PROSPECTS FOR REAL-GAS REVERSED BRAYTON CYCLE HEAT-PUMPS

Ideal-gas reversed Brayton cycles are shown to be intrinsically ineffi cient owing to the high level of turbomachinery losses. An appropriate selection of the cycle operating parameters leading to the location o f the expansion process in the vicinity of the critical point, where s pecific volumes and turbine works are small, allows the design of rege nerated gas cycles with efficiencies similar to those of conventional vapour compression cycles, at least in the generation of high-temperat ure heat. A number of working fluids are presented (both pure substanc es and mixtures) yielding a good conversion efficiency at various sour ce/sink temperatures. Basic optimization rules are given for fluids of different molecular structure. Fluids with a simple molecule (Xe, CO2 etc.) tend to produce heat at very high temperatures and with an exce ssive temperature change: compression staging is effective in correcti ng this trend. Moderate pressure ratios (2 to 4) are sufficient to yie ld a good cycle efficiency; however, operating pressures are intrinsic ally high, since a minimum pressure around p(cr) is in general request ed. The main features of the real-gas heat pump cycle can be summarize d as the large power density, the ability to operate at high temperatu re with a small pressure ratio, and non-isothermal heat generation. Wh enever such characteristics are of particular value, as, for example, in the production of heat for a long-distance conveyance, as needed fo r 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.