30 November 2007

Peak Oil: Meet Geothermal

With the urgent need to find energy sources that are renewable and don't emit greenhouse gases, geothermal energy is ideal — "the best renewable energy source besides the sun," Kennedy says. Accessible geothermal energy in the United States, excluding Alaska and Hawaii, has been estimated at 9 x 1016 (90 quadrillion) kilowatt-hours, 3,000 times more than the country's total annual energy consumption. Determining helium ratios from surface measurements is a practical way to locate some of the most promising new resources.

There are two Terran sustainable energy sources huge enough to completely displace fossil fuels and nuclear fission: solar (with advanced storage) and geothermal. Geothermal has a big advantage over solar, in that it is "always on."

Science is learning more about plentiful geothermal all the time:
Currently, most developed geothermal energy comes from regions of volcanic activity, such as The Geysers in Northern California. The potential resources identified by Kennedy and van Soest arise not from volcanism but from the flow of surface fluids through deep fractures that penetrate the earth's lower crust, in regions far from current or recent volcanic activity. The researchers report their findings in the November 30, 2007 issue of Science.

"A good geothermal energy source has three basic requirements: a high thermal gradient — which means accessible hot rock — plus a rechargeable reservoir fluid, usually water, and finally, deep permeable pathways for the fluid to circulate through the hot rock," says Kennedy, a staff scientist in Berkeley Lab's Earth Sciences Division. "We believe we have found a way to map and quantify zones of permeability deep in the lower crust that result not from volcanic activity but from tectonic activity, the movement of pieces of the Earth's crust."

Kennedy and van Soest made their discovery by comparing the ratios of helium isotopes in samples gathered from wells, surface springs, and vents across the northern Basin and Range. Helium-three, whose nucleus has just one neutron, is made only in stars, and Earth's mantle retains a high proportion of primordial helium-three (compared to the minuscule amount found in air) left over from the formation of the solar system. Earth's crust, on the other hand, is rich in radioactive elements like uranium and thorium that decay by emitting alpha particles, which are helium-four nuclei. Thus a high ratio of helium-three to helium-four in a fluid sample indicates that much of the fluid came from the mantle.

High helium ratios are common in active volcanic regions, where mantle fluids intrude through the ductile boundary of the lower crust. But when Kennedy and van Soest found high ratios in places far from volcanism, they knew that mantle fluids must be penetrating the ductile boundary by other means.
Berkeley Lab via Physorg and Eurekalert

If accessible geothermal energy comprises many thousands of years worth of energy--not counting solar--does anyone doubt that humans could develop clean fusion energy in that time? With the wealth of geothermal, solar, and fusion energies, I believe humans could learn how to travel between stars in large numbers. At that point, even the dimming of the sun would be only a footnote in the history of humanity.

Of course we need to get from here to there. It will take decades to develop geothermal "hot rocks", and almost as long to develop solar to the point it is not hampered by diurnal cycles of light and dark. Fusion may take 50 years or longer.

That means we will have to use cleaner and safer nuclear fission, vigilant conservation, cleaner fossil fuel technology, and wise use of biofuels--without displacing crops and cropland.

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