Geothermal energy is thermal energy that originates within the earth. This source of energy is explained as being produced by nuclear decay, friction and pressure and heat left over from the formation of the earth billions of years ago. At Earth's core - 4,000 miles deep - temperatures may reach over 9,000 degrees F. Energy from Earth's core continuously flows outward, heating the surrounding layer of rock called the mantle. When temperatures and pressures become high enough, some mantle rock melts, becoming magma. Sometimes the hot magma reaches all the way to the surface, where it is called lava. Bust most often the magma remains below Earth's crust, heating nearby rock and water, sometimes to temperatures above 700 degrees F. There are some places in the world famous for spectacular geothermal activities, such as Yellowstone National Park's Old Faithful geyser, where water and steam from deep inside the earth leaks out.

Geothermal resources range from shallow ground sources (low temperature) to hot water, steam and rock miles below Earth's surface (high temperature). High temperature geothermal resources are underground reservoirs of hot water or steam that can be tapped for electrical power production. Presence of volcanic activity is a good sign that there is high temperature geothermal power ready to be tapped. Developers drill wells into the geothermal reservoirs to bring the hot water to the surface. Geologists, geochemists, drillers, and engineers do a lot of exploring and testing to locate underground areas that contain this geothermal water, so they they will know where to drill geothermal production wells. Then, once the hot water and/or steam travels up the wells to the surface, they can be used to generate electricity in geothermal power plants or for energy saving non-electrical purposes. In geothermal power plants, steam, heat, or hot water from geothermal reservoirs provides the force that spins the turbine generators and produces electricity. The used geothermal water is then returned down an injection well into the reservoir to be reheated, to maintain pressures, and to sustain the reservoir.

There are three kinds of geothermal power plants:

• A "dry" steam reservoir produces steam but very little water. The steam is piped directly into a "dry" steam power plant to provide the force to spin the turbine generator. The largest dry steam field in the world is The Geysers, about 90 miles north of San Francisco. Production of electricity started at The Geysers in 1960, at what has become one of the most successful alternative projects in history.
• A geothermal reservoir that produces mostly hot water is called a "hot water reservoir" and is used in a "flash" power plant. Water ranging in temperature from 300-700 degrees F is brought up to the surface through the production well where, upon being released from the pressure of the deep reservoir, some of the water flashes into steam in a separator. The steam then powers the turbines.
• A reservoir with temperatures between 250-360 degrees F is not hot enough to flash enough steam but can still be used to produce electricity in a "binary" power plant. In a binary system the geothermal water is passed through a heat exchanger, where its heat is transferred into a second (binary) liquid, such as isopentane, that boils at a lower temperature than water. When heated, the binary liquid flashes to vapor, which, like steam, expands across and spins the turbine blades. The vapor is then recondensed to a liquid and is reused repeatedly. In this closed loop cycle, there are no emissions to the air.

Since the first geothermally-generated electricity in the world was produced at Larderello, Italy, in 1904, the use of geothermal energy for electricity has grown worldwide to about 7,000 megawatts in twenty-one countries around the world. The United States alone produces 2700 megawatts of electricity from geothermal energy, electricity comparable to burning sixty million barrels of oil each year!

Although Wisconsin lacks the geological properties that make high temperature geothermal energy sources available, there are many homes, businesses and schools that are taking advantage of low temperature geothermal energy. Low temperature geothermal sources use the relatively constant temperature of soil or surface water as a heat source and sink for a heat pump, which heats and cools buildings. Just a few feet below Earth's surface the ground temperature stays relatively constant between 50 and 60 degrees Fahrenheit throughout the year. Geothermal or ground source heat pump systems use a series of underground pipes or loops to take advantage of the constant temperature just six feet beneath the earth's surface. In winter, Earth's heat is transferred from the ground to the house or building. In summer, the process is reversed. Similar to how a refrigerator works, the warmer indoor air is pumped or drawn back into the cooler ground.

Most heat pumps use a closed loop system where fluid circulates through loops installed in the ground horizontally or vertically. In other situations, the loops are submerged in a pond or lake. Open-loop systems while the cheapest to install, have environmental regulations that limit their use. In open-loop systems, ground water is piped from and back into a well; during the process it passes through a building where its heat is transferred to the heat pump. Geothermal heat pumps are more efficient in the cooling cycle. A typical air conditioner takes the hot air from outside and cools it. With a geothermal system, the source of cooling is from underground and does not require as much energy making the geothermal system more efficient and cost effective. Since the systems are more efficient for cooling, if extensive cooling is not required a geothermal system may not be the best option.

In the U.S., the temperature inside over 400,000 homes, schools, and offices is kept comfortable by geothermal systems, and hundreds of thousands more are used worldwide. The U.S. Environmental Protection Agency has rated geothermal heat pumps as among the most efficient heating and cooling technologies.

Geothermal heat pump systems can increase comfort and decrease costs for Wisconsin schools. Many schools like this technology because it allows each teacher to control his or her own system for improved comfort in the classroom. Temperature control can be applied to heat or cool whole buildings for events in just one area. Fond du Lac and Evansville High Schools in Wisconsin utilize groundsource heat pumps to heat and cool their schools (using closed-loop pond and vertical ground closed-loop systems, respectively).

Direct uses of geothermal waters ranging from 50 degrees F to over 300 degrees F include health spas, greenhouses, aquaculture, and milk pasteurization. These waters can also be used in the space heating of individual buildings and of entire districts. Geothermal district heating systems pump geothermal water through a heat exchanger, where it transfers its heat to clean city water that is piped to buildings in the district. There, a second heat exchanger transfers the heat to the building's heating system. The geothermal water is injected down a well back into the reservoir to be heated and used again. The first modern district heating system was developed in Boise, Idaho. In the western U.S. there are 271 communities with geothermal resources available for this use. Modern district heating systems also serve homes in Russia, China, France, Sweden, Hungary, Romania, and Japan. The world's largest district heating system is in Reykjavik, Iceland. Since it started using geothermal energy as its main source of heat, Reykjavik, once very polluted, has become one of the cleanest cities in the world.