Energy Conversion In Power generation system

Energy Conversion

Energy Conversion Using Steam

The combustion of coal, gas or oil in boilers produces steam, at high temperatures and pressures, which is passed through steam turbines. Nuclear fission can also provide energy to produce steam for turbines. Axial-flow turbines are generally used with several cylinders, containing steam of reducing pressure, on the same shaft. 

A steam power-station operates on the Rankine cycle, modified to include superheating, feed-water heating, and steam reheating. High efficiency is achieved by the use of steam at the maximum possible pressure and temperature. Also, for turbines to be constructed economically, the larger the size the less the capital cost per unit of power output. As a result, turbo-generator sets of 500 MW and more have been used. With steam turbines above 100 MW, the efficiency is increased by reheating the steam, using an external heater, after it has been partially expanded. The reheated steam is then returned to the turbine where it is expanded through the final stages of blading.

In coal-fired stations, coal is conveyed to a mill and crushed into fine powder, that is pulverized. The pulverized fuel is blown into the boiler where it mixes with a supply of air for combustion. The exhaust steam from the low pressure (L.P.) turbine is cooled to form condensate by the passage through the condenser of large quantities of sea- or river-water. Cooling towers are used where the station is located inland or if there is concern over the environmental effects of raising the temperature of the sea- or river-water.

Despite continual advances in the design of boilers and in the development of improved materials, the nature of the steam cycle is such that vast quantities of heat are lost in the condensate cooling system and to the atmosphere. Advances in design and materials in the last few years have increased the thermal efficiencies of new coal stations to approaching 40%. If a use can be found for the remaining 60% of energy rejected as heat, fairly close to the power station, forming a Combined Heat and Power (or Co-generation) system then this is clearly desirable.

 Energy Conversion Using Water

Perhaps the oldest form of energy conversion is by the use of water power. In a hydroelectric station the energy is obtained free of cost. This attractive feature has always been somewhat offset by the very high capital cost of construction, especially of the civil engineering works. Unfortunately, the geographical conditions necessary for hydro-generation are not commonly found, especially in Britain. In most developed countries, all the suitable hydroelectric sites are already fully utilized. There still exists great hydroelectric potential in many developing countries but large hydro schemes, particularly those with large reservoirs, have a significant impact on the environment and the local population.

The difference in height between the upper reservoir and the level of the turbines or outflow is known as the head. The water falling through this head gains energy which it then imparts to the turbine blades. Impulse turbines use a jet of water at atmospheric pressure while in reaction turbines the pressure drops across the runner imparts significant energy.

Particular types of turbine are associated with the various heights or heads of water level above the turbines. These are:

1. Pelton: This is used for heads of 150–1500 m and consists of a bucket wheel rotor with water jets from adjustable flow nozzles. 
2. Francis: This is used for heads of 50–500 m with the water flow within the turbine following a spiral path. 
3. Kaplan: This is used for run-of-river stations with heads of up to 60 m. This type has an axial-flow rotor with variable-pitch blades.

Typical efficiency curves for each type of turbine are shown in Figure 1.5. Hydroelectric plant has the ability to start up quickly and the advantage that no energy losses are incurred when at a standstill. It has great advantages, therefore, for power generation because of this ability to meet peak loads at minimum operating cost, working in conjunction with thermal stations – see Figure. By using remote control of the hydro sets, the time from the instruction to start up to the actual connection to the power network can be as short as 3 minutes. The power available from a hydro scheme is given by

P = rgQH                    (W)

where

Q = flow rate (m3/s) through the turbine;

r = density of water (1000 kg/m3);

g = acceleration due to gravity (9.81 m/s2);

H = head, that is height of upper water level above the lower (m).

Substituting,

P= 9.81QH                       (kW)

Gas Turbines

With the increasing availability of natural gas (methane) and its low emissions and competitive price, prime movers based on the gas turbine cycle are being used increasingly. This thermodynamic cycle involves burning the fuel in the compressed working fluid (air) and is used in aircraft with kerosene as the fuel and for electricity generation with natural gas (methane). Because of the high temperatures obtained, the efficiency of a gas turbine is comparable to that of a steam turbine, with the additional advantage that there is still sufficient heat in the gas-turbine exhaust to raise steam in a conventional boiler to drive a steam turbine coupled to another electricity generator. This is known as a combined-cycle gas-turbine (CCGT) plant, a schematic layout of which is shown in Figure 1.6. Combined efficiencies of new CCGT generators now approach 60%. 

The advantages of CCGT plant are the high efficiency possible with large units and, for smaller units, the fast start up and shut down (2–3 min for the gas turbine, 20 min for the steam turbine), the flexibility possible for load following, the comparative speed of installation because of its modular nature and factory-supplied units, and its ability to run on light oil (from local storage tanks) if the gas supply is interrupted. Modern installations are fully automated and require only a few operators to maintain 24 hour running or to supply peak load, if needed.

Nuclear Power

Energy is obtained from the fission reaction which involves the splitting of the nuclei of uranium atoms. Compared with chemical reactions, very large amounts of energy are released per atomic event. Uranium metal extracted from the base ore consists mainly of two isotopes, 238U (99.3% by weight) and 235U (0.7%). Only 235U is fissile, that is when struck by slow-moving neutrons its nucleus splits into two substantial fragments plus several neutrons and 3  1011 J of kinetic energy. The fast moving fragments hit surrounding atoms producing heat before coming to rest. The neutrons travel further, hitting atoms and producing further fissions. Hence the number of neutrons increases, causing, under the correct conditions, a chain reaction. In conventional reactors the core or moderator slows down the moving neutrons to achieve more effective splitting of the nuclei.

Fuels used in reactors have some component of 235U. Natural uranium is sometimes used although the energy density is considerably less than for enriched uranium. The basic reactor consists of the fuel in the form of rods or pellets situated in an environment (moderator) which will slow down the neutrons and fission products and in which the heat is evolved. The moderator can be light or heavy water or graphite. Also situated in the moderator are movable rods which absorb neutrons and hence exert control over the fission process. In some reactors the cooling fluid is pumped through channels to absorb the heat, which is then transferred to a secondary loop in which steam is produced for the turbine. In water reactors the moderator itself forms the heat-exchange fluid.

A number of versions of the reactor have been used with different coolants and types of fissile fuel. In Britain the first generation of nuclear power stations used Magnox reactors in which natural uranium in the form of metal rods was enclosed in magnesium-alloy cans. The fuel cans were placed in a structure or core of pure graphite made up of bricks (called the moderator). This graphite core slowed down the neutrons to the correct range of velocities in order to provide the maximum number of collisions. The fission process was controlled by the insertion of control rods made of neutron-absorbing material; the number and position of these rods controlled the heat output of the reactor. Heat was removed from the graphite via carbon dioxide gas pumped through vertical ducts in the core. This heat was then transferred to water to form steam via a heat exchanger. Once the steam had passed through the high-pressure turbine it was returned to the heat exchanger for reheating, as in a coal- or oil-fired boiler.

A reactor similar to the Magnox is the advanced gas-cooled reactor (AGR) which is still in use in Britain but now coming towards the end of its service life. A reinforced-concrete, steel-lined pressure vessel contains the reactor and heat exchanger. Enriched uranium dioxide fuel in pellet form, encased in stainless steel cans, is used; a number of cans are fitted into steel fitments within a graphite tube to form a cylindrical fuel element which is placed in a vertical channel in the core. Depending on reactor station up to eight fuel elements are held in place one above the other by a tie bar. Carbon dioxide gas, at a higher pressure than in the Magnox type, removes the heat. The control rods are made of boron steel. Spent fuel elements when removed from the core are stored in a special chamber and lowered into a pond of water where they remain until the level of radioactivity has decreased sufficiently for them to be removed from the station and disassembled.

In the USA and many other countries pressurized-water and boiling-water reactors are used. In the pressurized-water type the water is pumped through the reactor and acts as a coolant and moderator, the water being heated to 315 C at around 150 bar pressure. At this temperature and pressure the water leaves the reactor at below boiling point to a heat exchanger where a second hydraulic circuit feeds steam to the turbine. The fuel is in the form of pellets of uranium dioxide in bundles of zirconium alloy.

The boiling-water reactor was developed later than the pressurized-water type. Inside the reactor, heat is transferred to boiling water at a pressure of 75 bar (1100 p.s.i.). Schematic diagrams of these reactors are shown in Figures 1.7 and 1.8. The ratio of pressurized-water reactors to boiling-water reactors throughout the world is around 60/40%.

Both pressurized- and boiling-water reactors use light water.1 The practical pressure limit for the pressurized-water reactor is about 160 bar (2300 p.s.i.), which limits its efficiency to about 30%. However, the design is relatively straightforward and experience has shown this type of reactor to be stable and dependable. In the boiling-water reactor the efficiency of heat removal is improved by use of the latent heat of evaporation. The steam produced flows directly to the turbine, causing possible problems of radioactivity in the turbine. The fuel for both light-water reactors is uranium enriched to 3–4% 235U. Boiling-water reactors are probably the cheapest to construct; however, they have a more complicated fuel make up with different enrichment levels within each pin. The steam produced is saturated and requires wet-steam turbines. A further type of water reactor is the heavy-water CANDU type developed by Canada. Its operation and construction are similar to the light-water variety but this design uses naturally occurring, un-enriched or slightly enriched uranium.

Concerns over the availability of future supplies of uranium led to the construction of a number of prototype breeder reactors. In addition to heat, these reactors produce significant new fissile material. However, their cost, together with the technical and environmental challenges of breeder reactors, led to most of these programmes being abandoned and it is now generally considered that supplies of uranium are adequate for the foreseeable future.