Geothermal power is the red-headed stepchild of renewable energy.

Unlike wind, solar, and biofuels, it rarely makes headlines or stirs up controversy. Politicians and pundits never hail geothermal as the Next Big Thing, and often fail to even mention it when talking about the importance (or, depending on their political slant, boondoggle) of green energy. The average citizen, meanwhile, doesn’t even know what geothermal energy is, beyond the suspicion that it has something to do with volcanoes and Old Faithful.

Here’s the thing about geothermal: it is, by far, the most promising renewable source for big time, base load, continuous (not intermittent, like wind and solar) energy.

Dig down deep enough pretty much anywhere on earth and you’ll find dry rocks heated by the decay of radioactive minerals and heat radiating from the earth’s molten core. Tapping the vast, virtually endless amounts of heat stored in these rocks could (at least theoretically) help solve many, if not most, of our energy problems. In a report published by MIT, geologists and other scientists estimate that the United States alone contains 200,000 exajoules of recoverable geothermal energy -- 2000 times the amount of primary energy the country consumes annually.

How do we harvest this bounteous resource?

The basic idea, known as enhanced geothermal systems (EGS) is simple: find hot rock within drilling range, sink wells, pump water down at high pressure to open a network of fissures within the rock, then pump cold water through the fissures to absorb heat, send the water back up through the second well, transfer its heat to a liquid with a relatively low boiling point, and use the resulting steam to power electricity generating turbines.

So, what are we waiting for? Dig the wells! Pump the water! Let’s start using the planet’s store of heat to make clean, emission-free electricity!

Yes, let’s -- but before we do, there’s just one thing to consider. After more than 30 years of enhanced geothermal research and development, beginning with the Fenton Hill project at Los Alamos National Lab in New Mexico in the mid ‘70s, scientists are still a little shaky on how best to make the elegantly simple idea of EGS work in the field.

This isn’t to say that the technology doesn’t work; it does. Scientists know that you can use subterranean hot rocks to produce net energy. But they don’t know how to make EGS work as efficiently as possible every time, everywhere. Because, as the MIT report documents in fine detail, when you start messing around with hot, dense rock buried several thousand feet in the earth’s mantel, there’s no telling what might happen.

For example, engineering the network of fissures and cracks is no cakewalk. Ideally, the fracture system will channel the water toward the extraction well, up through which the now hot water returns to the surface to give up its valuable heat. But as researchers have learned over the past several decades, giant slabs of rock tend to have minds of their own when it comes to fracturing. Almost all large rocks have fused networks of cracks and fissures already in place. This forces pressurized water down to re-open the system, and often has unpredictable and unintended consequences, such as broadening the network so much that the water meant to absorb and return heart to the surface spreads out and seeps away.

Geothermal engineers have made progress since the ‘70s. Advances in drilling, fracturing techniques, and mapping and monitoring what’s happening deep underground have helped inch the technology forward. Small-scale commercial projects are operating in France and Germany, and dozens of other pilot projects are in the works around the world.

Still, EGS is a long way from realizing its huge potential. What needs to happen for EGS to take the next step, to scale up and become a true power player in the global energy game? I’ll tackle that question in my next post. Stay tuned.

You'll find more of my writing about geothermal energy and other renewables at renewablebook.com.