Geothermal Energy: Unanswered Questions

Professor Paul Younger (left) and Professor David Manning from Newcastle University prepare and test thermal water from their 400-metre borehole at Eastgate in Weardale. Photograph: Mike Urwin

Professor Paul Younger (left) and Professor David Manning from Newcastle University prepare and test thermal water from their 400-metre borehole at Eastgate in Weardale. Photograph: Mike Urwin

The BBC reports today on the commencement of a Geothermal Energy project in Newcastle. The academic in charge, the esteemed Paul Younger, says that ‘geothermal energy operates 24/7’ and the reporter happily parrots the factoid that in July the borehole will produce ‘water with a temperature of 80 ºC’. And as I listened and read, I searched for the fact that would change this from PR for Newcastle University into reporting. The reporter should have asked ‘How much?’. As in:

  • How much water will be produced at  80 ºC each day?

My suspicion – and please forgive me for being suspicious – is ‘not much’. I can boil water with a magnifying glass on a sunny day, but not much! In order to make sense, such a scheme needs to capture not just hot water, but lots of it!

What is your point Michael?: Now I think this is a great research project and I am glad that we collectively are funding it. But except in a rather small number of locations, the amount of heat rising up through the Earth per unit area of Earth’s surface is tiny: somewhere close to 0.1 watts per square metre. In the UK the average is less than this, excluding Cornwall the measured values are in the range 0.058 ± 0.016 watts per square metre. So to produce an output of (say) 100 kW(thermal power) requires the collection of the heat from an area around 1 kilometre x 1 kilometre. This either requires a complex arrangement of boreholes, or the exploitation of water seepage through natural cracks in the rock. All in all it amounts to a complex engineering structure and it does not run itself. Water must be pumped, cleaned, and returned or disposed off.  If the thermal output were 100 kW at 80 ºC this would involve the production of 400 millilitres of water per second, or around 34 cubic metres of water per day.

Cost: My (optimistic) guess is that such a piece of infrastructure might cost on the order of £1million to construct. It could replace the heating of 33 standard 3kW immersion heaters (Capital Cost ≈ £500) or around 5 standard domestic gas boilers (Capital Cost ≈ £15000). I can’t guess the running the cost of the geothermal project, but if we imagine that the £1million must be repaid at 0% interest over say 10 years the cost is ≈£100,000 per year.  If we imagine we need to maintain the facility or manage it in some way or pay for the pumps then the cost would be more than this. For immersion heaters, the running cost is £175,000 per year assuming one pays 20 p per kWh. And for gas boilers the running cost is £40,000 per year assuming one pays 4 p per kWh. So the capital costs of a geothermal project are enormous, and the running costs are similar or higher than conventional options. If a council said it was going to take £1million of its residents’ money to invest in such a project, but that there would be no financial benefit to the residents, then I think we all know what would happen to the council.

So what?: Now the carbon costs of the geothermal project are difficult to estimate – drilling through rock is amongst the most energy intensive activities conceivable! – but I have gone through this analysis to make one simple point.

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

And it won’t make economic sense until energy prices rise way beyond current levels.



6 Responses to “Geothermal Energy: Unanswered Questions”

  1. Bernard Naylor Says:

    Michael: I was quite startled by what I thought was the grossly hyped account of the new Newcastle project. Geothermal energy has been an increasingly important factor in the energy consumption of Southampton since the late 1980s. The ramifications of the project are extensive and still expanding. It long ago ceased to be merely experimental or investigative. There is a write-up on the Greenpeace website. The biomass power station is a little behind the schedule suggested in the Greenpeace article but construction has started, I think. Try googling ‘Southampton geothermal energy’.
    I really enjoy your blog!
    Bernard Naylor

    • protonsforbreakfast Says:

      Bernard. Hello. I trust you are well.

      I saw some references to the Southampton project as a I read about the Newcastle borehole. I will check it out but my first response is this: The basic physics is very simple. In equilibrium one cannot extract more heat than is being released from the Earth which amounts to around 0.1 W per square metre of the Earth’s surface. If a scheme extracts more than this it simply cools a large block of rock – in which case this is not sustainable energy. If one cools one cubic kilometre of rock by 50 Celsius it will contract by a linear scale factor of around 50 cm: – expect cracks!

      All the best


  2. Bernard Naylor Says:

    Is it partly a matter of ‘in theory’ and ‘in practice’? In theory, the energy from the sun is finite; it wll burn itself out eventually. In practice we can treat it as though it is infinite, because the practical consequences of its ‘finiteness’ are so far in the future as to be meaningless for our practical purposes. There are no signs, so far, of earthquakes occurring in this area. Or are we like the man who fell from the 100-storey building and, as he passed the fiftieth floor, could be heard to say: ‘So far, so good’?

    I shouldn’t really tangle with a physicist, especially one as bright as you. But ho, hum; there you go.

    All the best


  3. Stephan Says:

    Hello Michael,
    interesting thoughts. It made me look up our local city works’ web page. They have three of those geothermal plants, just collecting hot water from deep under ground. The power is 9 MW each, where the hottest one delivers 5 MW electrical and 4 MW thermal. The others only deliver heat. And yes, it is just about cooling some large block of material, which means that each hole can only be used for about 30 years (says the company). And they do worry about micro-earthquakes.

  4. Geothermal Energy: More Unanswered Questions « Protons for Breakfast Blog Says:

    […] Protons for Breakfast Blog Making sense of science « Geothermal Energy: Unanswered Questions […]

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