“Renewable” is the battle cry of environmentalists and climate change activists. Yet, across the spectrum of renewables, some resources seem more “green” than others.

Geothermal 101

Geothermal energy is a renewable resource derived from the Earth’s heat. By mass, 99.9 percent of the Earth is hotter than 100⁰ C. The Earth’s available heat is estimated at equivalent to 42 million megawatts of power and doesn’t deplete like an oil or natural gas deposit, ensuring an inexhaustible supply of energy.

As a power source, geothermal is unsurpassed. Plants are capable of running 24/7, providing steady base load power with high capacity factors. It’s clean and sustainable with virtually zero carbon. Facilities have a small surface footprint; there are no vast installations of unsightly, noisy wind turbines or immense arrays of mirrors or solar cells.

Most geothermal power is conventional, exploiting naturally occurring pockets of steam or hot water close to the Earth’s surface. Heat from water is used to boil fluid and drive a steam turbine connected to a generator. Conventional geothermal power plants are located in rift zones or volcanically active parts of the world such as Iceland, along the Pacific’s “Ring of Fire,” Indonesia, Philippines, and on America’s west coast.

A newcomer is engineered geothermal systems. This approach, based on related principles, is designed to work in non-volcanic areas by drilling thousands of feet underground. Wells are bored and pathways created inside hot rocks, into which cold water is injected. The water heats up as it circulates and is then brought back to the surface, where heat is extracted to generate electricity.

In 2008 it appeared geothermal energy had finally hit the big time. Google.org, the philanthropic arm of Google, and private equity heavyweights Kleiner Perkins Caufield & Byers and Khosla Ventures announced investments in EGS. Funding was also supposed to go toward geothermal resource mapping, information tools, and a geothermal energy policy agenda. What happened?


EGS technology has a propensity to cause noticeable earth rumblings. It’s a sign the technology is working, propping open or enlarging existing cracks and fractures, where injected, high-pressure water causes small tremors. Although man-made earthquakes are not unique to EGS—they occur with oil-and-gas drilling, and damming and mining operations – the shaking unsettles peoples’ nerves. Opponents of EGS seize upon that fear, highlighting unknown geological risks, potential damage, and high costs. Proponents tell us opponents are hysterical, tremors are manageable, and EGS will greatly reduce the cost and availability of geothermal power.

Either way, the sci-fi nature of the geological risk is enough to keep most investors’ wallets closed for now. More work needs to be done to understand the true geological risks. Those risks then need to be rationally balanced against the drawbacks of other energy technologies, such as fossil fuels. The real question, in the end, is what people are willing to tolerate in return for a secure energy supply.

Conventional Geothermal

Which brings us back to conventional geothermal: the resource not far below our feet with the power to boil unlimited water and generate clean, renewable energy with proven safe technology. Conventional geothermal energy supplies more than 10,000MW to 24 countries worldwide, meeting the electricity needs of 60 million people. The United States boasts the largest geothermal market, with about 3,000MW of installed capacity. The Philippines, the world’s second biggest producer, generates 23 percent of its electricity from geothermal energy. Geothermal energy has also helped Indonesia, Philippines, Guatemala, Costa Rica, and Mexico. Iceland derives 17 percent of electricity and 87 percent of its heating needs from geothermal energy.

As these figures demonstrate, an operational geothermal power plant is quite reliable, offering continuously available base-load power with historic reliabilities in excess of 90 percent. Compare this to wind-generated power, with 25 to 40 percent reliability (the wind doesn’t always blow when needed), or solar-generated power, with 22-35 percent reliability (the sun sets each night). Despite its lower profile, geothermal power has staying power. In this particular popularity contest, it may yet be voted “Most Likely to Succeed.”