How a heat pump works;

Heat naturally flows from a high temperature to a low temperature i.e. in a house in winter with a small amount of energy (electricity or waste heat) A heat pump can in effect cause the heat to flow in the opposite direction. This is done by extracting the heat from low temperature sources such as the ground or water and by reversing the cycle transfer into the heating circuit of a building. This is to some respect a reverse operation of a refrigeration unit.

The below diagram shows the basic refrigeration process of a heat pump (source: EU Heat Pump Installer Training Course 'Grimsby Institute, 12-16 December 2005 page 69).

1. The refrigerant flows around the circuit and is forced to evaporate at a low pressure / low temperature and condense at a high pressure / high temperature. A typical refrigerant for heat pumps is R407 C, with a boiling temperature of -43-9o C.

2. The evaporator heat exchanger is the “heat absorber” of the heat pump. Inside the evaporator, cold, low-pressure refrigerant absorbs heat from the heat source (or heat transfer fluid such as brine) and boils. During the evaporation process the temperature and pressure remain constant. The heat source can be ground, groundwater, air etc. in the case of indirect evaporation, as shown above. The brine (most often a mixture of water and anti-freeze) circulates through the heat source and transfers heat to the evaporator. For ground source heat pumps, plastic pipes are placed horizontally in the ground or vertically sunk in a borehole. For outdoor air or ground water sources the brine circuit is omitted and the air or ground water drawn directly through the evaporator.

3. Compressor. The gas that forms in the evaporator is sucked into the compressor and then compressed resulting in and increase of pressure and temperature. In most cases the compressor is electrically driven. Most of the heat pumps for domestic applications combine the motor and compressor inside a single pressure casing. This is called a “hermetic” unit.

4. Condenser. The hot compressed gas from the compressor is fed into the condenser heat exchange where heat is transferred to the heating medium (usually air or water). As the gas loses heat it condenses back to liquid. The pressure and temperature remain constant.

5. Expansion valve. The expansion valve works as a throttle valve adjusting the refrigerant flows from the condenser to the evaporator. A part of the working fluid is turned from liquid to gas. In order to form gas, the heat is taken from the remaining liquid. This results in a sharp drop in temperature and pressure of both the liquid and the gas.

Other devices found on heat pumps are safety devices such as the high and low pressure switches and sight glasses.

Some heat pumps can reverse the heating cycle and can be used for both heating and cooling applications.