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Ground Heat Pump

How it works

There are three important elements to a ground source heat pump:

The ground loop This is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped around the pipe absorbing heat from the ground.The ground loop can be:

Vertical, for use in boreholes

Horizontal, for use in trenches

Spiral, coil or 'slinky', also for use in trenches

A heat pump In the same way that your fridge uses refrigerant to extract heat from the inside, keeping your food cool, a ground source heat pump extracts heat from the ground, and uses it to heat your home. A ground source heat pump has three main parts:

The evaporator, (e.g. the squiggly thing in the cold part of your fridge) absorbs the heat using the liquid in the ground loop;

The compressor, (this is what makes the noise in a fridge) moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit;

The condenser, (the hot part at the back of your fridge) gives up heat to a hot water tank which feeds the distribution system.

Heat distribution system This consists of under floor heating or radiators for space heating and in some cases water storage for hot water supply.

Ground Source Heat Pumps (GSHP)

Introduction

+ Highly efficient renewable heating and cooling system

+ Carbon savings of 30-35% (more if the pump is powered by renewable electricity)

+ Life expectancy of 40+ years

- Relatively high installation cost

- Electricity drives the heat pump

- large site area required for horizontal pipe installation

- the use of refrigerants

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. 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

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.

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.

Design notes

Ensure maximum insulation

Make an accurate assessment of the buildings likely heat loss

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

References; http://www.greenspec.co.uk/html/energy/GSHP.html

http://en.wikipedia.org/wiki/Geothermal_exchange_heat_pump

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