There are many descriptions of how a heat pump works, at times it can seem confusing. It is actually a simple and fantastic technology that produces more energy for your home than it uses.
In essence, a heat pump is like a fridge working in reverse. It absorbs naturally-occurring heat from the air, earth or water outside to heat your home and water. Since it transfers heat rather than generates it, heat pumps are one of the most efficient means of warming your house.
The science behind heat pumps
Heat naturally moves from a hotter area to a colder one. A heat pump works by reversing this process, using a small amount of electricity to do so. It does this by a cycle of compression and evaporation.
There are four stages to the evaporation and compression cycle of a heat pump, through which a refrigerant is circulated. This refrigerant acts as a medium, which transfers the heat from one stage to another.
Heat is brought into a heat exchanger – known as the evaporator – from outside. Depending on the type of heat pump, this low-grade heat can come from any number of sources. For instance, ground-source heat pumps absorb heat from the earth, air-source from the air, and water-source from a nearby lake or pond.
This heat causes the refrigerant to evaporate. Within the evaporator, the low-pressure, low-temperature refrigerant can absorb heat even in very cold conditions (down to -20oC).
The evaporated refrigerant is compressed, which drives the temperature up. In more technical terms, the low-grade heat is upgraded into a useably high temperature.
The refrigerant gas transfers heat into the central heating system. This causes the refrigerant to condense back into a liquid. This happens in the condenser, a second heat exchanger, where the cooler water from the central heating system absorbs the heat. This heat is then either circulated around an emitter system (radiators or underfloor heating) or it is used to heat water.
4: Expansion valve
The cooled refrigerant passes through the expansion valve, which decreases the pressure. This further decreases the temperature, before the refrigerant goes back to the evaporator so the cycle can start again.
The working principle here utilises the concept of ‘latent heat of condensation and vapourisation’. In short, by altering the pressure, we can control the boiling point of the liquid medium, and subsequently control the movement of heat.
Types of heat pump
There are two main types of heat pump: air-source ground-source. Whilst they both use the same method of heat transfer as mentioned above, the way they bring low-grade heat into the evaporation-compression cycle is slightly different.
Air Source Heat Pumps
An air source heat pump is the most popular type of system. Sitting outside your home, the heat pump looks quite similar to a standard air conditioner. In fact, the method of heat transfer for air conditioners is very similar to the one above, only in reverse.
Heat from the sun warms the air around us, which is then drawn into the heat pump unit by the fan. This heat is extracted from the air into the heat exchanger coil – the evaporator – and so the cycle begins. The fan is there to keep a constant flow of warm air coming into contact with the heat exchanger.
Ground Source Heat Pumps
A ground source heat pump uses the solar energy stored in the earth as the source of heat for the evaporator. It collects the heat through pipes laid underground known as a ground loop or ground array. A mixture of water and a special type of anti-freeze is then pumped through this network of pipes underground, absorbing the naturally-occurring heat below the frost line. The antifreeze-water mix then delivers the heat to the evaporator within the heat pump, and the same evaporation-compression process begins, as shown in the diagram below.
The most popular setup for a ground source heat pump is to lay the heat-absorbing pipes horizontally. You’ll need a large surface area outside to lay the pipes in this way, or if you don’t have that much space you could opt for a vertical loop system, which involves drilling deep boreholes underground.
You can find out about the differences between air source and ground source heat pumps here.
Geothermal heat pump or ground-source heat pump?
Although used interchangeably with ground-source heat pumps, the American term geothermal heat pump can be misleading. Ground-source heat pumps don’t use geothermal heat i.e. heat from the earth’s core. The heat used by a ground source heat pump actually comes from the sun, which is absorbed and stored in the earth.
Examples of geothermal heating can be found in areas like Iceland, where heat from the earth’s core rises up between the European and North American continental plates.
Heat pumps are an extremely energy-efficient way of heating your home, as they require a small amount of electricity to operate. Although they carry a higher upfront cost than other types of heating system, they come with a large number of benefits that more than make up for it. To name a few, they’re extremely efficient, they give off no harmful emissions locally, and by investing in a heat pump, you could recoup a large portion – if not all – of the money back from the Renewable Heat Incentive (subject to performance).