Temperature glide matching in a zeotropic mixture heat pump
The use of zeotropic mixtures in heat pumps should lead to a reduction of the exergy losses within the heat exchangers due to the temperature glide (TG) and thus to an improvement of the entire process. In a previous publication (ECOS 2016), the coefficient of performance (COP) as a function of evaporation / condensation temperature, compressor rotation speed and composition of a zeotropic mixture (isobutane / propene) was investigated. Contrary to theoretical analysis, it was shown that the COPs of the mixtures increase only slightly compared to the better performing pure fluid. The temperature change of the secondary fluid in the heat exchangers was relatively small. In the present work, it is experimentally investigated whether a better matching of the temperature glide in the evaporator raises the COP of a zeotropic mixture compared to the better performing pure fluid. The targeted utilization of the TG is achieved by matching the temperature variation of the mixture and the temperature variation of the heat source; this was adjusted by varying the heat capacity flowrate of the source. An isobutane/propene mixture (42:58) with a TG of 8.03 K is investigated as the working fluid, together with investigations of the pure fluids isobutane and propene, all for equal boundary conditions. The experiments were carried out for a fix evaporator inlet temperature and two compressor rotation speeds. The exergy losses of individual components, and for the whole cycle, like exergetic efficiencies and COPs, were determined and were compared. In case of an optimal temperature match between the heat source and the refrigerant in the evaporator, contrary to the expectation, the COP of the mixture decreases by 4.2 % while the second law efficiency of the entire processes increases by 18.15 %.