Ground Source Heat Pump (GSHP) - useful information
Ground Source Heat Pump (GSHP) systems utilise the low grade heat absorbed by the ground from solar irradiation. At a depth of 5m a seasonal fluctuating temperature of between 8-12oC is recorded. At 20m the temperature becomes constant and at additional depths of 33m the temperature rises by 10C. The most commonly found GSHPs utilise closed loops buried in horizontal trenches, of 1.5-2m, or vertical boreholes, up to 100m, with a working fluid of brine or water mixed with anti-freeze. The low grade heat from the ground is transformed into useful higher temperature heat using a refrigeration vapour compression cycle. This system can also be used in reverse for cooling, expelling the heat collected from the building into the ground.
Vertical Ground Loop full system example, source souththamesgas.co.uk
The efficiency of the GSHP is measured by its coefficient of performance (COP), which is the ratio of the heating capacity to the effective power input of the unit. For example, at a ground temperature of 100C the GSHP could achieve a COP of 4, which can be described as, for 1 unit of electricity feeding the GSHP 4 units of heat are generated. Higher COPs are achieved when there is a smaller difference between the ground source temperature and the desired internal comfort temperature. To maximise COPs, systems should operate at a lower flow temperature than the 65-800C utilised by a standard boiler system. Under-floor heating systems, with a flow temperature of 350C, and over sized radiators operating at a flow temperature of 450C, are ideally suited and can give good COPs with consequential CO2 emission savings against fossil fuel boilers. Hot water can be utilised at these lower temperatures as long as provision is made for legionella. This often takes the form of an electric immersion heater timed to come on at intervals to heat the water to the required sterilising temperature of 65oC. To maximise GSHP COPs they should be designed to operate at low temperatures over long periods.
|Horizontal Ground Loop system installation, source: Andrew Engineering Ltd|
Feasibility and Development
There are a number of stages involved in assessing the feasibility of GSHP systems:
Siting and design considerations
- Siting - Primarily a detailed calculation of the heating and cooling demand of the building is required alongside an investigation of the ground conditions. This will determine the length of the ground loops or depth of borehole required. Although actual required sizes will vary considerably, as an approximation, 10m coiled loop trench will be required per kW of heat pump capacity.
- Retro-fitting - GSHPs are suitable for a variety of buildings. To maximise CO2 emission savings it is recommended that the building’s insulation is upgraded substantially prior to GSHP installation. This reduction in space heating requirement will allow a smaller heating capacity (kWth) GSHP, using less electricity to power the motor, to maximise CO2 emission savings and minimise running costs.
- Installation disruption - It is recommended to combine the installation with other building work or as part of new developments as installation requires significant excavation and disruption.
- Costs – Typically GSHP systems cost approximately £800 for horizontal loop and £1250 for vertical loops per kW of rated capacity.
For domestic properties GSHPs fall under Permitted Development (PD) rights, which allows for minor changes to be undertaken without needing to apply for planning permission, excluding listed buildings.
For business premises advice should be sought from the Local Planning Authority.
Management and maintenance
GSHPs are a very low maintenance system. Manufacturers should provide a warranty and a service and maintenance contract should be agreed with the supplier.
- MCS - www.microgenerationcertification.com
- Renewable Heat Incentive - http://www.rhincentive.co.uk/
- Yorkshire and Humber Microgeneration Partnership - www.yhmp.org/
For larger commercial schemes, dependant on the availability of a geothermal resource at the site, an Aquifer thermal energy systems could be considered.
Borehole thermal energy systems have been used for residential and commercial developments whereby heat is extracted for space heating needs in the winter months and replenished by removing heat from the buildings in the summer months and depositing it in the thermal store.