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How Ground Source Heat Pumps Work

Just below the ground’s surface, 1 - 2m deep, the ground has a year round temperature of between 8°C – 12°C, varying only slightly between seasons

The Ground Source Heat Pump is a system that extracts heat from the ground. A water or water antifreeze mix is pumped around a circuit of buried pipe work taking the low grade heat from within the ground. This low grade heat is then upgraded to a higher temperature through the utilisation of a refrigeration gas within the heat pump circuit.

This gas is compressed and forced through the heat pump circuit before the pressure is released in the condenser along with the heat where it is required in the internal house pipe work circuit. The heat is now used for space heating and water heating.

The heat pump function can be reversed for cooling purposes; this is particularly useful for commercial applications like in offices where cooling is needed on hot summer days.

Ground source heat pumps are a highly efficient method of providing hot water and heating for your home.

Due to the constant ground temperature a high coefficient of performance (COP) can be achieved even at low outdoor temperatures. This results in a year round high seasonal performance factor (SPF.)
Manufacturers continually boast high figures in COP & SPF in order to sell their heat pumps, however the market is much more tightly regulated than it used to be and we have no reason to doubt that a well designed, well tuned heat pump installation could not achieve these figures.

Ground source heat pumps are considered more efficient than air source heat pumps and typically have a higher SPF

There are two different types of ground source heat pump installation:
Closed Loop System
Open Loop System
The more usual ‘closed loop’ Ground source heat pump

Two examples of a ground source heat pump can be seen below. The ‘closed loop’ installation comprises of plastic piping buried in the ground and connected to a heat pump.

A water-antifreeze mixture is passed around the looped pipe where it absorbs heat from the ground. The fluid flows into an electrically powered heat pump, comprising a compressor and a pair of heat exchangers before discharging back to the underground loop.
‘Horizontal loops’
Piping is installed horizontally in trenches. The depth of the trenches will vary according to the design and soil characteristics, but is generally 1.5 – 2m deep. Horizontal loops require much more surface area than vertical loops.

Around 200m of pipework is generally required for a single dwelling.

Video shows a horizontal loop system

‘Slinky coils’
The ‘Slinky’ is a variation of the ‘Horizontal loop’. Slinky coils are flattened coils of overlapping piping, which are spread out and laid either horizontally or vertically.

Their ability to focus the area of heat transfer into small volume reduces the length of the trenches and hence the quantity of land needed. A 10m long trench laid with a ‘Slinky’ coil will typically supply 1kW of heating load.


‘Vertical loops’
Most commercial and institutional projects using GSHPs use ‘Vertical loop’ systems. The advantage of a vertical loop system, which consists of pipe inserted into vertical bore holes, is less space is required.

Holes are spaced at around 5m intervals and usually vary between 15m and 60m according to the design and soil characteristics.

However, some vertical loop heat pump systems can be drilled as much as 200m down. Each system is designed to the specific building the heat pump is intended to provide heat for. Generally a series of shallower bore holes are preferred to one or two really deep ones.

Video shows a vertical loop system
 
The less common open loop ground source heat pump system

In an open loop (or water to water) ground source heat pump, ground water is abstracted, usually from an aquifer or possibly from an adjacent river and passed through the heat pump before being returned to the ground or river.

A constant supply of water is needed. This type of ground source heat pump system is highly efficient because of the relatively high temperature of ground water, but the installation can be more complex and require regulatory approval from the environmental agency.

Open loop ground source heat pump systems are usually confined to larger commercial buildings rather than domestic properties.

 
 


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