POTENTIAL PERFORMANCE OF REAL-GAS STIRLING CYCLE HEAT-PUMPS

Heat pumps based on the reversed Stirling cycle are shown to be positi vely influenced by real gas effects, provided they are designed to ope rate in a proper region of the fluid state diagram. A simplified model of a Stirling heat pump, aimed at understanding the basic cycle therm odynamics is presented, which allows a first optimization of real gas cycles. Provided the expansion process takes place in a proper narrow region close to the critical point, efficiencies much higher than thos e achievable with an ideal gas and similar to those of vaporization-co mpression cycles are obtained. A number of zero ODP, safe fluids are c onsidered (Xe, CHF3, C2F6, CHF3 + CF4 mixtures) allowing optimum opera tion 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 use r's temperature requirements. In order to roach good energy performanc e, high-pressure operation (around 200 bar) and an efficient internal regeneration of heat are needed. Graphs are supplied that reveal the h eat pump cycle performance for each fluid at a wide range of temperatu res, pressures and cycle compression volume ratios. Loss analysis show s that fluids having a simple molecule yield the best efficiency and t he minimum amount of heat regeneration. Stirling power cycles are also shown to benefit from real gas effects, with the result that at top t emperatures around 400-450 degrees C, which are probably acceptable fo r a number of organic fluids, a fuel to work conversion efficiency aro und 25-30% seems possible for a cogenerative prime mover. The performa nce of such motors, intended for heat pump drives, are given for C2HF5 and C3F8 fluids. Very high pressures are required to optimize the cyc le performance. Preliminary information on the prospective characteris tics of a fuel powered Stirling-Stirling low-grade heat generator is g iven. Copyright (C) 1996 Elsevier Science Ltd and IIR