Geothermal is like hydropower, economically speaking, but requires unusual geology. Basically the only place it can work on a large scale is in Iceland, home of a full third of the world’s active volcanoes.
Apparently, with advances in drilling technology, and developing the tech to fracture hot dry rock could greatly expand the range of possible geothermal sites.
There's at least one startup working on this:
http://www.greentechmedia.com/articles/altarock-breaks-new-g...
Apparently, with advances in drilling technology, and developing the tech to fracture hot dry rock could greatly expand the range of possible geothermal sites.