Introduction
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.
GSHP technology
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
‘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.
‘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 can vary between 15m and 60m according to the design and soil characteristics.
‘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.
Heating
Space heating
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.
Water heating
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.
Environmental impact
CO2 emissions
| System |
Primary Energy Efficiency (%)
|
CO2 emissions
(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 | 100 | 0.21 |
| Electrical heating | 36 | 0.9 |
| Conventional electricity + GHSP | 120 - 160 | 0.27 – 0.20 |
| Green electricity + GHSP | 300 - 400 | 0.00 |
(Source: Sustainable Energy Ireland)
Refrigerants
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.
