SUPERCRITICAL HEAT-PUMP CYCLES

Supercritical heat-pump cycles suited for high-temperature heat genera tion and in which heat is delivered in the form of sensible heat of a high-pressure fluid are examined and their energy performance is evalu ated. The main variables governing the energy efficiency of the proces s and the temperatures of the heat produced are recognized to be the f luid critical temperature, the molecular complexity, the top cycle pre ssure and the amount of internal regeneration of heat. Two cycle confi gurations are examined: one featuring fluid compression after a regene rative preheating and one that also includes turbine expansion of a fr action of the high-pressure fluid in order to achieve a more effective regeneration. General diagrams giving the operating characteristics o f a supercritical heat-pump cycle for any kind of fluid are reported. Some fluids are presented (SF6, C3F8, C2HF5, c-C4F8), which exhibit a high level of thermal stability and are thermodynamically suitable for supercritical cycles: for each one a detailed performance chart is gi ven. An example application in which a conventional high-temperature c ycle is compared with two supercritical solutions is presented. The fo llowing conclusions summarize the findings of the thermodynamic analys is. (1) In supercritical cycles high heat-output temperatures are achi evable with moderate compressor pressure ratios and with a comparative ly simple cycle arrangement, while conventional cycles require a large pressure ratio and a complex cycle organization. Sub-atmospheric pres sures, which may be required in conventional cycles, can be avoided. ( 2) As heat is available in supercritical cycles within a certain tempe rature range, applications implying the use of heat at variable temper ature could benefit from the natural matching between temperature avai lability and process requirements. (3) The comparatively high pressure s 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 size able amount of heat, which is requested in supercritical cycles, repre sents a definite cost item for this type of heat pump.