Stirling engines using working fluids with strong real gas effects

Real gas effects typical of the critical region of working fluids are a powerful tool to increase the energy performances of Stirling cycles, mainly at low top temperatures. To carry out the compression near the critical region the working fluids must have a critical temperature near environmental conditions and the use of organic working substances (pure or in suitable mixtures) as a matter of fact begins compulsory. The moderate thermal stability of the organic working fluids limits the maximum temperatures to 300–400 °C and as a consequence, the achievable cycles efficiencies result rather low. Carbon dioxide, with a critical temperature of 31 °C, is, among the traditionally inorganic gases, an exception and is considered here in comparison with organic substances. But the good thermodynamics of the cycles allows, in the considered cases, conversion efficiencies of about 20%, with good specific powers. The good energy performance of real gas Stirling cycles is obtained at the cost of high maximum cycle pressure, in the range of at least 100–300 bar. These high pressures nevertheless have large positive effects on the heat power transferred per unit of pumping mechanical power, and the low top temperatures have a positive influence on the material problems for the hottest engine parts.