Along with hydropower, geothermal power is an old form of renewable energy. The first geothermal power plant developed to generate electricity dates back to 1904, in Italy — though geothermal energy more broadly has been utilized in one way or another even longer. Though firmly established geothermal power doesn’t get the press, nor public mention, nor investment, as newer, flashier forms of renewable energy, such as solar power, wind power, or biofuels.
Nevertheless, geothermal power can, in the right places, be a very important part of a future low-carbon energy mix. Let’s take a look at how geothermal power is tapped to produce electricity, how much is being generated, and how much possibly could be generated with changes in technology currently being explored.
Some history and terminology
When people talk about geothermal power they may be talking about using the heat of the earth more or less directly for heat or cooling — as with a geothermal heat pump or, as is the case in Iceland, where the vast majority of buildings get their hot water supply from water heated by the Earth itself — or they may be talking about tapping into the heat generated inside the planet to generate electricity. Right now, we’ll be discussing geothermal electricity specifically, though the term geothermal power will be used most often.
As was said, the first commercial geothermal power plant was developed over a hundred years ago in Italy. Experimental power plants were developed in several countries over the next decade, but Italy remained the only nation generating electricity directly from the heat of the Earth until 1958, when New Zealand developed one, using a slightly different technology. Two years later, the first geothermal power plant was built in the United States, which is now has the world’s largest geothermal power capacity.
How does it work?
There are three types of geothermal power plants, defined at the most critical level by the temperature of geothermal resource they can use, all of which basically take water that has been heated deep within the Earth, that emerges as steam, and use it to turn a turbine and produce electricity.
Dry steam plants, the oldest design, takes the steam directly from a drilled well, turns the turbine, gets cooled in a compressor back into water, and then sends the cooled water back into the Earth via another well. These need temperatures of at least 150°C.
Flash steam plants, the most common type, need temperatures of 180°C or higher. In these highly pressurized hot water is extracted and fed into lower pressure tanks before being sent to turn the turbine.
Binary cycle plants can run at temperatures of just 57°C. Here the hot water, which is obviously well below the boiling point, is passed by a liquid that has a lower boiling point than water. This second liquid is the one that turns to steam, turning the turbine. Most new geothermal plants today use this technology.
Under development is a fourth geothermal power plant type, called in the US enhanced geothermal systems, that has the potential to radically expand how much electricity can be generated from geothermal energy, but that will get its own section farther along.
Regardless of the specific set-up, geothermal power has the decided advantage, compared to other renewable energy sources, that the power source is always on — the Earth is always heating. And though local depletion of the resource has to be managed, the amount of heat generated by the planet far exceeds the ability to extract it.
Though not a truly zero-carbon power source — there are some greenhouse gases that are generated in the process, as well as other environmental impacts to be managed — the average CO2 emissions from a geothermal power plant are just one-eighth those of a coal-fired power plant.
How much is being generated today?
There’s a bit over 10.7 gigawatts of geothermal electricity capacity in the world today, with roughly one third of that installed in the United States.
In total, 24 countries have at least one geothermal power plant. Geothermal provides over one-quarter of the electricity in the Philippines (which has the second-highest total amount of installed capacity), in El Salvador, and in Iceland (where 27% of the electricity comes from geothermal).
Nations where there’s over 100 megawatts of geothermal capacity, not already mentioned, include Indonesia (the third-highest amount at 1197 MW and vast potential, though under 5% of electricity used), Mexico, Italy, New Zealand, Japan, Kenya, and Costa Rica.