In countries with cold winters, 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.
- In 2009, 2.9 million heat pumps had been installed across the globe.
- Providing heat using a heat pump can cut the carbon emissions of heating by up to 40% compared with using gas.
- Installing a household ground-source heat pump typically costs between US$18,000 and US$25,000.
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 pre-heat domestic hot water. As well as ground source heat pumps, air source and water source heat pumps are also available. Air source heat pumps are less efficient than those using the ground or water, as the temperature of air drops more in the winter.
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. Some heat pump systems are designed to provide cooling as well as heating.
There are three important elements to a ground source heat pump system:
The ground loop
This is a length of pipe buried in the ground, filled with a mixture of water and antifreeze. This mixture is pumped around the pipe, absorbing heat from the ground. The ground loop can be:
- Vertical, for use in boreholes. If there is a sufficient supply of water, an “open loop” may be used, where water is drawn into the loop from a source and then returned after heat has been exchanged. No antifreeze is used in this case.
- Horizontal, for use in horizontal trenches.
- Spiral or coiled, also for use in horizontal trenches.
The heat pump
This has three main parts:
- The evaporator, which absorbs heat from the ground loop, making the refrigerant fluid evaporate;
- The compressor, which compresses the gaseous refrigerant, heating it up to the temperature needed for heating the building;
- The condenser, where the refrigerant condenses and transfers heat to a flow of water which feeds the distribution system.
The heat distribution system
This consists of under-floor heating or radiators for space heating and in some cases water storage for hot water supply.
The components of a ground source heat pump system are also shown in the diagram below, from Ashden Award winner Kensa Engineering, which designs and manufactures heat pump systems for installation throughout the UK.
An air source heat pump uses a heat exchanger to absorb heat from the air, and a closed loop water source heat pump uses a heat exchanger in a lake or river. An open loop water source heat pump takes a continuous flow of water from an external source directly to the evaporator (see above), and then returns the cooled water back to the river or aquifer.
Benefits of heat pumps
In countries with a cold winter many buildings require heating, and heat pumps reduce the amount of fossil fuel used to do so, or if the electricity used by the system is from a renewable source then they can eliminate fossil fuel use. The technology used in heat pumps is very reliable, based on that used in refrigeration for many decades, and requires 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 ranges from US$18,700 (£11,000) to US$25,500 (£15,000), while an air source version costs US$11,900 (£7,000) to US$ 23,800 (£14,000). 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. The main barrier to using heat pumps is the capital cost, which is higher than conventional forms of heating. A ground loop is a significant part of the capital 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 ground loop can be installed as part of the initial site work, and underfloor heating can be installed easily. For example, Geothermal International 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. 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.
The conventional method of heating a large building is often natural gas. With a modern, high-efficiency boiler, the GHG emission is about 0.22 kg CO2 per delivered kWh of heat. Providing heat using a heat pump with a COP of 3.8, powered by UK grid electricity (0.49 kg CO2 per kWh), gives emissions of about 0.13 kg CO2 per delivered kWh, a reduction of about 40%.
Global installed capacity in 2009 was estimated at 2.9 million heat pumps, with a combined capacity of over 35 GW, generating about 60 TWh of heat. (Source: IEA Renewable Energy Essentials: Geothermal)
Heat pumps require a reliable electricity supply, so are best suited to developed countries and urban areas of developing countries. The potential resource is effectively unlimited, but physical constraints of individual buildings will limit the number of installations that are economically viable. Heat pumps are an important technology for reducing carbon emissions from heating, and several countries are now offering grants, tax incentives or heat generation tariffs to encourage further heat pump installations.