How Hydroelectricty Works





Three Gorges Dam on the Yangtze River, China generates up to 22,500 MW.

Three Gorges Dam on the Yangtze River, China generates up to 22,500 MW. Image by Le Grand Portage. License: CC BY 2.0.

Hydroelectricity is the most widely used form of a renewable energy, providing about one-fifth of energy used worldwide. The use of hydroelectricity is also the most widely used means of generating energy, second only to fossil fuels. In contrast to fossil fuels, hydroelectricity is a renewable, sustainable, form of energy. With just a constant, reliable water supply you have the potential to provide the energy needed to fuel our modern world without depleting resources.

The Basic Physics of Hydroelectricity

Hydroelectricity is, compared with other sources of renewable energy, the easiest and most efficient way to generate power. Solar power, wind energy, and other renewable forms of energy may have more potential for efficiency in the future, but currently hydroelectricity trumps other methods – a result of the availability of water itself, and the way in which energy is gathered from it.

The reason that hydroelectricity is so efficient is a result of its two main ingredients – water and gravity – to be constant, reliable, and available. The volume and velocity of water are the two most important factors in harnessing usable energy. The more water and the greater the speed of the water, the greater the output energy. To illustrate this more clearly, one gallon of water falling 100 feet per second can generate 1 kilowatt of electric power. The energy generated increases in relation to 1) the amount of water used, and 2) the speed with which the water travels. Thus, larger water volumes and a greater change in elevation, a longer fall, will supply more energy. The way that gravity and water are used to produce energy is relatively simple: a water source, like a lake or river, is harnessed in creating a unidirectional flow that moves a turbine. The turbine powers a generator, which is essentially the ‘motor’ used to covert the force of moving water into electrical energy that can be captured and stored or used.

Methods of Hydroelectricity Generation

Hydroelectricity is implemented in many different forms, and in many different environments. Hydroelectricity generation facilities all use the same principals generate electricity but must take into consideration the specific needs of the location of the hydroelectric plant. The specific method of harnessing hydroelectricity that is implemented depends greatly on the environmental surroundings of the facility.

Dams and Reservoir Hydroelectricity

Diagram showing how hydroelectric dams generate electrcitiy. Image by Tomia. License: GNU FDL

Diagram showing how hydroelectric dams generate electrcitiy. Image by Tomia. License: GNU FDL.

Dams are probably the most common and well-known method of not only retaining water, but also manipulating it to produce energy. The electricity generated by dammed water is dependent on both and volume of water available, and the change in elevation from the source to the eventual outflow. This height, which determines the velocity and therefore power output of the water, is referred to as the hydraulic head, or simply head. The larger the head the greater the distance that gravity is exerted on the water as it falls, and therefore the faster the water’s velocity. Creating a dam for the water makes use of this principle by creating a larger head, thus making as large a change in elevation as possible. With very similar construction and principle, pumped-storage hydroelectricity makes use of reservoirs of different elevations to manipulate the water sources based on periods of low or high energy demands. During low demand, the water is pumped into reservoirs of higher elevation with the use of the excess energy generated. In contrast, during high-demand, the water is re-released into the lower-elevation reservoir passing through a turbine, in which the energy is captured. The mechanics between dams and pumped-storage methods are essentially identical. However, pumped storage facilities, lacking the higher volumes of water provided by dams, recycle the water through two related reservoirs of different elevations, allowing the same water to continue to be used. Lastly, underground power plants harness the power from water with extreme elevation differences, such as a waterfall or high altitude lakes. The construction of a underground tunnel to transport water from the high reservoir to an underground generator, where the hydroelectric power is captured. Having a reservoir is certainly important for continuous or controllable hydroelectric power generation, but is by no means necessary.

Hydroelectricity from Moving Water

Two additional means of generating hydroelectricity are through the use of run-of-the-river facilities, and by using the patterns of ocean tidewater. Run-of-the-river hydroelectricity facilities have either no, or very little, reservoir capacity. Run-of-the-river uses flowing water, usually from smaller rivers or streams, to capture energy from the moment of that water as it passes into, and through, the generation facility. One might expect the generated output of these facilities would be rather small in comparison to the larger dam and pumped-stored sites. While this is often true, as with all hydroelectric plants it depends on the environment and water supply available. However, there are run-of-the-river plants, such as one such plant in British Columbia, that have the capacity to generate power that may rival the hydroelectric generation of some large-scale dams. By utilizing constant predictable conditions, like rivers or the ocean’s tides, we are able to harness that energy to create hydroelectricity. Hydroelectricity can be generated from the rising and lowering of the ocean tides. A unique feature of tidewater hydroelectricity systems is that its construction can be very simple and, much like pumped-storage reservoirs, may also be constructed for storage which allows the systems to store energy and adjust to high or low demand periods.

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