Renewables

How Does a Heat Pump Work?

At the heart of a modern heat pump is a refrigeration system. Paradoxically, the refrigeration cycle is an efficient provider of heat as well as cooling and the basics of its operation are quite easily understood. Heat pumps use similar technology to that employed in domestic refrigerators or freezers, but in reverse.

There are two principle locations in the transfer of heat; the place where heat is absorbed, (the source), and where it is rejected, (the destination). The compressor in the refrigeration system also produces waste heat, and a significant proportion of this can be recovered.

The mechanical refrigeration cycle consists of an arrangement of heat exchangers; one that absorbs heat, the other that rejects it.

This heat absorbed is transported through a sealed system of pipes by a fluid, the refrigerant, circulated by a compressor. The refrigerant is a fluid that has a low boiling point. A metering device to control the flow of refrigerant completes the arrangement and it is all connected by pipes. As the refrigerant works under pressure, the whole system is sealed for life.

In order to absorb and release the heat into and from the refrigerant, we exploit the ability of the refrigerant fluid to boil from a liquid to a vapour and then to condense back into a liquid. This is a continual process while the compressor is running and circulating the refrigerant.

High pressure liquid refrigerant is fed through the metering device into the evaporator heat exchanger where it evaporates into a vapour by absorption of heat from the heat source (air, water, ground, other) passing through the heat exchanger.

The relatively cool return vapour is drawn back to the compressor. The compressor and the electric motor that drive it are constructed in a fully sealed hermetic shell. The cooled return vapour from the evaporator is passed over the compressor motor windings within the heat pump, thus cooling the windings of the motor.

Much of the energy absorbed by the electric motor driving the compressor is absorbed into the refrigerant.

The combined heat from the source, plus much of the waste energy from the electric motor is then compressed to a high temperature vapour and enters the condenser heat exchanger where it is cooled and condensed into a high pressure liquid ready to begin the cycle again.

The heat released during the process of condensing the refrigerant to a liquid is rejected via the heat exchanger directly into air or transferred to water to heat the building. The air or water temperature at this point could be 43ºC to 60ºC, depending on the design of the system.

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