Sustainability

It’s not often that innovative technology is specified on PFI schemes. “Stick to what you know” tends to be the mantra, which is hardly surprising when you consider that PFI consortiums have to carry the can for any building malfunctions for 30 years.

A police headquarters in the west of England is a notable exception. The Reliance–HBOS PFI consortium is specifying the UK’s largest geothermal heating scheme at Gloucestershire Constabulary’s new home at Quedgeley. The system will enable the police to slash their carbon emissions and cut the costs of heating and cooling.

Geothermal heating and cooling was adopted because multidisciplinary consultant McBains Cooper persuaded the police and the consortium that it was the best method of meeting the sustainable brief. The requirement was for a low-energy building that had comfortable internal temperatures throughout the year.

“We didn’t want to use natural ventilation as there was a risk that the temperature would exceed 24°C in the summer, leading to penalties under the PFI contract,” says Anthony Coumidis, director of engineering services at McBains Cooper. “To guarantee low temperatures we’d need air-conditioning, which would have made a high BREEAM rating difficult to achieve.”

The opportunity to save money exists because the system used the heat and coolness that are already contained in the ground. The system features a geothermal ground loop based on 180 boreholes. The loop consists of plastic piping containing water that absorbs the stored heat or coolness as it moves through the system (see technical diagram).

“Open” loops may also be used to draw water from a lake or underground aquifier. Coumidis says this method is only appropriate if there is an excess of water below ground such as there is in London.

The depth of the boreholes depends on the conductivity of the ground. The substance beneath the police headquarters, clay, is a poor conductor and so boreholes had to be sunk 98 m to ensure that enough heat was extracted.

The soil temperature below 1.8 m varies between 10° and 13° centigrade during the year. A heat pump upgrades this low-level heat energy extracted from the ground into higher level heat energy within the building. The concept is similar to a fridge, which pumps heat out of its interior at 5°C and dissipates it within the room at 20°C. With a geothermal heat pump, the heat is extracted at 10°C and transferred to the heating system at about 50°C. In the summer, heat is drawn from the building by the heat pump in reverse cycle and replaced by coolness from the ground.

The cool water is then pumped around the air-conditioning system.

When the Gloucestershire scheme is completed at the end of 2005, the system will deal with a peak heating demand of 646 kW and a peak building demand of 646 kW in the summer.

The IT room also requires a further 125 kW of cooling, which will be provided by another cooling system incorporated within the ground loop circuit.

In an air-conditioned office that used a heat pump as the primary source of heat generation, a geothermal system will generally produce 40% less carbon dioxide and consume 50% less energy than a good gas-fired system. This is because the amount of energy produced is much higher than the energy consumed by the heat pump.

The system also offers the advantage of reduced plant. Coumidis says the heat pumps in the Gloucestershire scheme occupied the space of nine filing cabinets, enabling McBains Cooper to design a curved roof free of chillers and flues.

The use of the geothermal heating system in Gloucestershire has attracted the interest of other clients. In Kent, McBains Cooper has recommended the use of a geothermal system for Northfleet Headquarters and Custody centre. The system incorporates boreholes in the building’s piles because contaminated land prevented drilling beyond the building’s footprint. Coumidis says this method is potentially cheaper but it requires close collaboration with the pile contractors and construction engineers.

Geothermal heat pump systems are more expensive to install than standard heating systems, but the lower running costs lead to reasonably short payback periods. The fact that these systems have been specified for reasons of cost as well as sustainability by risk-averse PFIs, suggests that geothermal systems could soon be as popular in the UK as they currently are in the USA.

Gloucester Police HQ project team

architecture, structural and civil engineering, M&E engineering, quantity surveying McBains Cooper
geothermal consultant Geothermal Heating
borehole subcontractor Gloucester Raeburn Drilling
main contractor Britannia Construction