Ground source heat pumps. Are you using the soil of your garden to its full potential?

This article consists of a brief introduction into Ground Source Heat Pumps (GSHP) and covers some basic information about how they work and their use within the home and garden.

One type of heat pump system that is available is the GSHP, which represents a near carbon zero and sustainable method of providing some (if not all) of your home’s heating requirements.  However, GSHP are still a relatively new technology and can only work as a carbon zero way of heating a home if they use a renewable source of electricity for the systems power.  In simple, GSHP comprise of a loop of pipes buried within the ground through which a liquid runs (usually a refrigerant) absorbing heat from the ground.  Through another process this liquid serves to warm another liquid (the standard is water) that in turn is pumped around the home to heat it.

In the same way that a mug of tea feels warm as a result of the heat energy from the hot liquid contained within the ground stores the heat energy given to it by the sun.  The potential of soil’s capacity of storing heat can be seen with the heat that is given off when the soil in a compost bin is turned over.  This ability to store heat is the effect of thermal mass, and the greater volume of mass the greater potential for storing energy.  GSHP make good use of the ground’s stored energy in order to heat your home.

The energy in the ground is harvested by a loop of underground pipes (usually placed between one and two metres deep).  These pipes can be arranged in different ways dependant on the available space; three ways that this can be done is:

• Placing pipes horizontal or vertically in trenches that have been dug out into the land (see examples below).

Figure 1: Ground source heat pump pipework installed horizontally.


Figure 2: Ground source heat pump pipework: Pipe loop positioned vertically in a trench, ideally suited to site with less available space as it is dug into the ground rather than across the ground.

• Laying pipes across the land covering a large area at around 1-2 metres deep.
• Feeding pipes deep into the ground (up to 30metres) via bore-holes which can be drilled into the ground.

Therefore, a pipe system could be installed hidden underneath your garden.  The quantity of pipe-work will be site specific considering factors such as type of soil, climate and heating requirements of the home.

GSHP consist of roughly the same system which is within your fridge/freezer.  A fridge works by extracting warm air from within thus cooling the temperature down.  The heat that is extracted is then released (outside into the room) through a series of small pipes located at the back of the unit.  In the case of GSHP, a liquid (usually a refrigerant) is pumped at a cold temperature from the house, through the pipe network before returning back into the house at a higher temperature as a result of the heat ‘extracted’ from the ground.   This liquid is not hot enough to heat the house and, instead, is used to warm another liquid (usually water) to be pumped around the home to heat it.  This process, which required electrical energy, is by means of a heat exchanger (an evaporator and condenser), similar to that within a fridge.

The GSHP will (usually) require a slightly different system to that of a normal central heating system run with a traditional boiler.  This is because the heating temperature produced by a GSHP is lower than that of a traditional boiler.  For this reason GSHP work well with underfloor heating systems due to the large internal area that they cover.  As a result it is easier to install GSHP in new builds where the whole heating system can be designed to suit the heating patterns produced by the GSHP.  Furthermore, the ground can be dug for underground pipe-work during the build with the benefit of little disruption.

Electrical energy is required to run the GSHP, in specific by the ‘heat exchanger’ system and to pump the refrigerant through the underground pipe loop.  But the electrical energy required this is significantly lower than the amount of energy that is generated to heat the home. Therefore, GSHP represent a sustainable way to heat the home as long as the heat harvested from the ground is not greater than the energy which is replaced by the sun throughout an annual cycle.  Moreover, if the electrical energy supplied is from a renewable source then the use of GSHP can represent a carbon zero way to heat the home.

The GSHP discussed are those of a closed loop system and this is one of many types of heat pumps which work on a similar basis; other types include:

• Open loop GSHP using groundwater or a lake/river to heat the refrigerant.
• Air source heat pumps using the temperature of the surrounding air.

In summary GSHP are an efficient and, perhaps, sustainable and carbon zero method of producing heat for the home.  One thing not mentioned is the price of GSHP and whether it is cost-effective to invest in such a system.  The installation costs can be high and a good design and designer is required to accrue a successful system, however with progressing technology, development and popularity, these costs should reduce.  On the other hand, the installation of the system is an investment which could be paid back by the money saved from the continuous use of a traditional heating system.  It is also worth considering that GSHP have a low impact upon the environment can lower your carbon footprint and help you to do your bit for the environment.

Lewis Williams   (March 2007)

Further reading and sources of information:

• Ground Source Heat Pump Systems - Benefits, drivers and barriers in residential developments, NHBC Foundation.
• Domestic heat pumps: performance and economics, P Grigg and M McCall, BRE Press.
• Natural Home Heating - The complete guide to renewable energy options, Greg Pahl, Chelsea Green Publishing Company, 2003.
• Ground Source Heat Pumps - A technology review, BSRIA (Building Services Research and Information Association (UK) (www.bsria.co.uk), 2004.
• www.heatpumpcentre.org - European Heat Pump Association.
• www.lowcarbonbuildings.org.uk and www.est.org.uk - Information about government sponsored grants to aid towards the installation costs.
• www.energysavingstrust.org.uk 

Lewis Williams