ELEC1104

Lecture 4:

ELECTRIC POWERELECTRIC POWER GENERATIONGENERATION

Primary Energy Sourcey gy

Fossil Fuel (oil, coal, natual gas…)Fossil Fuel (oil, coal, natual gas…)NuclearH dHydroGeothermalRenewables» Solar, Wind, Biomass, Tidal etc.

Thermal Power Plants

The conventional thermal power plantsThe conventional thermal power plants convert the chemical energy of the fossil fuels into heat through combustion.ue s to eat t oug co bust o .The heat energy will then, through the working fluid and steam/gas turbines beworking fluid and steam/gas turbines, be converted into kinetic energy.Th ki ti f th t ti t biThe kinetic energy of the rotating turbines will then drive the generators to produce electricitelectricity.

Coal Fired Plant

Combined Cyclesy

In the combined cycle plant, two heat cyclesIn the combined cycle plant, two heat cycles are combined to extend the temperature range – a gas turbine cycle and a steam a ge a gas tu b e cyc e a d a steaturbine cycle.The exhaust gas from a gas-turbine cycle isThe exhaust gas from a gas-turbine cycle is used to produce steam in a Heat Recovery Steam Generator for the steam cycleSteam Generator for the steam cycle.Overall efficiency much improved.

Combined-cycle Planty

Black Point Power Station

Gas TurbineFirst stage temp. 1288oCExhaust gas temp. 613oC.

Steam TurbineHP inlet temp 534oCHP inlet temp. 534 CLP inlet temp. 286oC

Net heat rate 6811 kJ/kWHNet efficiency 52.9%

Nuclear Energygy

According to Einstein, matter can be g ,converted into energy according to the formula

2E = m c2

where, E [energy], m [mass], c [the speed of 8light = 3 x 108 m/s]

Such an energy conversion occurs in a nuclear reaction.

Nuclear Reaction

There are two kinds of 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» The combination of two light nuclei into one,

e.g. 2 hydrogen nuclei (deuterium + tritium) to form 1 helium nucleus (+ release of 1 neutron).form 1 helium nucleus ( release of 1 neutron).

Nuclear Fission: Basics

When a nucleus fission occurs, there are:When a nucleus fission occurs, there are:» Two or more main fission products.» Certain neutrons are emitted» 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:

235U and 238UU and UWhen uranium is bombarded by a neutron, only 235U is likely to undergo fission andonly 235U is likely to undergo fission, and 235U is known as a fissile material.Th b bilit f fi i f 235U iThe probability of fission of 235U increases if the kinetic energy of neutron is low (th l t )(thermal neutrons).

Fission of Uranium 235U

235 236 144 89n+ U U Ba + Kr +3n+177MeV→ →92 143 92 144 56 88 36 53n+ U U Ba + Kr +3n+177MeV→ →

Chain Reaction

Thus there are a number of neutronsThus there are a number of neutrons released during the fission process which are capable of inducing further fissions –a e capab e o duc g u t e ss o schain 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 theIf on average less than one neutron from the fission will cause further fission, the chain reaction cannot be maintained and will eact o ca ot be a ta ed a d weventually 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 b bbombsCommercial use of nuclear energy requires controlled nuclear reactioncontrolled 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 reactiongenerated by the nuclear reaction.A moderator would also be needed to slow down the neutrons to the thermal leveldown the neutrons to the thermal level.There are also control rods (neutron b b ) t t l th ti tabsorbers) to control the reaction rate.

Nuclear Power Plant

Most nuclear power plants adopts theMost nuclear power plants adopts the pressurized water reactor (PWR) with light water used as the coolant as well as the wate used as t e coo a t as we as t emoderator.Steam is produced in a heat exchanger suchSteam is produced in a heat exchanger such that the steam and reactor coolant operates in two separate circuitsin two separate circuits.The steam produced would drive the steam t rbine as in a con entional po er plantturbine as in a conventional power plant.

Nuclear Power Plant

Uranium Fuel

Natural uranium consists mainly the isotopeNatural uranium consists mainly the isotope U-238, with only about 0.7% of U-235.In some reactor design the percentage of U-In some reactor design, the percentage of U-235 has to be increased (i.e. fuel enriched) to maintain the chain reactionto maintain the chain reaction.The U-238 in the nuclear fuel can absorb a

t t f l t i 239 fi ilneutron to form plutonium 239, a fissile material that can be readily fissioned by fast ne tronsneutrons.

Hydro Power Plantsy

There is usually a storage reservoir and theThere is usually a storage reservoir and the primary energy source is the potential energy of the water.e e gy o t e wate .

Potential energy →(water flow)→Potential energy →(water flow)→Linear kinetic energy →(turbine)→

Rotary kinetic energy →(generator)→Electrical energy.gy

Hydro Power Plantsy

Hydro-power plants do not have aHydro power plants do not have a generation cost in the sense of the thermal power plants.powe p a ts.However, it normally involves significant capital investmentcapital investment.The amount of energy it can supply depends

th t i t d thi ld bon the water in store and this could be seriously affected by seasonal factors.

Hydroelectric Plant

Hydroelectric PlantHydroelectric Plant

Pumped-hydro Plantsp y

Pumped-hydro plants release water toPumped hydro plants release water to generate electricity during peak loads and pumped water from lower reservoir to upper pu ped wate o owe ese vo to uppereservoir during light load. The overall efficiency of a full ‘pump-The overall efficiency of a full pump-turbine’ cycle is around 60~70%It ti k f th diff ti lIts operation makes use of the differential cost of thermal generation during peak load and base load periodsand base load periods.

Pumped Storage Hydro-plantp g y p

Power Systems in Hong Kongy g g

Installed capacity >10 000 MWInstalled capacity >10,000 MWHong Kong Electric

L P Pl t (C l) 3 350 MW» Lamma Power Plant (Coal) 3,350 MWChina 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 areGenerators are driven mechanically by some sort of by so e so t oprime movers, such as,s,» Waterwheel» Steam turbine» Steam turbine

Generator – steam turbines

Hydroelectric generator – water turbiney g

References

“Electric Utility Systems and Practices”, Ed.Electric Utility Systems and Practices , Ed. H.M. Rustebrakke, John Wiley, 4th ed.. 1983.983.“Electric Power Systems”, B.M. Weedy and B J Cory John Wiley 4th ed 1998B.J. Cory, John Wiley, 4 ed., 1998.“Electric Energy Systems”, S.A. Nasar, P ti H ll 1996Prentice Hall, 1996.

References

“Electric Power Engineering”, O.I. Elgerd g g , gand P.D. van der Puiji, Chapman & Hall, 2nd ed., 1998.“Transmission and Distribution Electrical Engineering”, C. Bayliss, Newnes, 2nd ed., 1999.“Basics of Electric Power Transmission”, A.J. Pansini and K.D. Smalling, Pennwell, 1998.