How a Heat Pump Works

A heat pump transfers (pumps) heat from a cold place to a warmer place. Although to do this, it consumes energy; it is a much more efficient process than converting that energy to heat directly. Heat pumps can be used for space cooling as in an air conditioning system, and space heating. There are various kinds of heat pumps which are usually defined by their heat source. For instance, air heat pumps extract heat from the air; water heat pumps extract heat from water, and ground heat pumps extract heat from the ground.How a heat pump works

As heat pumps are highly efficient compared with conventional space heating solutions such as gas-fired boilers and electric heaters, they provide a green solution for both heating and cooling. Firstly, we will look at the fundamental physics of heat pumps and explain why they work; then we will look at how this is put into practice. Finally, we will look at the economics and why they can make an attractive investment.

The thermodynamics of heat pumps

As we all know(?!), the second law of thermodynamics states that heat always flows from a hot body to a colder body and never the other way round, so how does this tally with heat pumps which appear to achieve the opposite?
To make heat pumps work, you need to input work (we will call this W), usually in the form of electrical power. The electric power runs the “machine” that transfers the energy. We give an example of such a machine in the next section.

• Qc is the thermal energy taken from the source, for instance, air or the ground
• Qh is the thermal energy the “machine” transfers to the dwelling
• W is the work we need to input to achieve this
Putting these together, the heat pump equation is:
Qh = Qc + W

So, why don’t we use W to heat our homes directly? The reason is efficiency. If we had a truly 100% efficient electric heater, then 1 kWh of electricity would supply your home with 1 kWh of energy. In practice, it would be significantly less than this. But with a heat pump, 1 kWh of electricity could transfer, say 4 kWh from the colder environment outside to the warmer environment inside your home.

The ratio of Qh/W is a measure of the heat pump efficiency., which we call the coefficient of performance and which depends on many factors including the heat pump design, the temperature of the heat source, and the output temperature. Typically, air-source heat pumps have COPs from 3.2 to 4.5, and ground source heat pumps have COPs from 4.2 to 5.2.
To recap, that means for just one unit of electrical power you can achieve from 3.2 to 5.2 units of heating or cooling.

A practical example of a heat pump

There are many kinds of heat pump “machines”. Here we will look at a simple air source heat pump. The principles are the same as for a standard refrigerator. When a refrigerant liquid vaporises, it cools down. When it condenses to a liquid under pressure, it heats up. By continually repeating the process, we can pump heat from one location to another. For instance:
1. A compressor circulates refrigerant around a closed circuit containing an expansion valve, an evaporator, and a condenser.
2. The evaporator extracts heat from the external air as the refrigerant liquid vaporises. Air is usually blown across the evaporator coils by a fan.
3. This vapour is compressed by the compressor and passes through a condenser. The condenser emits heat to the internal space as the gas is converted back to a liquid at high pressure.
4. The liquid passes through an expansion valve into the evaporator where it evaporates to form a cold vapour which extracts heat from the air.
Ground sourced heat pumps work similarly except the heat is sourced from the ground. Real heat pumps are far more sophisticated than this simple description, but they all use similar principles.

Economics of heat pumps

In a residential environment compared with an electric heater or gas boiler, a heat pump is an expensive investment. However, the running costs are very much lower. While air-source heat pumps are cheaper than ground source, expect to pay upwards of £11,000 for a residential including installation and up to £20,000 for a top-range ground source heat pump. However, heat pumps usually qualify for payments from the Renewable Heat Incentive (RHI), so you will get some of that back.

You will also reduce your energy consumption by about two thirds, so if your heating bills are £2,100 a year, you will save around £1,400 a year. Typically, you could recover your initial costs over ten years, and although you will need to spend some money on maintenance, there is no reason your installation shouldn’t provide 20 years of service. Specific payback calculations should always be undertaken and the figures in this article are for background information only.