Geothermal Power: Heat and Light from Below the Earth

Geothermal power generation is one of our most stable renewable energy resources. Heat generated below the Earth’s surface can provide an almost unlimited supply to power and heat homes. And while geothermal electricity only accounts for around one percent of global generation, that is set to at least triple by 2050.

The Race for Power

In the US, geothermal electricity accounts for only 0.4 percent of net generation, but there is potential for it to provide 100 Gigawatts or 10 percent of the national total. Tapping that resource means America would keep its place as world leader in the field, but it requires renewed investment in enhanced geothermal systems (EGS).

Geothermal plants are normally built where fault lines in the Earth’s crust give access to natural reservoirs of hot underground water. Drilling wells into these reservoirs, holding water hotter than 302 Fahrenheit (150 Celsius) can provide enough recyclable steam and water to drive generation turbines for decades.

The Geysers in California’s Mayacamas Mountains is the world’s largest geothermal electricity complex. It produces enough electricity from its 18 power plants for 725,000 homes, totaling 20 percent of the state’s renewable energy. Superheated ‘dry steam’ is funneled from a large sandstone reservoir heated by a large magma chamber more than four miles beneath the surface.

Not every geothermal plant has the helpful geology of The Geysers–highly volcanic, spacious reservoir and permeable rock–so technological innovation is needed. Just as fracking has exploited hidden gas reserves by pumping liquids at high pressure into rock to release them, EGS use water to fracture rock and increase through flow to generate electricity.

How it Works

Heat is captured from its passage through the rock and the heated water converted into electricity. Cooled water is then recycled and pumped back to gather more heat. EGS technologies will open up many more sites for geothermal energy. "You can effectively put a power plant anywhere,” said Will Pettit, director of the Geothermal Resources Council. “All you have to do is drill deep enough and you will find hot rock.”

Most geothermal plants actually use a flash steam technique, where hot water (at 360 F or 180 c) is drawn up, passed into lower pressure tanks and ‘flashed’ into steam to power a turbine. Binary cycle plants are the growth technology because they can operate at lower water temperatures and more diverse geographical locations. They use moderately hot water to heat a secondary fluid with a lower boiling point–as low as 135 F–to drive turbines.

Now a new supercharged technique that uses special properties of water at temperatures in excess of 750 F (400 c) is promising to release enormous energy reserves. Water at this temperature and pressure is found at depths greater than 1.9 miles (3 km). To access it requires upgraded drilling technology and materials, but at this depth the water acts as a supercritical liquid that can potentially produce ten times more energy than conventional geothermal wells.

Geothermal plants already emit 11 times less carbon dioxide per unit of electricity than the average US coal power plant, making them environmentally sound. Unlike solar and wind farms they can also operate 24 hours a day to provide a solid base load for homes and businesses.

There are drawbacks too. Seismic activity around drilling wells is a factor, as are potential for gas emissions such as SO2 (causing acid rain), and water pollution. High investment costs are another.

But the US government is backing the sector with multi-million dollar funds to push forward low temperature and advanced EGS research through its Geothermal Technologies Office. Accompanied by an uptake in households using ground-source heat pumps to heat and cool their homes, geothermal energy is set to play a big part in the low-carbon electricity future.