KINETIC ENERGY, HISTORICAL EVOLUTION OF THE USE OF

Historically humans have used three natural sources of kinetic energy: wind, water, and tides.

THE ANCIENT WORLD (TO 500 c.e.)

Although early humans often inadvertently tapped into the kinetic energy of moving air or water to do things such as separate grain from chaff or float downstream, the deliberate use of kinetic energy to power machinery came only in the historical era.

Only one machine in classical antiquity made deliberate use of the kinetic energy of wind: the sailing vessel. As early as 3000 b.c.e., paintings illustrated Egyptian vessels using sails. By the first millenium b.c.e., the use of sails was common for long-distance, water-borne trade. However, ancient sails worked poorly. The standard sail was square, mounted on a mast at right angles to the ship's long axis. It was effective only if the wind was dead astern. It was barely adequate if the wind was abeam, and totally inadequate in head winds. As a result, ancient mariners usually timed sailings to correspond with favorable wind direction and often averaged only 1 to 1.5 knots.

Very late in the ancient period the square sail was challenged by a more effective design: the triangular lateen sail, aligned with the vessel's long axis (i.e., fore-and-aft). The origin of this rigging is uncertain; unlike square sails, lateen sails operated as fabric aerofoils and permitted vessels to sail more closely into headwinds.

The only evidence for the use of wind in antiquity to power other machinery occurs in the Pneumatica of Hero of Alexandria dating from the first century c.e. Hero described a toy-like device with four small sails, or blades, attached perpendicularly to one end of a horizontal axle. Wind struck the blades and turned the device. Pegs mounted on the opposite end of the axle provided reciprocating motion to a small air pump feeding an organ. There is no evidence that the idea was expanded to a larger scale, and some experts are suspicious of the authenticity of this portion of the Pneumatica.

Water, the other source of kinetic energy used in antiquity, saw wider application to machinery than did wind. The first evidence of the use of waterpower comes from the first century B.C.E., simultaneously in both China and the Mediterranean region. In China the preferred method of tapping the power of falling water was the horizontal water wheel, named after the plane of rotation of the wheel. Around the Mediterranean, the preferred form was the vertical water wheel. The vertical wheel came in two major forms: undershot and overshot. An undershot wheel had flat blades. Water struck the blades beneath the wheel and turned it by impact. The overshot wheel emerged later—the first evidence dates from the third century C.E. The overshot wheel's periphery consisted of containers, called buckets. Water, led over the top of the wheel by a trough, was deposited in the buckets; weight rather than impact turned the wheel.

The diffusion of waterpower was initially slow—perhaps due to its relatively high capital costs, its geographical inflexibility, and the abundance of manual labor in both the classical Mediterranean world and in China. Only in the declining days of the Roman Empire, for example, did watermills become the standard means of grinding grain in some areas, displacing animal- and human-powered mills.

THE MEDIEVAL WORLD (500–1500 c.e.)

Knowledge of how to tap the energy of wind and water was passed by both the Romans and Han Chinese to their successors. In the conservative eastern realm of the old Roman Empire (the Byzantine Empire), little was done to develop either wind power or water power, although Byzantine vessels did make increased use of lateen sails after the eighth century. On the other hand, the western part of the old Roman Empire, perhaps due to labor shortages, saw a very significant increase in the use of both wind and water power, especially between 900 and 1300.

The expansion of medieval Europe's use of natural sources of kinetic energy was most significant in waterpower. Although many Roman watermills were destroyed during the collapse of centralized authority in the fifth through seventh centuries, recovery and expansion beyond Roman levels was relatively rapid. By the eleventh century, sparsely populated England had over 5,600 watermills, a level of dependence on nonhuman energy then unparalleled in human history. Nor was England unusual. Similar concentrations could be found elsewhere. The Paris basin, for example, had around 1,000 watermills in the same era, and canal networks around medieval European cities were often designed to benefit mills.

In coastal estuaries, European craftsman by the twelfth century or earlier had also begun to use water wheels to make use of the movement of the tides. The usual arrangement required impounding incoming tides with a barrier to create a reservoir. During low tide, this water was released through a gate against the blades of a watermill.

With few exceptions, waterpower had been used in classical antiquity for only one purpose—grinding grain. By the tenth, and definitely by the eleventh century, European technicians had begun significantly to expand the applications of waterpower. By 1500 waterpower was used to grind not only wheat, but mustard seed, gunpowder, and flint for use as a glaze. It was used to crush ore, olives, and rags (to make paper). It was used to bore pipes, saw wood, draw wire, full (shrink and thicken) wool, pump water, lift rocks, shape metal, and pump bellows.

Nor was Europe the only civilization to make increased use of waterpower in this era. Although not blessed with the same abundance of stable, easy-to-tap streams, the Islamic world also increased its use of waterpower. Medieval travelers mention numerous mills and water-lifting wheels (norias) along rivers near Islamic cities such as Baghdad, Damascus, Antioch, and Nishapur. China, too, saw the expanded use of waterpower, for powering metallurgical bellows, grinding grain, spinning hemp, driving fans, crushing minerals, and winnowing rice.

The wood water wheels used in the medieval period were, by modern standards, inefficient. Medieval undershot and horizontal wheels probably had an efficiency of about 15 percent to 25 percent, medieval overshot wheels about 50 percent to 60 percent. Commonly, their power output was only about 2–5 hp. But relative to the alternatives available at the time—human or animal power—they offered a very substantial gain in power.

Water provided a more reliable source of energy than the winds, but labor shortages in medieval Europe also encouraged the further development of wind power. The sailing vessel continued to be the most important wind-powered machine, and it saw significant improvement. Between 1000 and 1500, Chinese and European shipbuilders began to use multiple masts, stern post rudders, and deeper keels to tap larger volumes of air more effectively, and Europeans by 1500 were making effective use of combined square and lateen riggings.

The first solid evidence of the use of air to provide mechanical power to something other than sailing vessels comes from tenth-century Islamic travel accounts that describe wind-powered mills for grinding grain in eastern Persia. These were horizontal windmills, so named because the plane of rotation of their sails, or blades, was horizontal. The rotors were two-story devices, surrounded by walls with openings facing the prevalent wind direction. Wind passed through these orifices, struck the exposed blades, and exited through other orifices in the rear. In heavy winds they might produce as much as 15 hp, but more typically their output was about 2 to 4 hp.

Diffusion was very slow. The earliest clear Chinese reference to a windmill occurs only in 1219. The Chinese adopted the horizontal rotor used in Central Asia, but equipped it with pivoted blades, like venetian blinds. These windmills did not require shielding walls like Central Asian mills, and could tap wind from any quarter.

European technicians departed much more radically from Central Asian designs. Drawing from close experience with the vertical watermill, by 1100–1150 they had developed a vertical windmill called a post mill. The blades, gearing, and millstones/machinery of the post mill were all placed on or in a structure that pivoted on a large post so operators could keep the mill's blades pointed into the wind. European windmills typically had four blades, or sails, consisting of cloth-covered wooden frames. They were mounted on a horizontal axle and set at a small angle with respect to their vertical plane of rotation.

The European windmill diffused rapidly, especially along the Baltic and North Sea coasts. By the fourteenth century they had become a major source of power. Eventually, England had as many as 10,000 windmills, with comparable numbers in Holland, France, Germany, and Finland. In some areas of Holland one could find several hundred windmills in a few square miles.

Around 1300, Europeans improved on the post mill by devising the tower mill. In the tower mill, only a cap (containing the rotor axle and a brake wheel), mounted atop a large, stationary tower, had to be turned into the wind. The tower design allowed the construction of larger windmills containing multiple pairs of millstones, living quarters for the miller and his family, and sometimes machinery for sawing wood or crushing materials.

By the end of the medieval period, growing European reliance on wind and waterpower had created the world's first society with a substantial dependence on inanimate power sources.

THE EARLY MODERN ERA (1500–1880)

Dependence on wind or waterpower does not seem to have grown substantially in other civilizations after the medieval era, but this was not the case in Europe. By 1700, most feasible sites along streams convenient to some mercantile centers were occupied by water-powered mills, making it difficult for newer industries, such as cotton spinning, to find good locations. Moreover, European colonists carried waterpower technology with them to North and South America. By 1840 the United States, for example, had nearly 40,000 water-powered mills.

Pressure on available, easily tapped streams pushed the development of waterpower in several different directions simultaneously between 1700 and 1850. One direction involved the application of quantitative techniques to analyze wheel performance. In 1704 Antoine Parent carried out the first sophisticated theoretical analysis of water wheels. Erroneously believing that all wheels operated like undershot wheels, he calculated the maximum possible efficiency of water wheels at an astoundingly low 4/27 (15%). When John Smeaton in the 1750s systematically tested model undershot and overshot wheels, he discovered Parent's error. Smeaton found the optimum efficiency of an undershot vertical wheel was 50 percent (his model wheels actually achieved around 33 percent), and that of a weight-driven overshot wheel was much higher, approaching 100 percent (his model wheels achieved about 67–70%). Smeaton thereafter made it his practice to install overshot wheels where possible. Where impossible, he installed an intermediate type of vertical wheel called the breast wheel. With breast wheels, water was led onto the wheel at or near axle level. A close-fitting masonry or wooden casing held the water on the wheel's blades or buckets so it acted by weight (as in an overshot wheel) rather than by impact.

The mechanization of various elements of textile production, especially carding, spinning, and weaving, after 1770 created important new applications for waterpower. As textile factories grew larger, engineers modified the traditional wooden water wheel to enable it to better tap the kinetic energy of falling water. In addition to following Smeaton's example and using weight-driven wheels as much as possible, they replaced wooden buckets, with thinner sheet-iron buckets and used wrought iron tie-rods and cast iron axles to replace the massive wooden timbers used to support traditional water wheels. By 1850 iron industrial water wheels, with efficiencies of between 60 percent and 80 percent, had an average output of perhaps 15–20 hp, three to five times higher than traditional wooden wheels. Moreover, iron industrial wheels developing over 100 hp were not uncommon, and a rare one even exceeded 200 hp. The iron-wood hybrid wheel erected in 1851 for the Burden Iron works near Troy, New York, was 62 feet (18.9 m) in diameter, by 22 feet (6.7 m) wide, and generated around 280 hp.

Despite these improvements, in the more industrialized parts of the world the steam engine began to displace the waterwheel as the leading industrial prime mover beginning around 1810–1830. Steam was not able, however, to completely displace water-power, in part because waterpower technology continued to evolve. Increased use of mathematical tools provided one means of improvement. In the 1760s the French engineer Jean Charles Borda demonstrated, theoretically, that for a water wheel to tap all of the kinetic energy of flowing water, it was necessary for the water to enter the wheel without impact and leave it without velocity.

In the 1820s, another French engineer, Jean Victor Poncelet, working from Borda's theory, designed an undershot vertical wheel with curved blades. Water entered the wheel from below without impact by gently flowing up the curved blades. It then reversed itself, flowed back down the curved blades, and departed the wheel with no velocity relative to the wheel itself. Theoretically the wheel had an efficiency of 100 percent; practically, it developed 60 percent to 80 percent, far higher than a traditional undershot wheel.

In the late 1820s another French engineer, Benoit Fourneyron, applied the ideas of Borda and Poncelet to horizontal water wheels. Fourneyron led water into a stationary inner wheel equipped with fixed, curved guide vanes. These vanes directed the water against a mobile outer wheel, also equipped with curved blades. These curved vanes and blades ensured that the water entered the wheel with minimum impact and left with no velocity relative to the wheel itself. Moreover, because water was applied to the entire periphery of the outer wheel at once, instead of to only a portion of the blades, Fourneyron's wheel developed much more power for its size than a comparable vertical wheel could have. This new type of water wheel was called a water turbine. Its high velocity and the central importance of the motion of the water on the wheel (water pressure) to its operation distinguished it from the traditional water wheel.

By 1837 Fourneyron had water turbines operating successfully on both small falls and large ones. At St. Blasien in Germany, a Fourneyron turbine fed by a pipe, or penstock, used a fall of 354 feet (107.9 m), far more than any conventional water wheel could hope to. It developed 60 hp with a wheel only 1.5 foot (0.46 m) in diameter that weighed less than 40 pounds (18.2 kg).

Engineers quickly recognized that turbines could be arranged in a variety of ways. For example, water could be fed to the wheel internally (as Fourneyron's machine did), externally, axially, or by a combination. Between 1830 and 1850 a host of European and American engineers experimented with almost every conceivable arrangement. The turbines that resulted—the most popular being the mixed-flow 'Francis' turbine—quickly demonstrated their superiority to traditional vertical wheels in most respects. Turbines were much smaller per unit of energy produced, and cheaper. They could operate submerged when traditional wheels could not. They turned much faster, and they did all of this while operating at high efficiencies (typically 75% to 85%). By 1860 most new water-powered wheels were turbines rather than vertical wheels.

The continued growth and concentration of industry in urban centers, however, most of which had very limited waterpower resources, meant that steam power continued to displace water power in importance, even if the development of the water turbine delayed the process.

European engineers and inventors also made significant improvements in windmills between 1500 and 1850, but windmills fared worse in competition with steam than water wheels and water turbines.

The sails of the windmills that emerged from Europe's medieval period were inclined to the plane of rotation at a uniform angle of about 20°. By the seventeenth century, millwrights in Holland had improved the efficiency of some mills by moving away from fixed angles and giving the windmill blades a twist from root to tip (i.e., varying the inclination continuously along the blade's length) and by putting a camber on the leading edge, features found on modern propeller blades. Another advance was the fantail. Developed by Edmund Lee in 1745, the fantail was a small vertical windmill set perpendicular to the plane of rotation of the main sail assembly and geared so that a change in wind direction would power the fantail and rotate the main sails back into the wind. In 1772 Andrew Meikle developed the spring sail, which used hinged wooden shutters mounted on the blades, operating like venetian blinds, to secure better speed control. Other inventors linked governors to spring sails to automatically control wind speeds.

As with water wheels, engineers also began to carry out systematic, quantified experiments on windmills to test designs. In 1759 John Smeaton published a set of well-designed experiments on a model windmill. Since he had no means of developing a steady flow of air, he mounted 21-inch (53-cm) long windmill blades on a 5.5-feet (1.54-m) long arm that was pulled in a circle in still air to simulate the flow of air against a windmill. His experiments refuted prevalent theory, which recommended that the blades of windmills be inclined at a 35° angle. They confirmed that the 18–20° angle generally used in practice was far better. Smeaton's work also confirmed the even greater effectiveness of giving the blades a twist, that is, inclining sails at a variable angle like a propeller. By 1800 a large, well-designed Dutch windmill might develop as much as 50 hp at its axle in exceptional winds (normally 10–12 hp), although gearing inefficiencies probably reduced the net output at the machinery during such winds to around 10–12 hp.

Despite the empirical improvements in windmill design and attempts to apply quantitative methods to its design, the energy that could be tapped from winds was too erratic and had too low a density to compete with newly developed thermal energy sources, notably the steam engine. Following the development of an effective rotary steam engine by James Watt, and the introduction of mobile, high-pressure steam engines (locomotives) in the early-nineteenth century, that brought the products of steam-powered factories cheaply into the countryside, windmills began a rapid decline.

Only in limited arenas did the windmill survive. It continued to produce flour for local markets in unindustrialized portions of the world. It also found a niche—in a much-modified form—as a small power producer in isolated agricultural settings such as the Great Plains of North America. The American farm windmill of the nineteenth century was a vertical windmill, like the traditional European windmill, but it had a much smaller output (0.2-1 hp in normal breezes). Consisting of a small annular rotor usually no more than 6 feet (2 m) in diameter, with a very large number of blades to produce good starting torque, American windmills were mounted atop a high tower and usually used to pump water. By the early twentieth century several million were in use in America, and the technology had been exported to other flat, arid regions around the world, from Australia to Argentina to India.

THE MODERN ERA (1880–2000)

By the late nineteenth century, both wind and water were in decline as power producers almost everywhere. Steam-powered vessels had, in the late nineteenth century, begun to rapidly replace sailing vessels for hauling bulk merchandise. Only in isolated regions did sailing vessels continue to have a mercantile role. Increasingly, sailing vessels became objects of recreation, not commerce. In areas such as Holland, traditional windmills continued to pump away here and there, but they were on their way to becoming historical relics more than anything else. In the twentieth century even the American windmill went into decline as electric power lines belatedly reached rural areas and electric pumps replaced wind-powered pumps. Meanwhile, growing dependence on steam engines steadily reduced the importance of waterpower in all but a few favorable locations.

The emergence of electricity as a means of transmitting power late in the nineteenth century offered new life to devices that tapped the kinetic power of wind and water, since it offset one of the greatest advantages of steam power: locational flexibility. With electricity, power could be generated far from the point at which it was used.

Around 1890 Charles Brush erected a wind-powered wheel over 56 feet (17 m) in diameter to test the possibilities of wind as an economical generator of dc electricity. Coal- and waterpower-poor Denmark provided much of the early leadership in attempts to link wind machinery with the newly emerging electrical technology. The Danish engineer Poul LaCour, for example, combined the new engineering science of aerodynamics with wind tunnel experiments and new materials such as steel to try to develop a wind engine that would be a reliable and efficient generator of electricity. By 1910 he had several hundred small wind engines operating in Denmark generating dc electricity for charging batteries, but he had few imitators.

The development of the airplane stimulated intensive research on propeller design. In the 1920s and 1930s, a handful of engineers began to experiment with fast-moving, propeller-shaped windmill rotors (wind turbines), abandoning traditional rotor designs. For example, Soviet engineers in 1931 erected a 100 kW (about 130 hp) wind turbine on the Black Sea. In 1942 the American engineer Coslett Palmer Putnam erected the first megawatt-scale wind turbine designed to feed ac into a commercial electrical grid—a very large, two-blade rotor—at Grandpa's Knob in Vermont. It failed after 1,100 hours of operation, and cheap fuel prices after World War II limited further interest in wind-generated electricity.

The fuel crises and environmental concerns of the 1970s revived interest in the use of wind to produce electricity. Governmental subsidies, especially in the United States, supported both the construction of wind turbines and research, and led to the erection of large arrays of wind generators (wind farms) in a few favorable locations. By the 1990s, over 15,000 wind turbines were in operation in California in three mountain passes, generating a total of about 2.4 million hp (1,800 mW), but wind's contribution to total electrical energy supplies remains miniscule (<1%).

Waterpower fared better. When electrical engineers demonstrated the possibility of long-distance transmission of electrical power between 1875 and 1885, waterpower suddenly assumed new importance. Many of the best waterpower sites had been untapped because they were in remote regions far from manufacturing centers. As long as power had to be directly transmitted by mechanical shafts and belts, the locational flexibility of steam power made it the overwhelming first choice of industry. Electric power transmission made low-cost hydropower more competitive.

The first experiments with commercial water-generated electricity took place in the 1880s, but applications were limited since the predominant form of electricity generated, low voltage dc suffered large power losses in transmission. The introduction of high voltage ac around 1890, with its sharply reduced transmission losses, led to the first hydroelectric plants of significant scale. The spectacular success of the first hydroelectric plant at Niagara Falls in 1895, which used multiple turbine generator units, each developing 5,000 hp (3,750 kW) under a 136-foot head, and transmitting that power via high voltage ac some 22 miles to Buffalo, attracted enormous capital into the field. Between 1895 and 1930 large hydroelectric plants sprang up all across the United States and Europe. In some regions such as Norway and California, hydroelectricity became the dominant form of energy used.

The move toward gigantic hydroelectric developments accelerated in the 1930s, especially in the United States and the Soviet Union, as government-sponsored projects replaced privately funded projects. The Dnieprostroy hydroelectric plant in the Soviet Union, completed in 1932, was equipped with nine 85,000 hp turbine-generator units. The seventeen turbine-generators installed at Hoover Dam in the mid-1930s were even more powerful: rated at 115,000 hp (∼86 MW each). More recent hydroelectric plants have grown even larger. For example, Itaipú (1984–1991) on the Paraná River in South America has eighteen turbine-generator units, each with a capacity of around 940,000 hp (700 MW), for a total capacity of around 17 million hp (12,600 MW).

Although the continued growth of energy consumption and the fixed amount of energy available from waterpower has prevented it from returning to its position as humanity's primary source of inanimate power, waterpower, unlike wind power, continues to be an important producer of energy. Today, waterpower is responsible for nearly 20 percent of the world's electrical energy. In some regions where hydrological and terrain conditions are appropriate, for example in Canada, Norway, and New Zealand it supplies a much higher proportion.

The technologies that enabled waterpower to remain important in the twentieth century—turbines, high voltage ac, and large dams—were also applied to harness the tides. In a few select estuaries where tidal variations were significant and construction conditions appropriate, large dams with embedded turbine-generators impounded high tides and used the outflow during low tides to generate electricity. The largest of these was placed in operation in 1966 on the Rance River on the coast of Brittany in France, where an average tidal range of around 28 feet was available. At Rance, a 0.4 mile long dam equipped with twenty-four turbines, generates around 320 MW of electric power. The high capital cost of tidal plants, the very limited number of sites with sufficient tidal variation and suitable construction conditions, and environmental concerns sharply limit the use of this form of kinetic energy. Thus, tides do not produce a significant portion of the world's energy (much less than 1%).

Terry S. Reynolds

See also: Hydroelectric Energy; Smeaton, John; Turbines, Wind; Watt, James.

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