Energy and environmental impact analysis of medium and low pressure refrigerants in heat pumps operating at moderate and high temperatures for district heating and cooling networks

This study evaluates medium-pressure refrigerants (R-1234ze(E), R-450A, R-513A) for moderate-temperature heating and low-pressure refrigerants (R-1233zd(E), R-1336mzz(Z)) for high-temperature applications using a heat pump prototype under conditions representative of district heating and cooling networks (DHCNs). The analysis includes evaporating and condensing temperatures, simultaneous heating and cooling, and heat source configurations. A two-stage cascade (TSC) simulation combining low- and medium-pressure refrigerants is also assessed. Energy performance and the environmental impact, including the Total Equivalent Warming Impact (TEWI) and the formation of Trifluoroacetic Acid (TFA), are analyzed. The effect of an internal heat exchanger (IHX) on medium-pressure refrigerants improves only heating capacity, with other parameters largely unaffected. Among medium-pressure refrigerants, R-1234ze(E) demonstrates promising performance with a coefficient of performance (COP) ranging from 2.68 to 4.21 and the lowest TEWI and TFA formation (0.63 kg). R-513A achieves the highest heating capacity, while R-450A performs intermediate. For low-pressure refrigerants, R-1233zd(E) generally outperforms R-1336mzz(Z) with a maximum COP of 5.62 versus 4.25. However, R-1336mzz(Z) operates advantageously at very high temperatures due to its lower discharge temperature, and high critical temperature, and shows lower TEWI with comparable TFA levels. TSC simulations reveal similar results across refrigerant pairs, confirming the technical and energy viability of high-pressure rate operation, especially for simultaneous heating and cooling applications.