Welcome
  • Welcome

    When you have studied this course, you should:

    • understand energy units and their manipulation
    • be familiar with the topic of energy conversion
    • be familiar with the topic of bulk electricity supply
    • understand load-forecasting and load-matching issues
    • understand electricity generation from coal and gas
    • understand the principles of nuclear fission power plant
    • understand the principles of sustainable electricity generation; from several renewable energies; wind, wave, tidal, solar energy and biomass
    • appreciate the topic of distributed/embedded generation the costs involved in the generating of electricity

  • Introduction

    This lesson discusses energy conversion to electricity from sustainable and non-sustainable sources of energy. It discusses the principles of energy conversion, and describes the topic of bulk electricity supply.
    Electricity is a commodity which has to be made as it is required, so supply has to be matched to demand at any instant. Thus the importance of load forecasting is highlighted, as is the use of pumped storage plant to meet sudden surges in demand.

    Inefficiencies of energy conversion to both the generation and the use of electricity is explored. For example, the generation of electricity in large thermal stations coupled with transmission losses is usually less than 40%.

    Over 75% of the world’s electricity is generated from fossil fuels or by nuclear fission, so the advantages and disadvantages of electricity generation in coal, gas and nuclear power stations are also examined.

    These ‘conventional’ generation methods and ‘renewable’ electricity generation techniques will also be compared.

    Of the latter, while hydroelectricity supplies over 16 per cent of worldwide electricity demand, much of other renewables technology is in its infancy.

    The need both to conserve the reserves of fossil fuels and to reduce carbon dioxide emissions is leading to rapid developments in renewable technologies; wind, wave and solar photovoltaic electricity generation, and such techniques are also discussed.

    The cost of generating electricity from different sources is also considered.
  • Units of Energy

    The SI unit of energy is the joule (J).

    1 kJ    kilojoule    103 J
    1 MJ    megajoule    106 J
    1 GJ    gigajoule    109 J
    1 TJ    terajoule    109 J
    1PJ    petajoule    1015 J
    1 EJ    exajoule    1018 J

    Much of our energy consumption is based on the burning of fossil fuels, the quality of which varies, so values of energy content of coal or oil are approximate. They are usually given in tonnes of oil equivalent (toe or Mtoe). 1 toe = 42 GJ

    For example, the worldwide energy consumption in 2016 was estimated to have been 14 500 million tonnes of oil
    https://en.wikipedia.org/wiki/World_energy_consumption

    14500 Mtoe = 14500 x 106 x 42 x 109 J = 609 EJ

    UK consumption of primary energy in 2016 was 194 Mtoe (= 8.15 EJ). Electrical energy is more usually calculated in kilowatt hours (kW h) since the rate of energy consumption is the watt ( W), a joule per second ( J/s).
    1 kW h = 3600 kJ = 3.6 MJ

    In 2016, worldwide electricity generation was approximately 24 x106 GW h and UK generation about 500 000 GW h.

    Video: World Energy Outlook 2016 📹


  • 5. Electrical. The energy in joules is given by the product of power (P = VI watts) and time (s).
        E = power x time (Note: 1 J =1 W s.)

        Small quantities of energy may be stored in a battery.
        Electricity generation must balance demand second by second


    6. Radiation.

    Photon Energy E = Hf

    Where f = Frequency
    h = Planck’s constant h = 6.63 x 10-34

    Examples: solar photovoltaic (pv) conversion to electricity; photosynthesis of sunlight to biomass. Atomic energy may also be included in this list, where mass (m) is converted to energy in a nuclear reactor. Energy is release

    dE = mc2

    c is the speed of light
    Forms of Energy

    Energy comes in several forms.

    1. Energy = mass x gravitational acceleration x height
        E = mgh
        Example: stored energy in a reservoir.


    2. Mechanical. Kinetic energy (KE) of a moving object.
        Energy = 0.5 (mass x velocity2)
        E = 0.5 mv2


    3. Thermal. The energy ‘stored’ as heat in a body (of gas or liquid for example):
        Energy = mass x specific heat x rise in temperature
        Energy is mainly due to an increase in kinetic energy of atoms/ molecules.

        Examples: heat from burning fossil fuels or from a nuclear reaction;
        heat from solar thermal collector.


    4. Chemical. Energy released in chemical reaction.
        Example: burning of fuel – often given in kJ/kg.
  • Conversion of Energy

    Energy is not consumed, but converted from one form to another. This is the Law of Conservation of Energy embodied in the First Law of Thermodynamics. Most of the electrical and mechanical energy and the heat used in our homes, offices and factories is generated from fossil fuels – oil, coal and natural gas. Chemical energy is being converted.

    Part of our electrical energy comes from hydroelectric stations where gravitational potential energy is being converted. Part also comes from nuclear power stations where energy stored in atomic nuclei is converted.

    On the whole, energy is being converted from useful forms of energy to less useful forms of energy.

    The above can be illustrated by considering the car. The car engine burns fuel, delivering mechanical energy to propel the car. It is an inefficient process. About 80 per cent of the energy becomes waste heat immediately, releasing thermal energy with the exhaust gases. Only about 20 per cent of the chemical energy of the fuel becomes mechanical energy, some of which becomes electrical energy for instrumentation, motors etc. Much of the mechanical energy ends up as frictional losses – thermal energy which is dissipated into the environment.

    The poor efficiency of the engine is partly because of the Second Law of Thermodynamics, which states that the perfect heat engine is impossible. The efficiency of an ideal heat engine is:


    In 2016, worldwide electricity generation was 24 x106 GW h, UK generation was about 450 000 GW h, though this includes some 30 000 GW h used by the electricity industry with a further 30, 000 GW h in lossess. Therefore, total UK consumption was about 390,000 GW h
  • Load Forecasting and Load Matching

    Demand for electricity changes dramatically by the minute, by the time of day, and from summer to winter.

    Figure 2 shows a typical annual variation in demand for the UK. The average demand is around 40 GW, but the peak demand in winter is nearly 60 GW and the minimum summer night demand is 22 GW.

    This latter demand is termed the base load which, in effect, can be considered as a continuous demand on the system throughout the year, upon which a varying demand occurs.

    In the UK the highest ever load met was 60 118 MW on 10 December 2002.

    Electricity is a commodity that has to be made as it is required – supply has to match demand at any instant. As customers demand more power, more must be generated to meet that demand.

    Sources of that electrical power must also be optimized to deliver that supply. A competitive market exists in which suppliers of electricity compete to sell electricity to a central ‘power pool’.

    Electricity thus bought at the lowest available price at a particular time is then distributed through the National Grid to consumers.

    In general, power stations with the lowest running costs supply the base load. In the UK, these are typically a mix of nuclear, large-coal and gas-fired stations.

    Night-time demand in summer can be very low, so prices are correspond- ingly low. As demand increases, so stations with higher costs bid into the pool and electricity prices rise. Similar ‘power pools’ exist in other countries.

    It should be noted though that many factors bear on the operation and optimization of an electricity supply system and now include such factors as anti-pollution legislation and resultant ‘carbon emissions taxes’ which pro- mote the use of renewable forms of energy to generate electricity.
  • Matching Supply and Demand

    Matching supply and demand is complex.

    Detailed load forecasting is essential and entails:

       1. detailed weather forecasting, particularly the effect of wind on the effective temperature;

       2. effect of TV programmes, particularly commercial breaks and the mid- point and end of large sporting events;

       3. day-to-day variations due to work patterns, weekends, and so on.

    Generators must be controlled so that the input power – for example from steam supplied to a turbine/generator set or water to a hydro-generator – exactly matches the output power demand.

    If insufficient steam or water is supplied, the generator slows down and the frequency falls. A further consideration is that it can take many hours, from cold, for a large 660 MW steam turbine/generator set to be prepared to deliver full power. A turbine expands between cold and hot and heating can take place only slowly.
  • Meeting Sudden Peak Demands

    A sudden requirement to switch a generator on might be caused by, for example, failure of other generating plant or by the automatic disconnection of transmission/distribution plant due to a fault.

    A more likely cause of a sudden increase in demand is TV broadcasts as kettles and other electrical apparatus are almost simultaneously switched on during commercial breaks or at half-time in football matches.

    Figure 39.3 clearly illustrates this phenomenon. Several surges in demand occurred at all the major breaks in the footballing action – the largest being a surge of 2900 MW. For example, 1 million 3 kW kettles produce a demand of 3000 MW, for which almost the complete output from five 660 MW generators is required.


    Solution To Meeting Sudden Peak Demands

    1. 660 MW generators can be run on no-load but at full temperature, so that they can be brought on-load quickly. This is termed ‘spinning reserve’ and must be prepared well in advance.

    2. Small open-cycle gas turbine stations, while expensive to run and only about 30 per cent fuel efficient, can be brought up to full power in a few minutes. This plant may only be used for such ‘peaking’ duties.

    3. The use of pumped storage plant.
  • Pumped Storage Plant

    This is hydroelectric plant having large generators which can also act as pumps – see section . When generating they are driven by water and can be ‘switched on’ in about 30 seconds to meet a sudden demand increase.

    Otherwise they are used to pump water to refill a storage reservoir, in pre- paration for the next surge in demand. Since turbine and generating plant are very efficient, the process achieves about 80 per cent energy conversion efficiency.

    The UK has a total of about 3 GW of pumped storage capacity at three locations, in Wales and Scotland.
  • Generating Plant

    This section describes electricity generating plant, and covers generation both by conventional technology and by the new renewable sources of electricity that are being developed to reduce carbon dioxide emissions and, at least as importantly, to reduce the rate at which fossil fuels are being used.

    A study of conventional electricity generation is needed to provide the context for the renewable sources which might replace them, such is the vast scale on which electricity is generated worldwide.


    Energy Conversion from Coal

    Single generator sets of over 600 MW are now used in the UK, though there are many smaller generators in use. A 600 MW generator can supply the average needs of over 1 million UK households.

    Three or four such generators are typically installed in a single large coal-fired station which is often sited close to a coal mine, away from the city dwellers who consume the electricity.

    Such generators are usually driven by a compound arrangement of high- pressure, intermediate-pressure and low-pressure turbines, increasing in size as the pressure decreases. Modern turbines rotate in a speed range from 1500 to 3500 r.p.m., usually 3000 r.p.m. for the UK’s 50 Hz system.

    For large coal-fired plant the steam pressure could be 25 megapascals (MPa) with steam temperatures of 500–600 °C to improve the thermodynamic efficiency.

    In nuclear reactors, which operate under less demanding conditions, the steam is superheated to about 5 MPa and 300 °C. Modern water tube boilers are complex and have fuel-to-steam efficiencies of about 90 per cent which they achieve by several sets of heat exchangers. The rate of supply of steam is critical and must be carefully controlled to match exactly the often rapid variation in demand for electricity. Too much steam and the turbine speeds up, too little and the turbine slows down, consequently affecting the frequency of the electricity generated. The rate of rotation must remain constant so that the frequency is maintained with less than about 0.5 per cent variation.

    In order to achieve the best thermodynamic efficiency, the exhaust tem- perature of the steam is lowered by a condenser through which cooling water is pumped. When a plant is close to a river or the sea, such cooling water is returned over 10°C warmer. Otherwise the warm water is sprayed down a cooling tower through a rising air current to cool it. Such cooling methods are very wasteful, the wasted thermal energy being the main reason for the low overall energy conversion efficiency. Utilizing this warm water for local space heating in combined heat and power (CHP) schemes brings higher useful overall energy conversion efficiencies.

    Modern Power Station Boilers

    Modern power station boilers are designed to maximize the energy trans- fer from the burning fuel. Nonetheless, waste products – ash; oxides of sul- phur (SOx) and nitrogen (NOx); and carbon dioxide (about 1 kg per kW h of electricity produced from coal) – present major pollution problems. Many existing stations burn pulverized fuel, particles of which are less than about 0.1 mm, which are blown in a carefully controlled flow of air to the burner jets, ensuring complete combustion of fuel. Energy transfer efficiencies to the boiler can exceed 90 per cent in good conditions. Rapid fuel combustion reduces the production of NOx. SOx are increasingly removed from the ‘exhaust’ gases by flue-gas desulphurization (FGD) plant in which SOx are combined with limestone, typically in a spray. A disadvantage of pulverized fuel boilers is the fine ash, fly-ash, which has to be removed from the flue-gas exhaust to the atmosphere. In an electrostatic precipitator, the ash particles are charged as they pass through a grid of high-voltage wires and deposited on earthed collectors. FGD and precipitation plant can add 20 per cent to the capital cost of a power station.

    Modern stations use fluidized-bed combustion to solve some of the prob- lems of NOx, SOx and ash emissions but not of carbon dioxide. In a fluidized bed boiler, air is blown through fine sand creating a ‘floating bed’ into which the fuel particles are fed. Rapid and efficient combustion ensues, heat being carried away in the water tubes buried in the bed. Limestone particles are also fed into the bed to remove SOx at source. The bed temperature is lower so NOx production is reduced. Ash production is also lessened.


    Combined Cycle Gas Turbine (CCGT) Plant

    When a gas turbine is used to drive a generator directly, termed ‘open-cycle’ operation, the thermal efficiency is only about 30 per cent. However, since exhaust gases are at over 500 °C, they can be used to raise steam to drive conventional steam turbine/generator plant.

    A simplified CCGT schematic diagram is shown in Fig. X. Such combined cycle gas turbine plant in which two turbines drive one generator has a much higher thermal efficiency: over 50 per cent.

    This is a consequence of the Carnot efficiency,


    where, in combined cycle operation,
    Tin = 1200 °C (1473 K) and Tout = 25 °C (298 K)

    In the case of combined heat and power generation, efficiencies of up to 80 per cent are possible.

    Since the UK electricity supply industry was privatized in 1989, the pro- portion of electricity generated from CCGT power stations in the UK increased to nearly 40 per cent. It is now decreasing again as the UK becomes a net importer of gas largely at the expense of coal-fired generation. This has been mainly because CCGT stations are cheaper and quicker to build than coal or nuclear stations. This so-called dash for gas has had environmental benefits. Table 39.5 shows that CCGT generates only half of the carbon dioxide output per kW h compared with coal. Natural gas also has little sulphur content so sulphur dioxide (SO2) emissions are very small.
  • Energy Conversion from Coal

    In order to achieve the best thermodynamic efficiency, the exhaust tem- perature of the steam is lowered by a condenser through which cooling water is pumped. When a plant is close to a river or the sea, such cooling water is returned over 10 °C warmer. Otherwise the warm water is sprayed down a cooling tower through a rising air current to cool it. Such cooling methods are very wasteful, the wasted thermal energy being the main reason for the low overall energy conversion efficiency. Utilizing this warm water for local space heating in combined heat and power (CHP) schemes brings higher useful overall energy conversion efficiencies.

    Modern power station boilers are designed to maximize the energy transfer from the burning fuel. Nonetheless, waste products – ash; oxides of sul- phur (SOx) and nitrogen (NOx); and carbon dioxide (about 1 kg per kW h of electricity produced from coal) – present major pollution problems. Many existing stations burn pulverized fuel, particles of which are less than about 0.1 mm, which are blown in a carefully controlled flow of air to the burner jets, ensuring complete combustion of fuel. Energy transfer efficiencies to the boiler can exceed 90 per cent in good conditions.

    Rapid fuel combustion reduces the production of NOx. SOx are increasingly removed from the ‘exhaust’ gases by flue-gas desulphurization (FGD) plant in which SOx are combined with limestone, typically in a spray. A disadvantage of pulverized fuel boilers is the fine ash, fly-ash, which has to be removed from the flue-gas exhaust to the atmosphere. In an electrostatic precipitator, the ash particles are charged as they pass through a grid of high-voltage wires and deposited on earthed collectors. FGD and precipitation plant can add 20 per cent to the capital cost of a power station.

    Modern stations use fluidized-bed combustion to solve some of the problems of NOx, SOx and ash emissions but not of carbon dioxide. In a fluidized bed boiler, air is blown through fine sand creating a ‘floating bed’ into which the fuel particles are fed. Rapid and efficient combustion ensues, heat being carried away in the water tubes buried in the bed. Limestone particles are also fed into the bed to remove SOx at source. The bed temperature is lower so NOx production is reduced. Ash production is also lessened.
  • Combined Heat and Power (CHP) Plant

    CHP, also called cogeneration, is the simultaneous generation of heat and electricity. Since CHP systems make extensive use of the heat produced during the electricity generation process, overall efficiencies in excess of 70 per cent can be achieved.

    In contrast, the efficiency of conventional coal-fired and gas-fired power stations, which discard this heat, is typically 40 per cent and 50 per cent, respectively, at the power station.

    Efficiency at the point of use is lower still because of the losses, typically about 8 per cent, that occur during transmission and distribution. In contrast, CHP systems sup- ply customers with heat and power directly at the point of use, therefore helping avoid the significant losses which occur in transmitting electricity from large centralized plant to consumers. The high efficiency of CHP leads to a reduction in the use of primary energy and emissions of carbon dioxide and other products of combustion. Fuel cost savings vary, but can be between 15 per cent and 40 per cent compared with imported electricity and on-site boilers.

    Coal-fired station: Fuel 100% - end-user electricity 35% =loss 65%
                        CHP: Fuel 100% - electricity 27%, useful heat 48% loss =25%

    CHP systems can be employed over a wide range of sizes, applications, fuels and technologies. In its simplest form, it employs a gas turbine or a steam turbine to drive a generator. The resulting electricity can be used either wholly or partially on-site. The heat produced during power genera- tion is recovered, usually in a heat recovery boiler and can be used to raise steam for industrial processes or to provide hot water for space heating. The main design criterion is that, to make the investment worthwhile, there must be a need for both the heat and electricity produced by the CHP unit (the thermal output is typically 1.5 to 2 times the electrical output). Some industrial processes that use electricity and heat continuously, for example paper and chemicals manufacture and refineries, are particularly well suited to CHP.

    Community or district CHP schemes supply both electricity and heat to a local community, via a network of insulated hot water pipes that carry heat to both public buildings and private dwellings (see Fig. ).

    District heating based on the cogeneration of heat and electricity has become a cost- and energy-efficient system, costs being one third-less than the costs of separate generation of heat and electricity. Finland makes extensive of CHP and dis- trict heating. The first district heating plant began operation there in the 1950s and today accounts for about 45 per cent of Finland’s heating market. In Helsinki it accounts for more than 90 per cent, heating networks covering over 7500 km under the city. Government statistics show that about 6000 MW of CHP capacity operated in the UK in 2006. In the home, a microCHP unit resembling a gas-fired boiler could provide not only heat for space and water heating, as does a boiler, but also electricity to power domestic lights and appliances. MicroCHP units are a very new technology only recently appearing in the UK market, but the potential for them is large.
  • Nuclear Power

    Nuclear power stations utilize nuclear fission reactions to generate heat for steam generation. Fission is the splitting of the nucleus of an atom into lighter nuclei producing gamma rays, free neutrons and other subatomic par- ticles. Fission of heavy elements releases large amounts of energy as elec- tromagnetic radiation and as kinetic energy of the fission products which heats the power reactor vessel and the working fluid, usually water, which conducts heat away to a steam generator. The available energy contained in nuclear fuel is millions of times greater than that contained in a similar mass of a chemical fuel such as oil. Nuclear fuel is thus a vast source of energy. However, the fission products are very radioactive and remain so for many years, giving rise to a nuclear waste problem. Concerns over nuclear waste (and weapons) offset the desirable qualities of fission as an energy source which is largely free of carbon dioxide so there remains intense political debate over nuclear power. Electricity shortages, fossil fuel price increases, global warming from fossil fuel use, new passively safe nuclear plant, and national energy security may renew the demand for nuclear power.

    The most common fissile nuclear fuel is U-235, the isotope of uranium with an atomic mass of 235. Naturally occurring uranium consists of about 99.3 per cent U-238 and 0.7 per cent U-235. The concentration of U-235 is less than that required to sustain a nuclear chain reaction in nuclear power reactor cores. Consequently, enrichment is required, normally to about 3.5 per cent. The nuclear fuel cycle of mining, refining, enriching, using and ultimately disposing of these elements is important for its relevance to nuclear power generation.

    There are now over 400 nuclear power reactors in operation in the world, providing about 16 per cent of the world’s electricity. The United States produces the most nuclear energy, about 20 per cent of the total electricity demand. France produces the highest percentage of its electrical energy from nuclear reactors: 80 per cent as at 2006. In the European Union as a whole, nuclear energy provides 30 per cent of the electricity although some coun- tries, such as Austria and Ireland, have no nuclear power stations.

    There are many different types of reactor, one of the main differences being the working fluid and the way in which it cools and conducts heat away to the steam generator. The UK has favoured gas-cooled reactors. The most prevalent type of reactor worldwide, the type at the heart of the French and US nuclear power industries, is the pressurized water reactor (PWR). The principles of operation are depicted in Fig. X.

    Fuel rods, tubes of a corrosion-resistant zirconium alloy containing enriched uranium pellets, are placed in the core of the reactor. A typical PWR contains nearly 100 tonnes of uranium. The fuel in the reactor vessel undergoes a chain reaction, heating water, at a pressure of about 15 mega- pascals, in the primary coolant loop. Water is heated to over 300 °C, but remains a liquid at such a high pressure. It is pumped through the reactor to a heat exchanger, the steam generator, in which the secondary coolant boils. The transfer of heat is accomplished without mixing the primary and sec- ondary fluids, because the primary coolant is radioactive. The steam formed in the steam generator is allowed to flow through a steam turbine, and the energy extracted by the turbine is used to drive an electric generator. After passing through the turbine the secondary coolant (water–steam mixture) is cooled down in a condenser before being fed into the steam generator again. Boron control rods absorb neutrons. Raising or lowering them into the reactor coolant controls neutron activity correspondingly.
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