DESCRIPTION

Ground source heat pumps are used to extract heat from the ground to
provide space and water heating to both individual houses and any type
of non-domestic building. Heat pumps take in heat at a certain
temperature and release it at a higher temperature, using the same
process as a refrigerator. As the ground stays at a fairly constant
temperature throughout the year heat pumps can use the ground as the
source of heat. The ground temperature is not necessarily higher than
ambient air temperature in winter but it is more stable whereas air has a
vast temperature range. This makes system design more robust.
Water (or another fluid) is circulated through pipes buried in the ground
and passes through a heat exchanger in the heat pump that extracts heat
from the fluid. The heat pump then raises the temperature of the fluid via
the compression cycle to supply hot water to the building as from a
normal boiler.

Most heat pumps are electrically driven but other systems can use waste
heat or burn fossil fuel such as gas. The measure of efficiency of a heat
pump is given by the Coefficient of Performance (CoP), which is defined
as the ratio of the heat output, divided by quantity of energy put in.
CoPs of 3 or more should be achievable with ground sourced heat pump
systems, giving good energy and running cost savings.
The heat pump can replace the boiler in a single house but in larger non-domestic buildings it is likely to be one of a number of modular boilers,
depending on what proportion of the heat demand it is designed to
satisfy. The optimal use of the heat pump system is with underfloor
heating as this is run at lower temperatures making the operation of the
heat pump more efficient. Electrically driven heat pumps should be very
reliable but require maintenance to keep to full CoPs.

The ground pipe system can be horizontal or vertical. For horizontal
systems, a coiled pipe network is buried at around two metres depth
below ground level, thus requiring a large area of open space depending
on the size of the system. For vertical systems, the pipes are placed in
holes bored straight into the ground to a depth of 15 to 150 metres
depending on ground conditions and size of system. Vertical systems
thus require very little ground space but do require access for the drilling
rig at the construction stage, though this is unlikely to be greater than
for normal construction vehicles.

Whilst a ground source heat pump is clearly not a wholly renewable
energy source as it uses electricity, the renewable component is
considered as the heat extracted from the ground, measured as the
difference between heat output, less the primary electrical energy input.
It may be possible to combine the use of ground sources for both heating
and cooling, using the same mechanical equipment.

WHERE CAN GROUND SOURCE HEAT PUMPS BE USED?

Ground source heat pump systems can be used in individual houses,
blocks of flats with communal heating and almost any size of nondomestic
building. A particular use is where natural gas is not available
making the ground source heat pump more economic. Ground source
heat pumps cannot be seen from the outside of the building, so aesthetic
design is not an issue.

COST RANGES

The main additional cost in a ground sourced heat pump system is the
installation of the pipes in the ground, which depends on the ground
conditions and length or depth needed to be installed. The additional
cost for an individual house is around £2,500 and to supply 50% of the
heating demand of a 3,000m2 office building, it could be around
£50,000. Developers should contact individual suppliers for up to date
prices, and investigate the possibility of bulk discounts.

OUTPUT RANGES

In houses, ground source heat pumps are likely to supply most of the
heating and hot water demand but may only provide a proportion in
larger buildings due to ground pipe lengths required.

TECHNICAL FEASIBILITY

Technical feasibility will depend on access for the ground pipe system,
either an area for a horizontal system or access to drill for vertical pipes.
For a large individual house, a horizontal pipe system would require a
garden area of up to 100 square metres. Ground conditions will also
affect ease of construction and performance of the system. Areas of solid
rock will be more expensive to drill through. A detailed ground survey will
be needed to determine the suitability of any particular site, having
regard to ground conditions as well as to underground obstructions such
as sewers and tunnels.
Different temperature conditions will be suitable for different types of
heating, for example underfloor heating, medium temperature radiators
and hot water heating.

MAINTENANCE REQUIREMENTS

No maintenance is required for the ground pipes, and the heat pump
requires standard mechanical equipment maintenance to maintain the CoP.

ADDITIONAL BENEFITS OF TECHNOLOGY

Fuel cost savings are achievable depending on the CoP and the relative
fuel costs of electricity against the base heat fuel. With a CoP of 4, the
cost savings to the user would be in the range of 25 - 70% depending on
the comparative system.

PLANNING AND LEGISLATIVE ISSUES

Ground sourced heat pump systems cannot be seen from the outside of
the building so they should not be an issue for planners when looking at
the aesthetics of a potential development. A license may be required to
drill vertical systems in most locations.

SOURCES OF MORE DETAILED TECHNICAL INFORMATION

• Heat Pump Association, Federation of Environmental Trade
Associations, 2 Waltham Court, Milley Lane, Hare Hatch, Reading,
RG10 9TH, T: 0118 940 3416, W: www.feta.co.uk/hpa/

• ‘Ground Source Heat Pumps - A Technology Review’. BSRIA Technical
Note TN 18/99, 1999.

• ‘Closed Loop Ground-Coupled Heat Pumps’. IEA Heat Pump Centre
Informative Fact Sheet 2. 2002

• ‘Heat Pumps for Buildings: Key Points’. Roger Hitchin, BRE.
November 2003

• Energy Efficiency Best Practice Programme ‘Domestic Ground Source
Heat Pumps: Design and Installation of Closed Loop Systems’, March
2004 www.est.co.uk/bestpractice.

Back to full range