High temperature geothermal resources, usually above 300°F (149°C), are accessed by drilling production wells. From the wells, the steam or hot water is brought to the surface and piped to a geothermal power plant where its energy is used by a steam turbine to generate electricity. After the geothermal water does its work in the power plant, it is piped back into the geothermal reservoir – and the cycle continues. Geothermal power plants operate 24 hours a day, since the “fuel” source is always available; these plants are therefore valued for their ability to provide us with baseload electricity.

Around the world there are about 11,000 MW of geothermal electricity generated in 22 countries. Current U.S. geothermal electric power generation totals approximately 2200 MW (or about the same as four large nuclear power plants).
California's geothermal power plants have a dependable installed capacity of over 1,800 megawatts – producing about 5 percent of California's total electricity (more than all other renewables combined, excluding large hydro). It is estimated by the U.S. Geological Survey that California is likely to have more than 10,000 megawatts of untapped geothermal resources, using current technologies.

There are two primary types of geothermal resources that are used for power production: dry steam and hydrothermal. A third type of power production technology is under development throughout the world – this is called Enhanced Geothermal Systems (EGS).

Dry steam resources exist where the reservoir is at pressures and temperatures that super-heat the water in the rock so that it has literally completely boiled. The resulting steam is trapped by an impervious “cap” rock, resulting in a very hot steam system in which no liquid water exists. These reservoirs are the easiest to exploit for power production because the very hot steam is tapped by drilling through the cap rock, and the production wells then directly pipe the steam to power generating turbines.

Hydrothermal reservoirs are geological systems in which very hot liquid water, rather than steam, makes up the reservoir. These systems are usually recharged continuously with rainwater. Power is generated in these systems by drilling production wells into the zone of hot water, and allowing the hot water to flash to steam as it ascends from depth.

Geothermal power plants use two basic methods to extract energy, flash plants and binary plants. To learn more about each type, click the links below.

Geothermal heat pumps are used to heat and cool buildings by way of a heat exchanger that pulls heat or sinks heat from the relatively constant ambient temperature of the ground.  The easiest way to imagine how this might occur is by thinking about a refrigerator.  In a refrigerator, a heat pump is used to remove heat from the interior air of the refrigerator (heat source) into the air of the room in which the refrigerator is located (the heat sink). 

Geothermal heat pumps take advantage of the immense thermal mass of the Earth.  In regions where summer cooling is required, exterior daytime temperatures generally are above 26 degrees Celsius (80 degrees Fahrenheit).  Where winter heating is required, exterior temperatures generally are below 10 degrees Celsius (50 degrees Fahrenheit).  At shallow depths in the Earth, a consistent temperature of about 10 degrees Celsius to 13 degrees Celsius is maintained.  This intermediate temperature between the summer highs and winter lows, makes the Earth an excellent potential heat sink in the summer and a heat source in the winter. 
The basic configuration of these systems is generally a vertical loop or horizontal loop system.  The vertical features boreholes that are a few hundred feet deep, while the horizontal loop system is generally 10 feet deep or less but has a larger footprint to achieve a comparable loop area as that of the vertical system.  These loops continuously circulate a working fluid between a heat pump inside the building and the Earth.   

The CGEC is currently working on a return on investment study for these systems for all of California as well as the 30 most populous metropolitan areas in the United States.  To see the progress on this study, click here.

Coming Soon!