Though the amount of thermal energy within the earth is very large, useful resources of geothermal energy are very limited due to feasibility to access and extract heat.
Normally the thermal energy available inside the earth is at a depth of more than 80 km. The average temperature gradient in the earth’s surface is 30oC /km depth. Therefore, for power generation to attain temperatures up to 300oC, a hole has to be drilled in earth’s surface of about 10 km depth.
However, there are few sites in the world where this energy can be extracted at the depth of 0.5 to 3 km. The sites from where the thermal energy can be extracted are known as geothermal fields.
The earth’s interior having the mass of hot liquids, gases and steam are cooling slowly and the temperature of the earth’s core is about 4000oC. Below the solid crust of the earth, the molten mass which is known as magma is still in the process of cooling.
The hot magma near the earth’s surface causes hot gasses, hot springs, and geysers. We can utilize these hot gases and hot water to generate electricity.
There are different types of geothermal resources. Those geothermal resources, in which the water is heated by contact with the hot rocks are known as hydrothermal resources. These are being used to generate electricity presently since the technology for commercial utilization of other geothermal sources is not available.
There are three main types of geothermal energy power systems that are used to utilize geothermal energy and generate power.
- Dry Steam System
- Flash Steam System
- Binary Cycle System
Dry Steam System
It is a most common system. This system is used where geothermal resources deliver steam with little or no water. In this system, pressure and temperatures of the steam reaching to the earth’s surface are limited to 8 bar and 200oC. The block diagram of a dry steam power plant is shown in Figure.
Dry steam is extracted from the geothermal field. It contains some water and solid particles. These are removed in a centrifugal separator. Pure and dry steam is then fed to a steam turbine. The steam turbine drives an electrical generator coupled to it and generates electricity.
The exhausted steam from the turbine is condensed in a condenser. The condensate is then re-injected into the ground.
Flash Steam System
Flash steam plants differ from dry steam because they extract hot water, rather than steam, directly to the surface.
The hot water is extracted from the geothermal field from a depth of about 1 km at about 40 bar. Then hot water reaches to the wellhead at low pressure.
It is a throttling process due to which hot water is converted into a two-phase mixture having the steam of low quality and is fed to a flash chamber cum brine separator.
In the flash chamber, hot water flashes into steam. It results in dry steam, pure and dry steam is fed to a steam turbine and brine is collected from the bottom.
The steam turbine drives an electrical generator coupled to it and generates electricity.
The hot brine is re-injected into the ground along with the steam condensate from the condenser.
Binary Cycle System
This system is used where geothermal resources deliver hot fluid in the temperature range of 90oC to 170oC. This temperature is not sufficient for the production of steam. In order to utilize this geothermal heat, an organic compound of low boiling temperature like isobutene is used under pressure in a primary heat exchanger.
In this system, the water or steam from geothermal resource never comes in direct contact with the turbines. Instead, water from below the earth is pumped through a heat exchanger where it heats a second liquid (isobutene). The geothermal fluid is re-injected to the ground after extracting heat.
The isobutene vapor, generated in the primary heat exchanger, is passed through a turbine where it expands. The mechanical power of the turbine is converted into electrical power by a generator.
The exhaust of the turbine is passed through a regenerator (heat exchanger) where it is cooled and then condensed in the condenser.
The returned condensate is heated in the regenerator by the exhaust vapor of the turbine.
Advantages and Disadvantages of Geothermal Energy
- It is a reliable source of energy and is available throughout the year.
- It is independent of weather conditions.
- No thermal storage is required.
- Capital and generation cost is low as compared to conventional thermal power plants.
- Needs a very small land area.
- It is a low-grade heat energy since the temperatures are limited to 150o
- The geothermal fluids are corrosive and abrasive in nature due to the presence of salts. Therefore, the life of the plant is low as compared to conventional power plants.
- The geothermal fluids also bring dissolved gases like H2S, CO2, NH3 and other solvents which cause air and land pollution if not discharged into the ground properly.
- Continuous extraction of these fluids may affect the stability of land and may trigger earthquakes also.
- MHD Generator Working Principle
- Closed | Open Cycle MHD System
- Tidal Power Plant Working Principle
- Working Principle of Hydroelectric Power Plant
- Nuclear Power Plant Working Principle
- Wind Power Plant Working Advantages | Disadvantages
- Concentrating Solar Collector Types | Power Plants
- Solar Panel Working Principle
- How Geothermal Energy Works
- OTEC | Ocean Thermal Energy Conversion System Working