About half of the solar radiation received by the Earth is absorbed at the surface. As a result, the ground temperature shows seasonal fluctuations to depths of about 15 m where the temperature is approximately equal to the mean annual air temperature(8 - 11° C in the UK). Below this the ground temperature increases at, on average, 2.6o C per 100 m due to heat flowing from the interior of the Earth. Mean temperatures at 100 m depth in the UK vary between about 7 - 15°C. Hence, in winter, the ground temperature is higher than the air temperature, whilst in summer it is lower than the air temperature. These temperature differences can be harnessed by ground source heat pumps (GSHP) to provide heating in the winter and air conditioning in the summer.
GSHP systems use some electricity to run the heat pump, but most of the energy for heating is taken from the ground, thus they produce less greenhouse gas than conventional heating systems. If the heat pump is run using ‘green’ electricity (e.g. generated by wind power), the system can be ‘carbon neutral’. GSHP systems can also be more cost effective than fossil fuel systems where the conditions are suitable, due to low running costs, however they have a relatively high capital cost.
There are two types of ground source heat pumps, closed loop and open loop. In a closed loop system sealed pipes are placed either horizontally or vertically in the ground. Water (with antifreeze) is pumped through the pipes and takes up heat. This is then extracted by the heat pump and released at a higher temperature to drive a space heating system. If the system is used for air conditioning in the summer then it operates in reverse. A typical open loop system abstracts water from an aquifer, extracts the heat in a heat pump and then releases the cooled water back into the aquifer. A GSHP system will have a maximum capacity for heating and cooling depending on factors such as the size of the heat pump and the thermal properties of the ground. As the GSHP is removing heat from the ground (for heating) or adding it (for cooling), the operation of the system can affect its lifespan. For example, if heat is extracted from the ground more quickly than it is replenished by solar or geothermal heating (or groundwater flow), the effectiveness of the system will diminish over time.
In order to provide relevant information to assist in the design and installation of GSHP systems, the BGS provides site-specific data on thermal properties, geology and hydrogeology through the GeoReports enquiry service. This is supported by a report explaining the extent of available datasets and how the GeoReports are compiled.
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