Geothermal Energy


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.

Geothermal resources are classified as convective systems (hydrothermal), conductive systems and deep aquifers.

  • Hydrothermal systems include liquid and vapour.
  • Conductive systems include hot rock and magma.
  • Deep aquifers include fluids in porous media or in fractures.

The global installed geothermal capacity is over 12,000 MW. In total there are about 12,000 MW in the pipeline and about 30,000 MW of geothermal resources under development.


Types of Geothermal Power Plants

Geothermal power plants work as steam turbine thermal power stations. They use heat from the Earth’s core to produce electricity or heat. This heat is used to heat a working fluid that can be water or not. This working fluid is used to do a turbine work and then a generator produces electricity. Eventually, the fluid is collected and is returned to the heat source.

Power plant can be classified as dry steam, flash steam and binary cycle:


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.

Binary cycle

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.


Types of Geothermal Reservoirs

A geothermal system requires heat, water and permeability. Sometimes the magma can reach the Earth’ surface but the heat usually remains below the crust of the Earth heating rocks and water. Hot water and steam can be stored in permeable and porous rocks under an impermeable layer. If this happens, a geothermal reservoir is formed. A geothermal reservoir can be found because it can manifest on the surface as hot springs or geysers. However, most of them are underground.

Hot spring in Yellowstone´s National Park
Castel Geyser

Geothermal reservoirs can be classified as:


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.


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


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.


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.

Benefits to the environment

Coolmyplanet proposes Geothermal as a solution to combat climate change because:

  • It causes a minimal environmental impact.
  • It is a renewable energy.
  • Mitigation of the gases released in depth operations: current methods are effective and protect the air from emissions.
  • In a closed loop geothermal system, there are no CO2 emissions.
  • Small footprint: minimal equipment and relatively small area to work is required for injection and production wells.
  • Reduced CO2 emissions:  steam has carbon dioxide but it is less than 4% of what is typically released by fossil fuel plants. The average rate of CO2 emissions for coal power plants are 2200 lbs CO2/MWh. Geothermal systems, produce less emissions, approximately 197 lbs CO2/MWh.


It is estimated that the global geothermal industry could reach about 13,450 MW of installed capacity by 2017 if all the scheduled projects are carried carry out.

The EIA predicts that geothermal heat could contribute 5.8 EJ annually by 2050:


The largest potential for geothermal heat can be found in Europe, China and North America.

For 2014 an increasing price competition is expected for both Flash and Binary systems. Flash because of the lack of development and binary due to the fact of increasing competition.

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