Geothermal Energy and Ground Source Heat Pumps

Site Investigation

Ground Source Heat Pumps use pipes buried in the ground to transfer heat into a building to provide spare heating or to pre-heat domestic hot water. 

There are three elements to a GSHP: 

1) The ground loop. This is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.

2) A heat pump. This has three 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.

3) Heat distribution system. Consisting of under floor heating or radiators for space heating and in some cases water storage for hot water supply.

The ground loop can be:

1) borehole;

2) straight horizontal - trench costs less than a borehole, but needs more land area;

3) spiral horizontal (or 'slinky coil') - needs a trench of about 10m length to provide about 1kW of heating load.

Installation

A typical 8-12kW system costs £6,000-£12,000 plus the price of connection to the distribution system. This can vary with property and location.

Running costs

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.