History:

Water energy has been with us for a very long time. The first recorded usage of waterpower was a clock built around 250 BC. Since that time, humans have used falling water to provide power for grain and saw mills, as well as a host of other applications. The first use of moving water to produce electricity was a waterwheel on the Fox River in Wisconsin in 1882, two years after Thomas Edison unveiled the incandescent light bulb. The first of many hydroelectric power plants at Niagara Falls was completed shortly thereafter. Hydropower continued to play a major role in the expansion of electrical service early in this century, both in North America and around the world. Contemporary Hydroelectric power plants generate anywhere from a few kW, enough for a single residence, to thousands of MW, power enough to supply a large city. Because the early hydro-electric plants were much more reliable and efficient than the fossil fuel plants of the day, small-medium hydro-electric plants started popping up all over. But as electricity demands soared in the mid 20th century these smaller plants went out of favor and most hydro-electric activity focused on "mega projects." The majority of which flood large areas of land to create a reservoir for continued electrical production. (S. Baird)

How it works:

Hydroelectric power plants capture the energy released by water falling through a vertical distance, and transform this energy into useful electricity. In general, falling water is channeled through a turbine, which converts the water's energy into mechanical power. The rotation of the water turbines is transferred to a generator, which produces electricity. The amount of electricity that can be generated at a hydroelectric plant is dependant upon two factors. These factors are (1) the vertical distance through which the water falls, called the "head", and (2) the flow rate, measured as volume per unit time. The electricity produced is proportional to the product of the head and the rate of flow. (S. Baird)

Based on that, hydroelectric plants can generally be divided into two categories, "high head" and "low head" plants.

"High head" power plants are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in the consistent and reliable production of electricity, able to meet demand. Heads for this type of power plant may be greater than 1000 m. Most large hydroelectric facilities are of the high head variety. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system. (S. Baird)

"Low head" hydroelectric plants are power plants that generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam or weir to channel water, or no dam and simply use the "run of the river". Run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power. Hydroelectric facilities with a capacity of less than about 25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small hydro", although hydroelectric technology is basically the same regardless of generating capacity. (S. Baird)

Environmental concerns:

Hydroelectric power plants have many environmental impacts, some of which are just beginning to be understood. These impacts, however, must be weighed against the environmental impacts of alternative sources of electricity. Until recently there was an almost universal belief that hydropower was a clean and environmentally safe method of producing electricity. Hydroelectric power plants do not emit any of the standard atmospheric pollutants such as carbon dioxide or sulfur dioxide given off by fossil fuel fired power plants. In this respect, hydropower is better than burning coal, oil or natural gas to produce electricity, as it does not contribute to global warming or acid rain. Similarly, hydroelectric power plants do not result in the risks of radioactive contamination associated with nuclear power plants. (S. Baird)

The most obvious impact of hydroelectric dams is the flooding of vast areas of land, much of it previously forested or used for agriculture. The size of reservoirs created can be extremely large. The La Grande project in the James Bay region of Quebec has already submerged over 10,000 square kilometers of land; and if future plans are carried out, the eventual area of flooding in northern Quebec will be larger than the country of Switzerland. Reservoirs can be used for ensuring adequate water supplies, providing irrigation, and recreation; but in several cases they have flooded the homelands of native peoples, whose way of life has then been destroyed. Many rare ecosystems are also threatened by hydroelectric development. (S. Baird)

Large dams and reservoirs can have other impacts on a watershed. Damming a river can alter the amount and quality of water in the river downstream of the dam, as well as preventing fish from migrating upstream to spawn. These impacts can be reduced by requiring minimum flows downstream of a dam, and by creating fish ladders, which allow fish to move upstream past the dam. Silt, normally carried downstream to the lower reaches of a river, is trapped by a dam and deposited on the bed of the reservoir. This silt can slowly fill up a reservoir, decreasing the amount of water that can be stored and used for electrical generation. The river downstream of the dam is also deprived of silt that fertilizes the river's flood plain during high water periods. (S. Baird)

Bacteria present in decaying vegetation can also change mercury, present in rocks underlying a reservoir, into a form that is soluble in water. The mercury accumulates in the bodies of fish and poses a health hazard to those who depend on these fish for food. The water quality of many reservoirs also poses a health hazard due to new forms of bacteria that grow in many of the hydro rivers. Therefore, run of the river type hydro plants generally have a smaller impact on the environment. (S. Baird)

The Future:

The theoretical size of the worldwide hydropower is about four times greater than that which has been exploited at this time. The actual amount of electricity that will ever be generated by hydropower will be much less than the theoretical potential. This is due to the environmental concerns outlined above, and economic constraints. Much of the remaining hydro potential in the world exists in the developing countries of Africa and Asia. Harnessing this resource would require billions of dollars, because hydroelectric facilities generally have very high construction costs. In the past, the World Bank has spent billions of foreign aid dollars on huge hydroelectric projects in the third world. Opposition to hydropower from environmentalists and native people, as well as new environmental assessments at the World Bank will restrict the amount of money spent on hydroelectric power construction in the developing countries of the world. (S. Baird)

In North America and Europe, a large percentage of hydropower potential has already been developed. Public opposition to large hydro schemes will probably result in very little new development of big dams and reservoirs. Small scale and low head hydro capacity will probably increase in the future as research on low head turbines, and standardized turbine production, lowers the costs of hydro-electric power at sites with low heads. New computerized control systems and improved turbines may allow more electricity to be generated from existing facilities in the future. As well, many small hydroelectric sites were abandoned in the 1950's and 60's when the price of oil and coal was very low, and their environmental impacts unrealized. Increased fuel prices in the future could result in these facilities being refurbished. (S. Baird)

Conclusions:

Hydroelectric power has always been an important part of the world's electricity supply, providing reliable, cost effective electricity, and will continue to do so in the future. Hydropower has environmental impacts which are very different from those of fossil fuel power plants. The actual effects of dams and reservoirs on various ecosystems are only now becoming understood. The future of hydro-electric power will depend upon future demand for electricity, as well as how societies value the environmental impacts of hydro-electric power compared to the impacts of other sources of electricity. (S. Baird)

Stuart Baird, MEng., MBA "Energy Fact Sheet: Hydro Electric Power" Energy Educators of Ontario, 1993