Ground source heat pumps extract this energy, to use for space heating.So strictly speaking, the energy source that is used for ground source heat pumps is the sun and not the heat from the earth.

In the UK, the constant ground source heat temperature is around 12ºC and can be utilised to heat and cool buildings. Pipe is buried underground in a trench or in a vertical well and water is pumped through it and heated by the underground energy. A heat pump can extract the ground’s heat and transfer it into a building for space and water heating, or it can remove heat from a building and deposit it into the ground to cool the building during hot weather.

Heat Pumps do need some power to operate. However for 1 unit of energy they use they generate 4 units of power. The energy generated by a ground source heat pump can be considered 100% renewable if the power to operate the pump comes from solar electric panels or a wind turbine for example.

What are Ground Source Heat Pumps?

Ground source heat pumps provide a new and clean way of heating buildings in the UK. They make use of renewable energy stored in the ground, providing one of the most energy-efficient ways of heating buildings. They are suitable for a wide variety of building types and are particularly appropriate for low environmental impact projects.

They do not require hot rocks (geothermal energy) and can be installed in most of the UK, using a borehole or shallow trenches or, less commonly, by extracting heat from a pond or lake. Heat collecting pipes in a closed loop, containing water (with a little antifreeze) are used to extract this stored energy, which can then be used to provide space heating and domestic hot water. In some applications, the pump can be reversed in summer to provide an element of cooling, but these systems are not currently eligible for UK grants.

The only energy used by Ground Source Heat Pump systems is electricity to power the pumps. Typically, a Ground Source Heat Pump will deliver three or four times as much thermal energy (heat) as is used in electrical energy to drive the system. For a particularly environmental solution, green electricity can be purchased.

Ground Source Heat Pump systems have been widely used in other parts of the world, including North America and Europe, for many years. Typically they cost more to install than conventional systems; however, they have very low maintenance costs and can be expected to provide reliable and environmentally friendly heating for in excess of 20 years.

Ground Source Heat Pumps work best with heating systems which are optimised to run at a lower water temperature than is commonly used in UK boiler and radiator systems. As such, they make an ideal partner for under floor heating systems.

Ground source heat pumps use a buried ground loop which transfers heat from the ground into a building to provide space heating and, in some cases, to pre-heat domestic hot water. As well as ground source heat pumps, air source and water source heat pumps are also available.

The benefits

The system does not require any external fuel and is designed to heat a whole building.

The efficiency of a ground source heat pump system is measured by the coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used to drive the compressor and pump for the ground loop. Average CoP is around 3-4 although some systems may produce a greater rate of efficiency. This means that for every unit of electricity used to pump the heat, 3-4 units of heat are produced, making it an efficient way of heating a building. If grid electricity is used for the compressor and pump, then you should consult a range of energy suppliers to benefit from the lowest running costs, for example by choosing an economy 10 or economy 7 tariff.

Ground source heat pumps - How it works

There are three important elements to a ground source heat pump:

1. The ground loop

This is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped around the pipe absorbing heat from the ground. The ground loop can be:

• Vertical, for use in boreholes
• Horizontal, for use in trenches
• Spiral, coil or 'slinky', also for use in trenches

2. A heat pump

In the same way that your fridge uses refrigerant to extract heat from the inside, keeping your food cool, a ground source heat pump extracts heat from the ground, and uses it to heat your home. A ground source heat pump has three main parts:

• The evaporator, (e.g. the squiggly thing in the cold part of your fridge) absorbs the heat using the liquid in the ground loop;
• The compressor, (this is what makes the noise in a fridge) moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit;
• The condenser, (the hot part at the back of your fridge) gives up heat to a hot water tank which feeds the distribution system.

3. Heat distribution system

This consists of under floor heating or radiators for space heating and in some cases water storage for hot water supply.

Is it suitable for my home?

You should consider the following issues if you're thinking about installing a ground source heat pump.

• You will need space outside your house for the ground loop.
• The ground will need to be suitable for digging a trench or borehole.
• What fuel is being replaced? If it's electricity, oil, LPG or coal the payback will be more favourable than gas. Heat pumps are a good option where gas is unavailable.
• The type of heat distribution system. Ground source heat pumps can be combined with radiators but these will normally be larger than with standard boiler systems. Under floor heating is better as it works at a lower temperature.
• Want to be 100% renewable? Buy green electricity, or install solar PV or some other form of renewable electricity generating system to power the compressor and pump.
• Is the system for a new building development? Combining the installation with other building works can reduce costs.
• Have you installed insulation measures? Wall, floor and loft insulation will lower your heat demand and make the system more effective.