Energy from Beneath the rocks
Perhaps unexpectedly, the sedimentary basin of Hampshire is associated with geothermal energy; and Britain's first geothermal power is used to heat areas of Southampton, including the civic offices.
Geothermal heat
Geothermal power is available from a number of sources, which are mainly
- active geological plate margins (such as Iceland with its pools of bubbling mud),
- volcanic regions, and
- stable areas underlain by radioactive materials relatively near the surface.
For instance, it is known that there are "hot rocks" below Cornwall, which would provide a lot of heat, if only a practical means of tapping that heat could be devised.
However, there is also heat below other, non-active regions.
Temperature around the world increases with depth, toward the unimaginably-hot (5500ºC) core of the Earth, from where newly-generated radioactive heat, heat of pressure, and "residual" or "remnant" heat of the Earth's formation 4.6 millions years ago, slowly leak to its surface.
On average, the temperature near the Earth's surface increases by about 1ºC with each 35m depth - or about 30ºC per kilometre - but is of course very variable, dependent on many factors including rock type and tectonic activity.
In South African gold mines, temperatures increase by 10-15ºC per kilometre, with the deepest mine being 4km. (This spawned an ice-making industry next door, who tip their product down the mine shafts to keep temperatures tolerable!)
Semi-thermal geothermal heat
Principle areas of potential geothermal aquifers in the UK
Where heated water can be tapped from below the Earth's surface, it is simply "circulating groundwater that is warmer than average". It can be found in "hyperthermal areas", where the water is heated to above 100ºC, or semi-thermal areas, where it is still very hot, but below the potentially-explosive threshold of 100ºC - which is the boiling point at "normal atmospheric pressure".
The latter is found below Southampton, and also in the Paris Basin.
This groundwater has retained its heat by virtue of an insulating "blanket" of Tertiary sedimentary deposits over the aquifer. There is no exceptional heat source underlying the regions, simply an exceptional amount of the 'background heat' from the Earth's interior is retained.
Because of this, accessing that heat is considered to be "heat-mining"; i.e., once used, it is only be replaced on geological times-scales. It is thus not "renewable", as are volcanically-active sources of geothermal energy.
Paris has been using groundwater from 1-2 kilometre's depth at 55º-70ºC since the 1970's for domestic heating.
Southampton's geothermal heat
Launched in 1986, Southampton Geothermal Heating Company was the UK's first geothermal energy scheme.
Research
Research was initiated in the late 1970's by the Department of Energy following the oil-price boom of that decade. They were consequently looking into the potential for alternative energy sources in the UK, particularly wind wave and geothermal energy. Their motivation at that time was financial cost, while now the environmental cost of burning traditional fuels is a major consideration.In pursuance of geothermal energy, the Department of Energy began drilling a well at the Marchwood power station on Southampton Water, hoping to test geothermal resources held in the Wessex basin. Following initial successful trials, a further well was drilled in the centre of Southampton. The Department of Energy then decided that the resource was too small to develop, and withdrew from the project. The Southampton City Council were reluctant to let the project fail, so found a partner with whom to develop the scheme - Utilicom, a French-owned energy management company.
The Water Source
Within the well, water was found at a depth of nearly 1.8km and at a temperature of 76ºC. Its reservoir is the Triassic sandstones, between about 1720 - 1780 million years old. The water rises naturally within the well to within 100 metres of the surface, from where it is pumped to the station. By the time it reaches the surface, the temperature of this a hot, corrosive brine is 2 degrees cooler. At the station a heat-exchanger working in conjunction with an absorption pump to transfer heat energy to clean water. The cooled brine, now at 28ºC, is then released into the River Test and thence the sea.
The present borehole has a life of about 20 years, from its initiation in 1986. To extend the scheme the company will need to tap into the aquifer elsewhere. 'The whole of the Hampshire basin is basically in the aquifer, so it's not just a question of Southampton being able to tap into it. Other people could repeat this exercise if they so wish'.
The District Heating scheme in Southampton
The geothermal heat provided by the well is used as part of Southampton's District Heating scheme , where it works in conjunction with the Combined Heat and Power scheme. Geothermal energy provides between 15-20% of the total heat-input into this scheme. The combined heat and power generators use conventional fuels to make electricity. "Waste heat" from this process is recovered for distribution through the 11km mains network.
The district heating scheme in Southampton helps reduce energy bills by 25% and the city's CO2 omissions by 10kt a year. It closely resembles a huge domestic central heating system, with hot, treated water circulating underground from the heat station. It supplies heat to a growing number of customers in the city centre, before being returned for reheating.
The borehole is non-polluting (at the moment the used brine is being pumped into the River Test) and costs little to run. The scheme produced enough electricity from its own generator to fuel itself, and there is electricity left over to sell to the electricity board. It can be adapted to take heat and power from other sources, such as existing generators or waste incinerators. Ironically the CEGB power station on which the first test bore was drilled has now closed down for economic reasons.
In April 2001 Southampton Geothermal Heating Company were delighted to win the Queens Award for Sustainable Development for this scheme.
For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk