Getting deep to hot rocks is important, but reducing the thermal resistance around the borehole is also important. Some schemes fracture the rock between two b

Getting deep to hot rocks is important, but reducing the thermal resistance around the borehole is also important. Some schemes fracture the rock between two boreholes, but this requires fiddly positioning of the wells as well as the fractures. Another interesting approach is to increase the thermal conductivity of the rock around a single well. Much of the thermal resistance is in the rock close to the well (as can be seen by examining the relevant integral), so this doesn't have to affect too far out to have a significant effect. A company XGS Energy recently raised $20 M in series "A" funding for this. Their fluid (which is forced into fractures around the borehole) is proprietary, but is thought to contain graphite dust. Graphite can be three orders of magnitude more thermally conductive than rock, so incorporating it into even narrow fractures can have a major effect. https://news.ycombinator.com/item?id=40434975 https://news.ycombinator.com/item?id=40434975 https://jpt.spe.org/hot-rock-slurry-developer-of-emerging-geothermal-tech-readies-for-field-tests https://jpt.spe.org/hot-rock-slurry-developer-of-emerging-ge... (the field tests mentioned there must have been successful, as they raised that $20 M subsequently.) Because this technology involves a single well that remains sealed from the surrounding formation, it could be used in existing played-out oil or gas wells, some of which go quite deep (although that's in basins with low geothermal gradients or else the fossil fuels would have been destroyed.)