Underground energy

Architects on the hunt for renewable technologies are increasingly looking to harness the Earth's power to heat and cool buildings, writes Hattie Hartman

Geothermal energy systems are heating up, so to speak, evidenced by projects such as Fletcher Priest Architects' Eastbourne Terrace, in which several systems were used. Increasing implementation owes to both the popularity of renewables, and to recent policy changes including measures like the Merton Rule – a guidance originated in the London Borough of Merton and that requires new developments supply 10 per cent of their energy through on-site renewable sources.
For its refurbishment of Eastbourne Terrace – five 1960s office blocks opposite London's Paddington Station, the architect chose to use a series of geothermal boreholes (also called ground-source heat pumps), which see pipes circulating water sunk into the ground beneath the development, using the Earth as a heat source in winter and a heat sink in summer. Although the technology of these systems has existed for a long time – 'even Harrods has one', notes Fletcher Priest project director Stephen Barton – even as of a few years ago there were relatively few. As of October 2007, Environment Agency data shows that some 140 geothermal schemes were proposed, under construction or operational in London alone.

 

Geothermal systems are either closed or open loop. In open-loop systems, water is pumped directly from the aquifer, where it is returned after cycling once through the building's heating system. Closed-loop systems are like an underground radiator, exchanging temperature by direct contact of
the pipes with the earth. They use a single well, with the water constantly recirculated. 'These systems are now on everybody's list if you need 10 per cent on-site renewables – you get quite a lot for the money and effort relative to other systems', says Barton.
At Eastbourne Terrace, Fletcher Priest opted for both open- and closed-loop systems. 'Open loops are better value for money because they generate approximately four times as many KW per £1 invested as closed systems', says Barton. However, they cannot be installed too close to one another
because of potential environmental impact on the aquifer.
The open-loop system is made of four 300mm-diameter boreholes approximately 150m deep. Two 'abstraction' boreholes contain pumps which pipe 15l of water per second from the aquifer to the plant room, where it passes through plate heat exchangers before returning to the aquifer through the
other two 'recharge' boreholes. Heat pumps distribute the warmed or chilled water to airhandling
units and four pipe fan coil units within the offices. Water exits the ground at around 14°C and is returned at about 20°C in summer, and at approximately 10°C in winter.
The main benefit of this approach is the reduction in energy usage of approximately 40 per cent compared with air-cooled chillers. No additional boiler or cooling plant has been installed at Eastbourne Terrace, and services engineer Stuart East of John Noad explains that system monitoring since completion has shown occupant comfort to be satisfactory – though groundwater temperature has increased by approximately 2°C, perhaps due to an unusually hot summer and two mild winters and the fact that the loads are used primarily for cooling.
The primary risks of open-loop systems are variable conditions within the aquifer; the uncertainty of predicting the actual water flow until the boreholes are complete; and the complexities of drilling. Because the process is noisy, pumping times are often restricted to certain hours. Open systems require a license from the Environment Agency, which regulates the proximity and size of projects in order to control their impact on changes in water level and aquifer temperature.
Unlike 40 Eastbourne Terrace, subsequent phases of the project came after the housing and planning minister Yvette Cooper called on developers to meet the obligations of the Merton Rule in a speech in June 2006. Aclosed-loop system was specified at 10 and 30 Eastbourne Terrace to provide 7 per cent
of the projects' energy loads. The other 3 per cent will be provided by photovoltaics integrated on brises soleil of the tower at 20 Eastbourne Terrace.
The future for geothermal systems, like all renewables, is hard to anticipate. There is an issue of localised capacity of the aquifer where schemes are sited too close together, but so far in the UK that is only pertinent to a handful of sites in London. More to the point is the market share of ground-source heat pumps as the price of other renewables technologies drops. 'What actually happens is anyone's guess. The market is so young, it is difficult to predict,' says Ric Horobin, water and environment director of groundwater consultancy firm Zenith International.

Click here for labelled diagrams of open and closed loop geothermal systems