In the search for alternative energy sources, inspiration often comes from powerful natural phenomena. To meet the ideal criteria, an energy source must be renewable, efficient, cheap, and constantly available. We have looked to the ocean, to the wind, and to the sun; why not consider the energy contained within the Earth itself?
Geothermal energy is heat energy generated and stored in the earth (geothermal comes from the Greek words geo, meaning earth, and thermos, or hot), accessible to us in the form of hot water and steam. Deep within the Earth is a solid core thought to be as hot as the surface of the sun. It is so hot, in fact, that it melts rock, creating magma. Magma is less dense then the surrounding rocks, and so rises to just beneath the surface of the Earth, where it heats rocks and water to high temperatures upwards of 700°F. Sometimes the superheated water escapes through cracks in the Earth’s surface, creating geysers and hot springs; other times, the water remains under the surface in what are called geothermal reservoirs.
The use of water heated by geothermal forces is not a new idea. Hot springs have been used since Paleolithic times for a multitude of purposes. The Native Americans used them for cooking and medicine, and the people of Pompeii (in the shadow of a volcano) used the water to heat their houses. More recently, Romans built strategically-placed spas that took advantage of geothermally heated waters. Using steam and hot water to generate electricity, however, didn’t begin until the twentieth century.
The first geothermal power generator was devised in 1904 in the dry steam fields of Italy by Prince Piero Ginori Conti. He showed the effectiveness of this power source by successfully lighting four lightbulbs. In 1911, the world’s first commercial geothermal power plant was built in the same location, though it took until the 1950s for industrial geothermal power to catch on in other countries. The first geothermal power plant in the United States was built by Pacific Gas and Electric in 1960.
Today, there are three conventional types of geothermal power plants. In each, the first step of electricity production is to drill down into the geothermal reservoir in order to access the stored thermal energy of hot water and steam. The first type of power plant is called a dry steam plant, and in it, steam pumped from underground spins a turbine, which powers the generator. According to the Geothermal Energy Association, as of 2012 approximately 50% of installed geothermal capacity in the US was produced by dry steam plants, all of which are located in California.
The second type of geothermal power plant is a flash steam power plant. In this setup, the well pumps high-pressure geothermal water. The pressure drops as the water moves upwards, turning some of the water to steam, which is then diverted into a turbine and used to power the generator. Any water remaining is injected back into the geothermal reservoir.
The final kind of power plant is called a binary power plant. This design allows energy to be produced from geothermal sources at temperatures less than 300°F, which means that unlike the others, this plant is not restricted to a tectonic plate boundary. Binary power plants use what’s called an Organic Rankine Cycle (ORC) system. In it, water from the geothermal reservoir is pumped upwards through the well and into a heat exchanger, where it is used to heat another liquid (called the “working fluid”; often isobutane or pentafluoropropane). This working fluid has a lower boiling point than water, and quickly turns to steam when heated, spinning a turbine as before. Geothermal water is injected back into the reservoir.
So how does geothermal energy compare with other alternative energy sources? It is clean, because it doesn’t use any fossil fuels to generate, and emissions are much lower than fossil fuels (in the binary power plant, emissions are nearly zero). Additionally, transport costs are minimized, because energy is produced near the plant. Geothermal energy is efficient, renewable, and reliable; it can be produced at any time of the day or night, since the superheated reservoirs are always being heated and refilled. The main problems with this form of energy production are availability and cost. Despite the advances of the binary system, geothermal plants are still restricted geographically, and will remain so until we have the technology to drill deeper to access even more of the Earth’s thermal power. As it stands, initial costs to drilling a geothermal well are sometimes as high as $1 to $4 million; going deeper will doubtless cost even more. In theory, our geothermal resources are probably enough to sustain all human needs, but to make reliable geothermal energy a reality, we must devise a way to get to the stored potential that waits, just beneath the surface.