Hydropower is an extremely flexible technology from the perspective of power system operation. Its fast response time enables hydropower to meet sudden fluctuations in demand or to help compensate for the loss of power supply from other sources. Hydro reservoirs provide built-in energy storage, which helps Optimize electricity production across a power grid. The dividing line for Categorization of small-scale and large-scale hydro differs from country to country, but generally it ranges from 10 to 30 MW.
Small-scale hydropower is normally run-of-the-river design and is one of the most environmentally benign energy conversion options available because it does not interfere significantly with river flows. Small hydro is often used in autonomous applications to replace diesel generators or other small-scale power plants or to provide electricity to rural populations.
Large-scale hydropower projects can be controversial because they affect water availability downstream, inundate valuable ecosystems and may require relocation of populations. New less-intrusive low-head turbines are being developed to mitigate these effects. As hydropower usually depends on rainfall in the upstream catchment area, its availability is affected by weather variations. Therefore, backup capacity can be needed to ensure power availability, which adds to hydropower costs.
The IEAs Hydropower Implementing Agreement estimates the world's technically feasible hydro potential at 14,000 TWh per year, of which about 8,000 TWh per year is considered economically feasible for current development. About
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808 GW are in operation or under construction worldwide. Most of the remaining potential for development is in Africa, Asia and Latin America. The technical potential of small hydropower worldwide is estimated at 150-200 GW. Only 5% of global hydropower potential has been exploited through small-scale sites.
At present, OECD countries and the rest of the world produce roughly equal amounts of hydroelectricity. The share in non-OECD countries will likely increase, however, as most large hydro potential that is economically attractive and socially acceptable has already been developed in OECD countries, while untapped potential and pending projects remain in non-OECD countries. China will add some 18.2 GW of capacity by 2009 with the completion of the Three Gorges Dam.
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