Great question! It depends on what kind of geothermal power you are trying to use. Basically there are two ways we can use geothermal energy: for heat, called ‘direct heat’ originally enough; and for electricity, which is usually just called ‘geothermal power’. If you just want direct heat and no electricity you only have to dig a few meters down to build a shallow geothermal system called Ground Source Heat Pumps (GSHP). This uses the slightly warmer temperatures of the soil and shallow rocks converted to heat energy for your home. If you want to make electricity you have two choices – traditional geothermal, like they have in Iceland, New Zealand and the Philippines, often called ‘high enthalpy’ (which really just means high temperature, but also means the rocks have lots of water and a high enough pressure for the hot water to be forced out) or Enhanced Geothermal Systems (EGS), where the rocks are hot enough, but don’t have the water and/or the pressure.
In the project in Cornwall geologists are using EGS to get at the high temperatures of the granite, then creating a system to get the hot water out of the rocks because there isn’t enough pressure to force it out (luckily or we might have geysers all over Cornwall!!). In order to get to the rocks that are hot enough, geologists are having to dig two holes (called ‘wells’) deep into the granite, one deeper than the other. The shallower well is being dug to about 2.5km deep, the deep well will be around 4.5km deep. To give you an idea of how deep that is – Ben Nevis, the tallest mountain in the UK is around 1.3km high and the tallest building in the UK, the Shard, is only 310m high. So this well would be about 14 times the Shard deep!