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Electricity If it weren’t for electricity we’d all be watching television by candlelight ~ George Gobal CHAPTER 13 Electricity is another elusive word meaning different things to different people. Power companies use electricity to mean electrical energy; many trade books talk of electricity as the flow of electrons, while others talk of electricity as being synonymous with electric charge. Whatever electricity is, it is closely associated with the imbalance between positive and negative charges within matter. Unlike other forms of energy, electricity is not a source of energy, but rather a convenient and cost-effective way to carry energy between two points. For example, we can use the chemical energy of coal to operate a power plant. Electrical transmission wires carry the electricity that is produced by the plant to our homes, where it is used to provide thermal energy through an electric heater, give off light through an electric light bulb, produce power to run an elevator or operate a TV, or emit sound energy through a speaker. Along the way, a small portion of the energy is lost to the atmosphere as waste heat. The versatility of electricity, exhibited by its ability to carry out so many tasks, is one reason it is considered to be a very high quality form of energy. Worldwide, 36% of all the energy consumed is used to produce 14 trillion kilowatt-hours of electricity. In the United States, half of all energy consumed is in the form of electrical energy, for a total installed capacity of 900 gigawatts or about one quarter of the world’s total.1 In this chapter, we will become familiar with the laws governing the flow, production, storage, and transmission of electricity, as well as issues of electrical safety. Electricity and Magnetism Electricity The word “electricity” is derived from the Greek word electron, meaning amber. The Greeks knew that a piece of amber rubbed against a cat’s fur would collect charges. Furthermore, they observed that identical objects touching the amber repelled each other. The same thing happened if the amber and fur were replaced with another pair of materials, glass and silk. However, if glass was brought next to amber, or if fur was brought next to silk cloth, the pairs would attract. 1 E nergy Information Agency, EIA, US Department of Energy (2004 data). Benjamin Franklin (1706-1790) suggested that an imbalance results when objects having an excess of an invisible fluid (amber, glass) were brought next to objects deficient in this fluid (fur, silk); in an attempt to reach equilibrium, this invisible fluid would move from one object into the other and the two would attract. We had to wait another century until it was discovered that this fluid was, in fact, a stream of electrons that flows from one object to another when they rub. However, Franklin erroneously presumed a lack of the invisible fluid rather than a surplus of electrons; opposite to his assertion, electrons transferred from fur and silk to amber and glass.2 Today, we understand the phenomenon by noting the existence of a charge imbalance between different atoms. As we discussed in previous chapters, atoms consist of negatively charged electrons orbiting a nucleus composed of positively charged protons and neutrons with no charge. For an atom to be neutral, the total number of protons and electrons is equal, resulting in a net zero charge. We also know that, just like with energy, electric charges are conserved. Electric charge can transfer from one material to another, but no new charge is created or destroyed. Electric charges may remain stationary or flow through an electrical conductor. The study of charges at rest is called electrostatics. Electrostatic charges may be caused by friction (shuffling one’s foot across a carpeted floor), by contact (touching a metal door-knob), or by induction (lightning). Electrostatic charges are not of concern in this study and will not be discussed further. When motion of charges is involved, electric currents are produced that flows through circuits, powering devices ranging from simple battery-operated toys and home appliances to state of the art electronics. Electrical Circuits To have a current between two points, we need a difference in electrical potential. This electrical potential can be supplied either by a battery (between the two electrodes), or by an electric utility company (between the two slots of a house’s electrical outlet). As long as there is no path between the two points (open circuit), or there exists a path that is nonconductive (such as connecting two point charges with a glass rod), no charge is transferred and no current flows. When the path is established by flipping a switch or by inserting a load, such as an electric motor or a light bulb, electricity flows. Unless a supply of charges from an external source (a battery or electric outlet) maintains the electric potential, the current eventually stops. In a battery, electric potential is maintained as it discharges through a chemical reaction. Household electrical outlets maintain electric potential by electrical generators driven by wind, water, W hen Benjamin Franklin made his conjecture regarding the invisible fluid, he assumed that fluid flowed from glass to silk. In reality, silk has an excess of electrons and glass is deficient i n electrons. If we had designated electrons as having positive charges, the flow of current would have been from higher to lower concentration of electrons, in line with the assertion t hat excess fluid moved from one object to the other. 2 300 Chapter 13 - Electricity Electrical and Gravitational Forces T Digging Deeper ... here is a striking resemblance between the way electrical and gravitational forces function. Gravitational force follows Newton’s law of universal gravitation and is proportional to the product of the two masses and inversely proportional to the distance between them. Electrical force follows Coulomb’s law and increases with the product of two charges and is inversely proportional to the distance between the two. The same way gravity is responsible for a satellite orbiting the earth, electrical force is responsible for electrons orbiting nuclei. Unlike gravitational forces that are always attractive, electrical forces can be either attractive or repulsive. ELECTRON SATELLITE F F PROTON EARTH The electric force holds electrons in orbit. The gravitational force holds satellites in orbit. gas, or steam turbines. The amount of current that can flow through a conductor (wire) depends, not only on the electric potential across the wire, but also on its resistance along the path. This is given by Ohm’s Law, which is expressed as: V=I.R (13-1) where V is the voltage, I is the current, and R is the resistance. The unit of electric current is amperes (A), defined as the quantity of charge in coulombs (C) flowing in one second. One coulomb is equal to the amount of charge possessed by 6.2x1018 electrons. In other words, one ampere of current represents the flow of 6.2x1018 electrons per second. 1 A = 1 C/s Similar to gravitational potential energy, which is a measure of the work needed to raise a weight above the ground, the volt (V) is potential electric energy and is equal to the work needed to raise the level of one coulomb of charge from its ground level. 1 V = 1 J/C The resistance is measured in ohms (W) and is the attribute of a conductor that controls the amount of current that can flow. 1 W = 1 V/A More conductive wire material, bigger cross sectional area, and shorter length mean less resistance to electric flow and greater capacity to carry 301 a current. A current can either be uniform (direct current or DC) or varying in time (alternating current or AC). Direct current is the direct flow of electrons through a conductor. Alternating current results from the back and forth movement of electrons in a wire. The electrons themselves move very little, but their periodic motions result in the transfer of energy to adjacent electrons. This is similar to a longitudinal wave along a spring coil. Although atoms of the spring move only a little, a wave is propagated downstream, carrying the energy from one end of the spring to the other end. The electrical potential of an alternating current is created by moving a coil of wire in a magnetic field. Usually the voltage provided by an electric utility company cannot be easily modified. Domestic electric service delivered by utility companies to US homes is 240 volt/60 hertz.3 This voltage is required for powering electric cooking ranges, clothes dryers, air conditioners, and other large appliances. The 240 V service is split at the power panel into the familiar 120 V circuits used for most household appliances which are plugged into wall outlets. Europe and Asia use 220 volt/50 hertz and appliances that are designed accordingly. Electrical Power The electrical energy supplied by a generator or battery is either converted to work (as in an electric motor) or is dissipated to heat (as in an electric heater or an iron). The rate at which the energy is converted is called power, and is given by multiplying the rate at which electron flows (current) by the energy that is carried by each electron (voltage): (13-2) P=V.I Substituting for voltage from equation 13-1 we get: 2 (13-3) P = R I2 = V R These equations show that energy dissipated as heat by an electrical conductor increases as the square of the current.4 In a resistance heater, the goal is to maximize the amount of heat that can be dissipated, so we would prefer to draw high current. Since voltage is constant, equation 13-3 suggests we need to minimize resistance. The easiest way to control current in a particular circuit is to insert special components, called resistors, in Hertz is the unit of frequency (cycles/s). E quations 13-1 to 13-3 above are for direct currents. The same equations work with alternating currents, except current and voltage values must be substituted with “root-meansquared” values given by: V0 I0 Vrms = and Irms = 2 2 3 4 In this equation, V0 is the peak voltage, and I 0 is the peak current. Ohm’s law and average power dissipated through AC circuits are rewritten as: Vrms = Irms . R P = Vrms . Irms = V2rms R 302 Chapter 13 - Electricity various series or parallel configurations. Resistors are made of materials with low electrical resistivity but which can withstand high temperatures and are commercially available for a large range of resistances – from a fraction of an ohm to many megaohms.5 If electric potential and current are given in volts and amperes, power will be in watts. One watt equals the energy used, in joules, in one second. 1 W = 1 J/s One kilowatt is 1,000 W, and 1 megawatt is 1,000 kW or 106 W. When discussing electricity consumption and production, it is customary to express power in kilowatts and time in hours. Since energy is the product of power and time, electrical energy is often expressed in kilowatt-hours.6 Table 13-1 shows power ratings for several household appliances. Table 13-1. Power Consumption for Several Household Appliances Appliance Clock Radio Television (color) Freezer/Refrigerator Hair Dryer Washer (clothes) Vacuum Cleaner Home Computer Power, W 2 70 - 400 65 - 200 600 - 1000 1200 - 1800 350 - 500 1000 - 1400 100 - 400 Appliance Air Conditioner Coffee Maker Iron Dishwasher Toaster Microwave Oven Dryers (clothes) Oven range Power, W 800 - 1000 900 - 1200 1200 - 2400 850 - 1400 800 - 1400 750 - 1100 1800 - 5000 10000 - 12000 Source: US DoE Office of Energy Efficiency and Renewable Energy, http://www.eere. Example 13-1: Calculate the power dissipated by the lamp in the circuits shown on Ex 13-1. Solution: Case (a): I = V/R = 18/3 = 6 A; P = V x I = 18 x 6 = 108 W Case (b): I = V/R = 36/3 = 12 A; P = V.I = 36 x12 = 432 W Note that increasing the battery voltage by a factor of 2, from 18 V to 36 V, increased the current by a factor of 2 and power by a factor of 2 x 2 = 4. Example 13-2: A 100-W light bulb operates in an American household for 4 hours every night. a. How much current is drawn by the light bulb? b. What is the resistance in light bulb filament? c. How much power is consumed by the light-bulb? d. What is the monthly cost of operating this light bulb if the electricity is charged at a rate of $0.10/kWh? Solution: American electrical power is delivered at 120 volts. 5 Lamp Battery E=18V R=3Ω + - (a) Lamp R=3Ω Battery + E=36V - (b) EX 13-1 Note that resistivity is not the same as resistance. Resistivity is a property of material, whereas resistance represents the impedance to the flow of electricity and depends not only on the resistivity of the material, but also on size and geometry. 6 1 kWh = 1,000 W x 3,600 s = 3.6x10 6 J. 303 a. The current drawn is: I = P/V = 100 W/120 V = 0.833 A b. The resistance is calculated as: R = V/I = 120 V/0.833 A = 144W c. The power is calculated as: P = VI = 120 Vx0.833 A = 100 W d. Monthly energy consumption by the light bulb is: E = (0.1 kW)x(4 hr/day)x(30 days) = 12 kWh. The monthly electrical cost is 12x0.10 = $1.20 Question: In the example above, which draws more current: the wire leading to the light bulb, or the bulb filament? Which gets hotter? Answer: The flow of electrons is exactly the same in all parts of the circuit. As the electrons try to overcome the greater resistance in the thin filament, they heat it. The wires connecting to the outlet are significantly thicker than that of the filament and therefore barely warm as they allow the same amount of current to pass through. V1 I R1 V -V I= 2 1 R R = R1 + R2 R2 Series and parallel Circuits V2 Series R1 R2 Different electrical devices can be assembled in a circuit in series, in parallel, or in a combination of the two. In a series configuration, the output current of one device will be the input current of a second device, and so on; the same current passes through all devices (Figure 13-1a). Since the total load increases for the same voltage potential, according to Ohm’s law, the current must decrease. In a series circuit, resistances add up: Rtotal = R1+ R2+… Rn (13-6a) V1 I I1 I2 I1 V2 I2 V -V I = I1 + I2 = 2 1 R 1= 1 + 1 R R1 R2 Since the current passes through both resistances, the voltage across the two is the sum of the voltages across the individual resistances. V=V1+V2.+… Vn (13-6b) Parallel Figure 13-1 Series (top) and parallel (bottom) resistances. The main disadvantage of a series configuration is that if one of the devices burns out we have an “open circuit,” in which current stops and none of the devices in the series circuit will work. Open and Short Circuits Digging Deeper ... N either voltage nor current can provide power by itself. No current flows in an open circuit, although there is a finite voltage across the source terminals. Consequently, power is zero. Likewise, a circuit shorted by connecting terminals with a superconducting wire (zero resistance) draws a very large current even though there is no voltage difference, and again, no power is produced. Question: In a flashlight, a battery provides power to turn on a light bulb. What happens if the light suddenly burns out? What happens if two ends of the battery are accidentally connected with a piece of conducting material? Answer: In the first instance, the circuit will be broken (open) and the flow of electrons stops. Short-circuiting the circuit by directly connecting the poles of the battery with a piece of wire allows a sudden release of energy with no substantial resistance to slow down the flow. This is actually very dangerous, as the high current can result in an electric shock. The rapid discharge generates heat, which can cause the battery to explode. 304 Chapter 13 - Electricity Example 13-3: Typically, Christmas tree decoration lights are arranged in a series configuration. Assume a strand of 50 miniincandescent bulbs is connected to a 120 V outlet; calculate the current drawn by the strand and the total power consumed. A typical bulb has a resistance of 8 ohms. Solution: The voltage across each bulb is 1/50th the voltage drop across the strand, or 120/50 = 2.4 volts. The current drawn is I = V/R = 2.4/8 = 0.3 A. The power consumed is P = 50.VI = 50x2.4x0.3 = 36 W. As explained above, when the bulbs are put in series, if one bulb burns out the entire strand will go out. Today’s Christmas lights, however, will stay on even if one or more lights are burned out. The trick is that new bulbs have an internal shunt resistance connecting two poles. The shunt is coated with a high-resistance material that prevents shorting the filament. When a light burns out, the heat causes the coating to melt thus reducing the shunt resistance and allows the current to pass through the rest of the strand so the remaining lights will stay on. In a parallel configuration, the same voltage potential is applied to all devices. The current necessarily divides. Since the same voltage is supplied to all electrical devices, each device works independently of others and a burned out device does not affect the operation of other devices. In a parallel circuit, currents add up: Itotal = I1+ I2+…. In (13-7a) EX 13-3 In other words, parallel resistances act to divide the current. It is left as an exercise to show that several resistances placed in parallel can be replaced by an equivalent resistance equal to: 1 Rtotal = 1 1 1 + + .... R1 R2 Rn (13-7b) Example 13-4: A 1,200-W electrical heater, ten 100-W electric light bulbs, a 300-W refrigerator, and a 1,500-W microwave oven operate simultaneously. Assuming that the household circuit is 120 V and all appliances are placed in parallel, what is the total current drawn? Solution: Applying Ohm’s law (I = P/V) for each device and using power ratings given in Table 13-1, we can write: Electric Heater: Ten 100-W light bulbs: Refrigerator: Microwave Oven: Total current drawn: 1200/120 = 10.0 A 10x100/120 = 8.33 A 300/120 = 2.50 A 1500/120 = 12.50 A 33.33 A Fuse 120 V Plug EX13-4 305 This is a large current and may be a fire hazard. Houses are usually equipped with 10-20 A circuit protection. If current exceeds these values, a fuse will burn out or a circuit breaker will trip, disconnecting the circuit. In house wiring, several circuits are used, each equipped with a separate circuit breaker. In the example above, at least two circuits each capable of carrying 20 A maximum are needed if all appliances are to be used simultaneously. Indeed, US electric codes require a microwave, garbage disposal, and refrigerator to have a 20 A circuit separate from lighting circuits and wall outlets for other appliances. Example 13-5: A three-way light bulb can be constructed by putting two resistances in parallel. Whether the first, second, or both resistances are placed in circuit, the light can be dim, bright, or very bright. What are the values for the two resistances if the light bulb can be operated to dissipate 50/100/150 W of electricity? Hydraulic and Electric Analogy FYI ... T he flow of the electrons in an electronic circuit is analogous to the flow of water in a hydraulic circuit. Consider a hydraulic circuit in which water is mechanically pumped from a reservoir at a lower elevation to another at a higher elevation. The water is then used to drive a waterwheel, where it releases its energy in the form of shaft work. This is analogous to an electrical circuit, in which electrons are pumped up by a battery to a higher electric potential, flow through the wire with negligible resistance, and pass through a load to a lower potential when the circuit is closed. The drop in electric potential across the load is used to run an electric motor or to heat the filament of a light bulb to emit light. The greater the difference in elevation (gravitational potential) between the two ends of a pipe and the less resistance to the flow of water through the pipe, the more water can flow in a given time. Similarly, in a conductor (wire), the greater the electric potential between the two ends of a wire and the less the resistance to the flow of electricity through the wire, the larger the amount of charge that can be carried in a given time. In a pipe, we can reduce resistance by making the tubes shorter, smoother, and of larger diameter. In a wire, we can reduce resistance by making the wire shorter, making it from material with a lower electrical resistivity, and by using a thicker wire. Open electrical circuits can be viewed as analogous to closing the valve leading to a waterwheel; whereas shorting the battery would be analogous to removing the waterwheel and allowing all the potential energy in the water reservoir to be suddenly converted to the kinetic energy of a waterfall. (energy stored) high pressure 12 volts Waterwheel (energy released) Water 30 kV Pump low pressure Electric motor Battery 0 volts 306 Chapter 13 - Electricity Solution: The value of resistances and currents are calculated as: 1 1 1 = + = 0.0124 R R1 R2 R = 80.67 W 2 V2 P= = 110 = 150 W R 80.67 The total current is: I=I1+I2=0.45+0.91=1.36 A At the brightest position, we have R1 = R2 = V2 1102 V 110 = = 242 W; I1 = = = 0.45 A 50 P1 R1 242 V2 1102 V 110 = = 121 W; I2 = = = 0.91 A P1 100 R2 121 as expected. Magnetism Electricity and magnetism are two phenomena which occur hand in hand; we cannot have one without the other. This was demonstrated by Hans Christian Oersted (1777-1851), who showed that a compass needle brought near a wire carrying electrical current deflects. Similarly, if an electric current is passed through a looped wire (called an electromagnet), a magnetic field is created around the wire (Figure 13-2). The greater the current, the stronger the magnetic field will be. Electromagnets are used in a number of devices, including simple on/off switches and practically all electromechanical devices, from small toys and hair dryers to large electric motors in elevators and cable car trolleys. Motors and Generators (a) Figure 13-2 Electromagnet Electric motors are devices that convert electrical energy into useful mechanical energy. The principle of operation of an electric motor is simple. If the coil of an electromagnet is placed between the poles of a magnet, the positive side of the wire and the north pole of the magnet are of the same charge and repel each other. The same is true of the negative side of the wire and the south pole of the magnet. The result is that the magnetic field exerts torque on the wire, which is deflected by half a turn and stops (Figure 13-3a). The electromagnet can now be rotated only if the direction of the current in the loop reverses, i.e. the polarity of the battery is changed. This is commonly accomplished by placing a commutator between the battery and the loop (Figure 13-3b). A commutator is a ring split in half – each side is in contact with a brush. As the wire loop continues to spin the polarity of the brushes reverse to assure that the loop and magnet remain of the same polarity. As long as a current is applied, the loop spins continuously. The rotors of practical motors are made of not one, but thousands of loops wound around a soft iron armature. (b) Figure 13-3 Electrical motors. (a) Without a commutator, the loop is rotated by only half a turn and stops; (b) The loop can be made to spin continuously with a commutator. 307 Electric motors can be driven by either direct current (DC) or alternating current (AC). The difference between DC and AC motors is in the way the magnetic fields are created. In DC motors, this is done by means of an electromagnet or a permanent magnet. In AC motors, the magnetic field is created by passing an alternating current through a stator such that the polarity changes just when the armature is lined up with the poles of similar polarity. Because AC motors do not use brushes, they are simpler to construct and require little maintenance. One of the main features of electric motors is their ability to produce torque as soon as they start. This is in contrast to internal combustion engines, where no torque is delivered until the engine attains a certain speed. In fact, this is why all cars running on petroleum require starter motors to operate. Furthermore, with only one moving part, electric motors are much simpler and have a much longer lifetime than internal combustion engines. Because of their ability to deliver peak torque at or near stall, electric motors are widespread in trolley cars, elevators, cranes, forklifts, and electric railroad locomotives. We will discuss the application of electric motors in electric and hybrid vehicles in the next chapter. Electric generators are devices that convert the rotational energy of turbines or spinning shafts into electrical energy. Electric generators work opposite to motors - a magnet is turned by some external means to induce current through a wire. In a typical power plant, a turbine shaft is directly connected to a magnet that spins inside wire coils. Depending on their application, there are different types of generators on the market. In synchronous generators, the rotor turns at exactly the same frequency as the electric grid; for a 60 Hz grid, it makes 60 revolutions per second, or 3,600 rpm (50-Hz or 3,000 rpm in Europe). These generators are often used in coal, oil, or nuclear power plants where fuels can be burned at a controlled rate, making the turbine rotate at a fixed velocity. In asynchronous generators, the magnetic field of the rotor and the electric field of the stator do not synchronize, but the rotor falls behind (slips). Wind turbines take advantage of variable slip generators by adjusting the slip (adjusting the resistance in the rotor winding) to allow the turbine to run faster as wind speed increases. Pole changing generators allow operators to choose the number of stator poles in order to change rotational speed and power of the turbine. Generation, Transmission, and Distribution of Electricity The delivery of electricity from production to consumption requires efficient generating facilities, access to reliable networks, powerful transformers and relaying stations, accurate metering, and other procurement services such as scheduling and dispatching (Figure 13-4). 308 Chapter 13 - Electricity Figure 13-4 Electrical power distribution from generating plants to consumers. At all points, the quality of electricity must be assured to maintain its frequency and voltage stability. This is particularly important in large networks in which electricity is continuously added and consumed at various nodes within the grid. Without this, a potential failure can propagate quickly, damaging a large part of the network and causing a blackout over a large geographical area. 7 Generation The primary method of electricity generation is by power plants. No matter which energy source is used, the energy produced is transformed into the rotational energy of a turbine, which drives a generator that produces electricity. Wind farms utilize the kinetic energy of air streams to power wind turbines. Hydroelectric and tidal plants use the potential energy of falling water to run water turbines. The potential energy of ocean waves can be used either to rotate a water wheel directly or to compress a column of air and drive a gas turbine. The generation of electricity by thermal power plants is accomplished in three steps: the conversion of chemical or nuclear energy of the fuel into thermal energy (heat) inside a combustion chamber, the conversion of thermal energy into mechanical (rotational) energy, and the conversion of mechanical energy into electrical energy by an electric generator. In coal, oil, and natural gas power plants, the source of energy is the energy trapped in the chemical bonds of the fossil fuel. In nuclear power plants, the binding energy of the nucleus provides the energy. In solar-thermal and geothermal power plants, thermal energy is directly available. The largest power failure in US history occurred in August, 2003, following a problem resulting from a fallen tree in Ohio on an unusually hot summer day when demand was especially high. This created an overload that triggered a series of power failures across the grid. To prevent equipment damage, more than a dozen nuclear power plants and over 80 fossil fuel generating stations in the United States and Canada were automatically shut down within nine seconds. Over 50 m illion people lost power. 7 309 Besides mechanical and thermal power plants, electricity can be generated directly. Fuel cells produce electricity from electrochemical reactions between hydrogen and oxygen. Photovoltaic cells convert the energy in sunlight directly to a flow of electrons through an external circuit. Thermophotovoltaics work in a similar fashion except that, instead of using the visible light from the sun, they convert infrared radiation or heat from furnaces into electricity. Magneto hydrodynamic (MHD) power generators omit the intermediate step by converting the thermal energy of fossil fuels directly into electrical energy, without first converting it to mechanical energy. Hot gas from coal combustors are seeded with some seeding agent (usually potassium) to turn into a hot plasma gas. A strong magnet separates out the charges, which drift toward and are collected by electrodes on opposite channel walls. An external circuit can be setup to take advantage of the charge gradient and produce electricity. Because of the low level of heat rejection, efficiency is higher than that of conventional thermal power plants and the demand for cooling water is lower. The hot exhaust gas is used to boil water to steam and produce additional electricity by conventional methods. Example 13-6: According to the data published by the Energy Information Agency (EIA)8, the world’s total energy production in 2001 was estimated at 403 quads. The same reference gives total electricity generation at 14 trillion kilowatt-hours. What fraction of the world’s total energy is converted to electricity? (1 quad = 1.055 EJ = 2.93x1011 kWh.) Solution: Assuming a thermal power plant efficiency of 33%, the total energy needed to produce 14 trillion kWh of electricity is: 14x1012 kWh 0.33 1 quad = 145 quads 2.93x1011 kWh This is 36% of the world’s total energy production. In the United States, 52% of power plants use coal, 16% use gas, 3% use oil, 20% are nuclear, and the remaining 10% are hydroelectric, solar, wind, or geothermal (Figure 13-5). Worldwide, about 64% of all electrical power generation is from fossil fuels, 18% is hydroelectric, 16% is nuclear, and only 2% is from geothermal and renewable energy sources. As of yet, no commercial MHD plants are built and the volume of electricity generated by fuel cells is insignificant. Load and Capacity Figure 13-5 2004 U.S. Electricity Generation by Fuel Source. Source: EIA, Monthly Energy Review, March 2005. The demand for electricity, called load, is not always constant and changes with geological location, season, even time of the day. In the United States, for example, daily electricity demand usually follows a pattern called 8 E IA, Monthly Energy Review, March 2005. 310 Chapter 13 - Electricity “the load curve,” which is low at nights and early mornings, gradually increasing during the day. It peaks in the early afternoon when industrial activities are the highest and drops back in the late afternoon or early evening (Figure 13-6 top). On an annual basis, overall demand is higher in summer, when a large number of air conditioners are operating, than in winter. The opposite is true for Canada, where extreme cold winters demand more electricity for heating than cooling. Depending on climate, different regions may experience their peak loads during either summer or winter months. Many regions of the world show curves with two maximums, one in summer and another in winter (Figure 13-6 bottom). The annual load curve can be constructed by summing up the average daily consumption over each month and plotting it over the period of one year. Just as demand for electricity varies, so does overall capacity of electricity generation facilities. It depends not only on the number of plants that are operational, but also on whether or not they are operating at their full capacity. When electricity is produced from renewable energy sources such as solar or wind, environmental parameters such as temperature and humidity and factors such as time of day, cloud cover, wind speed, and wind direction also become important. One way to meet the maximum load is to design a large power plant capable of satisfying the peak demand. This obviously is not acceptable, because the cost is prohibitively high and much of the power plant’s capacity would remain idle most of the time. The other extreme is the baseload capacity; power would seldom drop below it and the plant’s capacity wouldn’t go unused. This is the minimum capacity a power plant must meet at all times and sets the low limit for power plant size. The optimum capacity would obviously be somewhere between the two limits. Power plants’ total electrical output is measured in megawatts and expressed as installed, peak, baseload, or reserve capacity. Installed capacity is the maximum electricity that can be generated if all power plants are operated simultaneously and at their full capacities. The installed capacity in the US was 900 gigawatts in 2002. Peak capacity is the maximum amount of electricity that is needed in a given period. In any single day the peak demand occurs in the early afternoon when industrial consumption is highest. The peak demand during a year usually occurs during the hottest summer or coldest winter months. Baseload capacity is the minimum amount of electricity delivered continuously at any time during the year. Reserve capacity is the additional capacity that is needed during periods of unusually high demand or during a period of maintenance where some equipment is not operating. Normally nuclear, coal, or hydropower (from run-of-river ) plants supply the baseload demands. These plants are not suitable for rapid shut downs 311 Figure 13-6 Average daily and annual electrical loads. and start-ups so, except for regularly scheduled maintenance, they remain operational continuously throughout the year. During periods of peak consumption, baseload capacity is not sufficient and additional power generating capacity is needed. Because peak demands last only a few hours at a time and often occur with little or no warning, the additional power should be brought on line or taken off line quickly. In these instances, gas turbines and diesels are much smaller in size and capacity, and any variation in demand can be easily satisfied by adding or removing one or more of these equipments. However, the cost of electricity production using these devices is higher than that of conventional power plants. Every year there may come times when demand exceeds the installed capacity (that of the baseload power plants and all available gas turbines and diesels combined). In such instances, the power company buys power from other large utility companies or from individual farmers, homeowners, and other independent generation producers. If total supply is still not enough and the shortage persists, the power company may be forced to stop or limit power delivery to some customers until demand stabilizes and full capacity is restored. To stabilize demand, power utilities adjust prices according to a predetermined schedule. Elaborate algorithms can be envisioned where prices are changed according to the time of day and the quality of power (e.g. the current in the line). The “real-time” prices could then be transmitted to appliances. The end user could program the gadget to schedule its operation. Nuclear Run of river hydro 0 1 year Peak Load Megawatts Hydro capacity Gas turbine Coal Base Load Hours Figure 13-7 Load Duration Curve. A common practice used to calculate power generation capacity requirements is to calculate the number of hours that power plants must meet a particular demand and plot it in descending order. A sample of such a graph, called the annual load duration curve (LDC), is shown in Figure 13-7.9 The abscissa is the time in hours (T = 8,760 hours or 1 year), and the ordinate is the total electrical load measured in MW. Example 13-7: A utility company must meet the electricity demand for a community with daily and annual loads represented in Figure 13-7. Calculate: a. The total daily energy production in kWh. b. The average daily power in MW. c. The baseload capacity in MW. Solution: a. Total daily electrical output can be calculated by adding hourly productions. Mathematically, this is the area under the average daily load curve. Referring to Figure 13-7 and adding the hourly loads, daily consumption is determined to be 1,520 megawatt-hours. 9 Thomas, B. G., and Hall, D., “Probabilistic production costing under integrated operation agreement and joint power agency financing,” Energy Economics, pp. 200-208, July 1992. 312 Chapter 13 - Electricity b. The average daily power is calculated by dividing the total daily consumption by 24; i.e., 1,520 MWh/24 h = 63.3 MW c. The baseload capacity is the power generation capacity that satisfies minimum demand, in this case 30 MW. Capacity Factor Power plants do not utilize 100% of their capacity all the time. In addition to occasional down-time for repair and maintenance, there are times that demand is not sufficient for a plant to run at full capacity. Other factors such as a lack of necessary resources, moderate weather conditions, or economic sluggishness can also affect the demand. The capacity factor represents the degree of utilization of a particular power plant over a certain period. It is defined as the ratio of the actual amount of electricity produced within a specified time and what would have been produced if the machinery had operated at full capacity during the same time period.10 Capacity Factor = Actual power produced in a given period Maximum power that could have been produced during the same period The actual amount of production is calculated as the area under the load curve. The maximum electricity that the plant could produce is calculated by multiplying the peak capacity by the number of hours in the period under consideration. In the United States, the average capacity factors for electricity generation by nuclear, coal, solar, and wind plants are 76%, 67%, 25%, and 40%, respectively.11 Example 13-8: A 500-MW power plant operates at full power 45% of the time, 80% capacity for 50% time, and is shut down for the remaining 5% of the time. What is the capacity factor for this power plant? Solution: The total power produced is: (500MW)[(1.00)(0.45)+ (0.80)(0.50)+(0.0)(0.05)] = 425 MW The capacity factor for this power plant is 425/500 = 0.85 (or 85%) Example 13-9: To meet the peak capacity demand, a main power plant is equipped with an additional 10-MW from gas turbines. Assume that on average each gas turbine operates at full power 10 hours a day for two months in summer and 8 hours a day for one month in winter. W hat is the capacity factor for each additional turbine? Solution: Capacity factor can be calculated by dividing the actual energy use by the energy use if turbines operated 100% of the time. Capacity Factor = (10 MW)(10 hours)(60 days) + (10 MW)(8 hours)(30 days) = 9.6% (10 MW)(24 hours)(365 days) I nstead of Capacity Factors, power companies often give the “Utilization Factor,” which is the ratio of the actual energy produced and the energy produced during the time the machines operated at partial load capacity, not counting the down-times. 11 H all, Darwin, Private Communications. 10 313 Optimal Size of the Power Plants Although at first glance it may seem that a power plant must be sized to meet the peak demand, from an economic standpoint, this is probably not the best option. Oversizing the plant will result in only short intervals when it utilizes its full capacity, thus leaving much of its capacity idle the rest of the time. It is therefore reasonable to size a plant to meet, at minimum, the baseload demands, with additional power generation capabilities to meet peak demands. To reduce stresses on the system it is common to reduce peak loads and redistribute them to off-peak hours. This approach, called load leveling, is done most effectively by restricting access while providing customers with incentives to cut demand. For example, a utility company may offer its customers a cheaper rate if they shift usage from peak hours to hours where demand is lower. Another option, energy storage, is attractive when the marginal cost of the energy storage system is lower than the marginal cost of constructing additional plants. This is usually done by storing the excess capacity during periods of off-peak production and using it during times of peak demand. The last and possibly best option is to reduce consumption through conservation. Energy conservation measures alleviate stress on the grid, reduce load and emissions, and indirectly reduce the chance of power blackouts. Federal and state agencies can encourage conservation by providing tax incentives, rebates, and subsidies to companies that invest in energy-saving practices or allocate funds for energy related research or to individuals who purchase cleaner, more energy efficient appliances. Transmission After electricity is generated, it must be sent through transmission wires. The transport of electricity across an electrical network is fundamentally different from transport of a good along a transportation network. In transportation networks, cargo moves from one location to another along specific routes, usually the most direct route. Any interruption along a particular cargo route will affect only that route and possibly a few routes in its vicinity. Unlike cargo, electrical signals travel along the paths of least resistance, not necessarily along the shortest line connecting the two geographical locations; sometimes many thousands of miles are covered before a signal reaches its destination. An interruption along an electrical transmission line can affect power delivery at a point that is a long distance away from the original failure. In addition, as different generating stations go on and off the grid, either by choice or due to an equipment malfunction, loads continuously change, possibly affecting the stability of the system and causing fluctuations in the frequencies and voltages at various points on the grid. Because wires link everything, all generators must spin at exactly the same rate and in complete sync with each other. To prevent the overload of power lines, a reliable control strategy and 314 Chapter 13 - Electricity power conditioning system must keep current in each line within safe limits. Also, it must allow for additional capacity to absorb extra flow in case of a sudden failure somewhere else on the network. A generator located in the wrong place will not be able to meet an increase in demand because to do so would push the flow on some lines over their safe limits.12 For the efficient transmission of power, electricity must be in the form of an alternating current at very high voltages. Early power stations generated and sold DC electricity to their customers. The major problem with DC power is its high transmission loss, which limits its application to short distances. As time and technology progressed, power stations switched to AC power, which could be transmitted long distances with little loss of power. Today, almost all electric power generated in the United States is in AC form.13 To minimize energy losses through the network, current must be reduced as much as possible. Resistance losses depend only on the type of material and the current passing through it, but not on the voltage. As we saw in Equation 13-5, for the same power, voltage and current are inversely related; therefore, to lower the electric current we must raise the voltage. This is commonly done using a step up transformer which raises voltages to 300 - 500 kV before power is transmitted through the grid. At the city of destination, voltages are dropped back to convenient levels (around 12-30 kV) using a reduction transformer and further reduced to 120V (220V in Europe) before being distributed among various end users. Superconductors may also play a role in reducing transmission losses. Superconductors are materials that lose their electrical resistances at temperatures below a certain limit. Most materials exhibit this behavior at temperatures very close to absolute zero, although some alloys of yttrium, barium, and copper become super conductors at much higher temperatures. If room temperature superconductor materials are found, much of the losses due to long-distance transmission would be eliminated. Distribution The distribution network refers to the last mile delivery of electricity to homes and offices. Many household appliances use AC directly, although there are instances when DC power is needed. Rectifiers are devices that convert AC to DC power. They contain diodes that work by allowing current to flow in one direction, but not the opposite. The digital world is a prominent example, as computers, telephones, digital satellites, and Blumstein, C., Friedman, L. S., and Green, R. J., “The History of Electricity Restructuring in California,” Center for the Study of Energy Market (CSEM), UCEI, Berkeley, CA. 2002. The conversion from DC to AC was not a simple process. Until the middle 1880s, Thomas Edison’s power company supplying DC power dominated the utility market over his arch competitor George Westinghouse. Westinghouse used the newer and superior AC technology invented by Nikola Tesla which could be transmitted across long distances with very l ittle losses. Rather than challenging him on technical merits, Edison started a smear campaign against Westinghouse using the argument that AC is dangerous and that its use should be avoided at all costs. He even set up stations using Westinghouse’s AC generator, burned and killed animals in public, and later lobbied the New York legislature to use AC power in designing the electric chair. Edison’s plan to bring about the demise of Westinghouse was eventually unfolded and AC technology was adopted in power transmission. 12 13 315 other communication devices all work by direct current. Automobile accessories and all other battery-operated devices also use direct current. Power generated by photovoltaics and fuel cells is in the form of DC and is therefore more convenient to use when operating these devices. On the other hand, when electricity is generated for transmission over commercial networks using photovoltaics and fuel cells, it must undergo DC to AC conversion using inverters before it can be connected to the grid. The electrical operator matches electricity consumption with production, mostly by bringing new generation capability online when needed and taking it offline when demand falls. The operators can also decide to get electricity from certain power companies to lower costs and better serve their customers. To attract customers, power and transmission companies find incentives to invest in technologies that improve their performance and make them more competitive. Storage Not all the energy produced by electric generators can be used immediately after it is generated. Electrical utilities take advantage of lower demand during off-peak periods by storing excess electricity and using it during peak periods, when load exceeds the generator’s capacity. This reduces the load on the system during peak hours, allowing the main generators to work at constant power, near full capacity, and under optimal conditions, thus reducing the average capital and operating costs by cutting the number of power generating stations. The important parameter in evaluating energy storage devices is their energy storage densities per unit mass (kJ/kg) and per unit volume (kJ/m3). The most common methods of storing energy are pumped storage plants, storage batteries, capacitors, and flywheels. In addition, electricity can be stored by producing hydrogen that can be recovered when it is burned or used to operate fuel cells. Figure 13-8 shows a comparison between energy and power densities for various energy storage devices. Pumped Storage Plants Figure 13-8 Specific power and energy for several storage systems. In pumped storage plants, the water downstream of a dam or hydropower generating station is pumped back into a storage tank at a higher elevation to be used to generate electricity when it is needed. Pumped storage plants were discussed in detail in Chapter 4. Compressed Air Instead of pumping water as is used in pumped storage plants, it is possible to compress air to a high pressure and store it in sealed underground caverns. During peak hours, air is mixed with natural gas in a conventional gas turbine combustion process to generate electricity. The McIntosh 316 Chapter 13 - Electricity plant in Alabama has the largest compressed air storage facility in the United States (Figure 13-9). Batteries Batteries are devices that store electric energy as chemical energy (during charging) and release it as electric energy during discharge. A battery consists of a number of voltaic cells. Each cell produces a certain voltage that depends on the type of cell and materials used. When several cells are arranged in a series configuration in a plastic casing, they are collectively called a battery. Each cell consists of two unlike metal electrodes immersed in an electrolyte which reacts with them. The positive electrode (cathode) produces negative ions (molecules with extra electrons in their outer shells), while the negative electrode (anode) produces positive ions (molecules which lack electrons in their outer shells). If the two ends of the electrodes are connected through a wire and external load, an electric current path is established that allows electrons to flow from the negative to the positive electrode (Figure 13-10). The circuit is closed by a flow of ions through the electrolytes. In small batteries, electrodes are small rods inserted in a pool of electrolyte. In larger batteries, electrodes are in the shape of thin metal plates. The most widely used battery is the lead-acid battery, where the electrolyte is a solution of sulfuric acid and water and electrodes are pure lead and lead-dioxide. Depending on whether cells can be recharged or not, batteries are classified as primary and secondary. In primary batteries (such as those used in most flashlights) the chemical reactions that supply current are irreversible. Once these batteries are discharged, they must be replaced. In secondary batteries (such as car batteries) chemical reactions are reversible; by supplying current in the opposite direction, the chemical reactions are reversed and depleted electrode materials are restored. Depending on their application, batteries can be classified as deep-cycle or shallow-cycle. Deep-cycle batteries, such as those used for golf-carts, electric vehicles, backup power, or renewable energy systems, are allowed to deplete up to 80% of their charge many hundreds of times. Automotive batteries (also called SLI, for starting, lighting, and ignition) are shallowcycle batteries and cannot be allowed to discharge more than 2-5% of their capacity without being damaged. In addition to deep- and shallowcycle batteries there are marine (hybrid) batteries used on boats and other marine applications which can be discharged up to 50%. Fuel Cells Figure 13-9 Alabama Electric Cooperative in McIntosh, uses air compressed and stored in an underground cavern to supplement electrical generation capacity during peak periods. External Load Electron Flow Electrolyte Porous Separator Negative Electronde Positive Electronde Figure 13-10 Lead-Acid Battery. Another electrochemical device capable of storing energy is a fuel cell. The principle of operation of fuel cells is given in Chapter 14 and will not 317 be repeated here. Ultracapacitors Unlike batteries which store energy by chemical reactions, ultracapacitors (also called super capacitors) store energy as static energy (Figure 13-11). A capacitor is made of two conducting plates filled with an insulator called the dielectric. The major differences between a battery and an ultracapacitor are their energy and power densities. Typically, batteries are best suitable for storing a lot of energy and little power, whereas capacitors can provide large amounts of power but store little energy. Because of their ease of operation, low maintenance requirements, lack of negative environmental impacts, and virtually indefinite number of cycles (charging and discharging), ultra capacitors have been of intense practical interest in hybrid and electric vehicles for capturing the energy produced during regenerative braking, which improves fuel efficiency by 10-15%. Flywheels Figure 13-11 Ultracapacitor. Image courtesy of Maxwell Technologies, Inc. Flywheels are large wheels that store energy by rotating at very high speeds. Traditionally, flywheels have been used for energy balance in internal combustion engines. Recent advances in material technology have made it possible to design flywheels from lighter composite materials that can run at much higher speeds - up to 100,000 rpm - making them ideal for storing surplus energy from power plants to be delivered later during peak electrical demand. The storage capacity of flywheels can reach those of batteries with the advantage that they can be energized and de-energized faster; the cost is higher however . Flywheels have also found applications in fuel cells, gas turbines, and other prime movers. Flywheels are advantageous over conventional lead-acid batteries because they have high efficiencies, much longer life, operate at any ambient conditions, and can deliver energy over a much shorter time, i.e., they have a very high specific power. If desired, flywheels can be used to charge batteries, making them suitable for providing steady power to electric and hybrid vehicles or to produce bursts of energy during acceleration and on steep grades.14 Hydrogen Strictly speaking, hydrogen is not a fuel source, but a carrier and a means of energy storage. To produce hydrogen, one requires at least as much energy as is produced when it is burned in a combustion process, or the electricity it generates when operating a fuel cell. In other words, we store the energy contained in other energy sources in the form of hydrogen, and then use the hydrogen to produce power. Issues related to hydrogen production and storage will be covered in detail in the next chapter. 14 R osen, H., and Castleman, D. R., “Flywheels in Hybrid Vehicles,” Scientific American, p. 75-77, October 1997. 318 Chapter 13 - Electricity Structure of the Electrical Power Industry How much control the government should have over the pricing and regulating of power companies depends on how one sees energy and other natural resources. Many countries consider energy to be a basic public good, vital to society and essential in everybody’s life; it cannot be replaced by other means and therefore cannot be traded like other commodities. It must be offered in a predictable manner and be made available to everybody based on need rather than profit. Energy regulations must therefore fall under government jurisdiction for setting prices, assuring its reliability and quality of service, and enforcing established environmental laws. In these countries energy sources are nationalized and all aspects of production from recovery, processing, production, distribution and sales are strictly controlled by their governments. The proponents of the free market economy favor complete deregulation of the energy industry and argue that, to increase competition, energy must be treated as a commodity that follows the same laws of supply and demand that other commodities are subjected to. Any attempt to regulate the market will stifle competition, killing incentives for power companies to invest in new and more energy-efficient generation facilities and to upgrade existing transmission infrastructure. Deregulation will allow power companies to buy and sell electricity in an open market, but also empowers smaller utilities and individual energy producers to compete with the larger utility firms. This provides a more effective load leveling and reduces stress on generators. For example, a rural farmer or a single family homeowner can install photovoltaic cells on his roof or operate a wind turbine in his backyard (See Chapter 3). In this scenario, the homeowner is both a buyer and a seller of electricity; he can buy electricity at night or during a period of low wind velocity and sell his excess power during the daytime or at high wind velocities. In one plausible scenario, even an electric or hybrid car can be a source of power generation when it is parked in the garage during peak time. As soon as power demand drops, the batteries can be recharged again. Power companies can also benefit from these small entrepreneurs because they do not need to install new expensive equipments during times when demand peaks. The savings can be transferred to customers, lowering the price of electricity. The opponents of the deregulation argue that when electricity is treated as a commodity, maximizing profits – not efficiency – becomes the ultimate motive. When profit is not a motive, electricity is allowed to move freely in the network along the path of the least resistance. With profit as the primary motive, depending on transmission rights and costs, distributors determine over which lines power is transmitted, even though this may not be the best route as far as demand over the entire network dictates; electricity might travel thousands of miles around the country before it reaches its destination. 319 Some have proposed a middle-ground where the government allows competition but sets a maximum price that can be charged to consumers. In other instances, some subsidies are provided to assure reasonable profits to the utilities. England, Wales, Norway, and Argentina have followed this approach and have rearranged the electricity infrastructure by breaking up the utility monopolies into separate generation, transmission, and distribution entities.15 Many Eastern European countries undertook similar restructuring as part of the privatization of state-owned assets. Electricity Infrastructure in the United States Until a few years ago, private firms and investor-owned utility companies had total control over the electricity market and over setting prices in specific geographical areas.16 These companies were normally vertically integrated; they produced electricity, carried it through transmission lines, and distributed it among customers. Except for the national transmission network that is shared by all utilities, these companies owned or controlled a major portion of generating stations, transmission lines to and from the national grid, and other ancillary services. The role of the government was to oversee their operation to assure the industry’s compliance with environmental and safety regulations and to limit their monopolizing power, thus preventing them from illegal practices such as hoarding and market manipulation. The task of overseeing US energy industries including interstate commerce, wholesale power transactions, transmission, and infrastructure regulation is assigned to the Federal Energy Regulatory Commission (FERC).17 The role of local and state authorities has been limited to levying taxes and regulating rates utilities can charge customers while giving the utilities a fair rate-of-return on their investments. Below we discuss the electricity infrastructure in California. Other states have infrastructures that function in similar manners. In California, electricity infrastructure consists of four principal components: electricity generating facilities, the interstate electricity transmission grid, transmission lines connecting the generating facilities to the grid, and the distribution lines that connect the electricity grid to the end electricity users. Californian receive their electricity service from one of three types of providers (See the text box for definitions of commonly used terms below): • Investor-owned utilities (IOUs), which provide about 2/3 of retail electricity service. The state’s three largest electricity IOUs are Pacific Gas and Electric, Southern California Edison, and San Diego Gas and Electric. Each IOU has a unique, defined geographic service area. The 15 16 17 Jokow, P., L., “Restructuring, Completion and Regulatory Reform in the U.S. Electricity Sector,” Journal of Economic Perspectives, 22, Number 3, Summer 1997, pp. 119-138. I bid. Federal Energy Regulatory Commission, FERC ( 320 Chapter 13 - Electricity Commonly Used Terms FYI ... FERC (Federal Energy Regulatory Commission) - In charge of overseeing US energy industries including commerce, transmission and infrastructure, as well as issuing permits to all hydroelectric generating facilities such as dams. EC (Energy Commission) - State agency in charge of enforcing, research, and implementing energy conservation progress. EC issues permits to large thermal electricity generation facilities, primarily natural gas-fired power plants. ESP (Electric Service Provider) - A company that provides electricity service directly to customers who have chosen not to receive service from the utility that serves their geographic area. IOU (Investor-Owned Utility) - A privately owned electric utility that provide electricity to a specific geographic service area for profit. The Public Utilities Commission regulates the IOU’s rates and terms of service. POU (Publicly-Owned Utility) - A local government agency, governed by a board—either elected by the public or appointed by a local elected body—that provides electricity service in its local area. PUC (Public Utilities Commission). The state agency that regulates various types of utilities, including IOUs and ESPs. These conditions on electricity rates and service are known as “terms of service.” RPS (Renewables Portfolio Standard) - Requirement that electricity providers increase their share of electricity from renewable resources (such as wind or solar power) according to a specified time line. rates that IOUs can charge their customers are determined by the California Public Utilities Commission (PUC). • Publicly-owned utilities (POUs), which provide 1/4 of retail electricity service. POUs are governed by a board elected by public or appointed by a local elected body. POUs are not regulated by PUC. R ather, they set their own terms of service. California’s major publicly owned electric utilities include the Los Angeles Department of Water and Power and the Sacramento Municipal Utility District. • Electric service providers (ESPs), which provide the remaining retail electricity service. These ESPs generally serve large industrial and commercial customers. The ESPs also provide electricity to some state and local government agencies, such as several University of California campuses and some local school districts. Like POUs, ESPs set their own terms of service. Energy From Renewable Sources Current law requires IOUs and ESPs to increase the amount of electricity they acquire (from their own sources or purchased from others) that is generated from renewable resources, such as solar and wind power. This requirement is known as the renewable portfolio standard (RPS). Each electricity provider subject to the RPS must increase its share of electricity generated from eligible renewable resources by at least 1 percent each year so that, by the end of 2010, 20 percent of its electricity comes from renewable sources. Current law does not require POUs to meet the same RSP that other electricity providers are required to meet. Rather, each POU set their own renewable energy standards and schedule to meet those standards. 321 Electrical Safety Although electricity appears to be the cleanest form for storing energy, there are a number of health and safety issues that must be considered. Electrical power lines produce a magnetic and electric field that may pose a health problem or interfere with surrounding electronic devices. Inappropriate use of electricity could also be dangerous and, at times, even life-threatening. It is therefore necessary to understand the harmful effects of electricity and the proper procedures for safe handling of electricity in a manner that can minimize long-term health problems and risk of electrical shock. Health Effects of Electromagnetic Radiation The health effects associated with overhead power lines and household electrical appliances have been one of the most controversial topics debated among scientists.18 Strong magnetic forces are associated with high currents, such as those near thick wires and equipments which draw high currents. A current passing through a wire creates a magnetic field that encircles the wire (Ampere’s law). The magnetic field’s strength is highest closer to the wire and drops off as the square of the radial distance from the wire. It is very difficult to shield the magnetic field. Strong electric fields are associated with the presence of strong electric charges, such as around high-voltage equipment. A person standing underneath a power line experiences an electric field perpendicular to the ground. The closer the power line is to the ground and the higher its voltage, the higher the electric field’s strength. For example, a single 115 kV cable running 15m above the ground causes an electric field of E = 500 V/m near the ground. Depending on the frequency of the AC line, the direction of the field lines reverses 50-60 times per second, causing rapid changes in the direction of the electric field through a body and exposing it to ionization radiation. Unlike magnetic fields, electric fields can be shielded by conductors such as metals. We discussed the effect of ionizing radiation and its ability to cause symptoms from headaches to cancer in detail in Chapter 12. Although the connection between power line fields and cancer is an area of continuing research, so far there are no scientific studies that point to a consistent, significant link between cancer and power line fields. A summary of health effects associated with electrical shocks is given in Table 13-2. Physiological Effects of Electricity Table 13-2. Health Effects of Electrical Shocks* Current through the body trunk Effect on average human < 1 mA No sensation 3-10 mA Tingling. Person can let go. 10-30 mA Muscle contraction, person cannot let go 30-50 mA Painful. Severe muscular contractions. Breathing difficult 50-100 Ventricular fibrillation, mA probable death > 100 mA Fatal * Center for Disease Control & Prevention website When current passes through living tissue, it experiences resistance and dissipates its energy as heat. Depending on the magnitude of current and tissues involved, electricity can manifest itself in several forms, from low heat and slight tingling to severe burns, paralysis, and even death. For more information on effects of EMF exposure on health see the Word Health Organization’s International EMP Project ( peh/emf/emf _home.htm). 18 322 Chapter 13 - Electricity The most significant hazard associated with electric shock is its damage to a person’s nervous system. The nervous system consists of a series of nerve cells called neurons that are responsible for coordinating all of the body’s movements – from the beating of the heart to the blinking of eyes – by passing information from various organs to the brain. This is done by releasing chemicals called neurotransmitters which generate tiny electrical signals that can travel the nervous system. Currents from external sources can be strong enough to override the neurotransmitters, preventing them from carrying out their normal functions. If this happens, volitional signals cannot be transmitted and affected muscles will involuntarily contract. This effect is particularly dangerous when a person touches a bare electrical wire. Fingers have the least resistance to current flow and can easily bend involuntarily, clenching into a fist that grabs the wire. The victim becomes immobilized and unable to let go of the wire, making the shock even more dangerous. The involuntary contraction of muscles is called tetanus in medical terminology. The condition persists as long as the current flows. Shock Severity The length of time, the type of tissues involved, and the magnitude of the current determine the severity of hazard associated with electrical shock and the extent of damage it causes. The best way to reduce the electrical shock from a live circuit is to add resistance to the path of the current. Rubber gloves and boots increase the resistance, thus reducing the current for the same voltage difference. Grounding makes an excellent means of protection by providing a path of least resistance through the ground (and not the body). The current passing through the victim’s body is determined by the body’s resistance, which varies greatly from one organ to another and whether the skin is dry or wet. For the same voltage difference, the current passing through wet skin or a sweaty hand will be higher. When the path of current is from hand to hand or from hand to foot, vital organs such as the heart, lungs, or spinal cord are affected, and the shock effect is the most severe. Question: A common phrase often heard in regard to electrical safety is, “It’s the current, not the voltage that kills.” Why then, are we often FYI ... Safety Tip: The Third Prong F aulty electrical devices can deliver dangerous and sometimes lethal shocks. In the most common scenario, frayed insulation causes the high-voltage wire of the device to short (become connected) to the case. If a person standing on the ground touches the case, he completes a path for the current to reach the ground. Since a person’s resistance is typically much less than the resistance within the device, the person presents the path of least resistance and will therefore draw a large current. This kind of hazard can be avoided by the use of a three-wire system. A wire to the case of the appliance connects to the third prong of a plug, while the third hole in the outlet is connected to the ground. In the event of faulty wiring in the appliance, the current is routed to the ground through the prong and not through the person because the case and the ground have essentially no resistance between them. 323 warned of the danger of high voltage? Answer: Strictly speaking the sentence is correct. It is the current that is dangerous. The voltage only pushes the current through bodily resistance. The question is where do all those currents come from? High voltages mean the potential for creating high currents for a given resistance path through a body - higher voltages can be directly translated into higher currents. Question: W hen is the effect of electrical shock most severe? With a 120 V or a 220 V? With direct current (DC) or alternating current (AC)? Answer: If everything else is the same, higher voltages result in higher currents; therefore voltages in the US outlets are safer than those in Europe. It is difficult to quantify whether DC or AC is more dangerous. Direct current is generally considered to pose less of a shock hazard, but it produces more severe burns. A person shocked with an alternating current is more likely to go into heart fibrillation. Example 13-10: In the diagrams to the left, determine which instance provides the safest situation for the bird or person involved. Solution: For current to flow through a circuit, an electrical potential is required. A bird sitting on the wire experiences practically no voltage drops between its feet and therefore is immune to potential danger, no matter how high the current is. On the other hand, if the bird were to touch both the high- and low-voltage cables at the same time, it would then draw a lethal current through its body. To assure birds’ safety, the separation distance between the power cables is chosen to exceed the wingspans of most birds. This is not true for the boy, however. In diagram (a) the boy holds the bare wire with one hand. Since he is standing on the ground (which by convention has zero voltage), there is a voltage difference between his hand and foot. As a result, electricity flows through his hands and body and eventually reaches his feet, closing the circuit and shocking the boy. The downed power line in diagram (b) causes a large electric potential between the points where wire touches the ground and the nearest pole where the transmission line is grounded. Thus there is a voltage differential between the feet of the boy standing somewhere in between, and he would be shocked. Probably the best way to avoid shock is to keep your feet close together, stand on one leg, or run away from the power line. Running has the same effect of having one foot on the ground, preventing a large voltage drop between the victim’s feet. Safety Tip: Circuit Breakers current through the earth (a) current through the earth EX 13-10 (b) FYI ... H ousehold circuit breakers are normally designed to trip at currents of around 15-20 amperes or higher, much more than the lethal limit of 100 mA. They are intended to prevent electrical fires, not protect you from shock. 324 Chapter 13 - Electricity Example 13-11: In the example above, would it make a difference if the boy were bare-footed? What about if he touched the wire with both hands? Solution: If the boy wears shoes with thick, insulated rubber soles, then he is protected from electric shocks. The problem arises if any moisture, dirt, or other conducting substances (such as a metallic strip) provide a path of least resistance and allow the electricity to bypass the sole directly to the body. Leather soles provide much less resistance and are not nearly as effective. Some ground surfaces are better insulators than others. Asphalt contains some oil, which makes it a better insulator than most dirt, concrete, and rocks. If the boy holds the wire with both hands, the contact area doubles and two parallel pathways are available for the current to flow. The overall resistance is only one-half of the resistance from one hand, and twice as much current would flow though the body. It is a good practice to keep one hand in a pocket when working around electrical devices! Example 13-12: A bird is sitting on a piece of bare copper wire carrying 100 amperes. It is estimated that the copper cable has a resistance of 20 ohms/kilometer. Assuming the bird’s feet are 10-cm apart, what is the voltage potential established through the bird’s body? Solution: The resistance between the bird’s feet is calculated as 0.10 m x 20 ohms/1000 m = 0.002 ohms, and the potential difference is DV = I x R = 100 x 0.002 = 0.20 volts; this is not enough to do any harm! As mentioned earlier, power cables are often setup so the separation between the high and low-voltage cables exceeds the bird’s wingspan. Summary Electrical energy is the driving force behind much of our technological innovation. Not only does it provide lighting and heating to our rooms, but it is also essential in running our household appliances as well as the heavy industrial machinery which powers modern societies. Most of the electricity generated today uses coal, nuclear, or hydro power. As demand for electricity continues to increase, new resources must be exploited. Designing more efficient appliances and better utilization of existing resources can also help extend the lifetime of our valuable fossil reserves and maintain the quality of the air and environment. Additional Information Books 1. Bureau of Naval Personnel, Basic Electricity, Dover Publishing Company. 2. The Environmental Effects of Electricity Generation, IEEE, 1995. 325 Periodicals 1. The Electricity Journal, Direct Science Elsevier Publishing Company, This journal addresses issues related to generating power from natural gas-fired cogeneration and renewable energy plants (wind power, biomass, hydro and solar). 2. International Journal of Electrical Power and Energy Systems, Direct Science Elsevier Publishing Company. 3. Home Power Magazine ( Government Agencies and Websites 1. Federal Energy Regulatory Commission ( 2. Energy Information Agency, Department of Energy (http://www.eia. 3. California Energy Commission ( electricity). Non-Government Organizations and Websites 1. National Council on Electricity Policy ( 2. Southern California Edison ( 3. Pacific Gas and Electric ( 326 Chapter 13 - Electricity Exercises I. Essay Questions 1. What is Ohm’s Law? How does power transmitted through a circuit change with current? With voltage across the battery? 2. What is the difference between a series and a parallel circuit? What are the advantages of one over the other? Which type of wiring is more convenient in a house? 3. Describe the principal of operation of an electric motor and an electric generator. When do you use an AC or a DC motor. A synchronous and an asynchronous generator? 4. Define installed, peak, baseload, and reserve capacities. What kind of power plant is suitable to meet the needs for each of these? 5. Identify and describe the major components of a coal-burning electric power plant. How does it differ from a nuclear or a geothermal plant? 6. Describe the advantages and disadvantages of generating electricity using renewable sources of energy such as wind and solar. 7. What causes an electric shock? What are the basic safety precautions for avoiding electric shocks? II. Multiple Choice Questions: 1. Which of the following statements is not correct? a. Atoms are made of negatively charged electrons and positively charged nuclei. b. In neutral atoms, there are as many protons as there are electrons. c. The total number of charges in the universe is constant. d. Charge and mass are two properties of matter. e. Unlike charges which are always attractive, two masses separated at a distance can attract or repel each other. 2. A neutral atom with 10 protons must have a. 10 neutrons b. c. d. e. 10 electrons 10 neutrons and 10 electrons 10 neutrons and any number of electrons 5 neutrons and 5 electrons 3. A live wire (a wire in which current flows) is placed directly over a magnetic compass. As a result, the needle of the compass will a. Not move b. Point in the direction perpendicular to the wire c. Point in the direction parallel to the wire d. Fluctuate about an axis perpendicular to the wire e. Fluctuate about an axis parallel to the wire 4. According to Ohm’s Law, the current in a circuit is a. Independent of the voltage b. Linearly proportional to the voltage c. Proportional to the square of the voltage d. Inversely proportional to the voltage e. Inversely proportional to the square of the voltage 5. According to Ohm’s Law, the current in a circuit is a. Independent of the resistance b. Linearly proportional to the resistance c. Proportional to the square of the resistance d. Inversely proportional to the resistance e. Inversely proportional to the square of the resistance 6. Two light bulbs, a 100-W and a 200-W, are connected in parallel to the two terminals of a battery. a. Both light bulbs receive the same amount of current. b. The 100-W light bulb receives more current. c. The 200-W light bulb receives more current. d. The entire current passes through the 100-W bulb. e. The entire current passes through the 200-W bulb. 7. Two light bulbs, a 100-W and a 200-W, are connected in series to the two ends of a battery. a. Both light bulbs receive the same amount of current. 327 b. The 100-W light bulb receives twice current. c. The 200-W light bulb receives twice current. d. The 100-W light bulb receives four times current. e. The 200-W light bulb receives four times current. the the the the a. b. c. d. e. Energy Power Either power or energy Torque Electric potential 8. Consider two lamps that are identical but for one exception; one has a thicker filament than the other. Which has the higher resistance? a. Thicker filament b. Thinner filament c. Both about the same d. It depends on the current e. It depends on the voltage across them 9. How many 100-W light bulbs can be simultaneously turned on in a household before the circuit breaker fuse burns out? The maximum current through the circuit breaker is 15 amperes and the supply line is at 120 volts. a. 1 b. 5 c. 6 d. 8 e. 18 10. Ten identical bulbs are placed in a series circuit. One light burns out. What will happen? a. The rest of the bulbs will be turned off. b. The rest of the bulbs will burn out. c. The rest of the bulbs get dimmer. d. The current in the circuit reduces by 10 percent. e. The rest of the bulbs keep glowing as if nothing has happened. 11. The kilowatt is a unit of a. Energy b. Power c. Voltage d. Torque e. Force 12. The kilowatt-hour is a unit of 328 13. Which of the following represent(s) energy a. Joule b. BTU c. kWh d. eV e. All of the above 14. Which of the following represent(s) power a. Joule/min b. Barrels of oil per day c. Kilowatt d. Horsepower e. All of the above 15. What is the unit used by utility companies when they sell the electricity used in our homes? a. Kilowatts b. BTU c. Volts d. Kilowatt-hours e. Therms 16. Electrical energy can be produced by a. Mechanical energy b. Chemical energy c. Radiant energy d. Magnetic energy e. All of the above 17. Electrical power can be calculated by multiplying a. Current by voltage b. Current by resistance c. Current by resistance-squared d. Resistance by voltage e. Resistance by voltage-squared 18. To maximize power dissipation through a given resistance heater, we must a. Increase current b. Increase resistance c. Decrease current Chapter 13 - Electricity d. Decrease resistance e. Increase both current and resistance 19. The relationship between voltage and current in an electric line is like a. Flow rate to volume of the fluid in a hydraulic line b. Pressure to flow rate in a hydraulic line c. Pressure to volume of the fluid in a hydraulic line d. Flow rate to pipe diameter in a hydraulic line e. Flow rate to pipe length in a hydraulic line 20. The relationship between electrical resistance and flow resistance is like a. Wire length to pipe length b. Wire diameter to pipe diameter c. Wire material to pipe material d. Current in the wire to flow rate in the pipe e. Energy carried by electricity in a wire to energy carried by water in a pipe 21. Most electrical power transmission is in the form of an alternating current because a. Direct currents cannot be transmitted in electrical wires b. It is cheaper to produce c. It is safer to use d. It can be transmitted with less transmission loss e. It can travel faster 22. In the United States, most electricity is generated by a. Burning fossil fuels b. Nuclear power c. Solar energy d. Hydroelectric e. Wind and biomass 23. The failure of California to deregulate energy was mainly due to a. Insufficient refineries and generating stations b. Rapid increase in number of trucks and sport utility vehicles c. Out-dated power plants d. Malicious conspiracy by energy suppliers, politicians, and operators e. All of the above 24. The agency responsible for regulating interstate electric commerce is a. Federal Energy Regulatory Commission b. Power Exchange c. Department of Energy d. Department of Commerce e. Independent System Operators 25. The baseload of daily energy consumption represents a. The fraction used in early morning b. Average energy consumption during the day c. The load supplied by nuclear or fossil fuel d. The load required to provide minimal needs of consumers e. None of the above 26. Baseload power plants are used a. Primarily during the nighttime b. Primarily during the daytime c. Day and night d. During peak times e. During emergencies 27. Gas turbines are best suited for power generation a. At all times b. Only during daytimes c. Only during nighttimes d. During peak hours and emergencies e. For baseload power production 28. In the US, when is the peak electricity demand in summer? a. 6:00 am - 9:00 am b. 9:00 am - noon c. Noon - 3:00 pm d. 3:00 pm – 6:00 pm e. 6:00 pm – 9:00 pm 29. The severity of an electric shock depends on a. The health and physical condition of the victim b. The amount of current and length of time it passes through the body 329 c. The path the current takes through the body d. The type of electricity e. All of the above 30. If you think of a battery as a pump charging electric current, then you can think of the battery voltage as a. The flow rate of fluid b. The flow velocity c. The pressure d. The water e. None of the above 31. Consider two lights, A and B, each screwed into 120-volt sockets. The two bulbs are identical except that the filament in B is thicker than the filament in A . Which statement is true? a. Both A and B have the same brightness. b. A will be brighter because it has the highest resistance. c. A will be brighter because it has the least resistance. d. B will be brighter because it has the highest resistance. e. B will be brighter because it has the least resistance. 32. The amount of current that passes through the body as a result of an electrical shock depends on a. The voltage in the line b. The resistance of the body and clothing c. The source of electricity d. The contact area and contact pressure with the live wire e. All of the above 33. Ultracapacitors are best suited when we need a. A lot of power over a very short time b. A lot of power over a long time c. A lot of energy over a very short time d. A lot of energy over a long time e. A lot of energy and power over a short time 34. A magnet that is moved inside a coil of wires produces a. A magnetic field b. Electricity 330 c. Gravity d. A spark e. Nothing 35. How much of the energy of coal burned in a power plant is discarded as heat? a. An insignificant amount b. 10-20 percent c. 20-40 percent d. 40-60 percent e. More than 60 percent II. True or False? 1. Lightning is the result of static electricity. 2. Even currents as low as 50-100 mA can cause death, especially if the path of current is through the heart. 3. An alternating current results from the back and forth movement of electrons in a wire. 4. Power companies usually charge electricity in kilowatt-hours. 5. For the most economical way of transmitting power, it is best to use extremely high voltages. 6. Base-load energy is cheapest to produce; peak-load electricity is the most expensive. 7. Fuel cells are efficient devices for the direct conversion of solar energy to electricity. 8. Photovoltaic and thermophotovoltaics are two names for the same device. 9. Ultracapacitors are best when we want a lot of energy, but little power. 10. Both high currents and high voltages present danger to shocks. Safety Quiz: 11. You should not touch a person who was electrocuted unless you have unplugged the appliance. Chapter 13 - Electricity 12. It is only current that can kill. Voltage does not matter. 13. During a storm, a tree standing alone in the middle of an open field is more vulnerable to lightning than a tree in the middle of a city. 14. You can get a shock by touching just one live conductor, or even a charged object such as a fence. 15. You need not worry about electric shock if you work around low voltage equipment. 16. The best approach to electrical safety is to add resistance to the path by wearing rubber gloves, boots, and other safety gear. 17. It is best to run extension cords under a carpet so no one trips over them. 18. You should not unscrew a light bulb with a wet hand, no matter the wattage. III. Fill-in the Blanks 1. The study of charges at rest is called _________ _________. 2. The product of electric potential across a resistance and the current passing through the resistance gives the __________ dissipated by that resistance. 3. When resistances are _________ are added. placed in parallel, a. b. c. d. e. f. g. h. b. c. d. e. f. g. h. i. j. Charge Power Weight Mass Electric Potential Electrical Resistance Current Heat Pressure ohm joule volt coulomb ampere pascal kilogram newton newton 2. Match letters on the diagram with components in the figure: O B I F G H A N D1 L E D2 C J D3 K M Ambient air Water Steam Boiler Condenser Pump Turbine Cooling tower i. Fan j. Generator k. Transformer l. Heat source m. Heat sink n. Low-voltage line o. High voltage lines IV. Project I – Electrical Power Generating Station The migration of new industries and increased population requires the construction of new power generation facilities with a capacity of 10 MWe near your city. A panel consisting of consumer groups, oil and gas company representatives, coal miners, economists, scientists, business leaders, regulatory agencies, electric utilities, and environmentalists are meeting to debate the merit of various proposals. Several options are being considered: a. Purchase necessary additional power from a neighboring state or country. b. Use a combination of renewable energy alternatives such as wind, wave, and solar. c. Expand the existing power plant facilities using 331 4. To transmit power to long distances, it is best to use ___________ current. 5. Shock severity depends on the length of time, the type of tissues involved and the magnitude of the ____________ passing through the body. IV. Match the List 1. Match the list of variables from the column to the right with the units given in the column to the left. a. Force watt coal, oil, or natural gas. d. Build a new fossil plant in the city suburbs, away from major population centers. e. Construct a 15 MW combined heat and power (CHP) geothermal power plant. f. Construct a nuclear fission power plant. You are assigned the task of assuming the role of a reporter who is to report on various arguments being made in favor of or against each option. Please detail the findings by writing a report summarizing the following: 1. What are the geographical considerations that preclude using one or more of these options? 2. Which remaining options make the most economic sense? Consider the initial cost of construction, cost of producing and distributing power, maintenance costs, and the potential increase in employment. 3. Which option makes the most environmental sense? 4. What are the political implications of this decision, if any? Project II - Health Hazards associated with Power lines and mobile phones. There has been a wide array of studies on possible risks associated with the long-term health effects from electrical systems which was previously assumed to be safe. Some studies have claimed an increase rate of cancer among those who live in proximity to the highvoltage power lines, or using mobile phones frequently. None of these studies are conclusive, in part because of the difficulty in carrying laboratory experiments, or isolating the effect from many other factors which may contribute to the health risks. In case of mobile phones, data are limited, as these devices found their way in our daily life only about a decade ago. In light of the long latency period for developing tumors, the linkage of these devices to cancer can only be marginally documented. In this project you are asked to search the internet for scientific studies that link or unlink the electromagnetic radiation to safety and health. Write a one-page report outlining the recent findings and answer the following questions: 1. How does electricity and magnetism interact with matter, specifically to various human tissues? 2. Would you expect a greater effect at higher currents? At higher voltages? 3. Are there needs for concern? If so, are there steps that can be taken to minimize the health risks, what are they? 332