Question: how far down do you have to put pipes down to efficiently extract power?

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  1. It depends on what temperature you need and something called the “geothermal gradient” – that’s how fast the temperature increases with depth in the Earth. So lets say you wanted to generate electricity and you needed water at 120 degrees C to do this. In most parts of the Earth, the geothermal gradient is about 25 degrees per kilometre. Lets say the temperature at the Earth’s surface is about 20 degrees (which is a reasonable average for Perth, where I live). So we need to dig far enough down to get to a temperature that is 100 degrees hotter than at the surface (120 = 100 + 20). With a geothermal gradient of 25 degrees per kilometre, that means we would have to dig down 4 km, which is a long way to put a pipe! That’s why geothermal electricity generation is only done in places where the geothermal gradient is much higher than average. This happens in volcanically active areas like New Zealand, where hot water actually comes to the surface in some places.

    In Australia we don’t have volcanically active areas, and the geothermal gradient is generally not high enough to do electricity generation. But we can use geothermal energy to do other things that do not need such high temperatures, such as heating and cooling of buildings. There are places where you can get high enough temperatures to do those things at just 1-2 km depth, which is much more reasonable. By the way, we wouldn’t actually put a pipe down the hole. We drill 2 holes, one to pump hot water out, and the other to pump the colder water back into the ground after we’ve used it. Its called an open system – no pipes involved.