Geothermal energy comes from the high temperatures that exist under our earth’s surface. This natural heat can be used in several ways to reduce our reliance on nonrenewable resources. Three of these include using geothermal reservoirs to generate electricity (electric power), using the heat straight from the ground to heat homes and buildings (direct use), and heat pumps. In the U.S., heat pumps can be used in most places, while direct use is available mostly in the west and electric power mostly in Nevada and parts of California. Let’s take a closer look at these options.
When rainwater enters the earth’s surface, it can be heated by the intense heat of the earth’s center and become steam. In some places this steam forms in a layer of permeable, porous rock, but becomes trapped there under a layer of non-permeable rock. This is when a geothermal reservoir forms. We can drill into these reservoirs and build power plants, including dry steam, flash, or binary. The largest single source of geothermal power comes from a dry steam plant at The Geysers in California. Dry steam plants have been being experimented with since 1904 in Italy. The most common type of plant however, is the flash plant. You can see a diagram of a flash plant below. The plants tap into the reservoirs and extract the steam. In dry steam plants, this steam powers the turbines to produce energy. In flash plants, the steam is “flashed,” creating vapor which powers the turbines and the cooled water is returned to the reservoir.
Direct use is available in more places than electric power because it does not require as high a temperature for its reservoir. The reservoir is tapped into and the hot water is pumped out and transported through a system of pipes. This hot water can have several uses, including space heating in small communities or buildings, water heating, and greenhouse heating.
Heat pumps can be used in most places because they do not require geothermal reservoirs, but instead rely on the constant temperature underground. These systems can be used in a home or small office building to either heat or cool the space, and can also provide hot water. A system of pipes is buried below the surface near the home, and is filled with a mixture of water and antifreeze. As the mixture flows through the pipes it either absorbs heat from or gives heat to the ground. The heat pump then transfers the heat or cold to a system of ductwork, which distributes it to the home.