A Study Guide of Pyramid Power Plant, Study Guides, Projects, Research of Engineering

Download A Study Guide of Pyramid Power Plant and more Study Guides, Projects, Research Engineering in PDF only on Docsity! HEN HENRY ADAMS, the one-of-a-kind American historian, wanted to contrast the incdieval with the modern, the emblems he chose for those two eras were the Virgin and the Dynamo.The earlier age, he said, expressed its highest aspi- rations in building cathedrals consecrated to a spiritual ideal; in our time we exalt the generation of electricity: electric cathedral certainly agree with this assessment. A textbook for power-plant another invisible but powerful essence. The votaries of the operators says of the gencrating station, “It is like a shrine or source of unfailing light which must be given ceaseless attendance once it is brought into being.” Adams encountered the dynamo at the Universal Exposition in Paris in 190. A century later the electric power plant is no longer a novel piece of machinery that can attract a crowd at a fair, but it is more central than ever to daily life. And the dynamo (or generator, or alternator—they are all terms for the same miachine) still seems an apt symbol of both the hopes and fears invested in industrial progress. Electricity has become the standard currency of the energy economy. It is not in itselfa natural resource that you can dig out of the ground or pump from a well, bur other forms of energy are converted into electricity for convenience of distribution and use, just as the body converts a variety of foods into a few simple sugars that cir- culate to all the tissues. Thus, the power plant doesn’t create energy; it merely trans- forms it.The chemical energy locked up in coal, for example, is captured in the heat and pressure of steam, then passed on to the kinetic energy of a spinning turbine, and finally converted into electric current in the generator. Three kinds of power plants are scattered around the American landscape. Fossil- fuel plants, which burn coal, oil, or natural gas, make up almost two-thirds of the nation’s generating capacity. Nuclear plants tap energy from the disintegration of ura- nium atoms. Hydroelectric plants are found only where the water is—or more specifically where the water runs downhill. POWER PLANTS The John E. Amos power plant {opposite page) presides over a moody moment on the banks of the Kanawha River, a few miles from Charleston, West Virginia. From left to right the major structures are cooling tow- ers; the blue-clad buildings that house boilers, turbines, generators, and other machinery; two smokestacks; and @ tower for the transmission line that delivers electric power, The coal-fired plant, operated by American Electric Power Company, has three units, each with its own boiler and generator. The total capacity is 2,900 megawatts, which makes the plant one of the 10 largest in the United States. 188 The coal pile at the Mayo plant could keap the boilers fired for a few weeks if supplies were interrupted, The fwo conveyors in the background carry coal to and from the stockpile; it is piled up by a device called a stacker, with a swiveling arm. The coal is retrieved from the pile by on underground auger and then is brought into the plant by the upward-sloping conveyor line in the foreground A pulverizer on one of the lower levels of the Mayo plant grinds coal to a powder fine enough that it can be transported by blowing it through a conduit. Almost all of the coal-handling equipment inside the plant is fightly sealed, so you can watk through galleries of machinery that process thousands of tons of coal each day and never see any sign of the coal itself. INFRASTRUCTURE; A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE comes out of the lowest tap in the distillation tower) and in the sense that it com- mands the lowest price (see Chapter 4). The major problem in handling heavy oil is that it won’t flow in cold weather; it needs to be melted. To run through the fuel pipeline, the oil has to be above 100 degrees Fahrenheit; to be sprayed as tiny droplets from a burner nozzle, it needs to be about 200 degrees. Thus, steam heating coils are installed in the storage tank. The fucl pipeline may also have steam tracer lines to keep the oil fluid. Natural gas is a less troublesome fuel and much cleaner, but also more expensive. It is burned mainly in urban power plants where air pollution Jevels allow no alter- native. A utility-scale plant takes its gas not from the municipal gas mains, which operate at low pressure, but directly from high-pressure transmission lines. The Firebox. When you think of a coal-burning furnace, it’s natural to imagine a bed of coals glowing on a hearth, but that’s the wrong image for an industrial-scale burn- er. The powdered coal is treated like a fluid, not a solid. It is sprayed into the fircbox. The ideal is to burn it all, every last particle. Complete combustion not only gets full value out of the fuel; equally important, it minimizes waste-disposal and pollu- tion problems, Anything that doesn’t burn will eventually have to be hauled away. The key to full combustion is maintaining the right ratio of fuel to air, and mak- es there are two air streams. ing sure they are mixed thoroughly. In most large furna A big fan blows the powdered coal into the firebox and starts the combustion ptocess. Then an even bigger fan adds secondary air, with an effect somewhat like that of the afterburner on a jet engine. The fans are usually of the centrifugal type, with a snail shape, like the blower im a hand-held hair drier. But the power-plant fans are built on a totally different scale. They are as big as a two-story house, and the duct work that carrics their output is big enough to drive a truck through. Plants of this kind run 24 hours a day, not so much because there’s always demand for electricity but because shutting them down and starting them up again takes hours. POWER PLANTS The startup process—called lighting off—is not just a matter of striking a match Burning a special ignition fuel, usually kerosene, warms up the firebox enough to estab- lish a stable flame pattern before the primary fuel is switched on. It’s a difficult process to manage. If the flame goes out, the firrnace has to be purged with air to get rid of unbumed fuel, which otherwise might explode on reignition. Once the furnace is lit, another 10 or 12 hours may pass before the plant comes up to full power. Flue Gases. Inside the furnace a pillar of fire rises 100 feet or more. Even where the flame zone ends, the gases remain extremely hot—up to 3,000 degrees Fahrenhcit. ‘The idea guiding the design of the plant ts to let none of this heat go to waste. The pathway of the combustion gases is arch-shaped: up through the boiler, then horizontally across the top of the furnace building, then partway back down again. All along this route the gases pass through devices that extract heat in various ways. First is the boiler, where—obviously enough—the heat is used to boil water and make steam. Then, near the top of the arch is a superheater, which raises the tem- perature of the stcam far above the boiling point. Farther along, on the downward arc, are rehcaters, which pour more heat into steam that has alzcady passed through the first stages of the turbine. Then comes the economizer, where water is preheat- ed on its way to the boiler. And even after all this superheating and reheating and preheating, the flue gases are still not quite done with their day’s work. Their last task is to preheat the air that blows the fuel into the firebox, Air Pollution Control. Once upon a time, the spent flue gases would have gone straight up the smokestack, carrying a substantial load of unpleasantness with them. The problem is not smoke, as it would be from a malfunctioning fireplace. Smoke Coal burners are lined up along one wall of the fire- box. The coal-and-air mixture ascends through silver- colored conduits from the pulverizers three floors below; it passes through a scrall-shaped fan housing and is injected into the furnace. The green pipe carries liquid fuel used only in starting up the boiler, 190 Air-handling equipment at the Maye plant includes a maze of immense ducts. The green machine at lower left is o fan vath intake filters that reach a height of chout 20 feet. This “forced-draft” fan pushes air inte the furnace. An “induced-drafi” fan—the smafler green device at ground level right of centar—sucks air and fiua gases out of the furnace. The effects of the two fans ore balaaced in such a way that the pressure in the firebox is slightly negative; thus, amy small feoks drow air inward instead of spewing fumes outward. Tha path through the maze of ducts proceeds upward from the forced-draft fan, through a regenerator, then to the left and downward into the boiler building. Flue gases emerge from the furnace in the uppermost duct. They con make @ circuit through a selective catalytic reactor ‘at the upper right, or they can bypass this device and descend directly through the regenerator to the induced-draft fan. Fram there they flaw on ta the stack, just visible in the background. The regenerator, which sits-at the intersection of the two streams of gases, is an gir-to-air heat exchanger. The housing, just ocbove the forced-draft fan, is an octagon; inside, a honeycomb disk some 30 fest in diameter rotates slowly. The disk absorbs heal frem the exhaust gases and gives it up to the combystion air. INFRASTRUCTURE: 4 FIELD GUIDE TO THE INDUSTRIAi LANDSCAPE comes mainly from incomplete burning of carbon, which isn’t tolerated in utility- scale power plants. But coal is not pure carbon. For one thing, it includes a mineral residue that just won't burn. The heavier part of this residue, called bottom ash, winds up in a pan at the beitom of the firebox. The lighter part is Aly ash, and it is carried along by the Que gases as a fine gray dust. Coal also includes at least a little sulfur, which burns to produce sulfur dioxide, the precursor of sulfuric acid and acid rain. Power plants today are required to capture nearly al! the fly ash before it escapes up the stack. The two main technologies are baghouses and electrostatic precipita- tors. The baghouse is just a filter. The bags are long and thin—maybe six inches in diameter and 20 fect long. Each bag is closed at the cop but open at the bottom, where the dirty gas flows in to keep the bags inflated. Gases pass through the fabric, leaving the ash behind as a dust cake on the inner surface. As with a vacuum clean- er, the bags need to be emptied from time to time. In most plants this is done by briefly reversing the flow of air, driving the dust out of the bag and down into a hap- per; some units also have a shaker that thrashes the bag back and forth. Dlectrostatic precipitators rely on subtler physics than a vacuum-cleaner bag: they work on static cling, the force that makes a toy balloon stick co the wall after you rub it on your clothes, and that sometimes makes your clothes stick to you. Inside the precipitator arc many parallel rows of vertical metal plates, with flue gas flowing hor- izontally through the lanes between them. Hanging down into the spaces betwecn plates are fine wires energized with several thousand volts of electricity. Electrons are repelled from the wires and flce to the metal plates; along the way they attach them- selves to passing particles of fly ash, which then stick to the platcs. A “rapper” anism shakes the collected dust loose, and it falls into a hopper below. moch- FOWER PLANTS inside when the boiler is shut down for an overhaul. All the rust and scale accumu- late there. The corresponding structure at the top of the boiler is the steam drum, which in fact is more than a drum; it’s a complicated piece of apparatus, with lots of machinery inside. The main business of the steam drum is to separate steam from water so the vapor can be drawn off and piped co the turbine, while che liquid is recirculated through the downcomers. To those of us whose experience of steam comes mainly from ceakettles, separating stcam from water secms casy: the vapor just warts off the top. But under the conditions inside a power-plant boiler—temperature 675 degrees Fahrenheit, pressure 2,600 pounds per square inch—steam and water are hard to tell apart. It takes a kind of centrifuge, or cyclone, to separate them. A feature of all boilets, required by law, engineering codes, and insurance regula- tions, is a pressure-relief valve. Farly in the age of steam, boiler explosions were a notorious technological hazard. Both railroad locomotives and the stationary boilers of factory steam plants were blowing up with enough regularity to inspire public dread comparable to modern worries about nuclear-power accidents. Those at great- est risk were the enginccrs who tended the boilers, and yet they resisted regulation of their work. Nevertheless, a safety measure was imposed and remains universal today: every boiler has a valve that automatically vents off steam at some preset pres- sure not Loo far above the normal working pressure. The valve is installed on the steam drum. It relies on the sirmplest kind of spring-loaded mechanism, which pops open if the internal pressure cver exceeds the strength of the spring. Modern boilers are cquipped with other valves, tied into the central computer control system, that allow finer regulation of pressure. Buc the mechanical safety valve is there in case the computer ever crashes or someone falls asleep at the switch, 193 A boiler hangs fram the roof, allowing it to expand and contract with changes in tempercture, The full weight af the boiler is supported by o forest of steel rods that hang from the uppermast girders of the frame and connect to the roof of the boiler structure. The cuplike. objects at the very top of the frame are steam vents used for either routine or emangency releases. The cups are filled with baffles meant to reduce the noise level of high-pressure releases, Downcomers form part of c loop of piping that allows water fo circulate through the boiler. The photograph looks upward long one side of the boiler. Behind the metal sheathing and a layer of insulation is the water- wall, where pipes called risers are heated by radiant energy from the furnace. Some of the water in tha risers turns fo steam, but the rest is recirculated through the downcamers. rd The steam drum (below) is where risers and downcam- ers join at the top of the boiler. Visible here is just one end of a long drum, filled with machinery for separa ing water and steam. Pressure-relief valves (above) are on the roof just above the steam drum. They are simple mechanical valves, which discharge steam whenever the pressure in the boiler is grent enough to compress a spring that holds the valve closed. INFRASTRUCTURE: & FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE The Turbine. The steam-driven turbine is a close cousin of a jet engine. In both machines, a hot, high-pressure gas spins a series of fanlike turbine wheels. In the process, the gas expands and cools. It’s a simple idea, but power-plant turbines rated at a billion watts of mechanical power are not simple to build or operate. The turbines and the generators they drive are the most expensive hardware in a power plant, Often, they are built atop their own special concrete-and-steel founda- tion, separate from the rest of the plant. This is done to control vibrations in the rotat- ing machinery and to maintain precise aligument in the bearings that support the long, spinning steel shaft that runs through both the turbine and the generator. A single stage of the turbine consists of a stator wheel (which is rigidly fixed to the frame of the tarbine) and a rotor wheel (attached to the rotating shaft), Steam is steered through vanes in the stator and then passes through the blades of the rotot, turning it by the same principles that run a windmill or a waterwheel. Vhe blades and vanes have graceful airfoil shapes, and chey are carefully machined from fancy steel alloys that can withstand extremes of Lemperature, pressure, and mechanical stress, as well as a corrosive envirotunent. A single blade breaking off would destroy the entire s the debris crunched through the downstream, rotors and slators. machine » Typically a turbine has three units, all mounted on, the same shaft. Stearn straight ater is fed into the high-pressure turbine, where it expands and cools somewhat. The steam t a temperature comes back up to about 1,000 degrees Fahrenheit, althougb the pres- sure is not restored to its original level. This warmed-over steam then goes through the intermediate-pressure turbine, where again it expands and cools. Finally, the from the bailer and superhe: goes back to a ter unit, where its steam passes into the low-pressure turbine. Note that the same quantity of steam— the same mass of water molecules—goes through all three turbines, but because the pressure drops in each umit, the volume of steam increases. As a result, the intermediate- pressure turbine has to be bigger than the high-pressure unit, and the low-pressure turbine ts the largest of all. Judging from their relative sizes, you might guess that the big low-pressure turbine is doing most of the work, but the truth is just the oppo- site. The little high-pressure curbine puts out 60 percent of the total horsepower, and the massive low-pressure unit supplies only about 15 percent. Turbines are so large and cornplex that their most mundane auxiliary equipment is more imposing than any of the machincs most of ns meet in everyday life, Pumps and motors larger than an automobile engine are needed just to keep the turbine supplied with lubricating oi]. The bearings and seals along the main shaft also require large accessory pumps. Another vital auxiliary is the governor that regulates uurbine speed. The classic speed-control mechanisin is the flyball governor, which became an icon of the indus- trial age and a textbook example of the concept of feedback control. The governor has owo weights (the flyballs) attached to binged arms that spin around a vertical shaft at the same speed as the turbinc. As the shaft turns faster, the balls are flung outward, and the hinged arms are lifted up. A linkage attached to the flyball arms then closes the POWER PLANTS steam valve a little, slowing the turbine and allowing the flyballs to sink back toward their resting position. In this way the turbine is slowed every time it tries to speed up and is sped up every time it tries to slow down, so a steady speed is maintained. The flyball governor was the world’s first version of cruise control. The feedback principle is still at the heart of turbine control, although now ivs all done by computer. The Condenser and Feedwater System. lo make a turbine spin, it’s not enough to push steam into the inlet port; you also have to let it out at the exhaust port. Lowering the pressure and temperature at the outlet is the job of the condenser, where the steam gives up the last of its heat. As the slearn condenses, its volume is greatly dununished, and so the pressure falls too. Indeed, the pressure in the condenser is less than atmo- spheric: there's a partial vacuum, which actually sucks steam out of the turbine, The water that collects in the bottom of the condenser is distilled water, which is usually considered the ultimate standard of purity. But the water needs further treat- ment, called polishing, before it can be returned to the boiler. Any minerals deposited inside the tubes of the boiler would clog up the arteries and could cause a dramatic kind of heart attack: the deposits would act as an insulating blanket, allowing the metal wall of the tube to overheat. Ifa tube splits open, everyone within a few miles of the plant hears it. To remove suspended solids, the water is filtered through sand or charcoal, and magnetic separators extract particles of rust. An ion-exchange column works just lke a residential water softener to eliminate troublesome magnesium and calcium com- pounds. Other chemical teatments adjust the pH—the acidity or alkalinity—and remove dissolved oxygen, which can attack metals. 195 The high-pressure turbine is the smallest but most pow- ertul of three turbine units at the Mayo plant. The tur bine itself is hidden under a thick blanket of insulction. Govemors and throttle valves mounted on the high pressure furbine control the speed and the power out- put of the entire turbine-generator unit.  128 The switchyard of the Gorden Evens Eneray Center in Calwich, Kansas, is a thicket of transformers, switches, circuit breakers, lighting arrasters, and other high- voltage devices. The main function of the switchyard is to raise the voltage to a level that can be transmitted long distances. INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE gen burns only in the presence of oxygen; the key to using it safely is to exclude all air. Before the generator is Sled with hydrogen, it is purged with carbon dioxide. Electricity is carried away from the generator on bus bars, heavy copper or alu- minum conductors rated to carry as much as 40,000 amperes of current. (The heav- iest wires you'll find in your home are limited to 100 or 200 amperes.) ‘he bus baes are as thick as tree limbs, and they may be encased in protective tubes that make them look even thicker. They lead to a switchyard outside the plant. The Switchyard. Although the generator puts out prodigious currents, the voltage level is only moderate by power-company standards—usually between 10,000 and 30,000 volts. Right outside the wall, a transformer boosts the voltage to a much higher level—often 230,000 or 345,000 volls, and in a few cases as high as 765,000 volts. The high voltage allows the power to be transmutted long distances with rcla- tively little loss along che way. The transformers and their related switches and circuit breakers are set up in a fenced-off area called the switchyard, which can be as large as the rest of the planc. The devices here are essentially the same as those in the substations at the other end of the transmission linc, where the power is brought back down to lower voltages for distribution to neighborhoods. This machinery is discussed in the next chapter. The switchyard brings power into the plant as well as providing a way out. A typ- ical generating station absorbs 4 to 7 percent of its own electrical output for running machinery such as fans and pumps. When the plant is starting up, much of that equip- ment has to be running before the main turbmes and generators are cut in. The start- up power is supplied by other stations on the power grid, and brought in over the same transmission lines that normally export the plant’s own output. ‘What happens if all the power plants in a systern are shut down at the same time? Until 1965 plant operators thought they would never have to answer this question POWER PLANTS because such an event seemed so unlikely. But on November 9, 1965, a blackout in the northeastern United States left some cities dark for more than 12 hours. One rea- son it took so long to restore power was that generating stations didn’t have enough elergency power to restart without help from their neighbors—who were, of course, in the same predicament. The utilities companies promise it won’t happen again. COMBUSTION TURBINES The demand for clectricity fluctuates by the millisecond. When you turn on the cof fee pot and the toaster in the morning, a power plant somewhere has to respond by opening the throtile a little. Demand also fluctuates on longer time scales. People use more clectricity during the day than at night, and in most places they use more dur- ing the summer than the winter. It would be a great convenience to have generat- ing units that could be run only at times of peak demand. Big coal-fired stations are A power plant in a different architectural style, with all the machinery fully enclosed, the Rovenswood Generating Station is a well-known landmark for Naw Yorkers; it occupies a conspicuous site in Queens, just across the East River from Manhattan. itis also o land- mark for power engineers, the home of © generator knowa as Big Allis (built by the Allis-Chalmers Corporation], the first generator capable of producing 1,000 megawatts. The plent burns natural gas, with oil as a backup fuel.  200 A combustion turbine at the Gardon Evans Energy Center supplements the capacity of the plant’s main steam units. The tan duct extending horizontally to the right is the air intake. It arches over the generating ul The turbine itself is in the square lan enclosure; it is fol lowed by a Flored horizontal exhaust duct, and then the large gray exhaust stack, which discharges vertically to reduce noise, INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE not well suited to this duty, because they take hours to start up and shut down. Hydroelectric plants arc much more flexible in this respect, but, on the other hand, you can't build Lloover Dam just anywhere. The solution adopted by many utilities is a machine known as the combustion turbine or gas turbinc. Power company employees call them jets, and for good reason: they evolved directly from the engines that power jet aireratt. A combustion turbine relies on the sane physical principle as the stearn turbine s expand against the vanes of a turbine in a coal-fired plant: hot, high-pressure gas wheel, exerting a force that causes the wheel to spin. But instead of steam, the hot gases are the products of combustion. Fuel and air are mixed, compressed, and ignit- ed inside the turbine, where they expand and thereby turn the rotor vanes. The fuel is usually natural gas. A single combustion turbine has a power output of 10 co 100 megawatts, but it’s easy to build clusters of them with larger aggregate power. Combustion turbines are less efficient than che best steam turbines, but they have compensating advantages. First, of course, they can be started and shut down in a s just by pushing a button in a distant control room. matter of minutes, sometim Furthermore, because they don’t require as much land as a full-scale power plant and because they burn cleaner fuel, chey can be put closer to cities, which relieves con- gestion on electric transmission lines. And jets can supply start-up power for larger conventional plants. For this last reason, many steam plants have a few combustion turbines on the site. The standard mode of operation is to keep the big, efficient “base load” plant runing all the ume, and start the jets only at Limes of peak demand. Combustion turbines vary in appearance, but a common feature is that the turbine itself is overshadowed by air intakes and exhaust stacks. One reason for the large  POWER PLANTS daunting, Al. nuclear construction is governed by special engineering codes, with elaborate schedules of inspection and maintenance. Every pipe and valve must bear a “Code N” stamp, which raises the price more than gold plating would. Someday, dismantling the plants may wind up costing even more than building them did. And there’s also the cost of dealing with radioactive wastes and spent fuel. Worldwide, there is considerable diversity in the design of nuclear plants, which may sigmify that engineers have nol yer built enough of thern to reach consensus on how best co do it. Just two designs dominate in the United States: the pressurized water reac- tor (PWR) and the boiling water reactor (BWR). Only those types are described here. The Pressurized Water Reactor. The distant ancestor of the PWR is the US. Navy’s program to develop nuclear power for ship propulsion. The defining feature is a reactor core fully immersed in liquid water, which is kept under so much pres- sure thar it cannot boil cven though the temperature reaches 600 degrees Fahrenheit. About owo-thirds of the operating American reactors are PWR types. The PWR. relies on an indirect, pwo-stage process to drive the turbine and gener- ator. Water heated in the reactor core is pumped to a steam generator, where it heats and boils water in an entirely separate circuit; it is the fluid in this secondary loop that drives the turbine. There is no exchange of fluids between the two loops; this is the safe-sex version of nuclear power, Because the steam that drives the turbine never enters the reactor, the chance of radioactive contamination should be slight. A PWR has a distinctive profile. The containment building, which houses the reactor, is a tall cylinder with a domed lid. Deep inside is a massive steel pressure ves~ sel, and inside that is the teactor itself. Also in the containment building are the steam generators and pumps to drive the circulation through the primary loop. The pumps stand three stories tall and are powered by electric motors of 4,000 to 7,000 harse- power. Each pump has a flywheel that will keep ic running for a few seconds after a powcr failure—long enough for other emergency cooling systems to kick in. The reactor vessel is shaped like a medicine capsule standing on end, 49 fect high with steel walls nine inches thick. It weighs close to a million pounds, which means it can only be shipped by barge or rail. (There are no 500-ton highway trucks.) The inner surface is clad with half an inch of stainless steel as a defense against corrosion. And corrosion is a serious worry. ln 2002 a work crew at the Davis-Besse nuclear plant near loledo, Ohio, discovered a spot on the lid of the reactor vesse] where acid had eaten away the entire thickness of the wall except for the stainless-steel cladding. cd into a volume about Within the reactor itself, several thousand fuel rods are pac! the size of a high-ceilinged bathrom. There are also control rods made of boron car- bide—a compound of two neutron “poisons,” or absorbers. With all the control rods in place, neutrons are blotted up quickly enough that a chain reaction can’t sustain itself. The control rods are lifted out through the top of the pressure vessel to start the nuclear reaction. In the cvent of a power failure or some other malfunction, the rods fall back into place by gravity. 203  204 INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE Adjacent to the containment building is the fucl-handling building—often larger in volume though less distinctive in shape. Here fuel-rod assemblies are stored, both fresh ones awaiting installation and depleted ones removed during refueling. The fuel rods can’t sunply be stacked om a shelf. They are kept immersed in water both as a radiation shield and as a coolant. The deep vat of water, known as the swimming THREE MILE ISLAND Sometimes an industrial accident seems to have the fatal momentum of a Greek tragedy. Terrible things keep happening, but nabedy understands why until it's too late. March 28, 1979, was a bad day on Three Mile Islend, in the Susquehanna River south of Harrisburg, Pennsylvania. In the small hours of the marning, a shift foreman and two other workers were deing routine maintenance in cone of the two nuclear power plants built side by side on the island. Both plants are pressur- ized water reactors. The maintenance work was in what would seem to be a noncritical section of the plant—the polishers that remove minerals from feedwater in the secondary cool- ing loop. But events in that obscure corner of the plant had consequences the whole couniry soon heard about. The work crew was blowing compressed air into one of the polishers, and apparently the pressure drove water info an instrument air line, one of many small pneumatic tubes used for sensing and controlling conditions in the plant: The clogging of this particular air line had the effect of closing valves that controlled the flow of feedwater through the polishers. With the supply of water cut off, the main feedwater pumps shut down automatically (or, in powerplant arget, “tripped"). Three emer: gency feedwater pumps immediately started up, but they were unable to deliver any water because cnether pair of valves had mistakenly been left closed. The improper position of Ihese valves was discovered and corrected eight minvies later, but by then a great deal else had happened less than a second after the main feedwater pumps tripped, the turbine and generator tripped in turn. In the naxt three seconds, the pressure within the reactor and the primary coolant sysiem rose to 2,255 pounds per square inch, at which point a relief valve opened up, draining steam and water from the reactor vessel into a tank at the battom of the coniginment building. After another five sec- onds, the reactor itself tripped, and control sods were automatically inserted to halt the nuclear reaction Although this fastpaced cascade af emer- gencies sounds quite dire, there was as yet no reason for alarm. Turbine and reactor shut- downs are not rautine events, but operators are trained to deal with them. In this case the fwo operators on duly in the control room immediately set out to perform what seemed to be the mos? urgent lasks—dovble-checking the status of the turbine and generator to be cer POWER PLANTS pool, is a beautiful sight. The water is still and clear; technicians test its purity and clarity by reading fine print at the bottom through binoculars. The fuel assemblies hang on racks 25 feet below the surface. The most active elements are enveloped in the haunting blue glow of Cerenkov light, which is emitted when electrons streak through the water f: tain that these expensive pieces of machinery would not be damaged. The all-knowing cho- rus in a Greek tragedy might have warned them that bigger worries were looming, but the operators at Three Mile Island did not have the benefit of such a warning. Over the next wa hours the scene in the conirel room grew more hectic; at one point 60 operators, supervisors, engineers, and oth- ers struggled to stabilize the system. The main focus of their attention was maintaining the right water level in the primory caoling system. Most of the time, the level seemed to ba too high. A vessel called the pressurizar is sup- posed to be kept half full of water ond half full of steam; the operators thought it was Filling with liquid water, which would make it hard to control pressure in the system. Hence, they throttled back emergency systems that were pumping water into the reactor. Actuolly, the water level in the pressurizer was never too high; it was dangerously low. The cperators had been misled by their instruments. The underlying source of the problem wes yet onather valve malfunction: the pressure-relief valve that hac! popped open three seconds after the stort of the accident should have closed just 10 seconds later, but it remained open, allowing a massive leak. The stuck valve wes not discovered until more than two hours later, by which time most of the primary coolant had boiled away. By now it was too late to avoid serious damage te the reactor. Although inserting the contrel rads had halted the nuclear chain reac- tion, radioactive decay was still producing ct than the speed of light in water. about 30 megawatts of heat, which cauld not be removed fast enough, Parts of the reactor core crumbled and melted. Also, the overheat ed zirconium-alloy cladding on the fuel rods reacled with steam to produce hydrogen gas, raising fears that a hydrogen explosion might rupture the containment building. The explo- sion never came; it turned out there was too lit tle oxygen present to create an explosive mix- ture, Throughout the accident there were only small releases of radiation. It toek a month to coax the reactor into a sale state, and it took more than 10 years to clean up the mess. Several commissions investi- gated the accident—Greek choruses chanting of catastrophe alter the fact. Factors cited as contributing causes included management poli- cies that allowed the plant Jo run with emer- gency feedwater valves closed, operator train- 205 ing that put too much emphasis on one kind of accident and neglected other possible failures, and a reactor design that may have been too skittish for reliable control. Most of all, the investigators criticized the man-machine inter- face, The operatars could easily have averted ihe damage if only they had knewn what was happening inside the containment building, but the hundreds of meters and gauges in the con- iol room failed fa communicate the information they needed. The indicator for the crucial reliet valve showed that it had been ordered to close but did not register its true position. What lessens should be learned from Three Mile Island? Opinions vary widely. Opponents of nuclear power interpret the accident as a demonstration of just how dangerous and uncontrollable the technology is, Proponents lock at the same evidence and argue that the accident shows the inherent safety of nuclear reactors, since just about everything that could have gone wrong did go wrong, and yet there was no serious harm done to public health. Bath sides would rather not see any further demonstrations of this kind. Today the emply shell of the failed reactor still stands on Three Mile Island, next to its older sibling reactor, which was shut down alter the accident but was restarted in 1986, (In the photograph on the opposite page, the active unit is on the right, the corpse on the left.) General Public Utilities, the operating company, built « visitor center and souvenir shop, where you could buy Three Mile Island tee-shirts and cookbooks. But af last report the visiior center was closed.  208 Fan-driven cooling towers at the Browns Ferry nuclear station have a trapezeidal farm, wider at the top, so that warm water cascading down the side will wash oway any ice buildup. Each of the 16 shrouds atop the cooling unit houses a large fan blade that draws air in through the sidas of she structure and discharges it upward INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE in be converted to the hydrogen and the oxygen atoms of the water molecule radioactive forms. Worse still, radiation can break apart a water molecule, so that the bydrogen and oxygen are in gaseous form. The gases separate from the steam in the condenser, and a whole subsystem of the plant is needed to capture and dispose of them. That’s what the tall stack is for. It is designed to launch che emissions well up into the atmosphere, where the gases disperse. The release of radioactivity is quite small. The Nuclear Regulatory Commission sets the maximum allowable amount at a level believed to be completely safe, and plants routinely stay below 1 percent of that stan- dard, But that doesn’t always sct the neighbors’ minds ar ease. COOLING TOWERS Ever since the accident at Three Mile Island, the cooling tower has been the sinister symbot of nuclear power. Television reports on nuclear issucs set the mood with a haunting image of the towers, often with a cloud of white vapor drifting above them, hinting at some toxic release. This choice of icon could not be less appropriate. In the first place, not all nuclear power stations have cooling towers, and not all cooling cowers are installed at nuclear plants. Second, most cooling towers look nothing like the tall, tapered chimneys that have acquired such menacing associations. Finally, the cooling tower is nat where the stinger is in nuclear technology. Nothing radioactive case of radiation would have to come from elsewhere. passes through it, and any ‘Lhere is another irony in the evil reputation of the cooling tower. The reason for building the towers is not that utility companies earn money from them. On the POWER PLANTS contrary, they are a concession to environmental preservation. Their main function is the protection of aquatic life, A perfect power plant would convert all the heat liberated by burning fuel or by a nuclear reaction into electricity. Real power plants fall short of that goal, For a coal- fired plant, only about 40 percent of the heat energy is captured in electric power; nuclear plants do even worse, with an efliciency of only abour one-third. All the zest is waste. A nuclear plant with an electrical output of 1,000 megawatts must get rid of 2,000 megawatts of waste heat. The flow of water needed to carry off that heat can amount to 500 nullion gallons per day. This is moxe than cnough warm watcr to provide a luxurious daily bath for the population of New York Cicy. It’s also enough to parboil the fish in a small river or lake. The cooling tower dissipates some of that heat to the atmosphere. Cooling towers come in two basic types: the fan-driven tower, which ix more common but less conspicuous, and the natural-draft tower, which is the one that has entered the public imagination. The choice between them is one of balancing oper- ating costs against capital costs, Fan-Driven Towers. The typical fan-driven cooling tower is a long, boxy structure, roughly 50 feer wide and 50 feer tall and as much as several hundred feet long. The end walls are of solid construction, but the long side walls consist of louvers to allow for the inflow of air. The warm water is pumped to the top of the Lower and falls 209 A variation on the design of the farced-draft cooling tower wraps the structure inte circle and puts the fans in the center, drawing air through from the perimeter. This tower is at the Coal Cresk power station in Underwood, North Dakota,  The notural-dratt cooling tower at Arkansas Nuclear One hos the classic hyperbalic form, tapering to a nar- row threct and then opening fo a slighty wider diame- ter. The shape is designed fo produce optimum oir low for a given temperature difference between the water ond the surrounding atmosphere. At the base of the tower (detail on opposite pagel, the water is broken up into fine draplets to maximize evaporative cooling, INFRASTRUCTURE A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE through a labyrinth of wood or plastic slats called fll. Meanwhile a fan pulls air through the fill inte a central void, and then cxhausts it upward. Thus, in the fill there 1s a cross-flow: water trickles downward while air is drawn inward. Seen from the end, the cooling cower has the form of an upside-down trapezoid. Jt is wider at the top than at the base, and so the louvered walls slope inward. This shape is chosen to control icing in the winter. Because of the inward slope, the tou- vers are continually washed by warm water. POWER PLANTS For lower heads and higher Hows, turbines of another type work better. The run- ner, which has curved vanes rather than buckets, is immersed in a stream of water that flows through it.The runner is mounted on a vertical shaft and enclosed in a spiral scroll case, shaped like a snail shell. Water enrers horizontally and flows inward to the runner, [hen makes a 90-degree turn as il is deflected by the vanes, and exits At the Shaste: Dam in northem Califarnia, five large penstocks emerge from the face of the dam to drive turbines in the powerhouse below.  A surge tank at the crest of a hill helps to smooth the flow of water through penstocks feeding a small hydra- elactric plant below the Fantana Dam in Nerth Carolina. INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE downward, parallel to the axis af the turbine shaft. Turbines of this type—called a reaclion-wheel turbine—turn much slower than the Pelton wheel The low rotation speed of the reaction-wheel turbine calls for a different kind of generator, one that can produce 60-hertz alternating current when turning at only a few hundred revohutions per minute. In the high-speed generators used with steam tur- bines, the rotor has a single pair of magnetic poles, much like an ordinary bar magnet. The generator produces one cycle of alternating current for each revolution of the rotor; thus, 60 cycles per second requires 60 revolutions per second, or 3,600 revolu- iens per minute. To generate the same output frequency with a machine that turns tore slowly, you need a rotor with more pairs of poles. If the rotor is a cluster of 12 pairs of north.and south poles, spaced equally around the perimeter, then on each rev- ohation the output current will go through 12 alternating cycles. The generator produces 60-hertz power when turning at only 5 revolutions per second, or 300 revolutions per minute. The generators employed in slow-curning hydroelectric plants have as many as 69 pairs of poles, yielding 60 hertz at a rotational speed of just 60) revoludions per minute. These generators are larger in diameter than the high-speed machines, in order to make room for the many rotor windings, but they can be shorter in the other dimen- s the form of sion. Because the generator is mounted on a vertical shaft, it at the tor and turbine. squat the iter that hans: cylinder on the powerhouse floor, with a small turre main thrust bearing supporting the shaft of botb gen Often the powerhouse of a hydroelectric project is built into the structure of a concrete dar, usually at the foot. Water drops down through passages within the body of the dam, turns turbines installed near the baseline, and then rushes out into the tailrace. In other cases the powerhouse is a structure separate from the dam, pos- sibly miles away. Water is conveyed from the reservoir to the powerhouse through a penstack, which is typically a welded steel pipeline 10 or 15 feet in diameter. Look for a surge tank above the penstock somewhere along the run. It is needed to smooth changes in the rate of flow as the load on the turbine varies.’Lhe tank is designed to be about half full during normal, steady-state operations. If the gates suddenly open wider, calling for more water, the surge tank is drawn down momentarily Lo help meet the demand. When the gates close suddenly, the surge tank is evert more impor- tant: it gives the moving water somewhere to go gs it decelerates, preventing the hard knock called water hammer. . The environment in the generator gallery of a hydroelectric plant is calmer than the turbine hall of a fossil-fuel plant. Gone is the shriek of steam. The noises are all low notes—hurns, buzzes, groanings, rhythmic vibrations that you feel rather than hear. Workcrs—if there are any—can converse as quietly as in an office. The control of a hydroelectric plant is also less hair-raising than chat of either a fossil-fuel or a nuclear plant. Power output is regulated by gates that control the flow of water through the penstock and into the turbine. An automatic governor system adjusts the gates to track variations in load and keep the generator turning at a constant speed. POWER PLANTS One of the features of hydroelectric plants most welcome to power dispatchers is thac they can he started up and shut down at a moment’s notice. It takes as little as two minutes Lo get a unit up lo speed and synchronized with the power grid. This makes hydroelectric power attractive as a means of satisfying short-term peak loads. When you come home in the evening and switch on the lighls and the TV, some- where a gale in a penstock has opened very slightly and sent a few gallons more down the penstack OTHER ENERGY SOURCES Fossil-fuel plants, nuclear reactors, and hydroelectric plants account for 99 percent of the electric power generated by utility companies in the United States. Everything else all the “alternative” energy technologies—amount to just 1 percent, and so they ate prelty marginal in economic ternis. But the alternative energy sources have a com- spicuous place in the landscape and in public consciousness, even if they don’t yet make much of a dent in the cnergy budget. And their contributions are growing. Three of these technologies are described here: wind power, solar power, and geothermal power, Blowin’ in the Wind, Wind power, likc waterpower, has a long history. The Old World windmill, with its broad cloth-covered blades, or sails, goes back at least 800 years. But wind technology has been evolving rapidly in recent decades, and mod- cra windmulls look nothing like their ancient prototypes. They are tall and spindly, with narrow bladcs like those of an airplane propellcr but on a vastly larger scale. ‘Generators at Haover Dam are mounted with the shalt vertical. Each generator has 60 pairs of magnetic poles and turns at 60 revalutions per minute to produce 60° hertz alternating current. The eight generators seen here are on the California side of the dam; there are nine more an the Nevada side  218 A wind form near Tehachapi Poss in California had more than 600 turbines spinning when this photograph was made in 1999, The machines are planted like orchard treas in a rectangular array; many other wind farms line the machines up along ridgas INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE expensive, and so a design with fewer of chem might be expected to reduce the cost of the machine. The minimum number, obviously, is one, and one-bladed rotors have actually been tried. They look funny, to say the least; even though 2 small counter- weight keeps the machinery in balance, the visual impression is of something dra- matically out of kilter. But that’s not the big problem with oue-bladed designs; more serious is that the onc-bladed rotor has to turn faster to produce the same energy output as a turbine with morc blades, and higher speed brings more strain and noise. Two-bladed rotors have a subtler problem. The balance of the blades is perfect, but trouble comes whenever the wind shifts direction and the turbine has to swivel—or yaw—to stay pointed into the wind. When both blades are vertical, there’s no resis lance ta yawing, but as the blades turn toward the horizontal, the inertia increases. This cyclic change in resistance to yawing—going from maximum to minimum twice in every revolution—creates vibration and stress, shortening the life of the blades. The vast majority of modern wind turbines have exactly three blades. Apparently three is just enough to solve the problems of speed, balance, and vibration; any more than three would be a needless expense. POWER PLANTS Stil another contentious issue in wind-turbine design is whether to mount the blades on the upwind or the downwind side of the machine. Letting the blades wail behind the rest of the nubine has one big advantage: the blades can act like a weath- er vane, automatically turning the machine to face into the wind. When the blades are mounted in front, some land of steering mechanism is necded to sense the wind and forcefully pivot the turbine whenever the dircction shifts. Nevertheless, the prevailing design has the rotor in the font, with a complex power-stecring unit to keep it prop- erly pointed. The reason is one J never would have guessed: with a rear- mounted rotor, the turbine blades pass through the “wind shadow” of the cower structure on every revolution. The result is a cyclic variation in wind force that can set the blades vibrating, thus creating yel another source of fatigue and premature failure, All this leads to a portrait of the typical wind turbine. Ir has three blades, cach about 50 feet long, made of carbon fiber or some other lightweight ultrastrong mate- rial. The blades are attached to a hub, which in turn pokes out the front of a stream- lined housing called a nacclle. Inside the nacelle, which is the sizc of a moving van, are the generator, a gearbox, and other machinery necded to control the turbine. The nacelle is mounted atop a hollow stecl pylon, 100 feet high, 20 feet in diarneter at the base, and tapering gradually coward the pinnacle. On top of the nacelle you might notice a small airplane-shaped weather vane—just like the ones you see on suburban lawns. This is the sensor for the mechanism that keeps the turbine facing into the wind. Elsewhere on the wind farm, scattered among the massive turbines, are uny, spinning cups of anemometcrs on tall masts. These instruments are there to keep records of wind speed for use in analyzing wrbine per- formance and also to shut the turbines down if winds approach dangerous levels. Every wind turbine is designed for a limited range of wind speeds. Too little wind, and it’s not worth starting up. Too much, and the machinc could destroy itself. On some turbines the blades cap be “feathered,” or twisted so that the wind won't spin the rotor, when speeds get into the danger range. Others have aerodynamic “spoil- ers” with the same purpose. The final line of defense is a mechanical brake that binds the main shaft—but the opcrators at a Tehachapi wind farm told me they're not cager to use that one. Climbing the tower in a gale to tighten down the brake is more excitement than they're looking for. In normal operation most wind turbines spin at a fixed rate. You might think they would speed up and slow down as the wind varics, but instead they are designed to adjust the pitch of the blades so that the speed stays constant even as the energy out- put changes. Running at a constant speed makes it easicr to maintain the steady fre- quency of the alternating current that the turbine supplies to the power grid. The speeds are slow enough that you can count the revolutions. At one big wind farm in northern California [ found that the turbines were making 40 turns per minute; at another farm down the road the speed was 72 revolucions per minute. Most wind turbines turn clockwise, as seen from the hub side of the retor. But there’s no fundamental reason for this, and a few machines spin the other way. The three-bladed propeller of the wind turbine drives a generator inside the nacelle, atop the mounting mast. Within the range of operating wind velocities, the tur- bine rotates at constant speed, adjusting the pitch of the propeller blades to regulate power oulput.  220 LONG BEFORE THE WIND FARM, The multibladed, pinwheellike farm windmill was an American invention in the middle of the nineteenth century that become an icon of American rural life, By the 1890s the windmills were a standard item in the Sears catalogue, and traveling salesmen peddled dozens of brands to farmers throughout the Midwest and Southwest. An estimated 100,000 of them are still at work in the United States, mostly pump- ing water on ranches in the western states. In aggregate they may put oul 250 megawatts. The most fomous brand of farm windmill is the Aermotor, designed by Thomas ©. Perry At the peak of production in the 1890s some 20,000 per year were being made. The com- pany is still in business, in San Angelo, Texas, where they manufacture about 500 windmills anaually, Most farm windmills are erected directly over o well shaft. A crank arm connected to the fan wheel operates a piston down in the well tube. Some later models have gearing to reduce the speed of the pump and increcse the force available. The windmills come in many sizes, but the most common ones have a rotor eight feet in diameter and can pump up to 10 gallons per minute. As with other styles of wind machines, the big challenge in building «i farm windmill is making sure it doesn’t fly to pieces when the INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE The most unusual of all wind-turbine designs is the vertical-axis machine devel- oped by D, G, M, Darrieus. Instead of an airplane propeller, it’s an eggbeater; a ver- tical shaft with two thin blades bent into bow shapes so that they can be attached at the top and the bottom. ‘The big advantage of the Durrieus design is that it responds equally well to wind from any direction, with no need to pivot when the wind shifts, Also, the generator can be mounted at ground level, which makes it more convenient for maintenance and allows a lighter structure. Nevertheless, the design seems to have gone out of fashion. In the northern California wind farms, the few Darrieus machines still running were looking pretty ticed and careworn when I last saw them. A large wind farm, with hundreds of turbines, makes a powerful visual impression. Froin a great distance, they look like cheerful daisies or sunflowers planted in neat THE FARM WINDMILL middle of the winter. A number of tricks were tried (including hinged towers that fold in half to bring the works down to ground level), but nevertheless there are a lot of squeaky old windmills out there. The Fairbonks-Morse New Eclipse model pic- tured below was still twirling cheerfully, despite @ peppering of bullet holes, when | photo- graphed it in northern California in 1999. wind blows too strongly. Over the years, designs were equipped with spring-loaded vanes or centrifugal weights or other contrap- lions to furl the blades or turn the fan wheel parallel to the wind whan the speed reaches dangerous levels. Another engineering issue is the need to grease the bearings of a windmill mounted atop a tower 20 or 30 feet tall. Nobody ever wanted to climb up there in the POWER PLANTS you can’t produce steatn to turn a turbine and generator. Most flat-place collectors are rooliop installations used for water heating and space heating. To reach higher termperatures, you have to cancentrate the sunlight, collecting over a wide area, and focusing it on a smaller patch. In principle, lenses might be used to do the focusing, but in practice it’s always done with mirrors. The ideal shape for a reflective solar collector is a parabola, because this curve has the property that parallel rays of sunlight striking the mirrored surface are all reflected to the same focal poinc. One style of collector is a long trough with a parabolic cross section, mirrored on the upward-facing surface so as to focus sunlight on a tube that runs parallel to the trongh at just the right position to receive all the concentrated light. The biggest installation of parabolic troughs in the United States is at Kramer Junction, a cross~ roads in the southern California desert near the city of Barstow. Each trough is about 15 feet across and 150 feet long, assembled from 300 curved glass panes. Altogether there are 546,000 panes, with a total area of one square kilometer (about 25: At the focus of each trough is a receiver pipe housed in a glass vacuum tube to retain heat. The receiver pipe is painted black for best absorption, but when the plant is operating and the sun is shining, the pipe glows bright white, like a fluorescent lamp. The receiver pipes are filled with oil, which is heated under pressure to more than 750 degrees Fahrenheit. The hot oil is pumped to a heat exchanger, where ic gener- ates steam at 670 degrees; the steam then runs a fairly conventional turbine and gen- erator. The Kramer Junction solar array is divided into five independent plants, each capable of producing 30 megawatts of electricity in full summer sun. It would be more efficient to run one big unit rather than five small ones, but at the time the plant was built, tax incentives for solar power were limited to plants of 30 megawatts ar less. acres). 223 Each trough collector at Kramer Junetion has a para- bolic cross section, which concentrates the sun’s rays on a tube installed at the focus of the parabola, The tube is painted black, but it glows white whan the collector is operating, Oil pumped through the collectar tubes gath- ers the solar heal and generates steam to run a turbine, 224 The Solar One and Solar Two projects at Daggett, California, achieved higher temperatures by focusing the sun’s light on e point rather than a line. The last of the California experiments was shut down in 1999, but the program continues in Spain, INFRASTRUCTURE: A FIELD GUIDE TO THE INDUSTRIAL LANDSCAPE ‘The woughs at Kramer Junction are aligned an a north-south axis, and during the course of the day they tilt, facing east in the morning, then din solar noon, and finally turning toward the west at sunset. The tracking is done auto- matically with a sensor that tries to keep the focused image of the sun centered on the receiver pipe, Standing near a trough, you can hear it adjust itself every few seconds. With 250 acres of glass in the middle of a dusty desert, washing the mirrors is a full-time job. It’s done at night, with high-pressure hoses. It takes about two weeks to wash off the entire field of collectors; then the washing starts over again. Even with a large parabolic tough, the temperatures are not as high as power- plant engineers would like to see for maximum efficiency. To reach still higher temn- peratures, the trick is to focus the sun’s light not on a line of pipes but on a single point. One way to do this is with a mirror in the shape of a paraboloid, like a satcl- hitc-dish antenna or a radio telescope, but building a really big paraboloidal mirror that can tilt ta track the sun is an engineering challenge. A better idea is to set out lots of small mirrors, called heliostats, which can be adjusted individually so they all reflect sunlight onto the sarne point. According to legend, the principle was invent- ed by Archimedes in 212 BC, when he had a troop of Greek soldiers at Syracuse use their bronze shiclds as heliostats to burn the ships of an invading Roman fleet. Heliostats were the basis of the Solar One project at Daggett, another town near Barstow in southern California. Some 1,800 flat mirrors, with a total area of 17 acres, were continually adjusted so that they all reflected the sun’s image onto a black receiver at the top of a 300-foot tower. Water purnped through the receiver boiled to produce steam at about 900 degrees Fahrenheit, which then drove a turbine and gen- erator at the base of the tower. Solar One opcrated as a pilot project in the 1980s. Later, a new receiver was fitted to the tower and the plant was recommissioned as Solar Two. Instead of boiling water directly, the solar energy was now absorbed into overhead at local POWER PLANTS molten salt, which could be stored for a few hours so the plant could continue gen- erating electricity even after sunset. Solar Two was shut down in 1999. A similar plant called Solar Tres is under construction in Cordoba, Spain. Photovoltaic technology has almost nothing in common with solar-thermal power beyond the basic fact that both rely on sunlight as the ultimate source of enex- gy. A photovoltaic device dispenses entirely with boilers, turbines, and generators; it converts light directly into electricity in one step. The transformation is accomplished with no moving machinery. All the complexity is hidden in the microscopic struc - ture of the photocells, which are high-tech products of the serniconductor industry. The photoelectric effect was first noticed more than 150 years ago, and the first good explanation came from Einstein in 1905 (that’s what he won his Nobel Prize for—not for relativity thcory). The key idea is that light comes in packets, or parti- cles, called photons, cach of which carties some definite cnergy. Ifa photon’s energy is great enough, it can kick an electron out of its stable orbit inside an atom, making the electron available to carry an electric current. These forced evictions are hap- pening all the time; a coin sitting in the sunshine is seething with liberated electrons But in most cases all the activity comes to naught because the electrons just wander around for a while and then fall back into ihe atomic orbits they came from. A photovoltaic cell is designed to capture the ejected clectrons and put them to use. Most photovoltaic cells are made of silicon, and if you can get a closeup look, yon may find them to be quite beautiful objects, with crystal facets like frost on a win- dowpanc, in various shades of blue. Stripes or grids of metal electrodes are laced across the surface to collect the electric current. The output of a photovoltaic collector is direct current (DC) rather than the alter- nating current (AC) of the national power grid, Also, the voltage produced by an individual cell is closer to that of a flashlight battery than that of a power-plant gen- erator. Thus, connecting a pancl of cells to the utility grid calls for special electron- ics to boost the voltage and to convert from direct to alternating current. Then there’s the rnatter of cost. Even though the fuel is free, photovoltaic power remains substantially morc expensive than electricity from caal-fred power plants. As a result, you are most likely to see arrays of photocells in places where utility lines haven't reached—powering emergency telephones along highways, powering the lights on marine buoys, powering remote homesteads. And most remote of all are the many spacccraft that have relied on photovaltaic power. The cost of photocells has been coming down steadily for two or three decades, and interest is finally growing in utility-scale projects. The pioneer in this field in the United States is the Sacramento Municipal Utility District in California, which oper- ates more than eight megawatts of photovoltaic collectors. One big array of photocells js next to a decommissioned nuclear plant, but most of the collecturs sre distributed around the utility’s territory on residential rooftops and in parking lots. It’s sometimes said that to run the country on solar powcr we'd have to pave the whole landscape with collectors. It’s not nearly that bad. According to one estimate, 225 Photoveltaic cells, which generate electricity directly from sunlight, are made of polycrystallina silicon. The array of cells above is installed at Montgomery College in Germantown, Maryland. & detail, below, shows the grain structure of the silicon material and the grid of metallic conductors laid dawn over it to collect current.