Transcritical vapor compression cycle device with a variable high side volume element

An apparatus for control of the high side pressure in a vapor compression cycle device operating with super-critical high side pressure, comprising a compressor, a heat exchanger, an expansion means and an evaporator connected in series in a flow circuit, said apparatus comprising at least one variable volume element having a compartment connected to and in free communication with the flow circuit at a location between the compressor and the expansion means, a movable partition means defining at least one side of the compartment, the partition means being displaceable between first and second positions, respectively defining first and second volumes of refrigerant within the compartment, and means external to the flow circuit for displacing the partition means between the first and second positions to thereby change and control refrigerant volume within the compartment.