Today, i’d like to entertain you with recent studies with improvements of geothermal technologies.
As you know geothermal energy use the heat from Earth crust/core to provide energy (heat, electricity or even chill). Most common applications for generating electricity are steam power plant (flash or dry steam) and binary power plant. In both cases, water is use as a thermal media, heated by the rocks and then pumped to the surface to be used directly in steam turbine or go through a heat exchanger and heat/vaporize a second fluid (working fluid), in the case of binary power plants.
If finding temperature on the deep ground is not a big problem (generally between 600m for volcanic regions and up to 6000 m for new generation of European deep geothermal projects), the key problem is to find water in the reservoir, and a good permeability so that the reservoir is not quickly depleted. Some technologies in development, known as EGS (for Enhanced Geothermal System) consider injecting water in some wells and pump it in others (see the project of Soultz, in East of France, a pioneer but costly pilot plant using this technology).
Recently, some researchers have been looking into using transcritical CO2 as a thermal media, instead of water. Transcritical means that the CO2 works about its critical point : at high pressure and high temperature it is not a gas, nor a liquid, but something in between and physical properties of the fluid are different from the two previous states. Some of the properties of transcritical fluids can be interesting, like low viscosity leading to low pressure drops.
CO2 is interesting for several reasons : it is a non dangerous, natural and cheap fluid ; its is abundant, not patented and free of use ; it is transcritical at relatively low temperature (but still high pressure); we would like to reduce its concentration in the atmosphere, and it is very likely that we would have to store some into the ground, to reduce global warming.
Some researchers at Berkeley National Lab in California (USA), have proposed an interesting technological solutions, where transcritical CO2 is used as thermal fluid to recover the heat from the deep (3200 m), then pump it and expand it directly in a CO2 expansion turbine
While costs of drilling cannot be neglected (you may consider roughly 5 Meuros per well), the system offer the possibility to store some CO2 in the process in the dead volume of the reservoir.
In area where the earth crust is hot, but very no/low water (typically where EGS would be useful) use of CO2 could be interesting.