Geothermal energy is derived from the thermal energy generated and stored in the earth. This energy originates from earth's formation and is continuously replenished through radioactive decay of materials. With the earth's core at approximately 7200 deg Fahrenheit and under 400 deg within a few miles of the surface, there is a significant temperature gradient between core and surface. This gradient results in a continuous conduction of thermal energy in the form of heat from the core to the surface. Since the earth's crust is not uniform in thickness, varying from about 20 - 40 miles thick, there are places where the hotter temperatures will be closer to the surface. Volcanoes, hot springs, and geysers are physical manifestations of a thin crust and are ideal locations to tap into this geothermal energy.
A typical geothermal power plant taps into this heat source through wells that are drilled to a sufficient depth to access rock that is about 350 degrees Fahrenheit. Dry steam stations (Figure 1) are the simplest and oldest design, producing geothermal steam directly that is used to drive a steam turbine to generate electricity. Flash steam stations (Figure 2) are the most common in use today. These plants produce high-pressure hot water from geothermal reservoirs and dump it into lower-pressure tanks. As the pressure decreases, some of the hot water boils into steam which is used to power the turbine generator. Remaining water and condensed steam are then injected back into the reservoir, where it is reheated to be used again. (See video.)
The major pros of geothermal power are:
- Almost entirely emission free with zero carbon
- Constant power (not subject to fluctuations as with solar or wind)
- Smallest land footprint of any major power source
- Virtually limitless supply of underlying energy (heat) available most places
- Inherently simple and reliable with low operating costs
- New technologies show promise to expand opportunities
The major cons are:
- Prime sites for conventional geothermal are location specific and often far from population centers
- Water usage
- Sulfur dioxide and other emissions in produced steam; however, processes exist to remove compounds before they are released
- High construction costs
- Requires difficult drilling into heated rock
- Minimum temperature of 350 deg F generally required
The United States leads the world in geothermal electricity production with approximately 3.5 GW of installed capacity from 77 power plants. This provided approximately
0.4% of the total electrical power generation in the US in 2022, or about 17 billion kWh. As of January 2021, the largest geothermal field in the world is The Geysers in California (see Figure 3) with a rated capacity of 900 MW spread among 14 separate power plants, followed by the Larderello plant in Italy at 769 MW and the Cerro Prieto plant in Mexico at 720 MW (see
Statista.com). As of 2021, Kenya was the country that produced the
largest percentage of it's electricity usage with geothermal at 43%, followed by Iceland at 29%.
With conventional geothermal technology there are relatively few locations where the required combination of temperature, steam/water, and permeable rock are adequate. However, new technology in the form of Enhanced Geothermal Systems (EGS), or "Superhot rock geothermal energy", provides the opportunity for essentially unlimited power. See
OWOE Cool Tech - Enhanced Geothermal Systems.
Iceland is pushing the boundaries of geothermal technology with the
Iceland Deep Drilling Project (IDDP), a multi-decade research project that is attempting to drill into a magma chamber under the Kafla volcano which last erupted 700 years ago (Figure 4). At this depth the extreme heat (close to 800 deg F) and pressure will turn water into a "supercritical" fluid, which is neither gas nor liquid. Engineers expect the resulting steam that will be used to power a turbine to generate electricity will provide the equivalent energy of 5-10 times that of a conventional geothermal well.
Using an entirely different approach than large thermal generating plants, a pilot program in Massachusetts will be a
first-of-a-kind geothermal system that will use the earth as a heat pump to pull the heat out of the ground to warm buildings in winter and pump heat from buildings back into the ground in summer to cool them. The local utility,
Eversource, has drilled a total of 88 boreholes (averaging 600 to 700 feet deep) in three locations around a neighborhood in Framingham, Massachusetts, consisting of 36 residential and commercial buildings. An environmentally friendly type of antifreeze, propylene glycol, will be pumped underground to absorb the earth's heat during winter. It will then be pumped through a network of pipes to the individual homes or businesses where it will drive heat pumps to heat the buldings. In the summer the system will reverse, pulling heat out of the buildings to be absorbed into the cooler earth (see Video 2). The system should allow the buildings to maintain an average temperature of about 70 degrees year-round.
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