Geothermal Energy: More Unanswered Questions

Mike Smith, Executive director of Southampton Geothermal Heating Company, in the engine room at the CHP plant

Mike Smith, Executive director of Southampton Geothermal Heating Company, in the engine room at the CHP plant

Having written yesterday about the unanswered questions regarding the Newcastle geothermal borehole, I was contacted by the eminent librarian and friend Bernard Naylor who said (paraphrasing):

Don’t be so skeptical – geothermal energy is alive and well and living in Southampton!

Suitably rebuked, I read about the Southampton scheme and the features that have made it successful. However I find myself suspecting – there goes my suspicious mind again – that the geothermal aspects of the Southampton project are actually irrelevant to its success.

The Southampton scheme is a Combined Heat and Power (CHP) Station. It is a small station generating around 2.5 MW of electricity from mainly gas and a little oil. It’s not as efficient to make a small power station as a large one – say 1000 MW, and the Southampton scheme appears to turn about 35% of the raw chemical energy of the gas and oil into electricity, compared with perhaps 45% or higher for large gas stations. However the Southampton system can play two trump cards. Firstly, it is in the middle of the City, not miles from town, so less energy is wasted (as heat in the wires) transporting the electricity. Nationally around 10% of electrical energy is lost this way. Secondly, the waste heat from electricity generation process is not wasted, but used to heat water to 80 °C which is circulated through insulated pipes to nearby hotels, shopping centres and flats. Overall, the CHP plant utilises something like 75% of the energy in the fuel. The scheme combines entrepreneurial spirit, local political infrastructure planning; and European Union and National funding. This combination appears to have nucleated a project which is an economic success, a source of local pride, and demonstrates a less carbon-intensive way to live in cities.

Where does the geothermal energy come in? Well scheme nucleated  around an old test borehole, but it is unclear exactly how much thermal power is extracted from the borehole. I have read claims that

If we guess these figures are approximately correct, this would imply an annual energy production of (say) 18% of 30,000 MWh which is 5400 MWh, equivalent a continuous heat output of around 600 kW. At an output temperature of 80 °C, this requires around 2 litres per second of water, or 172 cubic metres per day. If this really is the resource available, then I declare myself to be officially amazed. But I doubt that it is. It may been initially available from test boreholes but my guess – and it is a guess based on the British Geological Survey Estimate of the available heat flow outside of Cornwall – is that actual output is just a few percent of this value. If you have any information on this I would love to hear about it!

My ex-colleague Stephan, commented on yesterday’s story to say that in Germany/Munich they have local geothermal generation (possibly this scheme?) which initially produced 118 litres per second of water at 130 °C (presumably under pressure). This is a phenomenal energy output: around 50 MW! I am sure the output temperature would quickly decline as water was drawn from such a resource. As Stephan comments, this not harvesting a sustainable resource, but simply cooling a large block of rock. This report also comments that the output of the Southampton scheme will decline but I have not read whether the decline is over years, decades or centuries. My guess is years.

Is it worth it? Well this brings me back to the basic point I made yesterday.

  • However much we might like geothermal energy to be implemented, it will not happen until it makes economic sense.

If one has a hole in the rock out of which a large thermal resource flows, one would be bonkers not to use it. In Southampton, the geothermal element of the project may helped initiate the CHP project, but it is now (IMHO) largely irrelevant. The key feature of the Southampton scheme is that it is using precious carbon resources around twice as effectively as the rest of the UK. This is the real success that should be trumpeted and praised and, most importantly, copied.


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