Heat Pumps - How Do they work?

The following is a basic explanation of how a heat pump works.

Overview

Heat pumps move stored solar energy from the ground into the home to provide a total solution for Domestic Hot Water (DHW) and all heating needs.

Because heat pumps simply move energy, rather than creating it by burning fossil fuels, they are very efficient. For every 4KW of heat moved into the house only 1KW of electricity is consumed. This gives a perceived efficiency of 400% sometimes stated as a Co-efficient Of Performance (COP) of 4:1.

Collecting the Heat
Heat collection is achieved by installing a series of pipes in the ground. They are buried a metre deep and contain a water glycol mix at a low temperature. The surrounding soil is at a higher temperature, typically 10-13°C and gently warms the glycol mix as it is pumped around the ground loop.

A temperature increase of the ground loop fluid of just 3 or 4°C; is all the heat pump requires. The returning warmed liquid is fed into a heat exchanger/Evaporator.

The Evaporator
The purpose of the Evaporator is to take the collected heat out of the ground loop liquid and return it cooled to the pipe for the next cycle. It does this by using a refrigerant that boils at approximately -10°C, the act of boiling turns the refrigerant into a vapour which is then moved into the Compressor.

The Compressor
The Compressor does exactly what its name suggests; the vapour is compressed in volume and as its volume reduces its temperature increases. Temperatures of between 75 and 125°C are achievable and the gas is then fed through a heat exchanger.

Condenser/heat exchanger
Feeding the hot gas through a condenser allows the refrigerant to turn back into a liquid. As it condenses it cools and releases its heat through a heat exchanger into the DHW and central heating system.

The Expansion Valve
To complete the closed circuit of the heat pump the only thing which needs to be done is reduce the pressure of the condensed liquid and this is done via an Expansion Valve.

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