Geothermal power is energy generated from the heat stored beneath the earths surface. The inside of the earth is hot. Sometimes this heat finds its way to the surface in the form of volcanoes, geysers and steam vents. Some of the heat warms pockets of the oceans or atmosphere. The heat can be used to create steam to drive a turbine to create electricity. This is geothermal power.

Geothermal power, like many renewable sources of power, is more readily available in some geographical locations than others. For example, Iceland has a lot of surface level geothermal activity, and produces nearly 20% of its electricity and heats more than 85% of its homes using this resource. But Iceland is one of just over 20 countries around the world that utilize geothermal power.

Geothermal resources have been used for centuries for bathing and heating. It wasn’t until 1904 that the first geothermal power generator was tested in Italy. The first commercial geothermal power plant was built in the same location in 1911. New Zealand built the second commercial plant nearly 50 years later, in 1958. Most of these early power plants relied on existing steam vents. Hot Dry Rock (HDR) geothermal power was developed by pumping water down into the porous hot rock a few kilometers below ground. The resulting steam powers generators and is recollected as water to be pumped into the ground again. Enhanced Geothermal Systems (EGS) use cold water or chemicals to create the cracks and pores necessary to generate sufficient steam and therefore power. As EGS technologies continue to improve, the energy potential for geothermal can reach 2,000 ZJ as reported by an MIT study in 2006. The study predicted that this would be enough to sustain the worlds present energy consumption for several millennia.

Like solar and wind, geothermal power utilizes a free source of energy with very little harmful emissions. Unlike solar and wind, geothermal power is fairly constant and unaffected by the weather, allowing it to be easily used for base load power generation. It is considered nearly sustainable since the heat extraction is small relative to the total heat reservoir. Geothermal power is already economically competitive in some geographic locations and can often scale to provide large generation capacity. The largest dry steam field in the world is at The Geysers, north of San Francisco, CA, which can produce 1,360 MW of electricity. One of the power plants at The Geysers is pictured on the left.

There are some drawbacks to geothermal power. Geothermal fluids are corrosive and realtively low temperature (compared to steam). The lower temperature causes less efficient transfer to power. Trace amounts of toxic elements such as mercury and arsenic may also be present, so the fluids must be disposed of properly. Some systems such as EGS can cause land stability due to the injection of water. Some plants emit low levels of carbon dioxide, nitric oxide and sulfur, but at roughly 5% of the level of fossil fuel plants. Most of these emissions can be captured and sequestered back in the earth to drop the emission levels to less than 0.1%. While the overall geothermal source is relatively limitless, some local cooling may occur. Care must be taken to design plants at sustainable production levels, allowing their heat reserve to replenish from deeper in the earth’s mantle.

Like most of the renewable sources I have looked at, geothermal is probably not the single answer to the problem. However, it appears to be a viable solution for large base load power generation which can be supplemented with various other renewable sources. My next challenge is to start a head-to-head comparison of the various energy sources to really see how they stack up over a full life cycle assessment; or maybe I’ll take the easy way out for now and jump into transportation alternatives. Check back to find out!