Heat pumps offer a low-carbon alternative to using fossil fuels for generating heat. If the electricity used to drive the pump comes from renewable sources, then the heat supplied is completely renewable.
Most buildings in the UK and similar climates currently require heating for part of the year. The energy source for heating can be gas, electricity, heating oil, LPG, biomass or coal. Most heating energy in the UK is supplied by gas, which is problematic for reducing CO2 emissions – although methane can be produced from renewable sources, the quantities currently used for heating could not. Heat pumps provide an alternative, as they can use electricity generated from renewable sources and are more efficient than other electric alternatives, such as storage heaters.
The main barrier to using heat pumps is the cost, which is higher than conventional forms of heating. The pipes buried in the ground are a significant part of the cost, so many designs try to make the installation of this component easier. Ground source heat pumps are ideally installed in new building developments, when the pipes can be installed as part of the initial site work, and underfloor heating can be installed easily.
For example, GI Energy uses ‘Energy Piles’, where the ground loop of the heat pump is integrated into the concrete piles of a building during construction, saving significant time and money. Kensa developed heat pump kits that simplify installation inside of the pump and packages the pipes for burying in the ground so that they are quick to spread out when being buried in the ground. Air source heat pumps are easier to retro-fit to existing buildings, as the heat exchanger is simply a box mounted on the outside of the building.
Heat pumps reduce the amount of fossil fuel used to heat buildings. The technology used in heat pumps is very reliable, based on that used in refrigeration for many decades, and needs less maintenance than gas, oil or biomass-fuelled heating.
Costs and savings of heat pumps
The typical cost of a ground source heat pump installation for a household is £13,000, while an air source version costs £5,000 to £8,000. Heat pumps are generally a cost-effective option where the alternative heating source is oil, LPG, coal or direct electrical heating, in other words, when mains gas is not available.
How heat pumps work
Heat pumps operate in a similar way to refrigerators, moving heat from one location to another. While a refrigerator moves heat from its interior to the radiator at its rear or sides, a heat pump moves heat from a source outside to underfloor heating or radiators inside a building.
Ground source heat pumps take heat from the ground outside a building to provide space heating and, in some cases, to heat domestic hot water. As well as ground source heat pumps, air source and water source heat pumps are also available. An air source heat pump uses a heat exchanger to absorb heat from the air, and a closed loop water source heat pump absorbs heat from a lake, river or aquifer.
The efficiency of a heat pump is measured by the coefficient of performance (COP). This is the ratio of the units of heat output for each unit of electricity used to run the pump. The average COP of a ground source heat pump is typically three to four. This means that for every unit of electricity used to pump the heat, three to four units of heat are supplied, making it an efficient way of heating a building. Air source heat pumps are less efficient than those using the ground or water, as the temperature of air drops more in the winter.
Three elements of a ground source heat pump system:
- The ground loop, buried and filled with water and antifreeze, absorbing heat from the ground
- The heat pump, made up of the evaporator, the compressor and the condenser
- The heat distribution system, whish consists of under-floor heating, radiators or hot water supply
Heat pumps require a reliable electricity supply, so are best suited to more developed countries. The potential resource is effectively unlimited, but physical constraints of individual buildings will limit the number of installations. Several countries are now offering grants, tax incentives or heat generation tariffs to encourage further heat pump installations.