Ground Source Heat Pumps

Harness underground heat and benefit from renewable energy and lower heating bills

A ground source heat pump system harnesses heat from underground by pumping water through it. The heat pump then increases the temperature, and the heat is used to provide home heating or hot water. The pump needs electricity to run, but it uses less electrical energy than the heat it produces. The heat pump performs the same role as a boiler does in a central heating system, but it uses ambient heat from the ground rather than burning fuel to generate heat.

Just 2 meters below the surface the temperature of the ground is a fairly constant 11-12°C. We can capture this warmth and use it as a reliable, renewable heat source to run central heating systems for our homes. The soil, clay and stones found 2m underground may not feel warm to the touch, but there is enough heat in there – absorbed in the first place from the sun – for ground source heat pumps to utilise and to release into homes and other buildings. This is done by means of a buried network of fluid-filled pipes connected to a compressor and pump unit.

HOW A GROUND SOURCE HEAT PUMP WORKS

The most distinctive feature of a Ground Source Heat Pump system is the pipework, usually about 100m of it, which is buried in loops in trenches or in one or more vertical boreholes. Once the pipework is buried the surface of the ground can return to being a field, garden, drive etc, and you wouldn’t know it was there. A liquid – typically water with antifreeze – is pumped through the pipework and absorbs the warmth of the ground. A compressor in the main unit of the heat pump raises the the temperature of this fluid, and a heat exchanger transfers the warmth to a separate body of water which circulates around the central heating system. The now-cooled water is pumped back out to the buried pipework and the cycle begins again. The whole system is powered by electricity, so unless this comes from a renewable source such as a wind turbine, a ground source heat pump still generates carbon emissions, though less than those associated with conventional types of heating, and with no on-site emissions.

A ground source heat pump increases the temperature from the ground by between 1.5 and 4 times – so if the ground temperature is 12°C, the output would be between 18°C and 48°C.

VERTICAL BOREHOLES V HORIZONTAL LOOPS

In general horizontal ground loops are less expensive than vertical boreholes. However, you may feel that vertical boreholes are worth the additional expense, for a number of reasons:

Horizontal loops require more land than boreholes. Furthermore that land cannot be built on, and cannot be used for a year after installation.
The installation of boreholes is less disruptive than that of horizontal loops, since the installation disturbs a much smaller area of ground.
The collector length is shorter for a borehole than for a horizontal loop collector, since the temperature of the ground increases with depth.

IS MY PROPERTY SUITABLE?

If you are considering a heat pump it is very important to make sure your home is well insulated as heat pumps work best in buildings that require little energy to maintain a temperature once it has been reached. Heat pumps operate more efficiently the smaller the temperature difference between the collectors (the pipes in the ground) and the emitters (the heat distribution system). Consequently, heat pumps produce heat at a lower temperature than a conventional central heating system and so a larger area is required for the heat distribution. Underfloor heating is ideal but heat pump specific large radiators can also be used. Heat pumps also work more efficiently when there are gradual rather than sudden demands on the system.

The efficiency of a heat pump is calculated in a ratio called the ‘Coefficient of Performance’ (CoP). So, for example, if your heat pump produces 4 units (kW) of heat for every 1 unit (kW) of electricity used, the CoP will be 4. The CoP is dependent on factors such as correct sizing of the system, type of heat distribution system, energy efficiency of the property, ground temperature and required room temperature. As a heat pump will not always operate at its maximum efficiency the quoted CoP will always be higher than the actual efficiency of the system over the year.

A typical domestic pump is the size of a large fridge and you may wish to have it installed in an outbuilding or basement. It is preferable to site it close to the collector pipes. A good installer will make sure that the heat pump, collector and heat distribution system are sized correctly. Under or over sizing the component parts will lead to a drop in efficiency and could result in excess expense, frozen ground or a poorly heated home. Make sure that both your products and installer are registered with the Microgeneration Certification Scheme (MCS).

ADVANTAGES OF GROUND SOURCE HEAT PUMPS

A typical ground source heat pump could save you £180 (replacing oil-fired heating) to £480 (replacing electric heating) a year
Ground source heat pumps generate less CO2 than conventional heating systems
They require very little maintenance
Electricity is required to power the pump which circulates the liquid in the ground loop, but for every unit of electricity used by the pump, you get between two and four units of heat – making this an efficient way to heat a building.

DISADVANTAGES OF GROUND SOURCE HEAT PUMPS

Installing a ground source heat pump is expensive – typically £10,000 – £20,000, depending on the size of the system (not including the cost of fitting under-floor heating, if required)
Ground source heat pumps are generally not suitable for properties with existing gas-fired central heating as the technology works at lower temperatures, making it better suited to homes with underfloor heating.
The groundworks required to dig the trench can be expensive and disruptive – planning permission may be required if you need a borehole.
You still need to use electricity to drive the pump, so a ground source heat pump can’t be considered completely zero-carbon unless this is provided by a renewable source, such as solar power or a wind turbine.

MAINTENANCE

As the system is closed and hermetically sealed (airtight), very little maintenance is required. An annual service of the pump prior is advisable. Heat pumps last around 20 years and the ground collectors are expected to last for 70 years so little needs replacing. If something does go wrong, for example a leak of the refrigerant, you would require an engineer with F Gas certification.