ELEC1104 Lecture 4: ELECTRIC POWER GENERATION
Primary Energy Source Fossil Fuel (oil, coal, natual gas ) Nuclear Hydro Geothermal Renewables» Solar, Wind, Biomass, Tidal etc.
Thermal Power Plants The conventional thermal power plants convert the chemical energy of the fossil fuels into heat through combustion. o The heat energy will then, through the working fluid and steam/gas turbines, be converted into kinetic energy. The kinetic energy of the rotating ti turbines will then drive the generators to produce electricity.
Coal Fired Plant
Combined Cycles In the combined cycle plant, two heat cycles are combined to extend the temperature range a gas turbine cycle and a steam turbine cycle. The exhaust gas from a gas-turbine cycle is used to produce steam in a Heat Recovery Steam Generator for the steam cycle. Overall efficiency much improved.
Combined-cycle Plant
Black Point Power Station Gas Turbine First stage temp. 1288 o C Exhaust gas temp. 613 o C. Steam Turbine HP inlet temp. LP inlet temp. 534 o C 286 o C Net heat rate 6811 kj/kwh Net efficiency 52.9%
Nuclear Energy According to Einstein, matter can be converted into energy according to the formula E = m c 2 where, E [energy], m [mass], c [the speed of 8 light = 3 x 10 8 m/s] Such an energy conversion occurs in a nuclear reaction.
Nuclear Reaction There are two kinds of nuclear reaction: Nuclear fission» The splitting of a heavy nucleus into two or more fragments. Nuclear fusion» The combination of two light nuclei into one, e.g. 2 hydrogen nuclei (deuterium + tritium) to form 1 helium nucleus (+ release of 1 neutron).
Nuclear Fission: Basics When a nucleus fission occurs, there are:» Two or more main fission products.» Certain neutrons are emitted.» Energy released in the form of gamma rays It is noted the sum of the masses of these fragments is less than the original mass and the 'missing' mass has been converted into energy.
Nuclear Fuel - Uranium The common nuclear fuel is Uranium. Natural uranium has two main isotopes: 235 Uand 238 U When uranium is bombarded by a neutron, only 235 U is likely to undergo fission, and 235 U is known as a fissile material. The probability bilit of fission i of 235 Ui increases if the kinetic energy of neutron is low (thermal neutrons).
Fission of Uranium 235 U 235 236 144 89 n+ 92 U143 92 U144 56 Ba 88 + 36 Kr 53 +3n+177MeV
Chain Reaction Thus there are a number of neutrons released during the fission process which are capable of inducing further fissions s chain reaction. If on average just one neutron from the fission will cause further fission, the chain reaction can be maintained at a steady rate we have controlled fission (reactivity = 1).
Chain Reaction If on average less than one neutron from the fission will cause further fission, the chain reaction cannot be maintained and will eventually die out (reactivity < 1). If on average more than one neutron from the fission will cause further fission, the chain reaction will increase beyond control (reactivity > 1).
Nuclear Reaction Chain reaction Uncontrolled chain reaction results in atomic bombsb Commercial use of nuclear energy requires controlled nuclear reaction.
Nuclear Reactor In a nuclear power plant, the fission process takes place in a nuclear reactor. In the reactor core are the fuel rods. There are coolants to carry away the heat generated by the nuclear reaction. A moderator would also be needed to slow down the neutrons to the thermal level. There are also control rods (neutron absorbers) b to control the reaction rate.
Nuclear Power Plant Most nuclear power plants adopts the pressurized water reactor (PWR) with light water used as the coolant as well as the moderator. Steam is produced in a heat exchanger such that the steam and reactor coolant operates in two separate circuits. The steam produced would drive the steam turbine as in a conventional power plant.
Nuclear Power Plant
Uranium Fuel Natural uranium consists mainly the isotope U-238, with only about 0.7% of U-235. In some reactor design, the percentage of U- 235 has to be increased (i.e. fuel enriched) to maintain the chain reaction. The U-238 in the nuclear fuel can absorb a neutron to form plutonium 239, a fissile il material that can be readily fissioned by fast neutrons.
Hydro Power Plants There is usually a storage reservoir and the primary energy source is the potential energy e gyof the water. Potential energy (water flow) Linear kinetic energy (turbine) Rotary kinetic energy (generator) Electrical energy.
Hydro Power Plants Hydro-power plants do not have a generation cost in the sense of the thermal power plants. However, it normally involves significant capital investment. The amount of energy it can supply depends on the water in store and this could be seriously affected by seasonal factors.
Hydroelectric Plant
Hydroelectric Plant
Pumped-hydro Plants Pumped-hydro plants release water to generate electricity during peak loads and pumped ped water from lower reservoir to upper reservoir during light load. The overall efficiency of a full pump- turbine cycle is around 60~70% Its operation makes use of the differential cost of thermal generation during peak load and base load periods.
Pumped Storage Hydro-plant
Power Systems in Hong Kong Installed capacity >10,000 000 MW Hong Kong Electric» Lamma Power Plant (Coal) 3,350 350 MW China Light and Power» Castle Peak (Coal) 4100 MW» Black Point (Natural gas) 2500 MW» Penny s Bay (Gas turbines) 300 MW» Daya Bay (Nuclear, in Guandong) 1800 MW
Generator Generators are driven mechanically by some sort of prime movers, such as,» Waterwheel» Steam turbine
Generator steam turbines
Hydroelectric generator water turbine
References Electric Utility Systems and Practices,Ed., H.M. Rustebrakke, John Wiley, 4 th ed.. 1983. Electric Power Systems, B.M. Weedy and B.J. Cory, John Wiley, 4 th ed., 1998. Electric Energy Systems, S.A. Nasar, Prentice Hall, 1996.
References Electric Power Engineering, O.I. Elgerd g g, g and P.D. van der Puiji, Chapman & Hall, 2 nd ed., 1998. Transmission and Distribution Electrical Engineering, C. Bayliss, Newnes, 2 nd ed., 1999. Basics of Electric Power Transmission, A.J. Pansini and K.D. Smalling, Pennwell, 1998.