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In the present paper, the experimental results of the use of R1234yf and R1234ze(E) as dropin alternatives of R134a in a water-to-water heat pump are discussed. The heat pump is firstly tested with R134a, to establish the baseline performance, and, then, is tested with the two abovementioned refrigerants considering the same compressor shaft rotational frequency and the same outlet temperatures of the secondary fluids from the evaporator and the condenser. The obtained results show that, compared to the R134a, the use of R1234yf leads to a reduction of the heating capacity and of the COP up to 9.80% and 7.39% respectively, whereas with the use of R1234ze(E) the capacity reduction achieves a maximum of 33.82% while a COP variation in the range −12.27% to +4.32% is found. A second group of tests is carried out varying the rotational frequency of the compressor shaft with the aim of matching the baseline R134a heating capacity. The results indicate an increase in the shaft rotational frequency up to 16% with R1234yf and up to 50% with R1234ze(E) which, however, leads to a maximum reduction of the heat pump COP respectively equal to 7.38% and to 18.11%. SubscriptsCOMP compressor COND condenser EV AP evaporator or evaporating G water-ethylene glycol mixture INS T Instrumental IN inlet OUT outlet 2 R refrigerant S HAFT shaft S UP superheating W water

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