Ground-source heat pumps

Earth-power

Ground source heat pumps are devices that can extract geothermal energy from the ground at low temperatures and transfer it to the inside of a building – at a higher temperature – to be used for indoor space heating. In the summer, ground source heat pumps can reverse the process and cool down indoor spaces by extracting excess heat from the building and transferring it to the ground.

Heat pumps are a very efficient technology to provide heating for buildings using electricity. They can save substantial amounts of fuels and reduce GHG emissions in comparison with conventional systems. Although heat pumps require electricity to operate, most of the heat delivered to the building is renewable energy, as it is extracted from the air in the surroundings.

Ground source heat pumps are a commercially mature technology and due to the stable underground temperatures, they can be efficiently used in almost all climate zones. They are a particularly attractive technical option in rural areas that are not connected to the gas grid but have access to electricity. In rural areas houses tend to be larger and have higher heating (and cooling) demands. Heat pumps are particularly suitable for these situations. Moreover, electrically driven heat pumps could be supplied by locally generated renewable electricity (e.g. solar PV).

Investment costs for heat pumps are typically higher than for conventional boilers. However, depending on the energy demand of the building, the fuel savings can pay back the investment in a reasonable time. Once installed, ground source heat pumps require little maintenance, so the overall operation costs are usually very low.

General Info

What is it?

A ground source heat pump uses a system of underground pipes to extract heat from the ground (or ground water) using it as an energy source for space heating. The temperature of the ground remains fairly constant throughout the year so the system can extract sufficient heat, even in winter. In the summer, ground source heat pumps can also cool a building by using the ground as a sink for excess heat.

What are the benefits?

Low Carbon: Depending on the system replaced, heat pumps can reduce GHG emissions substantially compared to fossil- based alternatives.

Efficiency: Heat pumps are efficient systems to produce heat with electricity. Savings from 20% to 50% in primary energy (fuel) are common.

Cost savings: Ground source heat pumps can reduce energy bills substantially, especially compared with conventional electric heating systems.

How does it work?

A closed loop of pipes is buried in the ground. The ground is substantially warmer than the outdoor air in the winter. Water is circulated through the loop and into the heat pump which removes the heat from the water and delivers it to the heating system of the house.

There are also ground source heat pump installations available on the market with an open loop. Such systems use water from an aquifer (a water well or body of underground water) as the fluid for the heat pumps and re-inject the used water back into the aquifer.

Suitability/applicability

Ground Source Heat Pumps are particularly suitable for rural areas where houses tend to be larger and have a higher heating (and cooling) demand. In such situations they are most efficient. Moreover, electrically driven heat pumps are very suitable for locations not connected to the mains gas supply and could be supplied by locally generated renewable electricity (e.g. solar PV).

Due to the stable underground temperature, ground source heat pumps are suitable in most climate zones in Europe.

The area around the building needs to be suitable to dig trenches in which to bury the pipes of the ground source. There has to be sufficient ground space that can be accessed in order to install the horizontal pipes. The trenches are typically several meters deep. In case less space is available, a vertical loop is a good alternative but comes at higher cost. This makes ground source heat pumps particularly suitable for rural areas where sufficient outside space is available to dig the trenches.

A ground source heat pump system usually does not provide tap water that is warm enough. Hot tap water often needs to be >60° whereas for domestic systems, the maximum water temperature obtainable is 50-55°. A water heating strategy can be designed where the incoming water supply is preheated by the heat pump before reaching an ancillary heating source (this can be a small condensing boiler).

Since ground source heat pumps work most efficiently when producing heat at a lower temperature (<55°C) than conventional boilers, it's essential that buildings are well insulated. Systems also perform better with under floor heating systems or warm air heating than with radiator-based systems because of the lower water temperatures that are required.

Ground source heat pumps need electricity to operate. However most of the heat delivered to the building is renewable (geothermal) energy, as it is extracted from the ground in the surroundings. Electricity can come from the grid or can be locally generated by renewable energy sources such as wind and solar energy.

Heat pumps are equipped with a fluid compressor (like a refrigerator). This type of equipment produce levels of noise that may be too loud e.g. if the heat pump is located close to a bedroom. Appropriate siting is therefore important.

Detailed Info

Costs, Savings, Earnings

Costs, savings and earnings of installations depend on a number of factors:

  • The heat distribution system installed (to improve efficiency it is suggested to use low temperature floor or wall heating systems or low temperature radiators)
  • Required energy efficiency improvements of a building (if necessary)
  • The climate, country or region involved (some countries provide economic support for the use of heat pumps).
  • Current fuel costs: The economic savings will depend on the costs of the fuel saved. If the system substituted is a conventional electric heating system, monetary fuel savings can be high. Heat pumps are particularly effective in rural regions not connected to the gas grid and/or where heating fuels are expensive.

In many European countries, financial incentives are available in the form of production support or investment grants. Costs of heat pumps are expected to come down further through increased experience and economies of scale.

Environmental Impacts

If designed and installed properly, ground source heat pump systems can reduce greenhouse gas emissions for heating and cooling substantially. Depending on the energy source used to produce the electricity, this has an impact on the environment. The heat they extract from the ground (or water) is however constantly being renewed naturally.

Efficiency

In the discussion of the energy performance of heat pumps, the term coefficient of performance (COP) is used instead of efficiency. COP describes the ratio of useful heat that the system is able to ‘pump’ for each unit of input energy (electricity). Typical COPs for ground source heat pumps are in the order of 2.5 to 5. Ground-source heat pumps typically have higher efficiencies than air-source heat pumps – an alternative technology. This is because they draw heat from the ground which is at a relatively constant temperature all year round, compared to outside air temperatures that are less constant.

Commercial Maturity

Ground source heat pumps are a commercially available and mature technology. On-going technical efficiency improvements of the system are expected to reduce operating costs and increase efficiencies. Installers may not always be aware of the potential of the technology. It is important that the system is installed by a certified installer that has appropriate experience.

Level of Maintenance

LOW: Ground source heat pumps require very little maintenance once installed. Most suppliers provide warranties for 5 or more years on installations and often a service contract is included.

Technical Details

A heat pump works like a refrigerator in reverse. In the heat pump, the heat from the water in the ground pipes is absorbed at low temperatures by evaporating a refrigerant fluid. This fluid is then compressed and forced to condensate, releasing back the heat absorbed from the underground water pipes. This heat can then be used for space heating. The cooled water ground-loop passes back into the ground where it absorbs further energy in a continuous process as long as heating is required. The circulation pumps and compressors required to drive this cycle are usually powered with electricity. This process is depicted in the figure below.

Picture: main components of a heat pump. Source: IEA-RETD1

A ground source heat pump for domestic use will typically require an electric power supply in the range of 1.5 to 8 kW. In some cases three-phase power supply may be required.

As a rule of thumb, the space required to install the external ground pipes is approximately 1-3 times the floor surface of the building to be heated (depending on the level of insulation of the building).

Regional variations

In northern European countries heat pumps are used primarily for space heating purposes. As they are reversible systems, they are also used for cooling purposes in southern climates, extracting heat from the building and delivering it to the ground.

Trade associations

European Heat Pump Association

www.ehpa.org

Association of the European Heating Industry

www.ehi.eu