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Hydropower

In hydropower – or “water power” – systems, water is contained in reservoirs created by dams which contain hydraulic turbines and electric generators. Potential energy (the stored energy in the reservoir) becomes kinetic energy when channeled through an intake in the dam [1]. Electric power is produced from water by directing this column of falling water past the “fins” of a hydraulic turbine [2]. In a typical run-of-the-river system, the force of the river current applies the pressure to the turbine blades to produce electricity [3]. The efficiency of such systems can be close to 90% [1]. In some cases, the turbines and generators are located several miles downstream from the dam. In this later arrangement, water flows through a large pipe or “penstock” from the dam to the turbines [1, 2].

There are two main categories of hydroelectric power generation: the first, described above, is a conventional method which produces electricity via water flowing in one direction; the second is referred to as “pumped storage.” Pumped storage is a method by which water from a higher elevation reservoir flows to a lower elevation reservoir during peak demand periods, but is pumped back from the lower reservoir to the higher reservoir during non-peak periods [1]. It then flows back to the lower elevation reservoir during the next peak demand period – thus yielding a net gain in electricity generation.

Hydropower offers many advantages over other energy sources including the fact that it is fueled by water, and thus does not pollute the air like fossil power plants. Furthermore, hydropower is a domestic source of energy and is generally available as needed since the flow of water can be controlled at the dam [4]. However, recently hydropower has come under criticism for the impact that dams have on fish populations and water quality and flow. Also, since hydropower plants can be impacted by drought, during dry seasons hydropower plants cannot produce as much electricity [4].

The US hydro resource base is very large, assessed as 512 GW, which is equivalent to approximately 4,500 TWh/yr. The World Energy Council estimates that the annual technically exploitable capability is approximately 1,750 TWh, of which 501 TWh is economically exploitable. As of 2002, US hydro capacity was approximately 80 GW and had an annual capability of about 300 TWh [5].

In 2005, hydroelectric power plants generated 270 TWh, which was significantly lower than the peak year for hydroelectric production over the past decade (356 TWh in 1997). This is due to severe droughts from 1999 to 2004 which plagued the western states [6]. Furthermore, new dams have not been built and with no growth in capacity, the share of net generation from hydroelectric plants continues to decline every year, reaching only 6.6% of net generation in 2005, down from over 10% in 1997 [6]. Most analysts agree that although conventional hydroelectric power is the mainstay of renewable resources in the electric power industry today, due to the lack of additional large sites for hydropower, it is unlikely that the source will grow even if more favorable regulatory policies emerge [7].