Sometimes I astonish myself with how stupid I can be.
At Protons for Breakfast last autumn I stated that geothermal energy supplies could only ever extract around 0.1 watts per square metre of ground. Why? Because that’s the average rate at which heat rises through the Earth. This is a ridiculously low figure, and I couldn’t understand how it made any sense. And then it clicked.
- Geo-thermal energy isn’t sustainable, at least on a strict definition. It’s a one-shot operation that cools a reservoir over a decade or so, and then waits a few decades for the reservoir to heat up again.
- Ground Source Heat Pumps don’t capture Geo-thermal energy – they capture Solar Energy and there is on average over the Earth around 240 watts per square metre of solar energy available.
I realised that I had confused these two sources for years, and browsing the web makes me think a lot of other people have too. Let me explain.
Geothermal energy is the heat flowing outward from the centre of the Earth. It arises in part from the radioactive decay of elements within the Earth, and in part from heat left over from the formation of the Earth. The Earth’s crust is an excellent insulator and the heat only flows out slowly. Across the UK, the average heat flow is just 0.038 watts per square metre, which means that in order to generate the 10 kW one needs to heat a house, one requires an area of around 500 metres x 500 metres – and drilling down doesn’t increase this figure at all. However there are two situations in which we can extract this heat and use it.
- In areas of the Earth where the heat flow is stronger than average (e.g. Iceland) it would be perverse not to exploit the gift of heat.
- The second situation is more common and involves drilling down to around 3 km depth where the temperature is around 100 ºC. If the rocks are porous at this depth then we can pump cold water in to the rocks, and extract hot water. The porosity of the rock allows the water to accept heat from a large volume of rock. This is not strictly sustainable in that the block of rock will cool down over a few years, and we will need to leave it to warm again. But it will keep warming up for millions of years to come.
Both these schemes extract genuine geothermal energy, which originated on Earth, roughly one half it, nuclear in origin.
Ground Source Heat pumps collect heat in the top few metres of the Earth and here the temperature is strongly affected by the temperature of the surface. And this is determined by the local average heating from the Sun, typically around 240 watts per square metre. Even in areas with cold winters, if one digs down more than a few metres, the soil stays warm , and a heat pump can be used to extract this heat. The pump chills a fluid such as ammonia to around 3 °C and then passes it through the buried pipes. Being colder than the surrounding soil, the ammonia absorbs heat from the soil, and evaporates. When compressed, the heat is released at a temperature of around 20 °C which can be used to heat a house.
However the source of the energy is the Sun not the Earth, and so 100% of it comes from nuclear fusion.