Resource

Ground Source Heat Pumps (GSHPs) use the ground within the immediate proximity of a building as a heat source; they transform a large amount of low temperature heat into a small amount of higher temperature heat.

The earth absorbs around 50% of the sun’s energy, which means that the ground maintains a constant temperature of around 11 to 13°C. In the winter this temperature is warmer than the air above it. Ground source heat pumps are used to extract this heat. A typical system will provide 95% to 100% of a household’s heating requirements.

Thermal energy is transferred from the earth into fluid in a pipe; this is upgraded by passing to a water source heat pump. This heat pump is then used to heat a building.

There are several hundred heat pump installations in the UK. The first modern, closed loop borehole system was installed near Plymouth, Devon in 1994 in a new build domestic property. Cornwall has seen the first social housing installation at Marazion and the first use with a school at Charleston.

Technology

There are three important elements to a GSHP:

• The Ground Loop. This is made up of lengths of pipe buried in the ground. The pipe is usually a closed circuit (but can be an open loop system) filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground. The ground loop can be:
  • borehole;
  • straight horizontal – large trenches of approximately 1-2 metres deep will need to be dug. Needs more land but costs less than a borehole. Deeper trenches would be beneficial during long cold spells in mid winter.
  • spiral horizontal (or ‘slinky coil’) – needs a trench of about 10m length/1kW of heating load.
• The Pump: this has three main parts:
  • the evaporator – takes the heat from the water in the ground loop;
  • the compressor – moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit;
  • the condenser – gives up heat to a hot water tank which feeds the distribution system.
• Heat distribution system: consisting of under floor heating or radiators for space heating and in some cases water storage for hot water supply.

The efficiency of a GSHP system is measured by the coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used to drive the compressor and pump for the ground loop. Typical CoPs range from 3 to 4. The higher end of this range is for under-floor heating, as it works at a lower temperature (30-35ºC) than radiators. This means that for every unit of commercial energy put in, usually electricity, the system gives 3-4 units of heating. This will therefore save some 2/3 to ¾ of energy input with consequent cost savings.

Costs

Typical commercial systems cost around £1,000/kW of heat energy needed, and save significant percentages of heating energy compared to more traditional heating systems using fossil fuels.

Commercial GSHP systems suit new build and industrial projects with a large heat demand, preferably at a relatively low temperature: such as swimming pools, hospitals, care homes, offices (where cooling can be useful in the summer), etc.

The financial advantages of installing a GSHP are:

• provides security of energy costs when fossil fuel prices are rising rapidly
• allows future installation of renewable electricity supply to minimise future costs and maximise energy security - long life systems with little or no maintenance
• low space and low noise
• some grant assistance available

GSHP is most likely to be an option where there is no access to natural gas and so the alternative may be oil or direct electric heating (storage heaters). In the case of electric heating, financial savings could amount to around £640 per annum (assuming off-peak electricity). In the case of oil fired heating, the likely running and installation costs would be comparable.

There are few planning issues associated with closed loop ground source heat pumps as they produce no noise or visual intrusion.

There is little requirement for maintenance for heat pumps. The underground elements have a design life of 50 years, and have no moving parts, so do not necessitate maintenance.

Image: www.techstore.ie

Environmental impacts

Refrigerant Fluid

Inside a typical 8kW heat pump there is approximately 2kg of HFC refrigerant. It is a legal requirement to recover the refrigerant at the end of the heat pumps life. Refrigerants are now all 'ozone friendly'.

Electricity

GSHP require electricity to run the pumps so can generate pollution if using mains electricity; however it will work at a good CoP (Coefficient of Performance) level and so produce much less CO2 emissions then the most efficient condensing gas or oil boilers with the same output. To mitigate this:

• Purchase dual tariff green electricity
• Couple with other renewable energy production systems such as solar PV, small scale wind, or micro – hydro.
• Wall, floor and loft insulation will lower the heat demand making the system more efficient thus reducing costs.

Hydrology

The main issues occur during and as a result of borehole and trench installation:

• The drilling process could allow existing polluted groundwater or surface water to spread
• Disturbing the subsurface conditions
• Adverse temperature changes in the aquifer

The excavation of the ground and or the drilling of boreholes can have potential impacts on site archaeology. Open loop systems, which continually draw upon a reliable 'warm' water source such as shallow water wells could impact local hydrology. Projects must therefore be carefully designed to meet Environment Agency regulations. There is potential for the use of open loop systems in Cornwall in flooded mines.

Image: Carnon Contracting installing systems in Penwith - The picture shows the drill rig in action for vertical holes for gshp for a series of bungalows, replacing coal fires.