Ground Source Heat Pumps

Although the idea of using “heat pumps” may seem a little out of the ordinary, the technology is something with which we are all very familiar – the same principle drives our fridges and air conditioning. Just as these common-place examples remove heat from where it’s not wanted, leaving cool air behind, ground source heat pumps do the same thing – only in reverse.

It could hardly be simpler or more energy efficient; by moving warmth from the ground to where it is wanted to warm our homes and buildings, this approach generates three or four units of heat for every one unit of electricity used to drive the system’s circulating pump.

How Does It Work?

Unlike geothermal energy – which makes use of hot rocks and is restricted to particular parts of the world – ground source heating can potentially be used throughout most of the UK and is particularly suitable for projects aiming for very low-carbon heating. It relies on two things; firstly, the sun’s energy heating up the soil in the first place and secondly, the fact that the resulting temperature a metre or so below the surface remains fairly constantly around 10 degrees C, even in the depths of winter.

Ground source heat pumps enable us to tap into this renewable energy source to keep our houses warm. There are three main elements to the system – a ground loop, the heat pump itself and finally a heat distribution system.

The ground loop is simply a length of pipe buried either within a vertical borehole or a horizontal trench. Trenches are usually dug between one-and-a-half to two metres below ground level and the pipe is frequently laid in a spiral – often referred to as a “slinky coil” – to maximise the size of the ground loop. Although they need more land area than boreholes, for small individual systems, they are typically the less costly option of the two.

As a broad guide, a typical three bedroom, new-build house would probably need a total trench length of around 60 metres, while a borehole system would normally need to be dug between 15 and 150 metres down into the earth. However it is arranged, the loop is a closed circuit, containing a water/antifreeze mixture, which picks up heat from the ground as this liquid is pumped around the loop.

The heat pumps themselves are made up of three parts – an evaporator, compressor and condenser – which are collectively responsible for first extracting the heat from the ground loop and then subsequently transferring this heat to the water tank feeding the distribution system. This usually consists of either radiators or under-floor heating; ground source heat pumps are best partnered with systems which run at lower temperatures than used in conventional radiator/boiler combinations, making them particularly suitable for under-floor arrangements

Costs and Benefits of Heat Pumps

Dependent on the type of building and its location, a typical ground source heat pump installation – excluding the cost of the distribution system – would run to around £6,000 – £12,000. This is, clearly, far more expensive that conventional heating systems, but in their favour, heat pumps have remarkably low maintenance costs and an achievable lifespan of 20 years or more in which to provide sustainable and environmentally-friendly warmth.

According to the Energy Saving Trust, compared with heating homes with electricity, the use of heat pumps could achieve annual savings of nearly £900 and avoid the release of almost seven tonnes of carbon dioxide a year per household.

One of the best bits of news, however, is that ground source heat pumps can qualify for funding, under the UK’s Low Carbon Buildings Programme from the Department of Business Enterprise and Regulatory Reform (BERR).

Phase 1 of the scheme runs until 2010, replacing the previous DTI Clear Skies and Solar PV grant programmes and is open to households, public, not-for-profit and commercial organisations. There is a second phase of this programme, which opened in April 2008, but it is open to public and not-for-profit organisations only, allowing them to apply for 50 per cent of the installation costs.

Ground source heat pumps offer a remarkably efficient way to heat buildings – which becomes even greener when coupled with either green tariff power, solar PV or other renewable generation to provide the electricity needed to drive their circulating pumps. With the chance of grant aid for their installation, low running costs and the promise of significant year-on-year savings, it’s well worth considering heat pumps for any community energy project.