Low Energy House - Ground Source Heat Pumps - GSHP

As part of a system of heat collection, extraction and distribution, Ground Source Heat Pumps transfer heat from the ground into a building to provide space heating or to pre-heat domestic hot water

Ground Source Heat Pumps

Radiation from the sun heats the earth all the year round. In the UK winter, the temperature of the earth, close to its surface, is between 8° and 12°C. The technology used in the heat pump is the same as that used in a domestic refrigerator. A refrigerator extracts heat from its contents and pumps that heat to a radiator on the outside of the cabinet, where it is dispersed. Similarly, a ground source heat pump extracts heat from the earth and pumps it into a building, where it can be used for heating.

Ground Source Heat Pumps - System Efficiency

For every unit of electricity used to power a ground source heat pump system, between 3 and 4 units of heat are extracted and distributed. At this high level of efficiency, there are less carbon dioxide emissions than for a gas boiler system.

In many cases, it may also be possible to provide the electricity required to operate the system by means of renewable energy, eliminating the need for fossil fuel energy altogether. Alternatively, green electricity can be purchased, or Solar PV, or another form of renewable electricity generating system can be installed to power the compressor and pump.

How Ground Source Heat Pumps Work

There are three important elements to a Ground Source Heat Pump System: the Ground Loop, the Heat Pump and the Heat Distribution System.

The Ground Source Heat Pump Loop System

The ground loop is a pipe buried underground in either a vertical borehole or horizontal trench. The loop is not pressurised so plastic tubing can be used.

Boreholes

• Boreholes are drilled to a depth of between 15 and 150 metres (50 to 500 feet) and benefit from higher ground temperatures than trenches. Specialist equipment and good access are required for a borehole so they are usually reserved for larger projects.

Trenches

• For smaller domestic projects, trenches are usually cheaper than vertical boreholes, and more convenient, as an excavator will usually be on the site at some point in a building programme. The trench is excavated approximately 1.5 to 2 metres (5 to 7 feet) deep and a system of straight or coiled pipe is laid horizontally and the trench is back filled.

Pipe

• The pipe loop can be laid in a straight line but it is more common for it to be laid in a series of loose coils. These will reduce the length of the trench required but the pipe will still absorb the same amount of heat. The pipe is a closed loop system and is filled with a mixture of water and Glycol antifreeze which is pumped round the pipe and absorbs heat from the ground.

Ground Source Heat Pumps - Ground Area

The size of ground area required for trenches will vary with the location, the type of property and the output required. As a general indication, for a newly built 3 bedroom house with a heat loss of around 6 kilowatts, a trench of about 10 metre (30 feet) length is required to provide each 1kilowatt of heating load.

A Ground Source Heat Pump has three main parts:

Evaporator

• The heated water and antifreeze mixture from the underground loop is pumped to the heat exchanger that is housed inside the Heat Pump Unit. The refrigerant expands in the evaporator, changes from liquid to gas and absorbs the heat from the underground mixture.

Compressor

• The refrigerant is pumped to the compressor where it is pressurised and super-heated to the temperature required by the heat distribution circuit in the building.

Condenser

• The condenser transfers the heat to a hot water tank which feeds the distribution system. When the water and antifreeze mixture leaves the heat pump and returns to the ground it is at a much lower temperature. The loop is continuous and the mixture returns to the ground to be reheated.

GSHP System Controls

An electronic control system is incorporated in the Heat Pump unit to operate the temperature and timing functions.

Ground Source Heat Pump Heat Distribution System

The heat produced by a Ground Source Heat Pump System is at a low temperature relative to that normally required for space heating. Ground Source Heat Pumps can be combined with radiators, although the radiators have to be very large in area and are therefore not very practical.

The efficiency of the system is greatly improved if the temperature of the heated water is kept as low as possible, for this reason under floor heating is generally recommended. Alternatively, the heat produced by the Ground Source Heat Pump can be used to pre-heat the domestic hot water supply in a special storage cylinder or a thermal store.

Once a Ground Source Heat Pump is installed, there are no external fans or other visible external equipment. The system is quiet in operation and requires no regular maintenance