Energy alternatives : Geothermal

Earth's interior contains vast amounts of heat, some of it left over from the planet's original coalescence, some of it generated by the decay of radioactive elements. Because rock conducts heat poorly, the rate at which this heat flows to the surface is very slow; if it were quicker, Earth's core would have frozen and its continents ceased to drift long ago.

The slow flow of Earth's heat makes it a hard resource to use for electricity generation except in a few specific places, such as those with abundant hot springs. Only a couple of dozen countries produce geothermal electricity, and only five of those — Costa Rica, El Salvador, Iceland, Kenya and the Philippines — generate more than 15% of their electricity this way. The world's installed geothermal electricity capacity is about 10 gigawatts, and is growing only slowly — about 3% per year in the first half of this decade. A decade ago, geothermal capacity was greater than wind capacity; now it is almost a factor of ten less.

Earth's heat can also be used directly. Indeed, small geothermal heat pumps that warm houses and businesses directly may represent the greatest contribution that Earth's warmth can make to the world's energy budget.