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Control bifurcation issues of transcritical CO2 heat pump water heater systems.

Author(s) : SIENEL T., HUFF H.

Summary

The transcritical vapour compression cycle introduces an additional degree of freedom in control over subcritical cycles in the uncoupling of the high side pressure from the temperature on the high side. Pressure and temperature are normally coupled through the saturation properties in subcritical systems, but above the critical point this relationship no longer exists, requiring control over the high side pressure to be managed through the amount of charge in the high side of the system. It has been found that for some control schemes there is the possibility of two stable operating conditions existing, one efficient and the other not. One example of this behaviour is found in a heat pump water heating system where the high pressure and water discharge temperature are both regulated, one with a controllable expansion valve and the other with a variable speed pump. When the high pressure is regulated at the compressor exit, it is possible to satisfy both the pressure setpoint and the water temperature setpoint at two different stable conditions. One condition is with a relatively high water flowrate and a relatively low suction pressure, which gives high capacity and efficiency. The other can occur when the water flowrate decreases, leading to an increase in the exit enthalpy of the gas cooler and causing the suction pressure to rise which increases the mass flow rate of the compressor. The ultimate effect is an extremely inefficient cycle. Control strategies for preventing this cycle have therefore been developed.

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Pages: 2006-2

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Details

  • Original title: Control bifurcation issues of transcritical CO2 heat pump water heater systems.
  • Record ID : 2007-1734
  • Languages: English
  • Source: 7th IIR-Gustav Lorentzen Conference on Natural Working Fluids (GL2006). Proceedings
  • Publication date: 2006/05/29

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