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The CO2 Heat Pump


Carbon dioxide is a natural refrigerant and is denoted by R744. Some of the most important things about this refrigerant are given below:

Working cycle of R744

The figure shows different components of heat pump with R744

  • Refrigerant CO2 leaves the evaporator in Liquid/Vapour form after having its temperature raised through a heat exchanger with the outside air. The vapour then enters the compressor.
  • The liquid becomes a high pressure/high temperature vapour leaving the compressor and entering the gas cooler. The compressor requires an electrical input to increase the pressure of the CO2.
  • Heat transfer with the heat sink across the gas cooler. This lowers the temperature and the pressure. It is still above the critical line.
  • The working fluid is put through an expansion valve lowering the pressure of the liquid/Vapour. The CO2 is now at a lower temperature than the heat source and passes through another heat exchanger extracting heat from the outside air.

CO2 heat pump cycle

This transcritical cycle that makes CO2 unique, gives the refrigerant characteristics not seen in other refrigeration cycles. There is no sharp phase change on the heated side of the cycle where the condenser would normally exist in traditional heat pump cycles. The lack of phase change means that instead of a condenser in the CO2 cycle there is a "gas cooler". The working fluid is cooled through this gas cooler but doesn't drop below the critical point on the curve until after the expansion valve.


Temperature - enthalpy diagram

  • Point 1 - 2: Compressor
  • Point 2 - 3: Gas cooler
  • Point 3 - 4: Expansion valve
  • Point 4 - 1: Evaporator

The temperature drop through the gas cooler is known as the "temperature glide"!



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