Supercritical heat pump cycles

Abstract

Supercritical heat-pump cycles suited for high-temperature heat generation and in which heat is delivered in the form of sensible heat of a high-pressure fluid are examined and their energy performance is evaluated. The main variables governing the energy efficiency of the process and the temperatures of the heat produced are recognized to be the fluid critical temperature, the molecular complexity, the top cycle pressure and the amount of internal regeneration of heat. Two cycle configurations are examined: one featuring fluid compression after a regenerative preheating and one that also includes turbine expansion of a fraction of the high-pressure fluid in order to achieve a more effective regeneration. General diagrams giving the operating characteristics of a supercritical heat-pump cycle for any kind of fluid are reported. Some fluids are presented (SF 6, C 3F 8, C 2HF 5, c-C 4F 8), which exhibit a high level of thermal stability and are thermodynamically suitable for supercritical cycles: for each one a detailed performance chart is given. An example application in which a conventional high-temperature cycle is compared with two supercritical solutions is presented. The following conclusions summarize the findings of the thermodynamic analysis. (1) In supercritical cycles high heat-output temperatures are achievable with moderate compressor pressure ratios and with a comparatively simple cycle arrangement, while conventional cycles require a large pressure ratio and a complex cycle organization. Sub-atmospheric pressures, which may be required in conventional cycles, can be avoided. (2) As heat is available in supercritical cycles within a certain temperature range, applications implying the use of heat at variable temperature could benefit from the natural matching between temperature availability and process requirements. (3) The comparatively high pressures at which heat is produced in supercritical cycles could represent a drawback for small-capacity plants but are probably acceptable or even beneficial for large systems. (4) The internal regeneration of a sizeable amount of heat, which is requested in supercritical cycles, represents a definite cost item for this type of heat pump.