Introduction
The fundamental process of thermal power plants is based on the conversion of thermal energy into mechanical energy and, subsequently, into electric power, using steam as the working fluid. According to the U.S. Energy Information System, over 67% of the electrical energy in the United States is generated through thermoelectric power plants, which use the combustion of fossil fuels such as natural gas, coal and petroleum to generate the steam necessary for the process; moreover, thermoelectric power generation accounts for an approximate 80% of all the power generation worldwide.
Thus, the electricity consumed in most homes and industries is generated in these type of electrical centrals, for which their study and understanding is primordial. Among the largest thermoelectric power plants are the Taichung Power Station in Taiwan with an installed capacity of 5,500 MW, the Shoaiba Power Plant in Saudi Arabia which generates 5,600 MW and the Surgut-2 Power Plant in Russian with a capacity of 5,597 MW. These plants use coal, fuel oil and natural gas as their energy source, respectively.
This paper will analyze the philosophy of thermoelectric power generation, the main components of these type of centrals, the role of turbines in the generation process and the thermodynamic laws that apply to them.
Main components of a thermal power plant
The basic process of a thermal power plant is as follows: a primary source of energy, such as coal, natural gas or oil is burned in a furnace, and the thermal energy obtained in this process is used to heat water until it evaporates inside a boiler. Subsequently, the steam generated is fed to a steam turbine, which realizes the thermal-mechanical energy conversion. Finally, this turbine is coupled to an alternator, which generates electricity when rotated by the turbine. The diagram shown in Figure 1 illustrates the simplified process of thermoelectric generation.
Figure 1. - Basic Process of a Thermal Power Plant
Source: Mastrangelo (2008)
The previous diagram shows thermoelectric power generation is in its simplest form. However, there are several other elements involved in the process. The main components of a thermal power plant and their respective functions are:
Boiler
The boiler is a closed vessel that contains water. Generally, it has an incorporated furnace in which fuel is burned to generate heat which is then transferred to the water until it reaches its boiling point, subsequently creating high-temperature and high-pressure steam. According to Raja, Srivastava and Dwivedi, there are two categories of furnaces: chamber fired furnaces (used to burn pulverized, liquid and gaseous fuels), and grate fired furnaces (used to burn solid fuel). Moreover, there are two types of boilers: water tube boilers and fire tube boilers. In the former, water flows through tubes surrounded by the gases produced in the combustion process, while in the latter, the hot products flow through the tubes which are immersed in water. Water tube boilers are the most commonly used, as they generate steam faster and have higher combustion efficiency due to larger combustion space. For the process to be efficient, it is necessary for there to be a water pump providing a constant supply of high-pressurized water to the boiler, which should minimize fuel consumption. An industrial boiler is shown in Figure 2.
Figure 2. - Industrial Boiler
Super heater
Used to further heat the steam generated in the boiler to approximately 540 to 620 °C, using the heat recovered from the combustion process. It consists of a set of tubes, which add additional heat to the saturated steam, effectively removing the remaining moisture in it. The steam turbine’s efficiency improves as the temperature of the steam rises, which is why most thermal power plants utilize super heaters before feeding steam to the turbine. Mamidisetti states that super heaters can be divided into three categories, which are the radiant type, convection type, and hybrids. Figure 3 shows a boiler that contains a super heater.
Figure 3. - Boiler with super heater
Steam Turbine
Steam turbines are designed to convert thermal energy into mechanical energy through a rotating shaft. They are composed of sets of stationary and rotating blades, which are connected to the shaft and are built progressively larger (from inlet to exhaust point) to allow the steam expansion, effectively completing the energy conversion. The Rankine Cycle describes the performance of these machines. Figure 4 shows a steam turbine.
Figure 4. - Steam Turbine
How it works – Philosophy of Operation
Steam turbines are classified (according to principle of operation) in impulse turbines and reaction turbines. In impulse turbines, the high-pressure and low-velocity steam coming from the boiler is driven through a fixed nozzle, which converts its inherent pressure energy into kinetic energy, as the nozzle’s outlet is a flux of low-pressure and high-velocity steam. Subsequently, the steam flows through a set of rotary blades, which change the direction of the momentum of the flow, and the steam molecules exert a centrifugal force on the blades’ surface causing them to rotate. According to Raja, Srivastava and Dwivedi, a row of nozzles and a raw of moving blades constitutes a stage of the turbine. In these type of turbines, the steam enters and exits the blade-section of the turbine maintaining the same pressure level. Conversely, Bloch and Singh explain that in a reaction turbine the steam expands in both the stationary and moving blades, as the latter are designed to use the kinetic energy of the stationary ones to act as nozzles as well by generating further kinetic energy, thus producing a pressure drop across them. Figure 5 shows the shape of a turbine blade and direction of the exerted force, and figure 6 shows the basic difference between impulse and reaction turbines.
Figure 5. – Turbine Blade Figure 6. – Impulse and Reaction Turbine Comparison
Figure 7 shows a diagram that illustrates the steam pressure and velocity behavior as it flows through an impulse turbine’s nozzles, moving and fixed blades. As previously explained, steam enters the nozzle section at high-pressure, which is then drastically reduced while increasing its velocity. As steam goes through the blades section pressure remains constant, due to the constant cross-section area of the blade passage.
Figure 7. - Steam Pressure and Velocity
Thermodynamics of steam turbines
The first Law of Thermodynamics is an adaptation of the law of conservation of energy applied to thermodynamics systems, and states that “the change in energy of a system is equal to the difference between the heat added to the system and the work done by the system”. This law can be applied to the conversion of energy performed in the nozzles through the Steady Flow Energy Equation that derives from it, considering that the steam velocity is obtained through the reduction of the steam enthalpy. Considering that it is considered an adiabatic process and there is no external work being done, the difference of steam enthalpy between the nozzle’s inlet and outlet can be related to the difference in velocity through the following equation: h1-h2= V22- V122.
The second Law of Thermodynamics states that “in any cyclic process the entropy will either increase or remain the same”, and steam turbines ideally perform an isentropic process (constant entropy). However, in practice, reversible processes do not exist and, in steam turbines, several losses occur in the nozzles due to deflection of flow, influence of viscous forces, friction, among other factors. The rate of the actual turbine work to the ideal scenario determines the isentropic efficiency of the turbine through the following formula: ηt= h1-h2ah1-h2s; where h1 is the entrance enthalpy, h2a is the real enthalpy exit state and h2s is the ideal enthalpy exit state. The ideal process of a steam turbine is described by the Rankine Cycle.
Rankine Cycle
The Rankine Cycle is the ideal or hypothetical process that describes the desired operation of a vapor cycle, not accounting for heat or other losses. For steam turbines, power generation through steam turbines the cycle defines the scenario in which all four stages of the process are considered ideal: pump, the boiler, the turbine and the condenser. After applying the Steady Flow Energy Equation to the four stages, the efficiency of the Rankine Cycle is:
η=WnetQ1=WT-WPQ1=h1-h2- h3-h4h1-h4
Figure 8. - Rankine Cycle T vs. s
Figure 7 shows the T vs. s (Temperature vs. Entropy) graph of the Rankine Cycle, which according to Steinhagen and Gottfried is explained as follows:
Points 1 – 2 represent the turbine’s isentropic expansion process, where steam expands at constant entropy, producing work that can be converted into electricity.
Points 2 – 3 represent the isobaric heat rejection process of the condenser, which is ideally a reversible process.
Points 3 – 5 represent the isentropic compression of the water obtained through the condensation process using an external pump.
Points 5 – 4 – 1 represent the isobaric heat transfer where high-pressure liquid is fed to the boiler (5) and is heated until saturation temperature (4). Further heating causes the water to evaporate until it has fully converted to steam (1), not accounting for any losses in the process.
Thermal Efficiency of Power Plant
Thermal efficiency is the rate of the useful output energy to the gross input of heat energy of the cycle (η=outputinput). The previous section described the ideal and high-efficiency process of a steam turbine; however, in reality, all processes are subject to losses that reduce their efficiency. According to the U.S. Department of Energy, the thermal efficiency of coal-powered thermal plants in the United States ranges from 27% to 37%.
Efficiency in a thermal power plants is reduced by several factors, including leakage of air, water, steam and fuel flow, presence of moisture in the steam fed to the turbine, differential pressure between inlet and outlet of the turbine, friction, etc.
Several measures can be taken to improve the cycle’s thermal efficiency, for instance, decreasing condenser pressure and increasing inlet steam pressure, for which other elements are introduced to the power plant, such as super heaters and economizers, which will be described in the following sections of this paper. Moreover, some products of the process are reused to reduce fuel and water consumption and consequently improving thermal efficiency.
Generator
The generator is responsible for converting the mechanical energy it receives through the rotation of its shaft by the turbine into electrical energy. Typically, power plants utilize AC synchronous generators for this purpose, which are characterized by the fact that the rotation rate of the rotor (measured in RPMs) is equal to the one of the rotating magnetic field, which differs from the basic principle of induction machines. Figure 5 shows an AC synchronous generator. Figure 5 shows the stator of a synchronous generator.
Figure 9. - Stator of AC synchronous generator
Condenser
The condenser, as its name suggests, condenses the steam at the exhaust of the turbine, keeping the pressure below atmospheric level, which improves the efficiency of the power generation process. The difference of heat per unit weight of the input and output steam represents the heat that is converted into mechanical energy. According to Pestana and Peypoch (2008), the reason for the increases efficiency is that condensing the output steam reduces its pressure, creating a larger pressure difference between the inlet and exhaust of the turbine, which allows for more heat per unit weight of steam.
These are the main components of thermal power plants, but other elements may be involved in the process, such as:
Economizer: Extracts some of the heat from the gases generated in the combustion chamber and uses it to heat the water fed into the boiler. This is used to save fuel (10 to 12%), and improves the boiler’s efficiency.
Cooling towers: These are concrete or steel hyperbolic towers used to cool water resulting from the condensation process to reuse it, thus making the process less harmful for the environment. According to the U.S. Department of Energy (2011), “warm recirculating water is sent to the cooling tower where a portion of the water is evaporated into the air passing through the tower. As the water evaporates, the air absorbs heat, which lowers the temperature of the remaining water”. After passing through the cooling tower, water can be pumped back into the system.
Conclusion
Thermal Power Plants, such as the Taichung Power Station and Shoaiba Power Plant, generate electricity from the conversion of heat energy obtained from coal, natural gas or other fuels. The basic scheme of a thermal power plant involves a furnace, in which fuel is burned to generate hot gases which will be used to heat water in a boiler to obtain steam, which is then used to drive a steam turbine that mill subsequently make the shaft of a synchronous generator rotate, effectively producing electricity.
Steam turbines are the heart of this type of power plants, as it is the machine that performs the heat energy to mechanical energy conversion. Steam turbines, which can be impulse or reaction turbines, take advantage of thermodynamic principles and through the use of nozzles increase the velocity of the inlet steam to impact a set of moving and stationary blades, exerting a force on them causing them to rotate.
The ideal process performed in thermal power plants is described by the Rankine Cycle, which does not account for losses. However, in reality all four stages (pumping, boiling, turbine and condensation) are subject to losses that reduce the cycle’s efficiency, to an average of 27-37%.
This form of power generation, though one of the most used worldwide, is responsible for the emission of greenhouse gases which aggravate global warming due to the burning of non-renewable fossil fuels; because of this, renewable forms of energy generation are in constant development.
Works Cited
Bloch, Heinz and Murari Singh. Steam Turbines: Design, Applications and Rerating. New York: McGraw Hill, 2009.
Mamidisetti, Venkat. "Role of superheaters in thermal power plants." 06 June 2013. India Study Channel. 07 April 2016.
Mastrangelo, Sabino. "Conceptos de Generacion Termoelectrica: Combustibles Utilizados e Impactos Ambientales." Boletin Energetico (2008): 14-26.
Pestana, Carlos and Nicolas Peypoch. "Technical efficiency of thermoelectric power plants." ELSEVIER - Energy Economics November 2008: 3118-3127.
Raja, A.K., Amit Srivastava and Manish Dwivedi. Power Plant Engineering. New Delhi: New Age International Publishers, 2006.
Steinhagen, Muller and Hans Gottfried. "Rankine Cycle." 02 February 2011. Thermopedia: A-Z Guide to Thermodynamics. Web. 07 April 2016.
U.S. Deparment of Energy (Energy Efficiency & Renewable Energy). Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency. February 2011. Document.
U.S. Energy Information Administration. "Independent Statistics & Analysis - What is U.S. electricity generation by energy source?" 01 April 2016. U.S. Energy Information Administration Website. Web. 06 April 2016. <https://www.eia.gov/tools/faqs/faq.cfm?id=427&t=3>.
