Hydroelectric Energy

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The ocean is a body of water occupying about two-thirds of a world made for man - who has no gills. ~ Ambrose Bierce (1842 - 1914)

Oceans cover more than 70% of the earth’s surface, making them the world’s largest source of hydro energy. There are many different ways to extract energy from water. Seawater is the source of deuterium, the ideal fuel for nuclear fusion. Surface water stores a massive amount of solar energy that can be exploited to design thermal power plants. In addition, water contains mechanical energy that can be converted to useful work in the form of the kinetic energy of river streams, or the potential energy of waterfalls, tides, and ocean waves. According to some estimates, these resources have the potential to produce 1-2 terawatts of electricity. This amount is enough to cover the energy demands of the entire globe, but tapping into most of that potential is not yet economically feasible. In this chapter, we will investigate how the mechanical energy stored in water can be used to design practical devices. Thermal and nuclear conversion options are discussed in Thermal Energy and Nuclear Energy respectively.


Hydroelectric Energy

 Vertical water mills; (a) undershot, and (b) overshot
Figure 1: Vertical water mills; (a) undershot, and (b) overshot
 Water wheel configurations: (a) horizontal and (b) vertical
Figure 2: Water wheel configurations: (a) horizontal and (b) vertical

Egyptians harnessed energy from flowing water about 2,000 years ago by turning waterwheels to grind their grain. These primitive devices allowed the force of falling water to act on a waterwheel and provide rotational energy or shaft power. Through the centuries, mechanisms were designed to facilitate many other applications beyond the simple grain mills of the Egyptians. By the time of the Industrial Revolution, water power was driving tens of thousands of waterwheels. Today, hydropower is the most widely available renewable energy. It is exploited almost exclusively for generating electric power, providing about 20% of all electricity used around the world and 10% of the US electrical capacity.

Question: Two medieval varieties of waterwheels were undershot and overshot wheels (1), (2) (Figure 1). Undershot refers to a paddle wheel fixed to the bank of a river or hung from an overhead bridge. It is turned by the impulse of the water current. Overshot water mills work by bringing a stream of water through a pipe or canal and pouring it onto the wheel from above. Which of the designs seems to have a higher efficiency?

Answer: Overshot wheels. The weight of the water falling on the blades (gravity) forces the wheel to turn at a faster speed. Furthermore, undershot wheels require a steady wave current, so their operation could be interrupted when water levels drop during the summer months.

Question: Watermills come in vertical and horizontal configurations (Figure 2). What are the advantages and disadvantages of each configuration?

Answer: Most early watermills were used solely for grinding wheat, barley and other seeds using a flat rock (stone mill). Horizontal mills were easiest to operate as they could directly turn the stone mill. The vertical designs required gears to transfer power to the stone mill and thus were not very convenient.


(1) Hodges, H., “Technology in the Ancient World,” Barnes & Noble, N.Y., 1992.

(2) Reynolds, J. Windmills and Watermills, Praeger, New York, 1970.

Further Reading

Bose, N. and Brooke, J., Wave Energy Conversion, Elsevier, 2003.

Ross, D., Energy from the Waves, Oxford University Press, 1995.

Cruz, J., Ocean Wave Energy: Current Status and Future Perspectives, Springer Series in Green Energy and Technology, Springer-Verlag, Berlin, 2008

International Journal of Wave Motion, Elsevier Science Publishing Company.

International Journal of Renewable Energy, Elsevier Science Publishing Company.

External Links

National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center (http://www.csc.noaa.gov).

European Commission on Tidal Energy (http://europa.eu.int/comm/energy_transport/atlas/htmlu/tidal.html).

OTEC, U.S. DoE, Energy Efficiency and Renewable Energy (http://www.eere.energy.gov/RE/ocean.html).

Wave Energy Council: Survey of Energy Resources (http://www.worldenergy.org/wec-geis/publications/reports/ser/wave/wave.asp).