Geothermal energy is more efficient in volcanic since the geothermal gradient increases in these areas. Temperatures at the core-mantle boundary reach over 4000 ºC. The high temperature and pressure in Earth’s interior cause rocks to melt and solid mantle to behave plastically. Rock and water is heated in the crust, sometimes up to 370 °C.
This kind of geothermal power plant uses hydrothermal fluids which are underground hot water-rich fluids that are able to transport metals in solution. Hydrothermal fluids usually are steam that goes directly to a turbine. Then they are driven to a generator that produces electricity.
These power plants use hydrothermal fluids above 182°C to generate electricity. They have a flash tank. Fluid is sprayed into it at a lower pressure than the fluid, causing the fluid to rapidly vaporize. The vapor then drives a turbine, which drives a generator that generates electrical energy. If any liquid remains in the tank, it can be flashed again in a second tank (this is double flash) to extract even more energy.
A binary cycle power plant allows cooler geothermal reservoirs to be used than with dry steam and flash steam power plants. Systems in binary cycle power plants include Rankine vapor cycle, dual pressure and dual fluid. Most geothermal areas contain moderate-temperature water (below 400°F). Energy is extracted from these fluids in binary-cycle power plants. Hot geothermal fluid and a secondary fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines. Moderate-temperature water is the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.
HIGH TEMPERATURE RESERVOIRS (more than 150ºC)
That provide enough heat to generate electrical energy from steam. This kind of reservoir is located in areas of lithospheric thinness or active volcanism. Hot Dry Rock geothermal reservoir is an example of high temperature reservoirs and is exploited by Enhanced Geothermal System (EGS). EGS injects water into wells to be heated and pumped back out. The water is injected under high pressure to expand existing rock fissures to allow the water to flow in and out. The technique was adapted from oil and gas extraction techniques. The geologic formations are deeper and no toxic chemicals are used, reducing the possibility of environmental damage.
MIDDLE TEMPERATURE RESERVOIRS (100-150ºC)
They have a lower temperature than the high temperature ones, but they allow extracting enough heat to produce electricity using a volatile fluid. These reservoirs are located in areas with geothermal gradients higher than the average.Their main applications are in district heating systems and in industrial processes
LOW TEMPERATURE RESERVOIRS (100-30ºC)
The geothermal gradient is like the average in the region. Hot groundwater is pumped from the aquifer and re-injecting it after it has delivered the heat and is cold again. These are used for the same application than middle temperature reservoirs.
VERY LOW TEMPERATURE RESERVOIRS (<30ºC)
The underground is used as a exchanger of heat. Heat pumping in a closed circuit is used. Applications: domestic and agricultural air conditioning systems. These reservoirs are located wherever because their efficiency depends on the underground thermal inertia in average geothermal gradient conditions.