One of the applications of geothermal energy is the generation of electricity. A geothermal power plant is like any other power plant, except that steam is not produced by burning fossil fuels or other fuels, but is pumped from the ground. The additional steam treatment is the same as that of a conventional power plant: steam is fed to a steam turbine, which drives the rotor of an electric generator. After the steam turbine enters the condenser, it condenses to return the water thus obtained to the geothermal source. The term geothermal energy is the energy that can be recovered from the interior of the Earth and used for energy or other purposes.
Geothermal energy is considered a renewable and sustainable energy source, as is solar or wind energy, because heat extraction is low compared to the Earth's heat content. Greenhouse gas emissions from geothermal power plants average 45 grams of carbon dioxide per kilowatt-hour of electricity, or less than 5% of the amount of conventional coal power plants.
Conversion of geothermal energy into electricity
One of the most used applications of geothermal energy is the generation of electricity. The choice of technology for the production of electricity depends on the type of geothermal deposit.
Geothermal energy uses steam to drive the steam turbine. Steam (wet or dry) can be obtained directly from the reservoir, and it can also be produced artificially in hot and dry rocks, the so-called advanced geothermal systems.
In beds with lower fluid temperatures, steam to drive the turbines is obtained indirectly by heating the working fluid with a boiling point lower than the boiling point of water. Rankin's organic cycle is different, and it's called the Kalina Process. The difference is in the composition of the working fluid, Rankin's organic cycle uses organic ingredients such as toluene, pentane, propane and other hydrocarbons, while a mixture of ammonia and water is used in the Kalina cycle. The Kalina cycle is not a preferred approach precisely for the use of ammonia.
In any case, geothermal power plants can be divided into three basic types: dry steam plants, evaporation plants (single and double) and binary plants.
Dry steam geothermal power plants
Dry steam plants are the first type of geothermal power plants that reach commercial status.
Steam can be fed directly to the turbine from the production well and discharged into the atmosphere after expansion. Generally reheated steam is generated, and contains only small amounts of other gases. Such a direct condensation cycle is the simplest and cheapest option to produce electricity from this renewable energy source. They are used in cases where steam contains a large proportion of non-condensing gases.
In condensation plants, steam condenses at the turbine outlet and cools in conventional cooling towers. The resulting condensate can be used in the power plant cooling system and pressed back into the tray. In this way, the bearing is restored and the required pressure is maintained.
Evaporative geothermal power plants
In water dominant beds, the technology of evaporative geothermal power plants is applied. The energy in this case is pressurized water. Since the pressure in the well is generally lower than the pressure in the well, water under pressure in the well flows to the surface. As a result of the pressure drop, a certain portion of the liquid evaporates and the well produces hot water and steam at the same time, with water being the dominant phase.
The double evaporation plant is an improvement over the single evaporation plant, since it provides 15-25% more production, for the same geothermal fluid conditions. The plant is more complex, more expensive and more demanding in terms of maintenance, but the higher output power generally justifies the installation of such plants.
Binary cycle geothermal stations
The binary cycle geothermal power plants, from the point of view of thermodynamics, those closest to thermal power plants that use fossil fuels or nuclear power plants, in which the working fluid is taking a real closed cycle. The working fluid, selected for its favorable thermodynamic properties, receives heat from the geothermal fluid. Thanks to the laws of thermodynamics, this fluid evaporates, expands in the turbine, condenses and returns to the evaporator through a feed pump.
Binary plants allow the conversion of geothermal heat into electricity from hot water tanks at low temperatures (the so-called dominant water tanks) with temperatures above 85 ° C. In addition, this technology is suitable for the exploitation of renewable energy sources of medium temperature with wet steam with a high water / steam ratio at temperatures too low for the practical application of the evaporation system. Binary plants convert heat from medium temperature sources into electricity more efficiently than other technologies.
The use of binary plants has been improved with the introduction of Kalina technology. A mixture of water and ammonia evaporates within a finite temperature range, producing two-component steam (for example, 70% ammonia and 30% water), unlike the Rankin organic cycle based on pure fluids that evaporate at a given evaporation temperature.
Comparison of geothermal power plants with conventional power plants
Whether geothermal energy is used to generate electricity or directly, the characteristics of geothermal deposits determine the technology to be exploited. The geothermal fluid often contains large amounts of gases such as hydrogen sulfide and various chemical solutions that can be very toxic. Therefore, problems of corrosion, erosion and deposition of chemical compounds can occur, resulting in the failure of pipes and turbines, and even a decrease in plant efficiency. These problems are avoided by a combination of the use of corrosion resistant materials, fluid temperature control, steam purification and the use of anticorrosive agents.
Specificity of geothermal plants:
- In geothermal plants there is no combustion of fossil fuels, which reduces costs, but also minimizes environmental pollution;
- The low temperature and vapor pressure result in a low thermodynamic efficiency of the plant (typically ~ 15%) compared to fossil fuel power plants (35-38%);
- A long and complex commissioning process makes geothermal power plants more suitable for covering the base load than for covering the maximum load;
- geothermal power plants should be located as close as possible to production to avoid transport losses;
- A 100 MW geothermal power plant consumes about 80 tons / hour of steam. This flow is generally achieved by multiple production wells that pump the same bearing;
- The steam has a good amount of minerals, which cause erosion and corrosion of the turbine elements. This requires continuous and meaningful maintenance;
- The initial cost of a geothermal power plant is higher since, in addition to the power plant, it is necessary to build a well, which is actually the highest cost. However, over time, costs decrease as the availability of resources is stable and predictable. In addition, a geothermal power plant does not depend on market movements in energy prices.