Ground Source Heat Pumps (GSHP)

Introduction

In theory, Ground Source Heat Pumps should be an efficient means of generating energy. However, in practice, as it would seem with most developing domestic 'renewable' energy technologies, theory doesn't always match reality. Manufacturers of GSHP sytems have been routinely promising COPs of 3-4, however the small amount of data available suggests efficiencies of much less. Some engineers even believe that the simple installation of efficient condensing boilers will realise less overall outputs of CO₂. Until more substantial data is available, specifiers of GSHP should be cautious of claims.

+ 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 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 an efficient 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

(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.

Costs

Capital

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.

Running

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.