Method for controlling operation of a vapour compression system in a subcritical and a supercritical mode

A method for controlling operation of a vapour compression system, the vapour compression system comprising a compressor, a heat rejecting heat exchanger, a controllable valve, a receiver, at least one expansion device and at least one evaporator arranged along a refrigerant path having refrigerant flowing therein, the vapour compression system being capable of being operated in a subcritical control regime as well as in a supercritical control regime, the method comprising the steps of:

• measuring a temperature, TGC, of refrigerant leaving the heat rejecting heat exchanger,
• calculating a pressure reference, PGC, Ref, based on the measured temperature, TGC, and using a calculation formula being applicable to the subcritical control regime as well as to the supercritical control regime, and
• controlling an opening degree of the controllable valve and obtaining a pressure of refrigerant leaving the heat rejecting heat exchanger which is equal to the calculated pressure reference, PGC, Ref,
• wherein the step of calculating is performed using the calculation formula:
• wherein Psat represents a saturation pressure in the subcritical control regime, extrapolated into the supercritical control regime, ΔT is a desired subcooling of refrigerant leaving the heat rejecting heat exchanger in the subcritical control regime, T* is a transition temperature indicating a transition between a subcritical control regime and a supercritical control regime, and α and β are constants.