How Ground Source Heat Pumps Work
Ground source heat pumps work by absorbing heat from the ground and transferring the heat into buildings – to heat the buildings without burning fossil fuels.
The heat pump itself is located in the building and works on the same principles as a domestic fridge: the heat pump in a fridge transfers heat out of the fridge and uses a heat exchanger to disperse the heat from a small radiator at the back of the fridge into the room.
A ground source heat pump absorbs heat from the ground – by circulating water though piping in the ground – and transfers the heat into the building by circulating hot water though radiators, or underfloor piping circuits.
The heat pump is able to increase the temperature it receives from the ground before circulating it into the house. It does this by compressing refrigerant gases. When a large volume of gas is compressed into a small space the heat energy in the gas becomes concentrated – the gas becomes very hot. The heat pump uses a heat exchanger to transfer that heat to the heating circuit in the building.
After the high pressure gas has yielded up its heat, the pressure of the gas is released and it then becomes very cold. The heat pump uses a heat exchanger to transfer that cold to the ground loop circuit. As the cold water is circulated through the ground it absorbs heat from the surrounding ground and the cycle can begin again.
How efficient is a Ground Source Heat Pump?
A heat pump uses electricity to work its compressor and uses electricity to pump water through its circuits. However, most of the energy transferred into the building is transferred from the ground and this energy is free. Therefore a heat pump is very efficient at providing more heat energy than it uses to perform the work needed. A well designed ground source heat pump installation can provide four or five kilowatts of heat for the consumption of one kilowatt of electricity.
The ratio of heat provided to electricity consumed over the heating season depends not only on the efficiency of the heat pump itself, but also the properties of the building, the heat distribution system within the building and the size and efficiency of the ground loop circuit.
It also depends on the temperature available from the ground.
The natural temperature of the ground
The natural temperature of undisturbed ground in Britain is very close to 10°C at a depth below six metres. This is true both in summer and winter as heat moves only very slowly through the ground. However, if a ground source heat pump is absorbing heat from that ground then the temperature of the ground will fall.
The ground temperature will fall faster if large amounts of heat are being extracted from a small volume of ground. As the ground temperature falls the ground source heat pump becomes less efficient and the "Coefficient of Performance" ("CoP") may fall well below the level of 4 that can be expected from a well-designed system.
The CoP of a heat pump is largely determined by the difference between the input temperature from the ground and the output temperature delivered to the building. As this gap increases the CoP of the heat pump installation falls.
Ground Source Heat Pump Installation
A ground source heat pump installation consists of:
- a GSHP in your building
- a ground array for heat exchange with the ground (pipes in horizontal trenches or vertical boreholes)
- a heat distribution system within your building (underfloor heating, oversized radiators or air ducting)
- a control mechanism to transfer heat from where it is available to where it is needed.
The most critical component of a gshp installation is the design. A good GSHP will not perform well without an adequate ground array to extract heat from, and a well balanced heat distribution system within the building; it also needs a well tuned control mechanism to achieve the right temperature balance in the building throughout the winter. The same components can be designed to provide cooling in summer. If cooling is provided in summer – by allowing heat to escape to the ground – then this will improve the performance of heating in the following winter.
Ground source heat pumps – How they work better
ICAX has developed a mechanism for capturing solar heat in summer and storing heat efficiently in ThermalBanks™ in the ground: ICAX can store naturally occurring renewable heat energy between day and night – and between seasons. Solar recharge of the ground is a critical component of Interseasonal Heat Transfer and enables ICAX to double the efficiency of ground source heat pump installations.
Integrating Renewable Technologies
ICAX integrates solar thermal collection in summer with ground source heat pumps to double the performance of an unassisted ground source heat pump. The vital link is the ThermalBank in which ICAX stores summer heat for use in winter.
How Ground Source Heat Pumps Work to save money
Ground Source Heat Pumps save money. Heat pumps are much cheaper to run than direct electric heating systems. GSHPs are cheaper to run than oil boilers and can be cheaper than running gas boilers.
Because heat pumps can be fully automated they demand much less work than biomass boilers – this also saves you money.
Heat pumps save space. There are no fuel storage requirements.
Heat pumps are safe. There is no combustion involved and no emission of potentially dangerous gases. No flues are required.
GSHPs require less maintenance than combustion based heating systems. They also have a longer life than combustion boilers. The ground heat exchanger element of a ground source heat pump installation is an infrastructure investment with a design life of over 100 years.
Heat pumps save carbon emissions. Unlike burning oil, gas, LPG or biomass, a heat pump produces no carbon emissions on site (and no carbon emissions at all, if a renewable source of electricity is used to power them).
GSHPs are safe, silent, unobtrusive and out-of-sight: they require no planning permission.
Heat pumps can also provide cooling in summer, as well as heating in winter.
The owner of a ground source heat pump installation is also entitled to receive the Renewable Heat Incentive.
See also: Renewable Heating
See also: Renewable Cooling
See also: Banking on IHT
See also: Energy Efficiency in Buildings
See also: Professor David MacKay on Sustainable Energy
See also: Economic Renewable Energy
See also: IHT Systems Design variations
See also: Seasonal Performance Factor