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There are different types of renewable energy sources, one of which is water. Hydropower plants, sometimes called hydroelectric plants, use water to produce nearly a quarter of the world's total requirement of electricity. A hydropower plant takes very little to create: running water, a dam, turbines and a generator. These components, when put together in the right order, harness water power to create electricity.
A hydropower dam is constructed to hold back water to create a reservoir from which to draw a steady stream. When the gates of the dam open, an intake valve takes in water from this reservoir, and pressure or gravity move this water through a turbine. The turbine is connected to a generator, and while the blades of the turbine turn, the generator uses that energy to create an alternating current (AC). A transformer turns the current into high-voltage energy that is then carried away to be used by consumers. The water used to create the energy is then returned through outflow pipes to a river below the dam.
Hydropower is considered one of the most ecologically friendly forms of energy. This is partly because after the hydropower plant is constructed there is no direct waste, and there is a lower level of carbon dioxide emitted by the plants than those that process fossil fuels, such as petroleum gas. Another advantage of hydropower plants is that running them requires comparatively little in upkeep, since they require no imports and maintenance is minimal. In addition, since plants are usually fully automated, the cost of staffing a hydropower plant is also minimal. Most plants producing hydroelectric power today were built nearly 50 to 100 years ago, which is considerably longer than plants that process fossil fuels. When a dam is built for other reasons, like flood control, and a hydroelectric plant is added, this can create added revenue for the region where it is built.
The data concerning hydropower plant numbers is hard to come by, since they can be operated on a small scale as well as a large one, however there are estimated to be slightly more than 2,000 plants in the United States. China, Canada and Brazil utilize the most hydroelectric power and have many more plants than the US, however other countries, including Russia, Norway, India and Japan, also make use of hydroelectric power. As more people discover the advantages, both to consumers and the environment, more governments are pushing for and creating hydropower plants across the world.
@Iluviaporos - I think that is a concern for quite a few places. Also, the environmental impact of putting a dam into a river. Although more and more often they are designed with the local animal life in mind, it's impossible to cater for all of it.
One solution is to try and harness hydropower from the tides rather than from the rivers. The tides, after all, are regular, and aren't likely to dry up in a drought!
Plus, it is easier to mitigate environmental impact.
Unfortunately, it is also quite difficult to build something that will work properly. They are getting better, but at the moment, river dams are still more efficient.
I've lived in New Zealand for a while and a large percentage of our power comes from hydrodams. It makes sense for a small country without the resources of a larger one, to make use of renewable energy like that. Of course, it also fits in well with the clean, green image that New Zealand wants to project.
The only problem with it is that often the water begins to dry up over the summer, and during a drought cities sometimes start to suffer from "brownouts" where the power is interrupted.
This doesn't happen often though, and when it does, Kiwis tend to rally together and tighten their belts... and lessen their electricity usage.