Ground Source Heat Pumps
Ground Source Heat Pumps collect heat from the ground using a network of pipes buried either horizontally in trenches or vertically in boreholes.
The majority of ground source installations use a ground collector installed horizontally in trenches, if sufficient land is available this tends to be the most cost effective method. The ground heat collector pipe can be installed in long lengths using a digger or trenching machine to bury pipe 1m deep and around 1m apart.
Less digging is required if coils known as "slinkies" are used. These typically are installed in a trench 1m deep by 1m wide. The same area of ground is needed for a horizontal or slinky collector but as the slinkies work the ground harder these need to be further apart.
Boreholes provide an excellent solution where space is limited but are more expensive to drill. These can be anything from 60 to 150m deep and at least 6m apart.
The ground collector pipes contain an antifreeze solution (glycol) also known as brine. This is colder than the surrounding ground so heat is collected from the ground and transferred to the heat pump. The heat pump can be sited within the building, in a garage, an outbuilding or a suitable external enclosure.
At a depth of more than one metre (which is below the frost layer) the ground is at a constant temperature all year round. The ground works like a rechargeable battery and allows the system to work very efficiently for a long time. During cold weather the ground source heat pump draws heat out of the ground, during the summer the heat is replaced. The heat collected is mostly solar energy with about 1% being 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.
The heat pump collects low grade heat from the air, ground or water and concentrates it into usable heat. The heat pump 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. A refrigeration cycle uses the renewable heat that is extracted from the environment to boil a liquid refrigerant into a gas. These refrigerants have a boiling point around -20 degrees C allowing air source heat pumps to operate during very cold weather.
The gas produced is then
compressed, becoming hot and the heat is extracted. Extracting the heat and reducing the pressure turns the gas back into a liquid and the cycle starts again.
Each unit of electricity consumed powering the system typically generates 3 or more units of heat i.e. 300% efficient.
How reliable are heat pumps and how much maintenance do they need?
Heat pumps have been widely used in Europe and North America for decades. Because the refrigeration cycle is a tried and tested technology they tend to be extremely reliable. However, in order to ensure they continue to run as reliably and as efficiently as possible we always recommend servicing them annually - just as you would a conventional boiler.
Why are heat pumps becoming more commonly used?
Traditionally the UK had plentiful supplies of fossil fuels; oil and gas from the North Sea and extensive coal workings. However, there are two main issues with this:
1) Burning fossil fuels releases climate changing carbon dioxide that had previously been locked away underground.
2) Energy prices are rising rapidly as the UK increasingly imports oil and gas from abroad.
Heat pumps are a very efficient way of indirectly using electricity to heat a home by extracting available heat from the ground or air, and as such can provide low running costs and low carbon heat. Typically every unit of electricity used to drive the heat pump can provide three or more units of heat.
Temperature of the hot water produced
Heat pumps are very efficient at heating water to around 40 degrees celsius. As such they are ideal partners for underfloor heating systems which typically require a flow temperature of the same. Heat pumps work less efficiently when heating water to higher temperatures and as required by the direct hot water in your home, but they will still readily maintain a hot water cylinder to a temperature of around 55 degrees celsius - which is hotter than would be required for a bath or shower. Heat pumps also automatically run periodic legionella cycles to ensure that any bacteria that might still survive at 55 degrees are definitely removed.
Maintenance
Heat pumps require little by way of maintenance, ground source heat pumps require the least of all. The ground collector is a sealed system that can't get dirty (because it is buried) and doesn't require any kind of user maintenance. Also, the heat pump itself is a sealed system and doesn't produce any waste gases, ash or moisture. It is recommended that the heat pump be serviced annually by an accredited company to check things like the concentration of the glycol in the ground loops and that the system is still operating as efficiently as expected, but other than that it should be maintenance free.