The 2010 'Getting warmer: a field trial of heat pumps' by the Energy Saving Trust would have lent support for the installation of ground source (GSHP) and air source (ASHP) heat pumps in areas of application where they are most effective - particularly off-gas, in new build and where a house has been well insulated.
The slightly more efficient GSHPs work by extracting heat from the ground and upgrading it to a useful temperature. In most instances GSHP works best with the 'low grade' heat associated with space heating rather than higher temperature DHW.
Heat pumps are only effective where the heat gained is a significant multiple of the electrical energy invested in running the pump. The ratio between the two is known as the 'Coefficient of Performance' (COP). The 'System Efficiency' ratio is slightly more onerous in that electricity for the entire heating system is included as the input. It is important to realise that manufacturers publish idealised COP figures as produced under laboratory conditions to EN14511. Above 3.0 is generally regarded as acceptable.
The EST report revealed that for a small proportion of GSHP installations, system efficiency figures of above 3.0 were achieved, but 'mid-range' came in at 2.3 - 2.5 - figures well below industry hype. Further analysis of the weaker performers suggested that installations had suffered from inadequate design and installation along with poor user understanding of controls.
Critically, the report proved that systems sized correctly and designed for simple operation, are the more successful. The EST recommends that carefully monitored installations should be carried out by companies who provide the complete service.
The Ground Source Heat Pump (GSHP) is a system that extracts heat from the ground, upgrades it to a higher temperature and releases it where required for space and water heating. The GSHP function can be reversed for cooling purposes.
A GSHP can be a highly efficient form of space heater, particularly where deployed in conjunction with a low energy heating system such as underfloor heating. Manufacturers claim (but see above) that for each kW of electricity used to run the heat pump some 3 – 4 kW of heat are typically produced.
The more usual ‘closed loop’ GSHP installation comprises of plastic piping buried in the ground and connected to a heat pump. A water or 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.
The upgraded heat from the GSHP can be used for space heating and / or water heating.
An example of a GHSP powered space heating system
Ground loop configurations
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.
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 can vary between 15m and 60m according to the design and soil characteristics.
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.
Because GSHPs raise the temperature to around 40° they are most suitable for underfloor heating systems or low-temperature radiators, which require temperatures of between 30° and 35°. Higher outputs, such as to conventional radiators requiring higher temperatures of around 60° to 80° can be obtained through use of the GSHP in combination with a conventional boiler or immersion heater.
The GSHP system is inadequate in itself for directly heating hot water output. Hot water for taps needs to be stored at 60° whereas for domestic GSHPs the maximum water storage temperature obtainable is 50°. A water heating strategy can be designed where the incoming water supply is preheated by the GSHP before reaching an ancillary heating source. However, it might be determined that an immersion heater working off off-peak electricity is more economical.
Primary Energy Efficiency (%)
(kg CO2/kWh heat)
Oil fired boiler
60 - 65
0.45 – 0.48
Gas fired boiler
70 - 80
0.26 – 0.31
Condensing Gas Boiler + low temperature system
Conventional electricity + GHSP
120 - 160
0.27 – 0.20
Green electricity + GHSP
300 - 400
(Source: Sustainable Energy Ireland)
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 approaching the market.
Pros & Cons
Efficient renewable heating and cooling systems
Carbon savings currently appear to be marginal, but positive if sytems are driven by renewable electricity
Life expectancy of 40+ years
Relatively high installation cost
Electricity drives the heat pump
If heating hot water, an ancillary electrical coil is required
Large site area required for horizontal pipe installation
The use of refrigerants
Manufacturers' claims of CoPs (Coefficient of Performance) of 3-4 are not generally being realised in practice, where CoPs of around 2 are more common
The installed cost of a GSHP ranges from about £800-£1,200 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.
The efficiency of a GSHP system is measured by the coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used to drive the compressor and pump for the ground loop. Typical CoPs range from 2.5 to 4. The higher end of this range is for under-floor heating, because it works at a lower temperature (30-35 °C) than radiators.
Based on current fuel prices, assuming a CoP of 3-4, a GSHP can be a cheaper form of space heating than oil, LPG and electric storage heaters. It is however marginally more expensive than mains gas. If grid electricity is used for the compressor and pump, then an economy 7 tariff usually gives the lowest running costs.
• Ensure maximum insulation
• Make an accurate assessment of the buildings likely heat loss
• Obtain a statement from the manufacturer quoting the COP at your designed operating temperatures (including both space and hot water heating).
• Design for use with green electricity for maximum efficiency
• Make a thorough survey of the site to identify buried services etc – then record the buried loops following installation.
• Check the soil type – different soils have different heat transfer rates
• Identify a heat distribution strategy that ensures maximum efficiency. For space heating go for underfloor heating followed by low-temperature radiators. When retro-fitting go for low temperature-radiators followed by conventional radiators in conjunction with electrical heating in a buffer tank.
• Be aware of the heat store potential of the structure. Floor slabs can store underfloor heat during off-peak electrical supply periods.
• Consider the need for space cooling
A useful introduction from the manufacturers Worcester Bosch, contains good footage of installation and the options available. (Warning: film makes CoP claims!)
• Geothermal Heat Pumps: A Guide for Planning and Installing, Karl Ochsner.
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