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How do you assess the efficiency of a ground source heat pump?

Posted by Alex Barrett on 13 June 2017 at 10:57 am

Ground Source Heat Pumps (GSHP) extract heat from the ground, and use it to produce hot water. The temperature in the subsurface remains around 11-12oC throughout the year [1]. By burying pipes just a few metres below the ground you can take advantage of this readily available supply of thermal energy, which is constantly replenished by the light of the sun.

Ground source heat pumps consist of a series of pipes, called ground loops which are buried in the garden. A mixture of water and antifreeze is pumped through these loops and collects heat from the ground.

The store of energy which a pump taps into ultimately comes from the Sun. Sunlight contains a lot of thermal energy which warms up everything it shines on. Small objects will lose this heat fairly quickly, but the ground has a large thermal mass. This means that it retains a lot of heat, and remains fairly warm, even in colder months.

The efficiency of a heat pump is described by two numbers

Coefficient of Performance (CoP)

The Coefficient of Performance (CoP) is the amount of thermal energy you get back for every unit of electricity you put in [2]. To calculate this you divide the heat output by the electricity input. For example if you get three kilowatts of heat for every kilowatt of electricity you put in then you have a CoP of three.

The CoP of a ground source heat pump is usually in the range of two to four. This means that for every unit of electricity you use to run the system you are getting back anywhere between twice as much and four times as much heat [3].

Seasonal Performance Factor (SPF)

The second metric to consider is the Seasonal Performance Factor (SPF). This is somewhat more complicated.

Although the ground temperature is much more stable than that of the air it is not completely constant. This means that heat pumps will not always operate at their maximum efficiency.

During the winter the heat loss from the ground is higher. The thermal store will not be replenished as quickly and there will be less energy available for the heat pump to extract. It is possible for a badly designed heat pump to draw too much energy out of the soil, and actually freeze the ground around its pipes. All of these factors mean that heat pumps will be more effective at some times of year than others.

The result is that the CoP of a heat pump changes over the course of the year. The SPF takes this into account and produces an average efficiency value. Energy efficient systems have a high SPF, while less effective systems will have a lower value. This is worked out when the system is installed, and will depend on how the system is designed.

The SPF also takes into account the fact that electricity is used to extract heat from the ground. This is particularly important when applying for payments under the Renewable Heat Incentive (RHI). The electricity used to run the pump is deducted from the SPF total, so that only the renewable fraction is considered. A heat pump must have a minimum SPF of 2.5 to qualify for the RHI [4].

Considerations

A heat pump has to operate for long periods of time, and uses electricity to run. This means that when you install a heat pump your electricity bill will probably go up. Unless you are replacing electric heating it is likely that the savings that result from the pump will be in your gas bill rather than the electric one. Hopefully the lower cost in gas will more than cancel out the increased cost in electricity.

There are a number of advantages to installing this sort of system. It is more efficient to move heat around than to make it, whether by burning fuel or using electricity. Consequently heat pumps are very efficient. Unlike burning fuel, heat pumps don’t produce greenhouse gasses. CO2 has to be emitted in order to build the system, and the electricity that powers it could come from fossil fuels, but the pump itself does not cause any emissions.

Ground source heat pumps are expensive to install, so the upfront costs can be high. However they tend not to need much maintenance since they are buried underground and so not exposed to the elements. The pump itself may wear out, but the pipework is unlikely to need to be changed.

They are a good way to cut down on your overall energy use, and heat your house without using fossil fuels.

References

  1. Centre for Sustainable Energy
  2. ICAX Interseasonal heat transfer
  3. Which? Magazine
  4. Ofgem

More information about Heat Pumps on YouGen.

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