Analysis of natural refrigerant for different applications in heat pumps above 100⁰C
Analysing suitable natural refrigerant for use in high temperature heat pumps for heating demand requiring temperatures between 100°C to 200°C. Carbon Dioxide (CO2), Pentane and Steam have been identified as suitable natural refrigerant. Each refrigerant have their weakness and strength which makes them most suitable for different applications. It has been identified that CO2 can deliver up to 130°C, but only when the heat sink inlet temperature is less than 30°C is the efficiency high enough to make it an interesting option. Pentane is a flammable refrigerant and is not suitable for all environments. Pentane operate sub atmospheric pressure below 36°C saturated temperature, to minimise the risk of air entering the refrigerant circuit and creating a potential explosive mixture, a pentane heat pump should not operate below 36°C saturated temperature. Pentane has a low volumetric evaporation energy at low temperature so preferable the suction pressure should not be less than 80°C to make pentane heat pumps economically attractive. Pentane heat pumps are most suitable for pressurized hot water systems where there is a temperature difference between inlet and outlet of the heat sink. Steam has a very low density below 100°C which will require 4 – 8 times larger compressor and heat exchangers per kW than traditional refrigerants like ammonia. To minimize compressor and heat exchanger sizes heat lifts below 80°C should be done with traditional heat pumps and steam compression should only do the last lift from 80°C to the desired steam pressure. Steam compression is best suited for application requiring steam especially if the steam can be used directly without additional heat exchanger.