Geothermal Energy Joe Ziccarelli and Collin Heiry Heat transferred from the earth’s underground concentrations of dry steam, wet steam, or hot water trapped in fractured or porous rocks Hot dry rock resources usually occur at depths of 3 to 5 miles beneath the Earth's surface Access to these resources involves injecting cold water down one well, circulating it through hot fractured rock, and drawing off the heated water from another well.

Where does the Earth’s heat come from? The vast majority of Earth’s heat is constantly generated by the decay of radioactive isotopes, such as potassium-40 and thorium-232. As potassium-40 decays, its nucleus changes, emitting enormous amounts of energy (radiation). Radioactive decay is a continual process in the core. Temperatures there rise to more than 5,000° Celsius (about 9,000° Fahrenheit). Heat from the core is constantly radiating outward and warming rocks, water, gas, and other geological material.

Where does the Earth’s heat come from? (cont.) Earth’s temperature rises with depth from the surface to the core. This gradual change in temperature is known as the geothermal gradient. In most parts of the world, the geothermal gradient is about 25° C per 1 kilometer of depth (1° F per 77 feet of depth). Source: National Geographic

Dry Steam Saturated steam occurs at temperatures and pressures where steam and water can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation. Wet Steam A mixture of water and steam at the boiling point temperature of water at a given pressure.

How does it work? Today, power plants use steam from geothermal wells to make electricity. The steam is used to power turbines. The turbines spin magnets in coils of copper wire to convert the energy into electricity.

Uses and Statistics Geothermal energy can be used to heat homes and buildings and produce electricity Geothermal energy is used to heat about 85% of Iceland's buildings 22 countries are extracting energy from geothermal sites to produce 1% of the worlds electricity Japan, developing after the Fukushima nuclear reactor crisis, has made a concerted effort to look for renewable sources of energy including tapping into its geothermal base

Statistics (cont.) Geothermal power plants are producing over 8,200 megawatts of electricity in 21 countries, supplying about 60 million people with electricity.

Statistics (cont.)

Geothermal in America The Geysers, located near San Francisco, California, is the world’s largest operating geothermal site Dry steam reservoir Heat is being withdrawn 80 times faster than it can be replenished, converting this renewable energy source to a nonrenewable energy source Geothermal energy powered the whole city of Santa Monica, California making it the first city ever to have this distinction America uses about 2353.4 megawatts of energy each year

Other sources of geothermal energy Molten rock (Magma) Hot dry rock zone (where molten rock has heats subsurface rock to high temperatures) Warm-rock reservoir deposits (usually found 8-10 kilometers below the earth’s surface)

Advantages The US Environmental Protection Agency rated geothermal heat pumps as one of the most efficient heating and cooling technologies available today Moderate net energy at accessible sites Lower CO2 emissions than fossil fuels Low cost at favorable sites Low land use and disturbance Moderate Environmental Impact, very minimal effect on ecological systems

Disadvantages Scarcity of suitable sites Depleted if any sites used to rapidly CO2 emissions Moderate to high local air population Noise and odor Cost too high except at the most concentrated and accessible sources

Sources http://www.renewableenergyworld.com/rea/tech/geothermal-energy http://www.nrel.gov/learning/re_geothermal.html http://www.energy4me.org/energy-facts/energy-sources/geothermal/ National Geographic