Heat pump system

A method of operating a heat pump system for controlling an internal temperature of a refrigeration unit, the system comprising:

• a compressor, a condenser, an expansion device and an evaporator which are fluidly coupled together by a flow of refrigerant to define a refrigerant circuit, and a thermal energy storage means which is thermally coupled to the refrigerant circuit to exchange thermal energy with the refrigerant, wherein the refrigerant circuit comprises a high-pressure stage and a low-pressure stage which are fluidly coupled together by a phase separator, wherein the high-pressure stage comprises the condenser and the low-pressure stage comprises the evaporator;
• wherein the heat pump system is configured to operate in a cool charging mode and in an auxiliary cooling mode wherein:
• in the cool charging mode, thermal energy is transferred from the thermal energy storage means into the refrigerant and transferred from the refrigerant by the condenser to heat external ambient air, and
• in the auxiliary cooling mode thermal energy is transferred from the evaporator into the refrigerant to cool an internal area of the refrigeration unit and transferred from the refrigerant to the thermal energy storage means;
• wherein the method comprises, when operating the heat pump system in the cool charging mode, isolating the high-pressure stage and, when operating the heat pump system in the auxiliary cooling mode, isolating the low-pressure stage.