Modelling and evaluation of a high-temperature heat pump two-stage cascade with refrigerant mixtures as a fossil fuel boiler alternative for industry decarbonization

High-Temperature Heat Pump (HTHP) is becoming a feasible technology for decarbonization, being proposed as an alternative to fossil fuel boilers in several industrial, commercial, and urban applications. This work presents a semi-empirical assessment of a two-stage cascade cycle for HTHP applications to produce hot water up to 150 °C from a water flow at 35 °C and 25 °C. This work uses experimental results of two single-stage heat pump prototypes (R-1234ze(E) and R-1336mzz(Z)) with different temperature lifts as baseline inputs. The energy performance of the proposed two stage cascade HTHP is evaluated through a semi-empirical model, including several novel mixtures for both stages. Up to 14% of COP increase was reached respect to the baseline when using R-152a/600 (0.08/0.92) and R-1233zd(E)/161 (0.88/0.12) for the low stage and high stage, respectively. The VHC increases 30% with the selected combinations, but the discharge temperature also rises. Direct CO2e emissions were negligible by using low GWP refrigerants. The country proposed for the two-stage cascade HTHP greatly influences greenhouse gas indirect CO2e emissions. It is estimated that in countries carbon emission factor lower than 0.35 kgCO2e kWh−1 the mixtures selected would reduce the emissions compared to fossil fuel boilers for the same heating capacity.