1. 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.

2. 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.

3. 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

4. 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.

5. 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.

6. 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

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

8. Statistics (cont.)

9. 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 megawatts of energy each year

10. 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)

11. 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

12. 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

13. Sources http://www.renewableenergyworld.com/rea/tech/geothermal-energy
National Geographic