Energy conservation and transformation
The concept of energy conservation
A fundamental law that has been observed to hold for all natural phenomena requires the conservation of energy—i.e., that the total energy does not change in all the many changes that occur in nature. The conservation of energy is not a description of any process going on in nature, but rather it is a statement that the quantity called energy remains constant regardless of when it is evaluated or what processes—possibly including transformations of energy from one form into another—go on between successive evaluations.
The law of conservation of energy is applied not only to nature as a whole but to closed or isolated systems within nature as well. Thus, if the boundaries of a system can be defined in such a way that no energy is either added to or removed from the system, then energy must be conserved within that system regardless of the details of the processes going on inside the system boundaries. A corollary of this closed-system statement is that whenever the energy of a system as determined in two successive evaluations is not the same, the difference is a measure of the quantity of energy that has been either added to or removed from the system in the time interval elapsing between the two evaluations.
Energy can exist in many forms within a system and may be converted from one form to another within the constraint of the conservation law. These different forms include gravitational, kinetic, thermal, elastic, electrical, chemical, radiant, nuclear, and mass energy. It is the universal applicability of the concept of energy, as well as the completeness of the law of its conservation within different forms, that makes it so attractive and useful.
Transformation of energy
An ideal system
A simple example of a system in which energy is being converted from one form to another is provided in the tossing of a ball with mass m into the air. When the ball is thrown vertically from the ground, its speed and thus its kinetic energy decreases steadily until it comes to rest momentarily at its highest point. It then reverses itself, and its speed and kinetic energy increase steadily as it returns to the ground. The kinetic energy Ekof the ball at the instant it left the ground (point 1) was half the product of the mass and the square of the velocity, or 1/2mv12, and decreased steadily to zero at the highest point (point 2). As the ball rose in the air, it gained gravitational potential energy Ep. Potential in this sense does not mean that the energy is not real but rather that it is stored in some latent form and can be drawn upon to do work. Gravitational potential energy is energy that is stored in a body by virtue of its position in the gravitational field. Gravitational potential energy of a mass m is observed to be given by the product of the mass, the height h attained relative to some reference height, and the acceleration g of a body resulting from the Earth’s gravity pulling on it, or mgh. At the instant the ball left the ground at height h1 its potential energy Ep1 is mgh1. At its highest point, its potential energy Ep2 is mgh2. Applying the law of conservation of energy and assuming no friction in the air, these add up to form the following equations:
In this idealized example the kinetic energy of the ball at ground level is converted into work in raising the ball to h2 where its gravitational potential energy has been increased by mg (h2 - h1). As the ball falls back to the ground level h1, this gravitational potential energy is converted back into kinetic energy and its total energy at h1 again is 1/2mv12 + mgh1. In this chain of events the kinetic energy of the ball is unchanged at h1; thus the work done on the ball by the force of gravity acting on it in this cycle of events is zero. This system is said to be a conservative one.
Varying degrees of conversion in real systems
Although the total amount of energy in an isolated system remains unchanged, there may be a great difference in the quality of different forms of energy. Many forms of energy, in theory, can be transformed completely into work or into other forms of energy. This is true for mechanical energy and electrical energy. The random motions of constituent parts of a material associated with thermal energy, however, represent energy that is not available completely for conversion into directed energy.
The French engineer Sadi Carnot described (in 1824) a theoretical power cycle of maximum efficiency for converting thermal into mechanical energy. He demonstrated that this efficiency is determined by the magnitude of the temperatures at which heat energy is added and waste heat is given off during the cycle. A practical engine operating on the Carnot cycle has never been devised, but the Carnot cycle determines the maximum efficiency of thermal energy conversion into any form of directed energy. The Carnot criterion renders 100 percent efficiency impossible for all heat engines. In effect, it constitutes the basis for what is now the second law of thermodynamics.
Robert L. Seale Charles R. Russell The Editors of Encyclopaedia BritannicaHistory of energy-conversion technology
Early attempts to harness natural forms of energy
Early humans first made controlled use of an external, nonanimal energy source when they discovered how to use fire. Burning dried plant matter (primarily wood) and animal waste, they employed the energy from this biomass for heating and cooking. The generation of mechanical energy to supplant human or animal power came very much later—only about 2,000 years ago—with the development of simple devices to harness the energy of flowing water and of wind.
Waterwheels
The earliest machines were waterwheels, first used for grinding grain. They were subsequently adopted to drive sawmills and pumps, to provide the bellows action for furnaces and forges, to drive tilt hammers or trip-hammers for forging iron, and to provide direct mechanical power for textile mills. Until the development of steam power during the Industrial Revolution at the end of the 18th century, waterwheels were the primary means of mechanical power production, rivaled only occasionally by windmills. Thus, many industrial towns, especially in early America, sprang up at locations where water flow could be assured all year.
The oldest reference to a water mill dates to about 85 bce, appearing in a poem by an early Greek writer celebrating the liberation from toil of the young women who operated the querns (primitive hand mills) for grinding corn. According to the Greek geographer Strabo, King Mithradates VI of Pontus in Asia used a hydraulic machine, presumably a water mill, by about 65 bce.
Early vertical-shaft water mills drove querns where the wheel, containing radial vanes or paddles and rotating in a horizontal plane, could be lowered into the stream. The vertical shaft was connected through a hole in the stationary grindstone to the upper, or rotating, stone. The device spread rapidly from Greece to other parts of the world, because it was easy to build and maintain and could operate in any fast-flowing stream. It was known in China by the 1st century ce, was used throughout Europe by the end of the 3rd century, and had reached Japan by the year 610. Users learned early that performance could be improved with a millrace and a chute that would direct the water to one side of the wheel.
A horizontal-shaft water mill was first described by the Roman architect and engineer Vitruvius about 27 bce. It consisted of an undershot waterwheel in which water enters below the centre of the wheel and is guided by a millrace and chute. The waterwheel was coupled with a right-angle gear drive to a vertical-shaft grinding wheel. This type of mill became popular throughout the Roman Empire, notably in Gaul, after the advent of Christianity led to the freeing of slaves and the resultant need for an alternative source of power. Early large waterwheels, which measured about 1.8 metres (6 feet) in diameter, are estimated to have produced about three horsepower, the largest amount of power produced by any machine of the time. The Roman mills were adopted throughout much of medieval Europe, and waterwheels of increasing size, made almost entirely of wood, were built until the 18th century.
In addition to flowing stream water, ocean tides were used to drive waterwheels. Tidal water was allowed to flow into large millponds, controlled initially through lock-type gates and later through flap valves. Once the tide ebbed, water was let out through sluice gates and directed onto the wheel. Sometimes the tidal flow was assisted by building a dam across the estuary of a small river. Although limited in operation to ebbing tide conditions, tidal mills were widely used by the 12th century. The earliest recorded reference to tidal mills is found in the Domesday Book (1086), which also records more than 5,000 water mills in England south of the Severn and Trent rivers. (Tidal mills also were built along the Atlantic coast in Europe and centuries later on the eastern seaboard of the United States and in Guyana, where they powered sugarcane-crushing mills.)
The first analysis of the performance of waterwheels was published in 1759 by John Smeaton, an English engineer. Smeaton built a test apparatus with a small wheel (its diameter was only 0.61 metre) to measure the effects of water velocity, as well as head and wheel speed. He found that the maximum efficiency (work produced divided by potential energy in the water) he could obtain was 22 percent for an undershot wheel and 63 percent for an overshot wheel (i.e., one in which water enters the wheel above its centre). In 1776 Smeaton became the first to use a cast-iron wheel, and two years later he introduced cast-iron gearing, thereby bringing to an end the all-wood construction that had prevailed since Roman times. Based on his model tests, Smeaton built an undershot wheel for the London Bridge waterworks that measured 4.6 metres wide and that had a diameter of 9.75 metres. The results of Smeaton’s experimental work came to be widely used throughout Europe for designing new wheels.
During the mid-1700s a reaction waterwheel for generating small amounts of power became popular in the rural areas of England. In this type of device, commonly known as a Barker’s mill, water flowed into a rotating vertical tube before being discharged through nozzles at the end of two horizontal arms. These directed the water out tangentially, much in the way that a modern rotary lawn sprinkler does. A rope or belt wound around the vertical tube provided the power takeoff.
Early in the 19th century Jean-Victor Poncelet, a French mathematician and engineer, designed curved paddles for undershot wheels to allow the water to enter smoothly. His design was based on the idea that water would run up the surface of the curved vanes, come to rest at the inner diameter, and then fall away with practically no velocity. This design increased the efficiency of undershot wheels to 65 percent. At about the same time, William Fairbairn, a Scottish engineer, showed that breast wheels (i.e., those in which water enters at the 10- or two-o’clock position) were more efficient than overshot wheels and less vulnerable to flood damage. He used curved buckets and provided a close-fitting masonry wall to keep the water from flowing out sideways. In 1828 Fairbairn introduced ventilated buckets in which gaps at the bottom of each bucket allowed trapped air to escape. Other improvements included a governor to control the sluice gates and spur gearing for the power takeoff.
During the course of the 19th century, waterwheels were slowly supplanted by water turbines. Water turbines were more efficient; design improvements eventually made it possible to regulate the speed of the turbines and to run them fast enough to drive electric generators. This fact notwithstanding, waterwheels gave way slowly, and it was not until the early 20th century that they became largely obsolescent. Yet even today some waterwheels still survive; in the early 1970s there were more than 1,000 grain mills in use in Portugal alone. Equipped with submerged bearings, these modern waterwheels certainly are more sophisticated than their predecessors, though they bear a remarkable likeness to them.
