The efficiency of a heat pump is dictated by multiple different factors, the most significant being temperature. A higher source temperature, whether extracted from air or the ground, means a heat pump has to do less ‘work’ to produce the temperatures required to heat a home, and therefore less electricity is required, resulting in higher efficiencies.
The two most commonly used heat pumps in the UK are air source heat pumps (usually favoured from an initial set-up cost perspective) and ground source heat pumps (usually more efficient but more expensive to set up initially). It is these two that we will concern ourselves with for the purposes of looking at their performance and efficiency.
Heat Pump efficiency compared to heating alternatives
As an alternative source of heat for the home, heat pumps are highly energy efficient when compared with the fossil fuel burning options like electricity or gas. Ground and air source heat pump efficiencies can at times be greater than 300% since they transfer heat rather than actually generate it.
However, it can be difficult to maintain these efficiency levels, especially when external factors like temperature can have a bearing on performance. So, how is heat pump efficiency measured?
Heat pump efficiency is measured by what is known as a Coefficient of Performance (CoP). This is a measure of how many units of heat can be produced for every unit of electricity used, under the best possible conditions.
|Source||Coefficient of Performance (CoP)|
|Air source heat pump||4|
|Ground source heat pump||5|
|New gas boiler||0.9|
Of course, the best possible conditions are not consistent throughout the year, so these efficiency levels can fall below the amounts indicated. However, as both air source and ground source heat pumps absorb heat from renewable sources, and compress it into higher temperatures to be used in the home, they are an excellent alternative heating source for many homes.
Air Source Heat Pumps and efficiency
Since air source heat pumps use heat energy extracted from the air, efficiency varies across the seasons and time of day. Air source heat pump efficiency is most affected during the winter months when the outside temperature drops and heat pump efficiency can decrease sometimes to around 1.5 CoP, as they require more electricity to maintain normal room temperatures. They are having to work harder and demand electricity at times when prices are often at their highest
Where air source heat pumps have an advantage is the set-up cost which is far less than other heat pumps as you only have to pay for the air source heat pump unit itself and installation does not require as significant installation work as ground source heat pumps.
Ground source heat pumps and efficiency
The efficiency of ground source heat pumps is not affected by the seasonal change to the same degree as air source heat pumps and they can consistently deliver between 3 to 4kW of heat for every 1 kW of electricity consumed. The temperature underground is largely consistent throughout the year (at around 10-12°C) although the quality of the ground can impact the ability to conduct heat. For example, if the area where the ground source heat pump is located in heavy clay this can reduce efficiency.
The consistency of ground temperatures means that on the coldest days, the source temperatures can be up to 15°C warmer than the cold air coming into an air source heat pump. This results in a significant difference in the quantities of electricity required to maintain temperatures in the home, plus a ground source heat pump unit have the advantage of being able to be run overnight when electricity prices are lowest.
Where ground source units are at a disadvantage is the cost. Because they require additional groundwork and elements need to be buried at sufficient depth to insulate them from temperature fluctuations, this means the initial cost is usually far greater than air source units.
Choosing a heat pump
Both air source and ground source heat pumps are great renewable alternatives to fossil fuel burning heating options in the home. They do differ in terms of efficiency and running costs, as well as the initial set-up cost. While ground source heating systems achieve higher efficiency than other heating systems, they are not suitable for all locations and the set-up cost is usually higher.
Those considering heat pumps need to make a decision regarding their choice of system. A cheaper air source heat pump may be a more suitable option in the short term, but for those who are not planning on moving from their home for the foreseeable future, investing in a ground source air pump, and possibly improving the insulation in your home to reduce the heat load, could provide greater efficiencies in the long term.
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