What is Geothermal Energy?

Both ‘Geothermal Energy’ and ‘Ground Source Energy’ are terms used to describe energy which is sourced from the Earth. Only a few metres down the temperature of the subsoil remains constant at about 10-16°C and usually differs from the surface temperature by a few degrees. This differential between the subsoil temperature and the surface temperature is the principle on which geothermal or ground source heating works. The amount of temperature differential depends on the geology of the soil, climate and seasonality.

How it works

There are three important elements to a geothermal or ground source heating system:

1. The ground loop
This consists of lengths of pipe buried in the ground 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 around the pipe absorbing heat from the ground. The ground loop can be:

• Vertical, for use in boreholes
• Horizontal, for use in trenches

2. A heat pump
In the same way that a fridge uses refrigerant to extract heat from the inside, keeping food cool, a ground source heat pump extracts heat from the ground and uses it to heat a building. A ground source heat pump has three main parts:

• The evaporator which absorbs the heat from the liquid in the ground loop
• The compressor which moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit
• The condenser which gives up heat to a hot water tank which feeds the distribution system
  

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

There are reverse-cycle heat pumps that can deliver both heating and cooling.