Energy, exergy, economic, and environmental (4E) assessment of an experimental moderately-high-temperature heat pump for district heating and cooling networks

Abstract

Developing district heating and cooling networks (DHCNs) requires new heat pumps (HPs) technologies to improve energy efficiency and replace fossil fuel boilers. This study presents a novel R-1234ze(E) moderately-high-temperature HP integrated with a semi-hermetic reciprocating compressor and an internal heat exchanger (IHX) controlled by a solenoid bypass valve. The HP is evaluated in terms of energy, exergy, carbon footprint, and economic viability. Two main configurations are explored: (i) simultaneous heating and cooling for 4G (4th generation) district heating networks (DHNs) and district cooling networks (DCNs), and (ii) heat source from a 4G or 5G DHN to increase the evaporation temperature, reaching a maximum value of 48 °C at 85 °C condensing temperature (48 °C/85 °C). Six scenarios combine several evaporating and condensing temperatures, with and without the IHX. The experimental results show that the IHX improves the heating capacity from 4.8 % to 19.3 %. However, it has a limited effect on simultaneous heating and cooling efficiency. Scenarios 2 °C/45 °C and 30 °C/65 °C with IHX achieve the highest COP (4.31 and 4.92). Exergy analysis reveals more significant destruction in extreme operating conditions, varying the efficiency from 21.5 % (low suction temperature) to 10.5 % (high suction temperature), highlighting improvement possibilities mostly in the evaporator, compressor, and condenser. Heat source DHN scenarios involve higher equivalent CO₂ emissions per MWh, especially when the condensing temperature is higher. Economic analysis proves viability for simultaneous heating and cooling in all scenarios, with an optimum payback period of 1.96 years. It occurs similarly for heat source DHN in selected cases (30 °C/65 °C, 30 °C/80 °C with IHX, and 48 °C/85 °C without IHX), with an optimum payback period of 3 years.