Cycle

The efficiency of the ideal stirling cycle assumes, that the cylinder volumes are isothermal and that the temperatures in the cylinder volumes are identical to the temperatures in the heat sink and the heat source. The efficiency is then identical to the efficiency of the Carnot cycle. However, in a real stirling machine the cylinder volumes are nearly adiabatic instead of isothermal, and the heat is added or removed in heat exchangers. Then the efficiency of a stirling cycle will always be smaller than for a carnot cycle even with ideal heat transfer in.regenerator and heat exchangers. Especially for stirling based refrigerators, which have been discussed recently as an alternative to refrigerators using CFC-gases, the maximum obtainable coefficient of performance is important. Based on a simplified stirling cycle with ideal heat exchangers and regenerators, the maximum possible efficiency and dimensionless output for a stirling cycle as a function of dead volume, phase angle and temperatures in heat sink and heat source are calculated and compared with the efficiency of the carnot cycle. Also the influence of heat transfer in the cylinder volumes is analyzed, and the effect of "isothermalizers" are discussed.