Potential performance of real gas Stirling cycle heat pumps

Heatpumps based on the reversed Stirlingcycle are shown to be positively influenced by realgas effects, provided they are designed to operate in a proper region of the fluid state diagram. A simplified model of a Stirlingheatpump, aimed at understanding the basic cycle thermodynamics is presented, which allows a first optimization of realgascycles. Provided the expansion process takes place in a proper narrow region close to the critical point, efficiencies much higher than those achievable with an ideal gas and similar to those of vaporization-compression cycles are obtained. A number of zero ODP, safe fluids are considered (Xe, CHF3, C2F6, CHF3 + CF4 mixtures) allowing optimum operation in a wide range of heat source and heat production temperatures. Only mixtures, however, are recognized to permit a fine adjustment of the fluid properties to the heat source characteristics and to the user's temperature requirements. In order to reach good energy performance, high-pressure operation (around 200 bar) and an efficient internal regeneration of heat are needed. Graphs are supplied that reveal the heatpumpcycleperformance for each fluid at a wide range of temperatures, pressures and cycle compression volume ratios. Loss analysis shows that fluids having a simple molecule yield the best efficiency and the minimum amount of heat regeneration. Stirling power cycles are also shown to benefit from realgas effects, with the result that at top temperatures around 400–450°C, which are probably acceptable for a number of organic fluids, a fuel to work conversion efficiency around 25–30% seems possible for a cogenerative prime mover. The performance of such motors, intended for heatpump drives, are given for C2HF5 and C3F8 fluids. Very high pressures are required to optimize the cycleperformance. Preliminary information on the prospective characteristics of a fuel powered Stirling-Stirling low-grade heat generator is given.