Prospects for real-gas reversed Brayton cycle heat pumps
Abstract Ideal-gas reversed Brayton cycles are shown to be intrinsically inefficient owing to the high level of turbomachinery losses. An appropriate selection of the cycle operating parameters leading to the location of the expansion process in the vicinity of the critical point, where specific volumes and turbine works are small, allows the design of regenerated gas cycles with efficiencies similar to those of conventional vapour compression cycles, at least in the generation of high-temperature heat. A number of working fluids are presented (both pure substances and mixtures) yielding a good conversion efficiency at various source/sink temperatures. Basic optimization rules are given for fluids of different molecular structure. Fluids with a simple molecule (Xe, CO 2 etc.) tend to produce heat at very high temperatures and with an excessive temperature change: compression staging is effective in correcting this trend. Moderate pressure ratios (2 to 4) are sufficient to yield a good cycle efficiency; however, operating pressures are intrinsically high, since a minimum pressure around p cr is in general requested. The main features of the real-gas heat pump cycle can be summarized as the large power density, the ability to operate at high temperature with a small pressure ratio, and non-isothermal heat generation. Whenever such characteristics are of particular value, as, for example, in the production of heat for a long-distance conveyance, as needed for urban heating systems or for industrial heat networks, the real-gas reversed Brayton cycle should be examined as a possible alternative to conventional heat pump cycles.