Steam Turbines and Generators
The steam turbine, invented by Charles Parsons, exploits the velocity and kinetic energy contained in a jet of expanding steam to drive an engine. The steam turbine breaks down the expansion of this high pressure, high temperature steam into a large number of small stages, using turbine blades and nozzles to control the expansion without having to cope with excessively high steam velocities. The resulting turbine combines both reaction and impulse elements to gain the highest efficiency. Steam turbines for large steam plants, up to 1000 MW or more in capacity, are made up of high pressure, intermediate pressure and low pressure turbines. Steam may be reheated between turbines stages to improve efficiency. Steam turbines drive large generators to produce electricity from rotary motion. These may be up to 1000 MW in size and cooling is a significant problem. Small generators adopt air cooling but larger machines use hydrogen or a mixture of hydrogen and water cooling.
Keywords
Charles Parsons; steam turbine; impulse turbine; reaction turbine; nozzle; blade; high pressure turbine; intermediate pressure turbine; low pressure turbine; generator; hydrogen cooling; air cooling; water cooling; efficiency
When the electricity-generating industry began, the only type of steam engine available to drive a generator was a reciprocating or piston engine. These had reached a high level of development by the end of the nineteenth century but even the best of them was restricted as to the absolute mechanical power output it could produce. Physical restrictions on weight and size meant that larger machines were impractical.
The principle upon which the reciprocating steam engine operated was the use of the potential energy in high-pressure steam to drive a piston in a cylinder, venting the exhausted steam to the air via a valve while the inertia of the engine returned the piston to its starting position and the cycle restarted. Charles Parsons, inventor of the steam turbine, sought, instead, to use the velocity and kinetic energy of a jet of steam released from a high-pressure source to drive an engine. It was already well known that a jet of fluid could be used to drive a wheel by mounting blades upon the wheel, upon which the jet impinged. This is the basis for the simple waterwheel. The Greek engineer, Hero of Alexandria, had also demonstrated that allowing high-pressure steam to escape from nozzles mounted tangentially on a boiler would drive the boiler to rotate. His aelopile, sometimes called Hero’s engine, is the first example of a reaction engine as used in a rocket.
The problem with harnessing this principle to build a steam engine in which the steam drove a bladed wheel was the speed of the jet of steam. Even a moderately low-pressure steam jet can reach 750 m/s and high-pressure steam exiting a nozzle into a vacuum might reach 1500 m/s. In order to harness this efficiently to capture energy using a bladed wheel, the blade has to be traveling at around half the speed of the steam. This would require the wheel to rotate at an extremely high speed. Aside from the basic engineering problems this raised there was the question of the materials being able to withstand the centrifugal force to which they would be exposed.
The solution that Parsons adopted to this problem was to reduce the speed that the steam reached during expansion by breaking the expansion into a series of small stages. He achieved this by building an engine that comprised a rotor with a series of blades that fitted into a cylinder that also had blades fixed inside it. The rotor blades fitted into the casing between the fixed blades. When steam from a boiler was released into the front of the engine it would pass alternately between the fixed and moving blades. The fixed blades acted as nozzles through which the steam partially expanded and the velocity it acquired was then transferred to the next set of blades which were moveable, causing them to rotate. At the same time, passages between the moving blades also acted as nozzles and as the steam expanded through these, the moving blades were further impelled by the expansion, a reaction in the manner of the engine of Hero of Alexandria.
By using multiple stages of fixed and rotating blades, the expansion of the steam was controlled to take place in small increments and the absolute velocity of the steam in each stage was made manageable. This allowed an efficient steam turbine to be built. However, even this rotated at around 300 rev/s or 18,000 rev/m, much faster than the 1500 rev/m typical of a reciprocating steam engine of the day. As a consequence Parsons had to design a dynamo that could be driven by his new engine. The first of these, now at the Science Museum in London, produced 75 Amps at 100 V, an output of 7500 W.
A Parsons turbine utilizes the action of high-speed steam on the rotating blades and also the reaction on the blade of the steam expanding through the nozzle created by the moving blades. It is also possible to build a turbine that exploits solely the action of the high-speed steam, creating what is known as an impulse turbine. However, these are generally less efficient than a turbine that exploits both the impulse and the reaction effect.
The Parsons turbine were initially used for ships’ lighting. It required Parsons himself to build power stations, first in Newcastle upon Tyne in 1890, then Cambridge in 1892 and Scarborough in 1893, for the new technology to start to be widely adopted for land-based generation. By 1900 there were 1000 kW turbine generators available, and in 1912 the first 25,000 kW machine was built. Today single-rotor turbines with outputs of 250 MW are common. However, large power plants with generating capacities of 1000 MW to 2000 MW are generally equipped with multiple turbines to extract the maximum amount of energy most efficiently.
Steam turbines for large coal-fired power stations are generally broken down into three sets, high-pressure turbines, intermediate-pressure turbines, and low-pressure turbines. The high-pressure turbines are the smallest of the set, with the shortest blades and they receive steam from the boiler at the highest temperature and pressure. In a basic layout the steam from the high-pressure turbine then enters the intermediate-pressure turbine. This has longer blades and is optimized for steam at a lower entry temperature and pressure. The high-pressure and intermediate-pressure turbines are usually on a single shaft, driving a single generator.
The low-pressure turbine or turbines – as there may be more than one of these – may also be mounted on the same shaft as the high- and intermediate-pressure turbines. However, it is often preferable to operate these turbines at a low rotational speed as they have very long blades and the tip speeds could otherwise reach velocities beyond the limits of the materials from which they are made to withstand. Steam exiting the low-pressure turbines is condensed at as low a pressure as can be achieved in order to gain the maximum energy from the steam. Heat may be recovered from the condensed water to heat combustion air before the water is returned to the boiler and recycled. A 3D section of a typical large power plant steam turbine with three sections is shown in Figure 4.1.
A more complex refinement to steam turbine layout involves steam reheat between turbine stages. The most common way of implementing this is to take the steam from the high-pressure turbine and return it to the boiler where its temperature is raised again before it enters the intermediate-pressure turbine. This can convey a significant efficiency advantage. One reheat stage is common. Less common is a further reheat stage between intermediate- and low-pressure turbines. This can increase efficiency further, but at a cost.
The best modern coal-fired power plants with ultra-supercritical boilers can come close to 47% efficiency. If steam temperature is pushed to 700°C it should be possible to reach 50% efficiency and at 750°C it may be possible to achieve 55% efficiency. However, these temperatures push existing materials to the extremes of their capabilities and new, more expensive alloys are needed to be able to build plants capable of operating under these conditions.
Generators
The last stage in the energy conversion process that starts with coal and ends with electricity is the generator. This electro-mechanical machine converts the rotary motion produced by the power station turbine into electrical energy that can be delivered to the transmission and distribution system.
All modern generators are based on a phenomenon discovered by Michael Faraday at the beginning of the nineteenth century. If a conductor is passed through a magnetic field, this movement generates an electric current in the conductor. Faraday built a very simple electromagnetic generator, now called Faraday’s disk, which demonstrated the principle of continuous current generation. However, the first effective electric generator was built by the French instrument maker Hippolyte Pixii. His machine consisted of a permanent magnet which was turned by hand. The north and south poles of the magnet alternately passed a coil of insulating wire wrapped around an iron core and this produced a pulse of current in the coil. The pulses alternated in different directions but by designing a commutator that reversed the polarity of the wires from the coil where it connected to an external circuit, Pixii was able to produce a direct current output.
Other scientists experimented with rotating coils within the poles of a permanent magnet. However, all these systems produced a pulsed DC current with low average output. By replacing the two-pole rotating coil with a multi-pole coil and a series of synchronized commutators it became possible to produce a better, more continuous current.
Machines with permanent magnets were called magneto-electric machines. However, permanent magnets limited the capability of these early generators. When the permanent magnet was replaced by an electromagnet, with a magnetic field generated from an external (exciting) direct current source, much more powerful magnetic fields could be produced. Machines using these were initially called dynamo-electric machines but this was eventually shortened to dynamo.
The next important discovery was made independently by the Englishman Henry Wilde and the German Werner von Siemens. Both men discovered a way of allowing the dynamo to provide the electric current from a secondary generator to power the magnetic field of the electro magnet, allowing these machines to be self-exciting. Doing away with a separate electrical source for the excitation current enabled much larger dynamos to be built.
Early generators provided a direct current because that was what was required for the local lighting circuits that they supplied. As the industry grew, and with it the need to transport the electric power over longer distances, the advantages of an alternating current supply became apparent. The voltage of an alternating current supply can be raised or lowered easily using a transformer, something that is not possible with a direct current. For long-distance transport of electricity it is more efficient to use a high voltage and a low current because resistive losses are proportional to the size of the current flowing in a cable. This high-voltage supply can then be reduced to a lower voltage/higher current close to the point of use. This high-voltage transmission/low-voltage distribution forms the basis of the modern supply system topology with its separate transmission and distribution systems.
Alternating current generators, which did away with commutators, were called alternators and these became the standard generators for all power stations in the early years of the twentieth century. The common form of a large power station generator is to have a rotating magnetic field generated by a rotor which turns inside a fixed coil, called the stator. Most modern electricity supply systems have three AC phases at 120° to each other. These are generated by building three independent coils into the stator, each occupying one third of the circumference.
The rotor of a large generator can contain a single two-pole coil producing one north and one south pole that rotates within the rotor. However, depending upon the speed at which the rotor turns, there may be more poles in order to produce an output at the required frequency. The faster the rotor turns, the fewer poles are required. A rotor that turns at 3000 rev/m (50 rev/s) requires only two poles to be able to generate an AC current at 50 Hz. (For a 60 Hz system the equivalent would be 3600 rev/m.) This would be suitable for the high-pressure and intermediate–pressure turbines in a large coal-fired power plant. However, this rotational speed might be too high for the much larger low-pressure turbines and these may operate at half or one quarter of the higher speed. At 1500 rev/m the rotor would need four poles to provide a 50 Hz output and for 750 Hz the number required is eight poles.
Early generators were relatively small and problems such as heat dissipation were not a major issue. However, as the size of generators grew, so did the size of the heat problem. A modern generator can achieve an efficiency of more than 98% and some exceed 99%. However, with generators of hundreds or even more than 1000 megawatts, the amount of heat that must be dissipated is of the order of several megawatts.
Cooling both the rotor and the stator is the key problem for designers of large generators. There are three coolants in general use, air, hydrogen, and water. Air is the cheapest cooling to implement but it is the least effective of the three for carrying away heat. Modern manufacturers can rely on air cooling for generators of up to a maximum of around 350 MW. By optimizing the cooling flow with the generator, manufacturers expect to be able to push this to perhaps 400 MW in the near future.
Hydrogen is around three to four times as effective at cooling a generator as air. However, the gas is explosive so must be controlled carefully. Hydrogen-cooled generators are normally operated at a hydrogen pressure of 2–3 bar to improve the cooling capabilities and to ensure that air cannot enter the generator enclosure and produce an explosive mixture. However, leaks have to be carefully monitored. In addition, hydrogen requires a gas cycling system and an external heat exchanger to cool the hydrogen that has passed through the generator. A 3D cross-section of a hydrogen-cooled generator is shown in Figure 4.2.
For the very largest generators, water cooling is necessary. This is 50 times more effective than air but is much more complex to implement than either air or hydrogen cooling. Generally water cooling is only used for the stator of a large generator while hydrogen cooling is employed for the rotor.
The efficiency of power plant generators is already very high so making significant gains is difficult. One area where gains can be made is by reducing the magnetic losses within the cores of the rotor and stator and the electrical losses in the coils themselves. This can be achieved with higher-quality materials. However, these materials are significantly more expensive and may not be cost-effective. One radical change that may eventually become possible is the use of high-temperature superconducting materials for coils, thereby reducing coil losses significantly. This may add a few tenths of a percentage point to overall efficiency but again cost will be a major consideration.
A new consideration for coal plant generators is the need to be able to operate at variable output in order to support renewable generation on a system grid. Cycling the output of a generator can cause significant aging and wear problems and redesign of some components may be necessary to accommodate this type of duty cycle.