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Heat pumps are often the cheapest way to heat a property.  They must be carefully sized to the meet the needs of your building and you will get the most benefit from them if your property is well insulated.

The way heat pumps work can be described using water as an analogy.  Water flows downwards from a high point to a low point, but can be made to go the other way using a pump.  In the same way, heat will flow from a hot body to a cold body, but can be driven in the other direction by a heat pump.  Like a water pump, a heat pump needs some electrical energy to do its work.  Much more energy is needed to pump the heat when the temperature difference between the hot and cold bodies is very large.  (In the same way, pumping water up a very high hill is more work!)

Ground source heat pumps actually pump the heat from the ground (a cold body) into the building or warm space.  The temperature a few metres below the ground stays pretty much constant throughout the year which gives these systems a great advantage as the heat never has to be pumped over a very great temperature difference, so the heat pump does not have to work so hard and becomes more efficient.

Heat pump efficiency is measured by the Coefficient of Performance, or “CoP”.  The CoP refers to the amount of heat energy pumped to the hot space by one unit of electrical energy.  The reason heat pumps give a large energy benefit is that one unit of electricity can put out several units of heat, giving a high CoP.  (The higher the CoP, the more efficient the pump.)  The average CoP over the whole year (sometimes called the Seasonal Performance Factor) is the key figure to look at when comparing heat pumps.

A good CoP for a ground source heat pump is between 4 and 5, meaning the pump produces 4 to 5 times as much heat energy as the electrical energy that it uses.
 
How does this actually work?
Most heat pumps work on a “vapour compression cycle”, which is very similar to a fridge being run in reverse.  The refrigerant inside the pipes goes through a 4-stage loop, passing through an evaporator, a compressor, a condenser and a throttle.  The key point to grasp in order to understand how a heat pump works is that a liquid at high pressure will evaporate at a higher temperature than normal.

The 4 stages of the loop are as follows:

Stage 1 - cold, liquid refrigerant is passed through the evaporator, which is made up of long underground loops or boreholes.  As the refrigerant passes through these pipes, it absorbs enough energy from the ground to evaporate which means that it turns into a gas.

Stage 2 - A compressor is then used to force this gas into a smaller space at high pressure, heating it up.  This is the part of the process that uses the electrical energy.

Stage 3 - The gas then passes into the condenser where it changes back into a liquid, giving back all the energy it absorbed from the condenser and the compressor.  But, because it is at high pressure, the gas gives back its energy at a much higher temperature.  This energy is collected by the heat pump and used to heat the air or water in your house.

Stage 4 - Finally, the high-pressure liquid is passed through a little nozzle (or throttle) to allow its pressure to drop back to where it started.  This also cools the liquid, making it colder than the outside air and ready to start absorbing heat again in the evaporator.

Types of ground source heat pump
Ground source requires extensive pipe work to absorb the earth’s energy. There are two ways to efficiently absorb this heat:

Open loop – These use the energy within the water stored under ground. One pipe sucks water into a heat exchanger, this puts the heat into the refrigerant. The water, which is cold, flows back down another pipe to be reheated by the earth. This system is highly efficient but always requires deep vertical boreholes and a flow of water.

Closed loop – This type of system circulates refrigerant fluid through pipes in your garden. The fluid absorbs the heat directly from the earth. There are two main types of closed loop system:

• Horizontal loops – these pipes run underground in shallow trenches, but require quite a large surface area (depending on the size of the heat pump). They can also be placed in ponds or lakes. They are cheaper to install than boreholes because no deep drilling is required.
• Vertical boreholes – these are deep holes drilled into the ground. They are usually between 50-90 metres deep, but can go as far down as 150 metres. Vertical boreholes are used where there is not enough space to lay horizontal loops. They are considerably more expensive than horizontal loops, but they are also slightly more efficient at collecting the underground heat.

What could possibly go wrong?
Ground source heat pumps can go wrong when they are forced to work too hard.  If a mistake is made at the specification stage and not enough pipework is dug into the ground, then it is possible to extract heat from the ground faster than it can be replaced.  This means that over a period of time the ground around the pipes will freeze and the efficiency of the heat exchanger will drop severely.  This could happen very slowly, taking up to two years before the heat pump performance is affected. As a rough guide, in the UK, around 10m of trench are required per kilowatt-hour of heat output.  However, it is very important to take into account the soil type and depth of the water table, to design a system accurately and not to skimp on the digging work.