How a heat pump works
Converting heat from the environment for heating a building
The schematic below illustrates the three circuits to consider in a heat pump installation.
The heat source is the blue circuit on the left of the diagram. The heat source can be air, water or the ground. Most of the heat absorbed by a heat pump is solar energy rather than geothermal from the centre of the earth. The heating distribution circuit is the red circuit on the right and represents the hot water flowing around the heating system in the property. The refrigerant circuit is the green circuit in the middle. This links the external heat source with the heat distribution system and is contained within the heat pump.
The three circuits are connected within the heat pump by two heat exchangers. The energy equation on the diagram is important in understanding the reasons for a heat pump's efficiency. Typically for every unit of electricity input, three to four units of heat are output by the heat pump.
In a typical ground source heat pump installation fluid circulates around the ground in plastic pipes at a lower temperature than the ground, say at 5oC. This creates a heat gradient and draws heat from the ground into the fluid in the pipe. The fluid passes through an evaporator (heat exchanger) in the heat pump and the heat extracted from the ground is absorbed by the refrigerant which circulates around the heat pump. The refrigerant becomes a gas and is compressed in a compressor which causes further heat. The pressurized refrigerant then passes over another heat exchanger called a condenser where the heat is transferred to the building’s underfloor or radiator pipe work. Having given up its heat, the refrigerant passes through an expansion valve where its pressure and temperature further reduces returning it to a liquid state to start the cycle again. The main power consumer in a heat pump is the compressor, but the majority of the energy comes from the environment.
