Thermal power station
A thermal power station, also known as a thermal power plant, is a type of power station in which the heat energy generated from various fuel sources is converted to electrical energy. The heat from the source is converted into mechanical energy using a thermodynamic power cycle. The most common cycle involves a working fluid heated and boiled under high pressure in a pressure vessel to produce high-pressure steam. This high pressure-steam is then directed to a turbine, where it rotates the turbine's blades. The rotating turbine is mechanically connected to an electric generator which converts rotary motion into electricity. Fuels such as natural gas or oil can also be burnt directly in gas turbines, skipping the steam generation step. These plants can be of the open cycle or the more efficient combined cycle type.
The majority of the world's thermal power stations are driven by steam turbines, gas turbines, or a combination of the two. The efficiency of a thermal power station is determined by how effectively it converts heat energy into electrical energy, specifically the ratio of saleable electricity to the heating value of the fuel used. Different thermodynamic cycles have varying efficiencies, with the Rankine cycle generally being more efficient than the Otto or Diesel cycles. In the Rankine cycle, the low-pressure exhaust from the turbine enters a steam condenser where it is cooled to produce hot condensate which is recycled to the heating process to generate even more high pressure steam.
The design of thermal power stations depends on the intended energy source. In addition to fossil and nuclear fuel, some stations use geothermal power, solar energy, biofuels, and waste incineration. Certain thermal power stations are also designed to produce heat for industrial purposes, provide district heating, or desalinate water, in addition to generating electrical power. Emerging technologies such as supercritical and ultra-supercritical thermal power stations operate at higher temperatures and pressures for increased efficiency and reduced emissions. Cogeneration or CHP technology, the simultaneous production of electricity and useful heat from the same fuel source, improves the overall efficiency by using waste heat for heating purposes. Older, less efficient thermal power stations are being decommissioned or adapted to use cleaner and renewable energy sources.
Thermal power stations produce 70% of the world's electricity. They often provide reliable, stable, and continuous baseload power supply essential for economic growth. They ensure energy security by maintaining grid stability, especially in regions where they complement intermittent renewable energy sources dependent on weather conditions. The operation of thermal power stations contributes to the local economy by creating jobs in construction, maintenance, and fuel extraction industries. On the other hand, burning of fossil fuels releases greenhouse gases and air pollutants such as sulfur oxides and nitrogen oxides. Carbon capture and storage technology can reduce the greenhouse gas emissions of fossil-fuel-based thermal power stations, however it is expensive and has seldom been implemented. Government regulations and international agreements are being enforced to reduce harmful emissions and promote cleaner power generation.
Types of thermal energy
Almost all coal-fired power stations, petroleum, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as all natural gas power stations are thermal. Natural gas is frequently burned in gas turbines as well as boilers. The waste heat from a gas turbine, in the form of hot exhaust gas, can be used to raise steam by passing this gas through a heat recovery steam generator. The steam is then used to drive a steam turbine in a combined cycle plant that improves overall efficiency. Power stations burning coal, fuel oil, or natural gas are often called fossil fuel power stations. Some biomass-fueled thermal power stations have appeared also. Non-nuclear thermal power stations, particularly fossil-fueled plants, which do not use cogeneration are sometimes referred to as conventional power stations.Commercial electric utility power stations are usually constructed on a large scale and designed for continuous operation. Virtually all electric power stations use three-phase electrical generators to produce alternating current electric power at a frequency of 50 Hz or 60 Hz. Large companies or institutions may have their own power stations to supply heating or electricity to their facilities, especially if steam is created anyway for other purposes. Steam-driven power stations have been used to drive most ships in most of the 20th century. Shipboard power stations usually directly couple the turbine to the ship's propellers through gearboxes. Power stations in such ships also provide steam to smaller turbines driving electric generators to supply electricity. Nuclear marine propulsion is, with few exceptions, used only in naval vessels. There have been many turbo-electric ships in which a steam-driven turbine drives an electric generator which powers an electric motor for propulsion.
Cogeneration plants, often called combined heat and power facilities, produce both electric power and heat for process heat or space heating, such as steam and hot water.
History
The reciprocating steam engine has been used to produce mechanical power since the 18th century, with notable improvements being made by James Watt. When the first commercially developed central electrical power stations were established in 1882 at Pearl Street Station in New York and Holborn Viaduct power station in London, reciprocating steam engines were used. The development of the steam turbine in 1884 provided larger and more efficient machine designs for central generating stations.By 1892 the turbine was considered a better alternative to reciprocating engines. Turbines offered higher speeds, more compact machinery, and stable speed regulation allowing for parallel synchronous operation of generators on a common bus. After about 1905, turbines entirely replaced reciprocating engines in almost all large central power stations.
The largest reciprocating engine-generator sets ever built were completed in 1901 for the Manhattan Elevated Railway. Each of seventeen units weighed about 500 tons and was rated 6000 kilowatts; a contemporary turbine set of similar rating would have weighed about 20% as much.
Thermal power generation efficiency
The energy efficiency of a conventional thermal power station is defined as saleable energy produced as a percent of the heating value of the fuel consumed. A simple cycle gas turbine achieves energy conversion efficiencies from 20 to 35%. Typical coal-based power plants operating at steam pressures of 170 bar and 570 °C run at efficiency of 35 to 38%, with state-of-the-art fossil fuel plants at 46% efficiency. Combined-cycle systems can reach higher values. As with all heat engines, their efficiency is limited, and governed by the laws of thermodynamics.The Carnot efficiency dictates that higher efficiencies can be attained by increasing the temperature of the steam. Sub-critical pressure fossil fuel power stations can achieve 36–40% efficiency. Supercritical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures above and multiple stage reheat reaching 45–48% efficiency. Above the critical point for water of and, there is no phase transition from water to steam, but only a gradual decrease in density.
Currently most nuclear power stations must operate below the temperatures and pressures that coal-fired plants do, in order to provide more conservative safety margins within the systems that remove heat from the nuclear fuel. This limits their thermodynamic efficiency to 30–32%. Some advanced reactor designs being studied, such as the very-high-temperature reactor, Advanced Gas-cooled Reactor, and supercritical water reactor, would operate at temperatures and pressures similar to current coal plants, producing comparable thermodynamic efficiency.
The energy of a thermal power station not utilized in power production must leave the plant in the form of heat to the environment. This waste heat can go through a condenser and be disposed of with cooling water or in cooling towers. If the waste heat is instead used for district heating, it is called cogeneration. An important class of thermal power station is that associated with desalination facilities. These are typically found in desert countries with large supplies of natural gas. In these plants freshwater production and electricity are equally important co-products.
Other types of power stations are subject to different efficiency limitations. Most hydropower stations in the United States are about 90 percent efficient in converting the energy of falling water into electricity while the efficiency of a wind turbine is limited by Betz's law, to about 59.3%, and actual wind turbines show lower efficiency.
Electricity cost
The direct cost of electric energy produced by a thermal power station is the result of cost of fuel, capital cost for the plant, operator labour, maintenance, and such factors as ash handling and disposal. Indirect social or environmental costs, such as the economic value of environmental impacts, or environmental and health effects of the complete fuel cycle and plant decommissioning, are not usually assigned to generation costs for thermal stations in utility practice, but may form part of an environmental impact assessment. Those indirect costs belong to the broader concept of externalities.Boiler and steam cycle
In the nuclear plant field, steam generator refers to a specific type of large heat exchanger used in a pressurized water reactor to thermally connect the primary and secondary systems, which generates steam. In a boiling water reactor, no separate steam generator is used and water boils in the reactor core.In some industrial settings, there can also be steam-producing heat exchangers called heat recovery steam generators which utilize heat from some industrial process, most commonly utilizing hot exhaust from a gas turbine. The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator.
Geothermal plants do not need boilers because they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids.
A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and superheater coils. Necessary safety valves are located at suitable points to protect against excessive boiler pressure. The air and flue gas path equipment include: forced draft fan, air preheater, boiler furnace, induced draft fan, fly ash collectors, and the flue-gas stack.