By using an electrically-driven or gas-powered heat pump, ground-source heat pumps (GSHPs) can take low-level heat which occurs naturally underground and convert it to high-grade heat.
This heat can be used to provide space heating for a building. GSHPs can also be driven in reverse to provide comfort cooling.
The heat is collected either through a series of underground pipes laid about 1.5m deep, or from a borehole system. In both of these options, water is re-circulated in a closed loop and delivered to the heat pump, which is usually located inside the building.
Heat pumps cover a wide range of capacities, from a few kW to many hundreds of kW machines that heat or cool large, multi-storey buildings.
How do they work?
The ground stores the heat from the sun at a temperature of around 8-12ºC at about 1m below ground level. This heat store can now be tapped thanks to ground source heat pumps to heat buildings and even to provide hot water. The technology used is the same as that used in refrigerators. Just as a fridge extracts heat from the food and pumps it into the kitchen so a ground source heat pump extracts heat from the earth and pumps it into a building.
How efficient are Ground Source Heat Pumps?
For every unit of electricity used to power the heat pump system, approximately 3-4 units of heat are captured and distributed. In effect this means a Ground Source Heat Pump is 300-400% efficient in terms of its use of electricity. At this efficiency level there will be less carbon dioxide emissions than for a gas boiler heating system. In many cases it may also be possible to provide the required electricity by means of renewable energy, thus virtually doing away with any use of fossil fuels and reducing carbon emissions to zero.
What do they look like?
A Ground Source Heat Pump system comprises three basic elements - a ground loop, the heat pump itself, and a heat distribution system. The ground loop is a pipe buried underground in either a horizontal trench or a vertical borehole. Horizontal trenches are dug 1.5 - 2 metres below ground level and, although using more land than a borehole, are usually cheaper for smaller systems.
Site suitability
The installation of GSHPs requires a large amount of civil engineering works, such as sinking bore holes (50m+) or digging 1-2m deep trenches to house the collector pipe. The feasibility of doing this will depend on the geological conditions at the site. Connecting a GSHP into an existing heating system is often constrained by the requirement of the existing system to operate at temperatures higher than that delivered by the GSHP. This can often be overcome, but at an increased cost. GSHPs are generally best suited to new-build projects, where they can be included in the building design. A 5-10kW GSHP system would be large enough to heat a small office.
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 3 or 4 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.