A study by the German Aerospace Center, commissioned by Greenpeace and Europe's Renewable Energy Council, claims that alternative energy sources could provide nearly 70% of the world's electricity and 65% of global heat demand. The report, Energy Revolution: A Sustainable World Energy Outlook, considers variations in geographic conditions and energy sectors.
And another report prepared by a panel led by MIT, The Future of Geothermal Energy, maps how geothermal energy can be used on a large scale to generate electricity from steam produced at depths of 5,000 feet or more within the US. Ground source heat pumps for space heating are already a reality at the scale of individual homes or institutional buildings (for illustration, have a look at the UOIT thermal energy system in Ontario), but this proposal suggests it's feasible to generate significant quantities of electricity by drilling deep into hot rocks, fracturing them and pumping water in to produce steam.
Update on 26 February 2007: Switzerland is developing an energy source using this method, but it was recently reported that pumping water into the wells of the Deep Heat Mining project caused an earthquake reading 3.3 on the Richter scale.
