Besides the large temperature gradient needed for efficient heat extraction (which you can get anywhere if you drill deep enough), and an efficient thermal exchange at those depths (which you might get by fracturing rock), you also need a larger enough heat flux from below than the rate at which you are extracting it.

The typical heat flux towards the earth surface is .06 watts per sq. meter--much smaller than the solar energy flux. To generate 1 MW of power, you would need to collect 100% of the geothermal energy over an area of 6.4 square miles. That's a lot of deep drilling.

Abundant energy does not equal abundant power.  

Why might the coolness of the deep sea not be renewable?

It would seem to me that the cycle of radiant energy associated with the rotation of the earth, day and night, would create heating and cooling which gravity would then order such that the cool would be beneath the warm, provided temperature of water is above 39F

Sticking stuff underground (especially to get useable stuff up/out) has an obvious attraction.

When I was a kid I wondered why we couldn't just tip all our waste into volcanoes, so the lava would take it to the earth's core, melt it all down and separate it all back into its basic chemical and ores again. Wouldn't that be neat?

But I'm just wondering about the consequences of mucking about with the balance of flowing (virtue of heat that is proposed to be removed?) magma, which by the numbers seems so vast anything we do will be very small, but may still be significant.

As one who thought it a stretch when some laid the Asian tsunami at the door of global warming, I was nonetheless moved to at least wonder a tad by subsequent theories based on the consequences of climate change on the ocean floors, and even sucking vast volumes of oil out of the ground.

How sure are we that our demands won't divert enough energy from down there to up here to not have a consequence?

I guess after nuclear I'm just worried that in our desire for more sources of energy rather than using less, we tend to rush into things that may cause a few new problems down the line. But then I was the one who wondered what sucking the energy out of the wind at coastlines may do for inland climate balances.

Just asking. I'd love to be reassured.

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I do not know much about geothermal energy, I have attended a couple conferences on this.  Ground heat loss will not damage the Earth, though specific geothermal power sites will not be sustainable as they cool down.  The environmental footprint is the dirty caustic sulfuric affluent which is often injected into aquifers.  Sulfur makes geothermal energy stinks like rotten eggs.

Deep hot geothermal wells are not cost effective.  I have been asked about the prospects of increasing shallow geothermal temperatures with solar concentrators because the low grade heat from the ground is not efficient for generating electricity.

Geothermal energy has a niche for carbon-free energy and is one example why wind is just part of what needs to be done.

Most important is that the US has over-developed energy and more energy is not needed.  We could shut down 50% of our energy supply with conservation and efficiency improvements, a cleaner and more cost effective base-load source of energy.

Don't carpool alone.