chapter  6
8 Pages

Electricity from falling water

Turning an electrical generator reliably and at low cost requires a source of energy that is itself reliable and inexpensive. One source of cheap, reliable rotary motion is the waterwheel. Work provided by a waterwheel could be used to operate a generator. Two changes occur in such a system. In the waterwheel, water flows spontaneously from high to low gravitational potential energy. When this happens, some energy becomes available for doing work. The generator causes a non-spontaneous movement of electrical charge from low to high electrical potential energy-from low voltage to high voltage. Waterwheels used in medieval times were not very efficient at

converting kinetic energy into work, because the water still possesses significant kinetic energy even after it leaves the wheel. The quest for more efficient waterwheels began around the time of the Renaissance. Leonardo da Vinci recognized that water flowing through curved blades, rather than flat ones, bears against those blades with a force or impulse. Such a device was at first called an ‘impulse wheel.’ Nowadays it is generally known as a turbine. A turbine is a device that converts the kinetic energy of a moving fluid into rotating kinetic energy. The fluid used in the turbine is called the working fluid. For the kinds of turbine discussed in this chapter, the working fluid is water. Later we will meet an

extremely important family of turbines in which the working fluid is steam. In a turbine operated by water, the water flows through pipes or

ducts over curved vanes. To provide the impulse to the vanes, the turbine has to be enclosed in a casing. The higher the pressure of water supplied to the turbine-essentially, the higher the column of water, or its ‘head’— the faster the turbine will spin. The rotational speed of a waterwheel is limited by the speed of the stream. This is not so with turbines, where it is easy to get rotational speeds as high as 30,000 revolutions per minute. The development of the turbine during the mid-nineteenth century was a major engineering advance in providing a source of work for the electrical generator. In the case where the kinetic energy of water is used to drive the turbine/generator set, the technology is referred to as hydroelectricity or hydropower. The term ‘hydroelectricity’ does not imply that the electricity is somehow different from electricity generated in other ways. The prefix ‘hydro-’ is used to remind us that water is the fluid used to operate the turbine. The essential feature of the turbine that allows it to capture the

kinetic energy of the water is the flow of the water over curved vanes. The energy is captured by the ‘impulses’ or ‘reactions’ between the fast-flowing water and the passages created by the curvature of the vanes. These turbine designs will work only if water is confined, and therefore the passageways for the water created by the vanes are enclosed in an external, closely fitting casing. A well-designed and operated version of this turbine can convert more than 90% of the kinetic energy of the water into rotary motion, at high rates of revolution. The hydroelectric plant at Fully, Switzerland, illustrates some of

the remarkable features of these installations. At the Fully plant, water is led downwards from a lake through a pipe almost 5 km long. In that distance, the pipe drops in elevation by nearly 2 km. This very steep gradient results in the water, at the lower end of the pipe, having a pressure of 14 MPa-that is, a pressure of a 140 kilograms per square centimeter. Water leaves the nozzle at a speed of nearly 650 kilometers per hour. (To indicate some of the extraordinary properties of water under these conditions, a water jet coming out of a 7.5-cm pipe at a pressure of only one-quarter of the value in the Fully plant, 35 kg/cm2, could not be cut through

or deflected even by hitting it with a steel crowbar.) The turbine wheels are 3.5 meters in diameter. Water at this pressure and velocity spins the wheels at a rate of 500 revolutions per minute. The world’s largest hydroelectric plant, the Three Gorges plant

in China, represents the largest engineering project of any kind ever undertaken anywhere. It has an intended generating capacity of 22,500 megawatts (MW). To put this figure into perspective, the output of the Three Gorges hydroelectric plant is some twenty times larger than that of a large coal-fired or nuclear electrical plant. At the other extreme of scale from plants like Fully and Three Gorges are many small-scale hydroelectric plants. Such installations have various names, intended to convey a notion of their small size. Customarily, small hydro plants generate less than 10 MW; minihydro, less than 1 MW; and micro-hydro, less than 100 kW. All use the same concepts of turbine design and operation. Small-scale projects that don’t require constructing dams to form large reservoirs of water represent an aspect of hydropower that is growing in popularity around the world. Small hydro plants that can feed electricity into a regional or national grid represent a source of relatively inexpensive, renewable energy. This can be attractive in countries that mandate the generation of a set fraction of electricity from renewable resources. Some countries in the developing world find the installation of ‘mini-hydro’ plants to be an economically attractive alternative to expanding nationwide electricity grids to connect a few large, expensive electricity-generating stations. Micro-hydro projects are popular because their output is sufficient for a small community or small commercial installation. At the small end of the micro-hydro range are units that would serve one, or a few, homes. Micro-hydro units can be set up to generate electricity in rural regions that have no connections to an electric grid. They can also be installed used by those who choose to disconnect from the electric grid. Vendors in several countries now offer virtually complete, commercial packages for those who want a source of electricity, or who want to live ‘offgrid.’ Many can be installed as a do-it-yourself project. Worldwide, hydropower provides about 16% of electricity

demand, equivalent to about 4% of total energy demand. World electricity production from hydropower plants is about 3,400 billion kW. Hydropower has a number of advantages. Compared with fossil fuels, hydropower is renewable; that is, highly unlikely to be

depleted on any human time scale. While we will likely someday experience shortages of fuels, there is little likelihood that rivers will dry up. Water will continue to flow through turbines. Hydropower is highly efficient. About 90% of the kinetic energy of water is converted to electrical energy (as compared, for example, with 30-35% when electricity is generated in coal-fired power plants). Hydropower is highly reliable mechanically, since the only moving parts are the turbine and the generator; as a result, maintenance costs are low. Unlike fossil-fuel or nuclear plants, there are no waste products and virtually no pollution, including no carbon dioxide emissions. Another contrast to fossil-fuel or nuclear plants is that the energy source, the water, is free-there is no recurring cost to buy fuel. If opportunities exist for the development of hydropower, regions or countries that do not have indigenous fossil fuel resources can still develop heavy industries on a large scale. From the plant operator’s perspective, an important advantage of hydropower is that it can easily be ramped up and down, or even switched on and off entirely (by diverting water onto or away from the turbines), making it an excellent choice for providing extra electricity at times of peak demand or in energy emergencies. Another virtue of hydropower is economic. In most places,

hydropower has always been one of the least expensive sources of electricity. Although the initial capital investment required to build a new hydropower installation can be very high (though estimated to be slightly less than a typical coal-fired plant), once the plant is built and running, the recurring costs for operations and for maintenance are usually quite low. The potential exists for using hydropower to produce about five

times the total output of all the world’s present power stations. While this may sound very attractive and promise a great future for hydro, many potential sites for new development are places that are inaccessible, far from potential consumers and markets for electricity, and often both. In many developed countries, there is growing public concern about the environmental effects of constructing dams to provide hydroelectric sites. In some developing nations, construction of dams and their large reservoirs could cause many people to have to be displaced from their homes. With these issues taken into consideration, it is likely that no more than about half of the world’s potential hydro capacity will ever really be developed.