Geothermal Power Generation
Geothermal energy is the energy which lies embedded within the earth. Geothermal electric power plants are built on the edges of tectonic plates where high temperature geothermal resources are available near the surfaces. From the center of the earth, there are different zones such as
- solid metallic core
- A molten core
- A mantle of solid rock 3400Km thick in layers
- A thin crust i.e. outer shell about 35 Km thick
The base of the mantel and the core has the temperature ranging from 3,000 to 4,000ºC.There is the tremendous heat energy which makes its presence felt in the eruption of volcanoes and in the spouting of hot springs and geysers. Mainly there are seven types of geothermal resources .Dry steam fields, wet steam fields and hot water are hydrothermal reservoirs .The types of geothermal resources are:
- Dry steam fields
- Wet steam fields
- Hot water
- Geopressured fields
- Magma deposit
- Hot dry rock
Dry steam fields
This is the most desirable form of geothermal energy. The steam is clean and easy to convert into electrical energy. Steam from the well is collected ,filtered to remove abrasive particles and passed through the steam turbines coupled to electric generators.
In dry steam plants, the steam is directed onto a closed water flow which on external contact becomes steam which in turn is used to move the blades of turbine. The spent water steam is then condensed back into water by condenser and then exposed again to the heat of dry steam. So ,this is a closed cycle system.
Wet steam Fields
Wet steam plants gives the mixture of hot water and steam under high pressure. The steam is separated and expanded in turbines to generate electricity. The hot water is used for removal of its minerals and then used for agricultural or municipal purposes. The hot water is also used for the Desalination plants, air conditioning and refrigeration, heating of buildings ,district heating, animal husbandry and industrial processes.
Types of Geothermal Power plants
For the conversion of geothermal energy into electrical energy, heat must be extracted first for its conversion into usable form. Mite-or-more-deep wells can be drilled into under ground reservoirs to tap steam and very hot water that drive turbines coupled to electric generators.
There are basically four types of geothermal power plants.
- Flashed steam plants
- Dry steam Plants
- Binary power plants
- Hybrid Power plants
1.Flashed steam plants: The extremely hot water from drill holes which released from deep reservoirs high pressure steam i.e. flashed steam is released. The force of steam is used to rotate turbines. The steam gets condensed and is converted into water again, which is returned to the reservoir.
- Steam with water extracted from ground
- Pressure of mixture drops at surface and more water “flashes” to steam
- Steam separated from water
- Steam drives a turbine
- Turbine drives an electric generator
- Generate between 5 and 100 MW
- Use 6 to 9 tonnes of steam per hour
2.Dry steam plants: The main sources of dry steam are geysers. The geothermal reservoirs producing mostly steam and little water are employed in electric power generation schemes. As steam from the reservoir shoots out ,it is used to drive a turbine, after sending the steam through a rock-catcher. The rock-catcher protects the turbine from rocks which come along with steam. Dry” steam extracted from natural reservoir
–180-225 ºC ( 356-437 ºF)
–4-8 MPa (580-1160 psi)
–200+ km/hr (100+ mph)
- Steam is used to drive a turbo-generator
- Steam is condensed and pumped back into the ground
- Can achieve 1 kWh per 6.5 kg of steam
–A 55 MW plant requires 100 kg/s of steam
3.Binary power plants: The geothermal water is passed through a heat exchanger where its heat is transferred to a secondary liquid. Because of double-liquid heat exchanger system, it is called binary power plant. The secondary liquid i.e. working fluid should have lower boiling point than water. It turns into vapor on getting required heat from the hot water. The vapor from the working fluid is employed for rotating the turbines. There is minimum chance of heat loss in this system.
- Low temps – 100o and 150oC
- Use heat to vaporize organic liquid
–E.g., iso-butane, iso-pentane
- Use vapor to drive turbine
–Causes vapor to condense
- Typically 7 to 12 % efficient
- 0.1 – 40 MW units common
4.Hybrid power plants: In this system of power generation, the flashed and binary systems are combined to make use of both steam and hot water. But the efficiency of hybrid power plants is less than that of dry steam plants.
- Similar to single flash operation
- Unflashed liquid flows to low-pressure tank – flashes to steam
- Steam drives a second-stage turbine
–Also uses exhaust from first turbine
- Increases output 20-25% for 5% increase in plant costs
Applications of Geothermal Power plants
- Low temperature(300°F or 149°C) geothermal resources are typically used in direct-use applications like district heating, greenhouses, fisheries, mineral recovery ,and industrial process heating.
- Geothermal heat pumps can be used for space heating.