Heat pump system

A heat pump system comprising:

• a heat-source-side refrigerant circuit having
• a variable-capacity heat-source-side compressor arranged to compress heat-source-side refrigerant,
• a first usage-side heat exchanger operable as a radiator of the heat-source-side refrigerant, and
• a heat-source-side heat exchanger operable as an evaporator of the heat-source-side refrigerant;
• a usage-side refrigerant circuit having
• a variable-capacity usage-side compressor arranged to compress usage-side refrigerant,
• a refrigerant/water heat exchanger operable as a radiator of the usage-side refrigerant to heat an aqueous medium, and
• the first usage-side heat exchanger operable as an evaporator of the usage-side refrigerant by radiation of the heat-source-side refrigerant, and
• a controller configured to control
• a capacity of the heat-source-side compressor so that a heat-source-side discharge saturation temperature becomes a predetermined target heat-source-side discharge saturation temperature, the heat-source-side discharge saturation temperature corresponding to pressure of the heat-source-side refrigerant in a discharge of the heat-source-side compressor, and
• a capacity of the usage-side compressor so that a usage-side discharge saturation temperature becomes a predetermined target usage-side discharge saturation temperature, the usage-side discharge saturation temperature corresponding to pressure of the usage-side refrigerant in a discharge of the usage-side compressor,
• the controller being further configured to vary the target usage-side discharge saturation temperature according to a predetermined target aqueous medium outlet temperature, the predetermined target aqueous medium outlet temperature being a target value of temperature of the aqueous medium in an outlet of the refrigerant/water heat exchanger.