Hot springs have been used for bathing at least since Paleolithic times. The oldest known spa is a stone pool on China's Lisan Mountain built in the 3rd century BC. In the first century AD the Romans used the hot springs at Aquae Sulis to feed public baths and under floor heating. Geothermal energy originates from the heat retained within the Earth since the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. Most high temperature geothermal heat is harvested in regions close to tectonic plate boundaries where volcanic activity rises close to the surface of the Earth. In these areas, ground and groundwater can be found with temperatures higher than the target temperature of the application. However, even cold ground contains heat and it may be extracted with a geothermal heat pump.
Economics of Geothermal Energy
Geothermal energy is a type of renewable energy that encourages conservation of natural resources. . In addition to being highly reliable they are built to last for decades. According to the U.S. Environmental Protection Agency, geo-exchange systems save 30-70% in heating costs, and 20-50% in cooling costs, compared to conventional systems. Geo-exchange systems also save money because they require much less maintenance
Power Station Types
Geothermal power stations are similar to steam turbine thermal power stations; heat from a fuel source is used to heat water or another working fluid. The working fluid is then used to turn a turbine, which in turn turns a generator to produce electricity. The fluid is then cooled and returned to the heat source.
Dry steam power plants
Dry steam plants are the simplest and oldest design. They directly use geothermal steam of 150°C or more to turn turbines.
Flash steam power plants
Flash steam plants pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines. They require fluid temperatures of at least 180°C, usually more. This is the most common type of plant in operation today.
Binary cycle power plants
Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 57°C. The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor, which then drives theturbines. This is the most common type of geothermal electricity plant being built today. The thermal efficiency is typically about 10%.