Ground Source Heat Pumps (GSHP)

GSHP's (Ground Source Heat Pump) take latent solar energy stored in the ground and compress it to heat your house. The heat is extracted from the soil by means of buried plastic tubing. An environment-friendly non-freezing liquid circulates in the tubing and delivers the collected heat to the heat pump. In the heat pump the heat is converted into high-grade heat for space heating and if required, to produce hot water.

GSHP's operate on the same principles as a domestic fridge, except the heat is retained and the cold recycled. The temperature of the earth at one metre below ground remains between approximately 9°C to 12°C all year round depending on geographical location. Contrary to popular belief this is solar heat stored in the ground, not geothermal heat from the centre of the earth. GSHP's (Ground Source Heat Pump) take latent solar energy stored in the ground using a collector system; this heat is then compressed to allow both space and/or hot water heating. GSHP consist of a heat pump, circulation pumps, a control system and an additional heat source should it be required. Generally GSHP's have built-in circulation pumps, making it easy to connect to the heating medium and collector circuits.

The basic principles of the GSHP begin with the absorption of heat from the heat source (rock, ground or lake). This is through a closed collector system containing water mixed with vegetable based antifreeze. The collector fluid then emits its heat to the refrigerant in the heat pump's evaporator. The refrigerant vaporises and is then compressed. The refrigerant, with its increased temperature, is led into the condenser where it emits its energy to the heat medium circuit, and therefore provides heating to the home or the hot water cylinder or both.

For horizontal ground loops, of all the closed collector heat sources mentioned, using a lake as the heat source is the most preferable option, as it has the best heat transfer qualities. Installing the collector in a lake or any permanent water source will also drastically reduce the necessary collector pipe length. In short, the wetter the better. Installs using closed loop collector pipes in the ground also follow these rules; collectors in wet sandy soil or wet clay are always positive. It is dry media, which is the least suitable; as the least effective transferor of heat the pipe lengths required can increase hugely. This is because they take longer to disperse the cold and gain degrees from the earth.

When a lake or sufficient ground for loops is unavailable, drilling boreholes becomes an option. In the UK drilling can be an expensive exercise due to the unpredictable make-up of ground. Drilling companies have to be prepared for many eventualities and are reluctant to provide fixed price quotations.

                                                       

Basic GSHP Principals

A heat pump can genuinely reduce energy bills by around 70% compared to oil.  When run efficiently at about 45°C, they can produce four or even five times as much power as they use, this is called Co-efficient of Performance (COP) and means that for each kilowatt of energy provided to the heat pump 5 kilowatts of energy will be provided to your house.  Quite amazing really!  Unfortunately, with older properties, the ideal level of insulation required for a GSHP to be fully effective during the very coldest days of the year, cannot always easily be achieved.  The harder a GSHP has to work the less efficient it is.  Therefore, the COP will drop and in some cases we suggest supplementing the worst days with an auxiliary system, an existing oil boiler for example.

GSHP docked to a secondary heat source

Below is a chart showing the typical comparisons of emissions and costs of heating a well-insulated house of 100 square metros by fuel type.

Heating and HWS System	Annual Fuel cost	Annual CO² emissions in tons
GSHP	£305	1.6
Condensing Gas Boiler	£450	3.0
Normal gas Boiler	£500	3.5
LPG	£1055	4.5
Oil	£1200	4.8
Electricity (storage & panel)	£960	6.2

The above figures were taken during a controlled experiment by Powergen and utilise the current fuel cost as of the 1/1/06. Any increases in energy costs since that date has not been included.

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