Energy Generation and Storage Using Water  Print page


Energy from falling water

Falling water possesses energy. Look at the photo of a water wheel driven by falling water. When water falls downwards, its gravitational potential energy is converted into kinetic energy. The kinetic energy is transferred to the water wheel and the water wheel rotates. If the water wheel is connected to a generator, electricity can be generated. This is the basic principle of hydroelectricity: using falling water to generate electricity.

 
Fig. 1   A water wheel driven by falling water.   Fig. 2   Hydroelectricity is an important renewable energy source.

Of course, the design of a hydroelectric power plant is much more advanced than that of a water wheel. For a typical hydroelectric power plant, a dam is built to store water at a higher level. Water is controlled to flow through the dam, driving the turbine generators near the base of the dam to generate electricity.

The animation below shows the operation of a hydroelectric power plant.

Flash animation: Hydroelectric power plant


Water cycle

How does water get to high places in nature, allowing us to make use of its gravitational potential energy to generate electricity? The answer lies in the water cycle of the Earth.

Let's take a look at the water cycle illustrated in the figure below. When the sun heats up the water in the sea, some water is evaporated. The rising water vapour forms clouds in the sky. The water vapour condenses and falls to the ground as rain. As water flows down from high places as river, its gravitational potential energy can be used to produce electricity

Fig. 3   The water cycle: (1) solar radiation heats up the sea, (2) sea water vaporizes, (3) water vapour forms cloud, (4) water vapour condenses as rain and falls to the ground, (5) water flows down from high places as rivers, returning water to the sea. Some water is retained by the land.


The Three Gorges Project

Fig. 4   In a hydroelectric power plant, water from the upper reservoir is controlled to flow through the dam to turn the turbine generator.

China is currently working on the Three Gorges Project, which is the largest hydroelectric project ever built in the world. The dam site of the project is situated on part of the Yangtze River in Sandouping, Yichang City, Hubei Province. The main dam is about 2,309 m long and 185 m tall at its highest point [1]. When the project is finished, a reservoir will be formed with a normal water level 175 m higher than the water level on the other side of the dam [2]. The water from the reservoir is controlled to flow through the dam so as to turn the turbine generator units installed behind the dam to generate electricity. The Three Gorges Project will provide a total generating capacity of 18,200 MW [2]. In comparison, the Castle Peak Power Station of CLP Power has a generating capacity of 4110 MW [3].

Hydroelectric generation does not consume any fuel and does not produce air pollution or greenhouse gas emission. It is also a source of renewable energy. The electricity produced by the Three Gorges Project could save the burning of huge amounts of coal and help improve the air quality in China. Hydroelectric power, however, also has drawbacks. The building of dams is very expensive and dams may disrupt the natural flow of water. This disturbs the ecology and blocks the movement of fish and other organisms. The reservoirs formed often flood large areas of land, destroying the habitat. Heavy metal like mercury and other contaminants may accumulate in the reservoir, causing environmental problems. Although hydroelectric generation is considered a source of renewable energy, large-scale hydroelectric projects may not be sustainable in the long run because of their impact on the environment [4]. For large-scale projects like the Three Gorges Project, there is also the need to resettle large population to make room for the dam and the reservoir.

Do you want to make a model of a hydroelectric turbine generator that can actually produce electricity? This activity will show you how.

Activity: Construction of a hydroelectric generator model


Energy storage using water

Fig. 5   The Guangzhou Pumped Storage Power Station (image courtesy of CLP Power)
 
Fig. 6   The structure of a pumped storage power station

Water can also be used, in a sense, to store energy generated by power plants. The electrical energy from power plants is used to pump water to a place of high altitude. The electrical energy used for pumping water is thus stored as the gravitational potential energy of water. During peak hours when the demand of electricity is high, water is allowed to flow back to a lower position. Turbine generators are used to convert the stored gravitational potential energy of water back to electrical energy. The facility that performs this energy storage process is called a pumped storage power station. See how it operates in this animation.

Flash animation: 
  Pumped storage power station

The Guangzhou Pumped Storage Power Station is an example of such an energy storage facility. By design, the water levels of the higher and lower reservoirs have a height difference of 535 m [5]. The facility has 8 turbine generator units each has capacity of 300 MW [6]. The turbine generators are designed to be reversible. That is, they can use falling water to generate electricity as well as make use of electricity to pump water to a higher position.

The total efficiency of the facility is 76 % [5], this means on the whole the facility consumes electricity during its operation. The facility is not a generation facility, but rather, it is an energy storage facility.

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