Sustainable Building, Eco Development Techniques

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Ground Source Heat Pumps

A Ground source heat pump works by using pipes buried in the ground to extract heat.  Underground, the temperature remains constant - summer and winter - and as most heat is required in the winter, this temperature differential is used to warm water, usually for underfloor heating.  It can also be used to raise the temperature of the incoming water to a conventional boiler so that less energy is used in heating it to a required temperature.

Is it worth installing a GSHP?

The efficiency of a ground source heat pump is measured by a coefficient of performance (CoP) - the amount of heat it produces compared to the amount of electricity needed to run it. A typical CoP for a ground source heat pump is around 3.2 without any reductions for the type of distribution system.

Manufacturers of GSHP sytems promise COPs of 3-4. The Renewables Advisory Board quotes an average of 3.2 for space heating and only 2.24 for domestic hot water.   Electrically driven heat pumps are required for the system to work and as this electricity is usually from the mains supply, it could be that high efficiency condensing boilers will release less Carbon Dioxide than GSHP systems.  We require more data before making a decision.  If you really want to cut your Carbon emissions, you will need to power the system using PV solar panels.

What is certain is that GSHP systems work best for low temperature underfloor heating methods.  They cannot supply water at the temperature needed for normal radiators and additional boilers are required.

How a ground source heat pump works

A ground source heat pump pipes a mixture of water and antifreeze around a loop of pipe - called a ground loop - which is buried in the ground. When the liquid travels around the loop it absorbs heat from the ground. There are three major types of closed loop geothermal systems:  horizontal loops, vertical loops, slinky coils.  Normally the loop is laid flat, (horozontal loop) or coiled in trenches about two metres deep (slinky loop - see photo) but if there is not enough space in your garden you can install a vertical loop to a depth of up to 100 metres - see image on the left.

The length of the ground loop depends on the size of your home and the amount of heat you need - longer and deeper loops can draw more heat from the ground, but a lot dependes on what type of ground you have.  The photo on the right shows slinky coils laid horizontally.

The liquid flows into an electrically powered heat pump, comprising a compressor and a pair of heat exchangers before discharging back to the underground loop.

The pumps are required to draw the heated water through the system - it cannot be operated by gravity as it is not a continuously upward flow of heated water.

Environmental impact of Ground Source Heat Pumps

CO2 emissions

SystemPrimary Energy Efficiency (%)CO2 emissions
(kg CO2/kWh heat)
Oil fired boiler60 - 650.45 – 0.48
Gas fired boiler70 - 800.26 – 0.31
Condensing Gas Boiler + low temperature system1000.21
Electrical heating 360.9
Conventional electricity + GHSP120 - 1600.27 – 0.20
PV panel + GHSP300 - 4000.00

(Source: Sustainable Energy Ireland)

Is a ground source heat pump sustainable?

This is a good question.

If the electricity used to power the system is from a solar PV panel, then the answer is yes, however, if you are using electricity from the grid, then the answer is no.

If you look at the table above you will see that when a GSHP is using conventional electricity, then it produces more emissions than a condensing gas boiler.

What can the heat be used for?

Heating the Home

GSHPs can only raise the temperature to around 40°.  For this reason they are most suitable for underfloor heating systems or low-temperature radiators, which require temperatures of between 30° and 35°.   Conventional radiators need temperatures of around 60° to 80° and this can be only obtained through use of the GSHP in combination with a conventional boiler or immersion heater.

Heating the Water

GSHP systems cannot directly supply hot water.  Hot water for taps is at around 60°.  The incoming water supply can be preheated by the GSHP before reaching the boiler or immersion heater. but it is possible that an immersion heater working off off-peak electricity is more economical.

Antifreeze

Refrigerants are present in GSHP systems and so present the threat of HCFCs and toxicity. However, new types and blends of refrigerant (some using CO2) with minimal negative impacts are being examined.

How much will a GSHP system cost?

GSHP installed prices ranges from about £900-£1,300 per kW of peak heat output, excluding the cost of the distribution system. Trench systems are cheaper so tend to be at the lower end of this range.  As a normal house requires betwwen 8 to 12 kW at peak times, it is expensive to install.