I'm fascinated by odd-ball energy systems; geothermal, gravitational, and even nuclear though it is well developed in some places and seems poised for a comeback from a prematurely presumed death. The focus on solar energy in all its forms - everything from fossil fuels to bio-fuels as well as wind and PV - neglects some massive pools of energy that seem almost available.

Some recent news about geothermal energy caught my eye.

Some 90% of all homes in Iceland are heated by geothermal energy; and a number of power stations are also producing electricity from steam at around 240C, extracted from boreholes between 600 and 1,000m deep.

But now, the plan is to go much deeper. Omar Friedleifsson of the Iceland Geosurvey is leading the consortium of energy companies in the Iceland Deep Drilling Project.

Last year, they drilled down to a depth of 3,082m and since then have been conducting flow tests.

Later this year, they will put a pressure lining into their borehole and drill on down to more than 4km deep.

At that depth, they hope to encounter what is called supercritical water: water that is not simply a mixture of steam and hot water but a single phase which can carry much more energy.

Engineers on the project have calculated that increasing the temperature by 200 degrees and the pressure by 200 Bar will mean that, for the same flow rate, the energy extracted from such a borehole will go up from 5MW to 50MW.

Power station manager Albert Albertsson predicts that, by the end of the century, "Iceland could become the Kuwait of the North". The vision is to use this cheap and carbon-free energy to split water, to yield hydrogen that could be despatched around the world in tankers.

Iceland isn't the only spot where the surface is closer to the molten interior of the earth, so these technologies and applications may find other homes as well. Magma intrusions aren't that rare. There are issues, not least of which is that such places are subject to high amplitude excursions such as volcanoes and earth quakes which could wreck facilities.

Why is the earth's core hot? What kind of energy is this?

. . . the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle. These isotopes radiate heat as they shed excess energy and move toward stability. "The amount of heat caused by this radiation is almost the same as the total heat measured emanating from the Earth."

Radioactivity is present not only in the mantle, but in the rocks of Earth's crust. For example, Marone explains, a 1-kilogram block of granite on the surface emanates a tiny but measurable amount of heat (about as much as a .000000001 watt light bulb) through radioactive decay.

That may not seem like much. But considering the vastness of the mantle, it adds up, Marone says.

Sometime billions of years in the future, he predicts, the core and mantle could cool and solidify enough to meet the crust. If that happens, Earth will become a cold, dead planet like the moon.

Long before such an occurrence, however, the Sun will likely have evolved into a red-giant star, and grown large enough to engulf our fair planet. At that point, whatever heat is left in the mantle will hardly matter.

It's interesting that our star is not our only nuclear furnace, though they are of different types: one is fusion and the other is fission. And it's interesting to reflect on the dynamic nature of the earth, its provision of its own heat in addition the the weak but continuing input of the sun. A civilized society ought to be able to make use of that heat beyond taking an occasional dip in an earth powered hot tub.