Originally posted by: BrownTown
Pretty simple really, it takes energy to push water (or other working fluid) through thousands of feet of piping, at a point the amount of energy to pump the working fluid is more then the energy being brought up. Also of note is the fact that the heat will be relatively very low, so the temperature difference between the heat source (underground) and heat sink (cooling tower) meaning very poor efficiency. Also, as soon as you get away from teh heat source you are still pumping the water up a considerable distance through COLD rock, so again you are losing energy. ITs not necessarily a physical limit as to how far you can go down until you reach hot enough rock, but the farther you go the cost will increase exponentially (since you are using more and more energy to get less and less efficiency).
Right, right.. thermodynamics.. however from this MIT report it seems technology (current and past like The Geysers built in the 60's) is sufficient enough to make this viable and extremely cost-effective. You shouldn't dismiss it so readily.
