Heat transfer system and method for operating a heat transfer system

Heat transfer system, comprising

• a) a refrigeration circuit comprising a first heat transfer fluid, a compressor, a condenser, and an outdoor heat exchanger;
• b) a cooling circuit comprising a second heat transfer fluid, which comprises or consists of a phase change material, a pump for circulating the second heat transfer fluid around the cooling circuit, a heat exchanger for transferring heat from the second heat transfer fluid to the first heat transfer fluid, and at least one indoor heat exchanger for transferring heat from a space to be cooled to the second heat transfer fluid;
• c) in the cooling circuit, a sensor system comprising or consisting of a temperature sensor and an electrical resistance sensor, wherein the temperature sensor and the electrical resistance sensor are realized as two separate sensors or as one combined temperature-and-electrical-resistance sensor;
• d) a controller configured to obtain temperature information and electrical resistance information from the sensor system;
• wherein the controller is configured to
• determine a combined state of phase value (SOPflow) based on the obtained temperature information and obtained electrical resistance information,
• control an operation of the heat transfer system based on i) the determined combined state of phase value (SOPflow) by controlling a speed of the compressor or controlling the speed of the compressor in dependence of a highest cooling load of an at least one indoor heat exchanger in the system;
• or
• ii) a determined state of phase difference (ΔSOP) across the at least one indoor heat exchanger by controlling a rotation speed of a fan of the at least one indoor heat exchanger, or by controlling a flow rate of the heat transfer fluid through the at least one indoor heat exchanger, or by controlling the flow rate of the heat transfer fluid by controlling a pump rate of the pump, or by controlling an opening degree of a valve in fluid connection to said at least one indoor heat exchanger.