PowerSwitch

See:
http://pesn.com/2008/09/04/9501492_Rase ... -turbines/

Extracts:
Instead of using electricity to turn a heat pump to cool a liquid, Raser runs a warmed liquid through a heat pump to generate electricity. With their rapid, low-cost deployment capability, they could maximize the U.S. geothermal capacity within ten years.
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Raser Technologies, Inc. (NYSE Arca:RZ) of Provo, Utah, USA is making huge strides in cost-effectively harvesting geothermal energy.
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This same technology by Raser can be used to harvest waste heat from power plants, whether nuclear, natural gas, coal, or even solar. The left-over, low-grade heat will work just fine to drive the UTC generators manufactured for Raser. This otherwise unused source of energy could probably answer for yet another five percent of the U.S. energy portfolio

In suitable location this has much to commend it, however in the UK such locations are rare.
To generate electricity, a substantial temperature difference is required.
The tradditional way is to use hot sub-surface rocks to boil water into steam, which is then used in a steam turbine to turn an alternator, this has been done for decades in areas with rocks that are hot enough and near enough to the surface.
In some systems naturally occuring sub-surface water turns into steam which only requires piping to the surface.
In other cases the rocks are hot and dry, and water must therefore be pumped down a well, where it turns to steam and emerges from a second well.
Both systems, though apparently green tend to upset NIMBYs since they consider that the subsurface water that is brought up to ground level is toxic.

The system proposed above is similar to the traditional approach but makes use of hot, not boiling water.
This is used to boil refrigerant gas, which works an engine and is then condensed by cool air or water, and used again.
This would no doubt work, but may not make economic sense since the lower temperature difference reduces the energy that can be extracted.
The additional equipment would add to costs, and the expensive and potentialy harmful refrigerant would no doubt eventually leak.