Thermal Power Engineering
Thermal Power Engineering
the branch of heat engineering that deals with the conversion of heat into other forms of energy, mainly mechanical and electric. Mechanical energy is generated from heat in heat engines, which power, for example, machine tools, automobiles, and conveyors; the mechanical energy from heat engines also drives certain types of electric generators. The devices in which heat is converted into electric power without the use of an electric generator are known as direct-power generators. Such devices include magnetohydrodynamic generators, thermoelectric generators, and thermionic power generators.
The conversion of heat into mechanical energy in heat engines is based on the ability of a gaseous or vaporous substance to perform mechanical work during a change in volume. The working substance (gas or vapor) must complete a closed sequence of thermodynamic processes, that is, a cycle. During the cycle, a certain quantity of heat Q1 is withdrawn from one or more heat sources, and a lesser quantity of heat Q2 is returned to one or more heat sources; here, the difference Q1 – Q2 is converted into mechanical work Atheor. The ratio of the work produced to the heat expended is known as the thermal efficiency of the cycle:
In the simplest case, a cycle can be carried out with a single heat source at a temperature T1 that imparts heat to the working substance and a single heat source at a temperature T2 that receives heat from the working substance. For this temperature interval T1 – T2, the highest efficiency ηc = 1 – T2/T1 is that of the Carnot cycle, that is, ηc ≥ ηr. An efficiency equal to unity, that is, a total conversion of the heat Q1 into work, is possible only when T1 = ∞ or T2 = 0. Of course, neither of these conditions is possible. It must be stressed that under terrestrial conditions the temperature T2 in the apparatus used in thermal power engineering must at best be assumed equal to the temperature Te of the environment (air, body of water). A heat source having a temperature T2 < Te can be created only by using a refrigerating engine, which in general requires the expenditure of work for its own operation. The impossibility of a total conversion of heat into work where all substances participating in the conversion return to their original state is established by the second law of thermodynamics.
Since the processes that occur in actual devices for converting heat into other forms of energy are accompanied by various losses, the actual work Aact done proves to be less than the work Atheor that is theoretically possible. The ratio of actual to theoretical work is called the relative effective efficiency of the device ηre, that is,
From equations (1) and (2), we see that
Aact = Q1ηtηre = Q1ηe
where ηε = ηt ηre is the effective efficiency of the device. Other conditions being equal, the efficiency of converting heat into work depends on the temperature at which the heat is transferred to the working substance. The maximum work obtainable from a quantity of heat Q withdrawn at temperature T1 with a temperature Te of the surrounding medium is called the efficiency of the heat, or energy, la, that is,
We see from equation (3) that when T1 = Te, the heat energy is equal to zero.
In its most complete form, an engine for converting heat into mechanical work (heat engine) includes a working substance that is carried through a closed sequence of thermodynamic processes (a cycle), systems for supplying heat to the working substance from some source of heat energy, one or more machines that either receive work from or perform work on the working substance, and a system for transferring heat from the working substance to the environment. A distinction is made between engines in which heat is supplied to the working substance from an external source (in a heat exchanger) and engines with an internal supply (with the working substance in the form of combustion products).
Nonnuclear thermal power plants. The heat engines in steam-electric power plants are the basic units of modern (1975) thermal power engineering. These plants comprise a boiler unit and a steam turbine. In the USSR, more than 80 percent of all electric power is produced (1975) in such plants. District heat and power plants are usually constructed in large cities, while condensation electric power plants are favored in regions where fuel is cheap.
District heat and power plants differ from condensation plants in that they provide the consumer not only with electric power but also with heat, via the feedwater, which is heated in boilers to temperatures up to 150°–170°C. The water is fed through pipes to apartment complexes, where it is either used directly or passed through intermediate heat exchangers to provide space heating and to heat water for the building. In addition to extractions for purposes of regeneration, there may be one or more controlled extractions for heating systems from the turbines in district heat and power plants. The operation of the turbines depends on the demand for heat, and during the colder part of the year almost no steam reaches the condenser. Space heating from district heat and power plants is more economical than from individual boilers or even central boilers because the feedwater at district heat and power plants is preheated by the spent steam, whose temperature (and, thereby, energy) is only slightly above the temperature of the feedwater. The heat used in the boiler rooms is at the maximum combustion temperature of the fuel in order to improve the efficiency.
A simplified schematic diagram of a condensation steam-turbine electric power plant is shown in Figure 1. The fuel (coal, mazut, natural gas) is burned in the combustion chamber of the boiler unit. The air required for combustion, which is first heated by gases leaving the boiler unit in a recuperative air heater, is fed to the combustion chamber by a forced-draft fan. The combustion products give up their heat to both the water and the steam in various elements of the boiler unit and then pass at a temperature of 130°–150°C through an ash collector to an exhaust fan, which discharges the products through the chimney.
Steam is the working substance that converts the heat into mechanical work. Superheated steam passes from the superheater into a steam turbine. The steam pressure prior to entering the turbine in large electric power plants reaches 35 meganewtons per sq m (MN/m2) at a temperature of 650°C. Within the turbine, the steam passes through fixed nozzles into channels formed by curved blades mounted on a rotor and, by giving up its energy, turns the rotor. The mechanical energy of the turbine’s rotor is converted into electric energy in the generator. The steam turbine usually requires two or three housings. Steam from the part of the turbine in the first housing is passed to the part in the second housing, sometimes being returned to the steam generator for intermediate reheating in the superheater. Spent steam from the turbine passes to the condenser, where a pressure of 0.003–0.005 MN/m2 and a temperature of 25°–29°C are maintained. The condensate obtained is pumped into a system of regenerative preheaters, where it is heated to 230°–260°C by steam taken from the turbine, and is then pumped to an economizer. From the economizer, the water enters the steam drum, from which it passes through boiler tubes on the walls of the combustion chamber. The water is partially evaporated in the tubes, and the steam-water mixture is returned to the drum. There the saturated steam is separated from the water and passed first to the superheater and then to the turbine; the water is returned to the boiler tubes. To generate steam having supercritical parameters (pressures above 24 MN/m2), flow-through boilers are used.
Cooling water is supplied to the condenser from natural or artificial bodies of water, to which it is returned after being heated by several degrees. The temperature of the cooling water is ultimately restored to the previous level owing to evaporation of part of the water. When there are no bodies of water of sufficient size, the cooling water is circulated through a closed loop and subjected to air cooling in evaporative coolers of the tower type called cooling towers. In regions where sufficient water is lacking, dry cooling towers (Hellert towers) are used in which the cooling water transfers its heat to the air through the wall of a heat exchanger.
An important trend in thermal power plants is the increase in the output of the turbine-generator units, making possible rapid growth rates in the power available per worker in the national economy. As of 1976, units in the USSR with an output of 800 megawatts (MW) were being used in condensation power plants, and a 1,200-MW unit was under construction. In district heat and power plants, 250-MW units were in operation.
In gas-turbine power plants, the heat engine is a gas-turbine engine. Here, fuel (natural gas, mazut) and air compressed to several MN/m2 are fed into the combustion chamber. Combustion of the fuel involves a high excess of air coefficient (2–4), which acts to lower the temperature of the combustion products entering the gas turbine. After leaving the turbine, the combustion products either give up part of their heat in a regenerator to air introduced into the combustion chamber or, in simpler systems, are discharged through the chimney. The mechanical energy of the turbine’s rotor is converted into electric energy in the generator, but part is expended in driving the compressor. Gas-turbine electric power generating plants are used to supply power on gas pipelines (where there is fuel gas under pressure) and to supplement the supply of power during periods of peak demand. As of the mid-1970’s, the total capacity of gas-turbine power plants in the world exceeded 2.5 gigawatts (GW).
Steam-gas turbine installations, in which there is a combined cycle involving both gas and steam turbines, hold great promise. Depending on the design for heat flow, these installations fall into two categories. In the first, steam at a pressure of 0.6–0.7 MN/m2 from a high-pressure steam generator is directed into the steam turbine; the combustion products enter the gas turbine, which is used to drive the air compressor and electric generator. In the second type of installation, the hot exhaust gases from the gas-turbine unit either enter the furnace of the steam boiler in order to increase the temperature or serve to heat the feedwater in the boiler’s economizer. Compared with steam-turbine plants having the same capacity and parameters, a steam-gas turbine installation consumes 4–6 percent less heat.
In diesel power plants, the electric generators are driven by diesel engines rather than by turbines, as in most thermal power plants. Diesel plants are used to supply electric power in regions distant from transmission lines; they are also used when it is not possible to build hydroelectric plants or other types of thermal power plants. The capacity of these plants can exceed 2.2 MW.
Nuclear power plants. In the vast majority of cases, nuclear power plants use steam turbines. They differ from the thermal power plants discussed above in that they have a nuclear reactor instead of a steam generator with a furnace. In the reactor, the energy liberated from the fission of uranium nuclei is converted into heat, which is transferred to a coolant, usually water, that circulates in a loop passing through the reactor vessel. The coolant in turn transfers the heat in a heat exchanger (steam generator) to the working substance (water) circulating in a second loop. The heat causes the working substance to evaporate, and the steam thus obtained is directed into a steam turbine. In some cases, particularly when the reactor is cooled by a liquid metal, a third loop with a heat-transfer agent is introduced between the first and second loops for safety reasons.
The world’s first nuclear power plant, having a capacity of 5,000 kilowatts (kW), was built in 1954 in the USSR. By 1964 the total capacity of the world’s nuclear plants was 5 GW, and by 1974 it was approximately 40 GW. It is predicted that by 1980 approximately 10 percent of the world’s electric power will come from nuclear power plants. Even though construction costs for nuclear plants are approximately 80 percent higher than costs for nonnuclear plants of the same capacity, operating costs turn out to be equal. Moreover, with the expected increase in cost of non-nuclear fuels in the future, nuclear power plants will become increasingly attractive.
Power plants for transportation. The engines used in motor transport are mainly internal-combustion piston engines with the mixing of fuel and air occurring either outside (engines with carburetors) or inside (diesel engines) the combustion chamber. The fuel’s combustion products serve as the working substance. All the processes needed to convert heat into mechanical energy are carried out in the cylinders. The fuel-air mixture is drawn into the cylinder and then burned; the combustion products thus formed perform work as they expand, and the work is transmitted by the piston to external mechanical apparatus. The combustion products are then forced out of the cylinder by the piston into the atmosphere. The diversity seen in piston engines is due primarily to the various thermodynamic cycles and is manifested in different engine designs. In railroad transportation, the prime mover until the mid-20th century was the steam engine—a piston engine operating on steam generated in a separate boiler. By the 1970’s, diesel and electric locomotives predominated in all industrially developed countries. Gas-turbine locomotives are presently being developed.
All the thermal power plants discussed thus far, from small engines used in automotive transport to steam-turbine units producing tens of MW, figure in ship construction. In aviation, piston aircraft engines, which supply mechanical energy to an airscrew, are used, as are turbo-propeller engines, which produce thrust principally through an airscrew but also (8–12 percent) through the discharge of combustion products. Aviation also makes use of jet engines, which produce thrust by discharging at a high velocity the working substance, that is, combustion products, from an exhaust nozzle.
Direct-power generators. The heat engines discussed above convert heat into mechanical energy, which in turn is either converted into electric energy at power plants by generators or is expended in powering various types of transport vehicles. However, it is also possible to convert heat directly into electric energy by means of direct-power generators, of which the most promising is the magneto-hydrodynamic generator (MHD generator). The thermodynamic cycle of an electric power plant with an MHD generator driven by the combustion products of an organic fuel is similar to the cycle of a gas-turbine unit. Fuel and compressed air that has been preheated to the highest possible temperature or enriched with oxygen are fed into a combustion chamber. This preparation is necessary to achieve in some way the theoretical products of the fuel—around 3000°K. At such a temperature, the combustion products, to which a certain quantity of ionizing substance, such as an alkali metal (usually potassium), has been added, change into the plasma state and become fairly good electrical conductors. The kinetic energy of this plasma in the MHD generator’s channel is converted directly into electric energy as a result of the interaction between the moving plasma and a stationary magnetic field. After leaving the generator, the combustion products are cooled, purified of ionizing additives, and discharged through a chimney. As of 1975, the capacity of MHD generators using combustion products was of the order of tens of MW. Because the temperature of the gases upon leaving the generator is very high (over 2000°K), MHD generators are efficient when used in conjunction with conventional steam-electric power plants. In this case, the heat removed from the gases is used to produce steam for the steam-turbine unit. The efficiency of such a combined MHD and steam-electric power plant can reach 50–60 percent. This increase in efficiency is also very important from the standpoint of reducing the heat discharged into the environment. Thus, if it is assumed that the efficiency of a thermal power plant is approximately 40 percent, an increase in efficiency to 60 percent reduces the quantity of heat discharged by a factor of approximately 2.3 (with the same plant capacity).
For small special-purpose power plants, such as the electric-power sources on spaceships, thermoelectric and thermionic power generators have been developed. A thermoelectric power generator comprises two dissimilar materials—n-type (electron) and p-type (hole) semiconductor materials. At one end, these materials are connected by a jumper, while the free ends are fitted with electric contacts to make connections to an external circuit. If the ends (junctions) of the materials are held at different temperatures, a thermoelectromotive force arises that is proportional to the temperature differential between the ends. When the circuit to which the ends of the materials are linked is closed on an external resistance, an electric current commences; during the flow of charge, an absorption of heat begins at the hot junction, and a liberation of heat at the cold junction. If Joule-heat losses in the circuit and the return flow of heat owing to the thermal conductivity from the hot to the cold junction are disregarded, the efficiency of the thermocouple proves to be equal to that of a Carnot cycle for temperatures corresponding to the junction temperatures. The actual efficiencies of thermocouples and of thermoelectric power generators made with thermocouples is substantially less, and for temperature differences between the junctions of 400°–500°K the efficiencies are at best several percent. The low efficiency, together with the high cost of the thermocouples, explains the limited use of thermoelectric power generators despite such advantages as extreme simplicity and absence of moving parts.
The simplest thermionic power generator is analogous to a two-electrode electron tube (diode). If the cathode and anode of the tube are maintained at different temperatures by supplying heat to the cathode and withdrawing heat from the anode, then the electrons liberated from the cathode as a result of thermionic emission move to the anode, imparting a negative charge. If the anode and cathode are connected to an external circuit through some resistance, a current will begin in the circuit because of the potential difference. If irreversible energy losses are disregarded, the efficiency of a thermionic power generator is close to that of the corresponding Carnot cycle. The actual efficiency, however, is no more than 7–8 percent, primarily because of the large heat loss by radiation between the cathode, which has a temperature around 2000°K, and the anode, which is around 1000°K. Thermoelectric and thermionic power generators are of interest when considered in combination with nuclear heat sources, a combination forming a steady-state, self-contained source of electric power.
REFERENCESFavorskii, O. N. Ustanovki dlia neposredstvennogo preobrazovaniia teplovoi energii v elektricheskuiu. Moscow, 1965.
Alekseev, G. N. Preobrazovante energii. Moscow, 1966.
Ryzhkin, V. Ia. Teplovye elektricheskie slantsii. Moscow-Leningrad, 1967.
Margulova, T. Kh. Atomnye elektricheskie stantsii, 2nd ed. Moscow, 1974.
Magnitogidrodinamicheskii metod polucheniia elektroenergii, fasc. 3. Moscow, 1972.
V. A. KIRILLIN and E. E. SHPIL’RAIN