Geothermal Energy Is Either Unsustainable Or Insignificant

I've started noticing news stories about the potential of geothermal energy in the UK. In Chapter 16 "Geothermal" of his excellent book Sustainable Energy - Without the Hot Air, David MacKay dismissed geothermal as either unsustainable or insignificant.

I would advise anyone who imagines that geothermal could be part of the UK's sustainable energy solution to read chapter 16 "Geothermal" starting on p109 of the SEWTHA PDF. The following points are noteworthy:

• 80% of geothermal energy comes from radioactive decay in the crust; 20% comes from heat flowing from the Earth's hot core.
• The amount of energy we can harvest sustainably is limited by the rate at which heat flows through rock and the (fixed) rate of radioactive decay.
• Typically 50mW/m² of heat arrives at the Earth's surface and the theoretical maximum we could capture is 17mW/m², if we drill holes 15km deep.
• If we harvest more than that the rock will cool down, diminishing how much we can harvest.
• To get anywhere near the theoretical maximum, we will need to fracture the deep rocks, aka fracking and minor earthquakes.

How Little Sustainable Geothermal Energy Is There?

The UK land area is 242,495km², ie 240x10⁹m².

So the theoretical maximum harvest-able geothermal energy across the total area of the UK is 4.1x10⁹W.

The UK population is about 65m: that gives us 63W per person available from sustainable geothermal energy, ie 1.5kWh per person per day.

https://en.wikipedia.org/wiki/Energy_in_the_United_Kingdom says the "Total energy consumption in the United Kingdom was ... 1,651 TWh in 2019".

Divided between 65m people over 365 days gives 1,651x10¹² / (65x10⁶) / 365 = 70kWh of energy consumed per person per day.

The theoretical maximum that geothermal energy could deliver sustainably in the UK is about 2% of our energy use.

Southampton Geothermal

To underline this result, Southampton District Energy Scheme supplies energy to 1,000 residential properties.

That's a small fraction of Southampton's total population of 250,000. Only 18% of the energy supplied comes from geothermal; 70% comes from natural gas; see https://en.wikipedia.org/wiki/Geothermal_power_in_the_United_Kingdom.

Unsustainable Geothermal

It's possible to extract geothermal energy unsustainably by mining it in an area until the rock is cold and then moving on. But that won't power the UK in the long-term.