Ground Source Heat Pumps
A network of pipes either sited vertically in boreholes or horizontally in trenches provide the heat collection system for Ground Source Heat Pumps.
In most installations, and as long as there is enough land available, the most cost effective method of heat collection is via ground heat collector pipes installed horizontally, by digger or treating machine, in 1 metre deep and approx 1 metre apart long length trenches.
Coils known as ‘slinkies’, installed in a 1 metre deep by 1 metre wide trench need less digging to install. Even though the same overall area of ground is needed as the horizontal pipes, these slinky collectors work the ground harder, so they need to be positioned further apart.
Where space is limited, vertical boreholes can be a good solution, but need to be between 60 to 150 metres depth, and at least 6 metres apart, so are a more costly drilling process to install.
Heat from these ground collector pipes is transferred to a heat pump, which can be positioned in a garage, outbuilding, external enclosure or even within the building. The heat transference occurs due to the glycol or brine (antifreeze solution) within the ground collector pipes, which is colder than the surrounding ground.
Due to the pipe depth beyond 1 metre (which is below the frost layer of the ground) this lends itself to a very efficient system for a long time, as the ground is at a constant temperature in all seasons at this depth.
It is as if the ground is functioning as a rechargeable battery.
During Winter months the ground source heat pump draws heat out of the ground, during the summer the heat is replaced.
The heat collected is mainly solar energy: the sun warms the ground with heat being transferred between the soil, rock or ground water into the collector pipes.
Around 1% of the heat collected is geothermal from the earth's core. The sun warms the ground with heat being transferred between the soil, rock or ground water into the collector pipes.
By collecting low grade heat from the air, ground or water and concentrating it into usable heat, heat pumps can provide all the space and water heating needed for a building,
They can also operate in reverse to provide cooling.
A heat pump uses the same principal to transfer heat that a fridge uses, in which a refrigeration cycle uses Once the renewable heat has been extracted from the environment it is used to boil a liquid refrigerant into a gas. Because these refrigerants have a boiling point around -20 degrees C ,this allows air source heat pumps to operate during very cold weather.
Gas is produced which is then compressed and becomes hot. The heat is extracted and the pressure is thus reduced, turning the gas back into a liquid. Then the cycle starts again.
The system is 300% efficient, with each unit of electricity used to power the system typically generating 3 or more units of heat.
Temperature of the hot water produced
Heat pumps heat water to around 40 degrees celsius, which is the flow temperature at which underfloor heating systems work. Heat pumps will maintain a hot water cylinder at a temperature of around 55 degrees celsius - which is hotter than would be required for a bath or shower, though would less efficient at producing higher water temperatures than this. Heat pumps also automatically run periodic legionella cycles to ensure the eradication of any bacteria that might still survive at 55 degrees.
Why are heat pumps becoming more commonly used?
Heat pumps are a very efficient way of indirectly using electricity to heat a home by extracting available heat from the ground or air. In doing so they can provide low running costs and low carbon heat.
Traditionally the UK had fossil fuels aplenty, with oil and gas from the North Sea and extensive coal extraction processes. The two main issues with this, however are firstly the unsustainable burning of fossil fuels releasing climate changing carbon dioxide that had previously been locked away underground. The second issue is the cost, with rapidly rising energy prices whilst the UK imports oil and gas from abroad.
How reliable are heat pumps and how much maintenance do they need?
In Europe and North America heat pumps have been used for decades. The refrigeration cycle on which they function is a tried and tested technology, so they tend to be extremely reliable. However, in order to ensure they continue to run as reliably and as efficiently as possible, an annual service- just as with a conventional boiler- is recommended.
Because the ground collector is a buried sealed system in a ground source heat pump, and no waste gases, ash or moisture can be produced in its sealed system. they require the least maintenance of all. The only recommendation is that the heat pump be serviced annually to check the glycol concentration in the ground loops.