Geo (Earth) thermal (heat) energy is an enormous, underused heat and power resource that is clean (emits little or no greenhouse gases), reliable (average system availability of 95%), and homegrown (making us less dependent on foreign oil). Geothermal resources range from shallow ground to hot water and rock several miles below Earth's surface, and even farther down to the extremely high temperatures of molten rock called magma. Geothermal heating and cooling systems provide space conditioning--heating, cooling, and humidity control. They may also provide water heating--either to supplement or replace conventional water heaters. Geothermal heating and cooling Systems work by moving heat, rather than by converting chemical energy to heat like in a furnace. Geothermal systems are much more efficient than competing fuel technologies. They are an average of 48% more efficient than the best gas furnaces on a source fuel basis, and over 75% more efficient than oil furnaces.
Earth's energy can be converted into heat and electricity. The three technology categories are geothermal heat pumps, direct-use applications, and power plants.
Geothermal Heat Pumps Use Shallow Ground Energy to Heat and Cool Buildings
Almost everywhere, the upper 10 feet of Earth's surface maintains a nearly constant temperature between 50 and 60 degrees F (10 and 16 degrees C). A geothermal heat pump system consists of pipes buried in the shallow ground near the building, a heat exchanger, and ductwork into the building. In winter, heat from the relatively warmer ground goes through the heat exchanger into the house. In summer, hot air from the house is pulled through the heat exchanger into the relatively cooler ground. Heat removed during the summer can be used as no-cost energy to heat water.
In the U.S., most geothermal reservoirs are located in the western states, Alaska, and Hawaii. Hot water near Earth's surface can be piped directly into facilities and used to heat buildings, grow plants in greenhouses, dehydrate onions and garlic, heat water for fish farming, and pasteurize milk. Some cities pipe the hot water under roads and sidewalks to melt snow. District heating applications use networks of piped hot water to heat buildings in whole communities. .
Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and very hot water that drive turbines that drive electricity generators. Three types of power plants are operating today:
- Dry steam plants, which directly use geothermal steam to turn turbines;
- Flash steam plants, which pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines; and
- Binary-cycle plants, which pass moderately hot geothermal water by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to steam, which then drives the turbines.
The three technologies discussed above use only a tiny fraction of the total geothermal resource. Several miles everywhere beneath Earth's surface is hot, dry rock being heated by the molten magma directly below it. Technology is being developed to drill into this rock, inject cold water down one well, circulate it through the hot, fractured rock, and draw off the heated water from another well. One day, we might also be able to recover heat directly from the magma. We're standing on a resource that could easily supply the energy needs of the entire world for centuries.