Hydroelectric Systems

Hydroelectric systems are the oldest (and considered the most reliable) type of electric generation in service today (Carless, Jennifer 1993). The major drawback of this form of generation is the adverse effect a drought has on the electrical output. An example of this is the drought during the summer of 1988, which caused a reduction of 25 percent in the U.S. hydropower production (Carless, Jennifer 1993). Conventional hydropower accounted for the largest portion of electrical production (71,533 megawatts) provided by renewable energy resources in the U.S. in 1991 (Carless, Jennifer 1993). Ten to 14 percent of the nation's electric generation is supplied by hydroelectric sources (Carless, Jennifer 1993). The global scale of hydropower production is much larger, with water sources providing more than one-fourth of the world's electricity (Carless, Jennifer 1993). The mechanical power of falling water is a tool passed down through the ages.

The Ancient Greeks used it to turn water wheels that ground wheat into flour as well as milling and pumping operations over two thousand years ago (Bureau of Water Reclamation, 1999). The United States, however, did not make use of the technology until the mid-1800s and these projects were somewhat small (Carless, Jennifer 1993). Niagra Falls was the first major facility to be completed, and it was completed in 1878. This set the stage for the next few decades of power production and by the 1930s, hydropower comprised 40 percent of the nation's power supply (Carless, Jennifer 1993). Meanwhile construction of large dams was taking place in other countries, such as Russia and India. As the years passed, other energy sources revealed themselves as more prevalent, and the demand for hydroelectric power has slowly decreased. "In 1965 this amount had decreased to approximately 20 percent, and this d! ownward trend has continued to the present" (Carless, Jennifer 1993, p.58).

Hydropower systems around the globe, through the 1970 s, were typically large-scale dams used to satisfy the national power grid (which is the overall electrical consumption of a nation) (Carless, Jennifer 1993). Today, the new hydroelectric plants are generally smaller, breaking the large-scale project trend set many decades beforehand. This down-sizing is a direct result of a lack of suitable sites to build a large dam and the ever-increasing environmental and economic concerns (please refer to "Economically Speaking" or "That Little Thing Called the Environment" for more information). Hoover Dam (Bureau of Water Reclamation, 1999) Hydroelectric systems are defined both by their size and by the way in which they capitalize on the water's motion.

If a plant is capable of generating 30 megawatts of electricity, then it is considered a "large" project, whereas any system that produces below that amount is considered "small" (Carless, Jennifer 1993). Most hydropower systems facilitate the use of a dam or similar structure to trap and hold water hostage until it can be released through a turbine (Carless, Jennifer 1993). A second, less commonly used method is to install turbines in pipes and place them in the middle of a river's flow, therefore eliminating the need to build a dam and stop up the river. "Most small projects are the run-of-the river type; that is, they are designed to use as much as possible of the river flow" (Wilson, Eric. 1991). The construction costs and environmental impacts of these run-of-river devices are much less than that of damming the waterway, providing for the ascension in popularity for this type of power generation (Carless, Jennifer 1993). The third and final form of hydroelectric power is the pumped storage method. Pumped storage systems differ from conventional hydroelectric systems in that they normally pump water from a lower reservoir to an upper reservoir when demand for electricity is low (Federal Energy Regulatory Commission, 1998).

Water is stored in an upper reservoir for release to generate power during hours of peak demand. For example, in the summer there is a high demand for electricity to power air conditioning units-during these times is when the water in the upper reservoir is released (Federal Energy Regulatory Commission, 1998). The electrical demand then decreases at night, at which time the water is pumped back to the upper reservoir for use the next day (Federal Energy Regulatory Commission, 1998). The pumped storage water projects are ideally suited for generating power on demand, acting as a reserve supply complementing the output of large fossil-fuel burning and nuclear steam electric facilities (Federal Energy Regulatory Commission, 1998). Start-up time of this type of plant is almost immediate, making it capable of supplying power much faster than fossil-fuel facilities that need a considerable more amount of time to increase output (Federal Energy Regulatory Commission, 1998). Like their hydroelectric cousins, pumped storage plants utilize the kinetic energy of falling water to generate power, but they use reversible turbines to pump the water back to the upper reservoir (Federal Energy Regulatory Commission, 1998). Thus, the operation serves the same function as a large battery; holding a charge, (the upper reservoir) until is needed (Gulliver, John. 1991). Pumped storage is particularly effective at sites that have high heads (large differences in elevation between the upper and lower reservoir) (Federal Energy Regulatory Commission, 1998). The two deciding factors in determining the amount of energy a hydroelectric plant can produce are the head and the flow of the water. The head (sometimes referred to as gross head) is the "difference between headwater elevation and tailwater elevation" (Warnick, C.C. 1984, p.10). (The headwater is the water stored behind the dam the tailwater is the water released from the dam.) The flow (or discharge) is the speed at which the water flows through the dam and into the tailwater (Warnick, C.C. 1984). The most economical combination in a hydroelectric contraption is high head and low flow (Warnick, C.C. 1984, p.10).