Real gas effects in Stirling engines

Real gas effects in the working fluid critical region are shown to be a powerful tool to increase the energy conversion efficiency of Stirling cycles, mainly at low top temperatures. Organic working substances are shown to represent the only practical choice to optimize real gas effects for applications at given minimum cycle temperatures. The moderate thermal stability of organic compounds limits the cycle top temperature to 350-450°C and, consequently, the achievable top efficiency. However the good intrinsic thermodynamics of real gas cycles allows conversion efficiencies in excess of 25% for power only and in excess of 20% for combined heat and power plants. Adequate working fluids, both as pure substances and as mixtures are shown to be practically available. An optimization procedure is illustrated allowing to select the best cycle performance for a given cycle top pressure and temperature