Operation above critical pressure
The COP, means the efficiency, of vapour compression systems decreases when the condensing temperature moves closer to the critical temperature, at constant subcooling temperature difference, as the available enthalpy difference between dew and bubble line decreases. In this way, the refrigeration capacity at constant mass flow would decrease.
Above the critical pressure, there is no two phase area and thus no condensation. Here the pressure is no longer linked to the temperature, but has to be controlled additionally.
To reach similar efficiency there, further system design or control measures are necessary.