Wednesday, June 4, 2008

Natural Heat

Measuring the planet’s nuclear energy

Heat from the decay of radioactive elements deep within the planet could meet Earth’s energy needs almost three times over. The problem, though, is how to take advantage of that immense source of energy.

Witness the explosive power of a volcano or the eruptions of geysers and hydrothermal vents, and it’s clear that Earth has a prodigious source of internal heat. Most of that heat is generated by the radioactive decay of elements such as uranium, thorium and potassium, says David S. Chapman, a geophysicist at the University of Utah in Salt Lake City.

Only in a few places, such as Iceland, is the heat flow through Earth’s surface concentrated enough to efficiently provide geothermal power. Data gathered at more than 20,000 sites worldwide show that typical heat flow from the ground is relatively feeble: At sites on land, heat flow is about 65 milliwatts per square meter, Chapman says. Put another way, the heat flowing up through an area the size of a football field, if it could be harnessed, would power only three 100-watt light bulbs.

At and near mid-ocean ridges, where molten material flows to the surface from deep inside Earth, average heat flow is about 140 milliwatts per square meter, the data suggest. Through ancient ocean crust, heat flow typically measures around 100 milliwatts per square meter.

Overall, the average heat flow through a square meter of Earth’s surface is about 87 milliwatts per square meter, Chapman reported May 27 in Fort Lauderdale, Fla., at a meeting of the American Geophysical Union. That average, however, when added up across Earth’s entire surface, totals 44 terawatts, he notes (one terawatt is 1 trillion watts). For comparison, one recent study placed global energy consumption in 2005 at 15 terawatts.


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