DOI: 10.1615/AtoZ.h.hydro_power

Hydro power is now almost exclusively used for the generation of electricity. In 1994, it accounted for approximately 18% of the world's electricity generation. Hydro power generation world-wide far outstripped any other renewable resource and it is likely to do so in the foreseeable future. Because of its very nature, hydro power is not evenly distributed world wide. For example in 1994, 95% of electricity generation in Brazil was from hydro whereas in England it only accounted for 0.1 %. The largest remaining areas for economically feasible pure hydro power development are in China, Brazil and Russia, respectively.

Pure hydro power exploits the potential energy of water in one of two ways: either by impounding it by means of a dam, and hence concentrating and releasing the potential energy at a chosen point, or by circumventing a region of large losses in potential energy, for example, a waterfall, rapids or river horseshoe. In many instances, there is insufficient water at one site for economic development, in which case the water is diverted and collected from diverse sites by channels, pipelines and tunnels to a common point. Water thus collected or impounded is directed by pressure pipelines (called penstocks) to the power-houses hydraulic turbines. Pure hydro power projects are often staged, the outfall of one facility being the head water of the next. For low head applications power-houses are typically situated at the base of dams. Power-houses in high head facilities are often underground.

The water channels and pipelines have to be designed for the hydraulic transients resulting from the change of flow rate of the water. Surge tanks, overflows and pressure relief valves are used extensively. Hydro facilities have to accommodate maximum predicted floods and are normally equipped with spillways, often gated. Water flow in complex scheme is controlled with hydraulic valves and gates, which are also used for isolation during maintenance. Depending on climate and location, the facility may be equipped to deal with ice floes and large amounts of vegetation. Where water-borne silt is a problem upstream, settling beds are provided.

Pumped storage hydro projects have little or no natural water inflow. Water is pumped from a lower to a higher reservoir during periods of low electrical demand and released through turbines during periods of high demand. As such, pumped storage is the only practical method of substantial energy storage, permitting thermal power stations to operate at base load. The upper and lower reservoirs of pumped storage facilities are often completely man made. A large percentage of pumped storage facilities use reversible pump turbines. However, there are also many facilities where the pump and turbine share the same shaft, which in turn is connected to the motor/generator. During turbine operation, the pump is dewatered and vise versa during pump operation. Alternatively, the two are isolated with mechanical couplings. For high head pumped storage schemes, multi stage pumps or pump turbines are used.

Because of the relatively fast response of hydro units, hydro power can be very effective as spinning reserve and peaking duty on a large generating system. In some instances hydro spinning reserve can be advantageously coupled with synchronous condenser operation of the associated hydro generators, for electrical system compensation. In a micro or macro form, relatively small hydro units can be installed to serve isolated communities, obviating the need for long and expensive transmission lines.

Power from a pure hydro facility is not constant and varies according to the availability of the water. Water flows tend to be seasonal (spring runoffs and monsoon) and, although the creation of large reservoirs alleviates this problem, they cannot completely eliminate it. Dependability of hydro power is also reduced if the scheme is multi purpose, providing water for irrigation or acting as flood control. In both cases the release of water could be dictated by factors other than the power demand (see also Water).

The initial capital outlay for a large hydro scheme can be substantial and difficult to arrange. The amount of construction work is usually considerable. Both of these factors give hydro projects a substantially longer lead time than an equivalent thermal generating facility. The development of large river schemes involving the flooding of substantial tracts of land upstream of the dam is disruptive to both the local environment and populace, especially if large communities have to be relocated. High head schemes are also environmentally contentious as they tend to be located in areas of natural beauty. Accordingly, it is increasingly difficult to obtain both approval and financing for the creation of new hydro power facilities and recent efforts have concentrated on the refurbishing of existing hydro developments.


Gummer, J. H., Barr, D. M, and Simms, G. P. (1993) Evaluation Criteria for Upgrading Hydro Powerplants, International Water Power and Dam Construction, v. 45-12.

International Water Power & Dam Construction Handbook, (1994) Reed Business Publishing, London, ISBN 0617 00548 6. DOI: 10.1016/0148-9062(95)99254-U

Mosonyi, E. (1991) Water Power Development, Akademiai Kiado, Budapest, ISBN 9630542706.

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