How can geothermal energy be extracted?

Question

How can geothermal energy be extracted?

Answer 1

Geothermal energy uses the heat contained in our earth crust for valuable purpose.

Please note that it is also possible to do the contrary and cool down using the cold (low heat level) of the ground. e.G.: A so called "canadian well" let the hot air in the summer circulate in the ground to cool it down before letting it enter in a building. Under 1m or 2m, the earth temperature is no longer influenced by seasons and has the average of the year, for example 13°C. The same way, you can let preheated air at this temperature in instead of opening a window and letting air ar 0°C or below enter.

The geothermal energy can be of four kinds:

- solar heat of the top part of the ground (with a temperature depending on the season)

- the almost constant temperature through the year at a low depth (under 1-2m): resulting from thermal inertia

- the high temperature at low depth resulting from radioactive decay of the radioactive elements contained in our earth crust

- volcanic activities and magma close to the crust.

Now TO EXTRACT IT:

You need a working fluid to transfer the heat to (water, refrigerants, alcohols, etc...).

You need a difference of temperature for the heat exchange to take place: -> You need to circulate the working fluids, for example in pipes in a well.

If you want a large harvest of the energy, you will need a long path for the water which you don´t have with a single well of a diameter around 1m. This is the reason why most geothermal fields of large power output have an injection well and an extraction well. Even if you had a source like the geysers in Yellowstone you would need to "refill with water" at some point if you don´t want to deplete the source from its hot water.

So now how to circulate the water between both wells ? if no natural geological cracks exist to create a path for the pressurized water you will need to create one, which means rely on TNT or others to create a mini-earthquake and break cracks in the rock layers.

Another problem: if you decide to directly harvest some natural hot springs, you might face high corrosion for some sources... therefore a binary system is often needed (two circuits in order not to pollute the one that has the expensive equipments like a steam turbine etc...), just  like in a nuclear power plant with a contaminated circuit and a clean one and a heat exchanger between both.