Geothermal: Case Studies
Ground-source heat-pump system - Buntingsdale Infant School, Shropshire
A ground-source heat pump installed at Buntingsdale Infant School removes heat from the soil and delivers it to the school in an upgraded form. A brine solution, chosen for its anti-freeze properties, is pumped around an underground loop of plastic pipework to extract heat from the ground. This heat is then upgraded to a useful temperature by the heat pump before being distributed around the building by fan-coil units. The overall system coefficient of performance is three, meaning that three units of heat will be delivered to the building for each unit of electricity consumed.
Local geological conditions meant that installing the underground pipework in boreholes was not possible, so fifteen 50-metre-long trenches, 1.3 metres deep by 1.5 metres wide, were dug in a farmer’s field adjacent to the school. The pipework was arranged into ‘slinky coils’ 1.2 metres in diameter and laid horizontally at the bottom of the trenches. Eighteen fan-coil units of varying sizes are used to distribute heat around the building. These units vary in size according to the heating requirements of each room and are far smaller, less intrusive, more controllable and quieter in operation than the electric night storage heaters they replaced.
Heat pumps can use synthetic refrigerants or water to enable heat from one source to be upgraded to a useful temperature and distributed as required. Some synthetic refrigerants have a high global warming potential, but are in the process of being phased out. The heat pump installed at Buntingsdale uses propane as its refrigerant, which is derived from liquefied petroleum gas. Propane does not damage the ozone layer and has a negligible global warming effect. It can therefore be classed as a green refrigerant.
Geothermal combined heat and power (CHP) scheme - Southampton: Geothermal energy
In the 1970s, a Department of Energy research programme identified possible sites of deep geothermal aquifers that contained water at a temperature high enough to provide heating for a number of buildings. One such site was at the Marchwood power station on Southampton Water. A second well in the centre of Southampton itself found water at a depth of nearly 1,800 metres and at a temperature of 76°C. However, the size of the resource was deemed too small to develop the planned large-scale district heating scheme. The project was abandoned by the Department of Energy. At this point, the City Council went into partnership with Utilicom, a French-owned energy management company with experience in this area. They developed the scheme, which is now a thriving and expanding £4 million multi-source heating and chilling system.
The original well, which currently provides about a fifth of the system’s heat input, operates alongside CHP generators. These use conventional fuels to make electricity. The waste heat from this process is recovered for distribution through the 11-kilometre mains network. Southampton’s scheme also has conventional boilers for top-up and standby needs. More than 20 major consumers in the city centre are now served by the district heating scheme, including the Civic Centre, four hotels, the Royal South Hants Hospital, Southampton Institute of Higher Education and an ASDA superstore. Recent developments include a district chilling system using the latest absorption chilling technology.
The geothermal well remains at the heart of the scheme. However, the current rate of flow is expected to decline eventually. The scheme delivers more than 30,000 megawatt hours of heat each year, alongside 4,000 megawatt hours of electricity sold from the generating plant plus 1,200 megawatt hours of power providing chilled water on tap.