DeCarbCH Wiki Technologies - Heat Pumps
Refrigerants for vapor compression heat pump
A crucial step in designing a high temperature heat pump is the selection of a refrigerant appropriate for the intended application. The considerations for selecting the refrigerant include thermal and chemical suitability, efficiency, safety, availability, and environmental compatibility etc. The selection criteria is presented in the following table:
The upper temperature limit of subcritical heat pump cycles is determined by the critical temperature of the refrigerant (see tool).
The pressure level of the heat pump system determines the material efforts of the equipment. The higher pressure should be kept below a practical value of about 25 bar, although compressors are available for R32 and R410A which can withstand up to 50 bar. The lower pressure should not be below one atmosphere at standstill to avoid foreign gas entry. To minimize compressor power the pressure ratio should be as low as possible. The volumetric heating capacity of a refrigerant is important for the compressor design as it determines its size and cost, and it influences the achievable experimental coefficient of performance (COP).
In the past, industrial high-temperature heat pumps mainly used the chlorinated hydrocarbon R114, as it allowed condensating temperatures of up to 130°C. Due to their high ODP, CFCs were replaced by fluorinated hydrocarbons in the 1990s. R245fa, R245ca, and R365mfc offer zero ODP and relatively high critical temperatures of 154.0, 174.4, and 186.9°C at moderate pressures of 36.4, 39.3, and 32.7 bar. However, with regard to the increasing restrictions imposed by the Kyoto Protocol and the F-Gas regulation, R365mfc, R227ea, R245fa, and R134a will be phased down in the foreseeable future due to their high GWP. This drives research and industry towards the evaluation of alternative refrigerants with lower GWP. Hydrofluoroolefin (HFO) refrigerants are considered as environmentally friendly (low GWP) alternatives to replace HFCs.R1234yf and R1234ze(E) are regarded as substitutes for R134a. The critical temperatures and pressures are comparable. The major application areas are automotive air conditioners, refrigerators and freezers.
Natural refrigerants suitable for high temperature applications are R718 (water), R744 (CO2), R717 (ammonia), and hydrocarbons. CO2 heat pumps are common in smaller sizes. They have also been commercialised in the larger range.
In spite of the low critical temperature of 31 °C and high critical pressure of 73.6 bar, CO2 heat pumps also achieve sink temperatures of 90 to 120 °C in transcritical cycles. R744 is feasible as an HTHP fluid if the inlet temperature of the heat sink is not too far above the critical temperature. The high transcritical temperature glide in the gas cooler makes R744 a particularly suitable refrigerant for domestic hot water heating and other processes with very large temperature differences in the sink.
Ammonia is widely used in industrial heat pumps up to about 90 °C heat sink temperature. Beneficial is its high VHC compared to other refrigerants. For higher temperatures, existing compressor technology is limited due to high discharge pressures. With special cast steel construction, NH3 compressors are able to withstand pressures of up to 76 bar and 110 °C. However, certain safety precautions must be implemented due to the toxicity of ammonia (B2L).
The hydrocarbons n-butane (R600) and pentane (R601) are refrigerants without ODP and very low GWP. They are relatively cheap (18 and 49 CHF per kg) and have high critical temperatures of 152 °C and 196.6 °C at 38.0 and 33.7 bar, respectively. R600 is considered as a suitable medium in HTHPs with liquefaction temperatures up to 120 °C. These temperatures can be achieved in standard compressors. On the other hand, due to the high flammability (A3), special safety measures have to be implemented and HCs are therefore recommended for small systems with small charging volumes. According to EN 378, the maximum capacity of HCs for laboratory equipment is limited to 150 g, for supervised commercial systems with corresponding explosion protection up to 2.5 kg.
In Switzerland, refrigerants are regulated by Annex 2.10 of the Chemical Risk Reduction Ordinance (ORRChem). The Federal Office for the Environment (FOEN) is in charge of the refrigerant control in Switzerland.
The upper temperature limit of subcritical heat pump cycles is determined by the critical temperature of the refrigerant (see tool).
Refrigerant effects on the equipment
The pressure level of the heat pump system determines the material efforts of the equipment. The higher pressure should be kept below a practical value of about 25 bar, although compressors are available for R32 and R410A which can withstand up to 50 bar. The lower pressure should not be below one atmosphere at standstill to avoid foreign gas entry. To minimize compressor power the pressure ratio should be as low as possible. The volumetric heating capacity of a refrigerant is important for the compressor design as it determines its size and cost, and it influences the achievable experimental coefficient of performance (COP).
Refrigerant for high-temperature heat pump
In the past, industrial high-temperature heat pumps mainly used the chlorinated hydrocarbon R114, as it allowed condensating temperatures of up to 130°C. Due to their high ODP, CFCs were replaced by fluorinated hydrocarbons in the 1990s. R245fa, R245ca, and R365mfc offer zero ODP and relatively high critical temperatures of 154.0, 174.4, and 186.9°C at moderate pressures of 36.4, 39.3, and 32.7 bar. However, with regard to the increasing restrictions imposed by the Kyoto Protocol and the F-Gas regulation, R365mfc, R227ea, R245fa, and R134a will be phased down in the foreseeable future due to their high GWP. This drives research and industry towards the evaluation of alternative refrigerants with lower GWP. Hydrofluoroolefin (HFO) refrigerants are considered as environmentally friendly (low GWP) alternatives to replace HFCs.R1234yf and R1234ze(E) are regarded as substitutes for R134a. The critical temperatures and pressures are comparable. The major application areas are automotive air conditioners, refrigerators and freezers.
Natural refrigerant
Natural refrigerants suitable for high temperature applications are R718 (water), R744 (CO2), R717 (ammonia), and hydrocarbons. CO2 heat pumps are common in smaller sizes. They have also been commercialised in the larger range.
In spite of the low critical temperature of 31 °C and high critical pressure of 73.6 bar, CO2 heat pumps also achieve sink temperatures of 90 to 120 °C in transcritical cycles. R744 is feasible as an HTHP fluid if the inlet temperature of the heat sink is not too far above the critical temperature. The high transcritical temperature glide in the gas cooler makes R744 a particularly suitable refrigerant for domestic hot water heating and other processes with very large temperature differences in the sink.
Ammonia is widely used in industrial heat pumps up to about 90 °C heat sink temperature. Beneficial is its high VHC compared to other refrigerants. For higher temperatures, existing compressor technology is limited due to high discharge pressures. With special cast steel construction, NH3 compressors are able to withstand pressures of up to 76 bar and 110 °C. However, certain safety precautions must be implemented due to the toxicity of ammonia (B2L).
The hydrocarbons n-butane (R600) and pentane (R601) are refrigerants without ODP and very low GWP. They are relatively cheap (18 and 49 CHF per kg) and have high critical temperatures of 152 °C and 196.6 °C at 38.0 and 33.7 bar, respectively. R600 is considered as a suitable medium in HTHPs with liquefaction temperatures up to 120 °C. These temperatures can be achieved in standard compressors. On the other hand, due to the high flammability (A3), special safety measures have to be implemented and HCs are therefore recommended for small systems with small charging volumes. According to EN 378, the maximum capacity of HCs for laboratory equipment is limited to 150 g, for supervised commercial systems with corresponding explosion protection up to 2.5 kg.
Swiss regulation
In Switzerland, refrigerants are regulated by Annex 2.10 of the Chemical Risk Reduction Ordinance (ORRChem). The Federal Office for the Environment (FOEN) is in charge of the refrigerant control in Switzerland.
Tool
List of refrigerants with important properties:
Critical temperature: ... °C
Critical pressure: ... bar
Saturation temperature at 1 atm: ... °C
Saturation pressure at 100°C: ... bar
Critical temperature: ... °C
Critical pressure: ... bar
Saturation temperature at 1 atm: ... °C
Saturation pressure at 100°C: ... bar
References
[1]: Arpagaus, C., Bless, F., Uhlmann, M., Schiffmann, J. and Bertsch, S.S., 2018. High temperature heat pumps: Market overview, state of the art, research status, refrigerants, and application potentials. Energy, 152, pp.985-1010.
[2]: Reissner, F., Gromoll, B., Schäfer, J., Danov, V. and Karl, J., 2013. Experimental performance evaluation of new safe and environmentally friendly working fluids for high temperature heat pumps. European Heat Pump Summit: Nuremberg, Germany.
[3]: Spoelstra, S., 2014. Industrial heat pumps. Petten: ECN.
Index
Refrigerant effects on the equipment
Refrigerant for high-temperature heat pump
Natural refrigerant
Swiss regulation
Tool
Reference
Related pages
Experts
DeCarbCH experts on this subject: OST-IES
Other Swiss experts: Swiss Admin Page, ETHZ-epse, HEIG-VD-IE