There is only one goddess Gaia and Al Gore is her only prophet...

[--Pete]

Hi,

Actually, using the surface heat may be an advantage in some areas. What you are looking for with heat pump technology is a large differential. So for example, where I live the ground a few dozen meters deep is a constant 58 degrees, while the surface fluctuates between -40F to 120F. But, generally as you go deeper, the radiative energy increases. On average, the geothermal gradient is approximately 75 degrees F per mile. The same principles that work for the small home heat pump, can be scaled larger and possibly deeper for larger installations. The amount of energy you can suck from the earth is relative to the surface area of the coils you've buried. From there, condensers will aggregate the accumulated heat which can be used directly, or converted into electricity if needed.

Whoops. You are speaking of heat pumps, and what you are saying is true -- for heat pumps. But the discussion is about geothermal electricity generation. You are still right about looking for the greatest temperature differential you can get, since the thermodynamic efficiency of a heat engine is determined by the difference in temperature between the hot reservoir and the cold reservoir. The cold reservoir is basically the atmosphere, and not much can be done about that. So, the deeper we can go, the more electricity we can generate.

You might want to go back and look at the link I posted earlier. Also, you might want to review heat pumps and geothermal generation. They are considerably different concepts, united mostly by both being the subjects of thermodynamics.

--Pete