The refrigerant’s temperature rises due to the compression process and the electrical energy input to the compressor.

This added energy heats the refrigerant as it circulates.

Instead of allowing the hot gaseous refrigerant to enter the condenser for conventional cooling, it’s redirected back into the evaporator via a discharge bypass valve (DBV or HGBV), allowing the system to provide heat.

The Reverse-Carnot Cycle, fundamental to refrigeration and heating processes, involves four key stages: compression, condensation, expansion, and evaporation. In the context of providing heat:

Compression – The refrigerant is compressed, increasing its pressure and temperature.

Condensation – As the compressed refrigerant passes through the condenser, it releases heat and changes from a gas to a liquid.

Expansion – The high-pressure liquid refrigerant then expands through an expansion valve, reducing its pressure and temperature, but in the heating mode, this process is adjusted to focus on utilising the heat generated in earlier stages.

Evaporation – Finally the refrigerant absorbs heat in the evaporator, completing the cycle.

In heating applications, the cycle is manipulated, particularly at the hot gas bypass stage, to harness the heat generated during compression and condensation, rather than focusing on the cooling effect produced during expansion and evaporation. This allows the chiller to act as a source of heat, utilising the intrinsic energy conversion processes of the refrigeration cycle.

The compressor generates heat both from the isentropic compression of the refrigerant and the inefficiency of the electric motor, contributing to the system’s ability to provide heat.

The HGBV operates based on suction pressure to ensure that the compressor does not pull a vacuum on the low side, which could lead to motor failure. It controls when hot gas is allowed to bleed through, contributing to heating.

In heating mode, the system’s hot gas bypass valve allows hot high-pressure vapour to be drawn through the evaporator from the system’s high side when cooling is not required, effectively converting the system to provide heat.

To accelerate heating, a heater can be installed in a flow-through water circuit. This method reduces heating time compared to relying solely on the heat of compression.