Energy Generation and Storage Using Water [Print]

Electricity generation using water

Have you seen photos of the Niagara Falls in southeastern Ontario, Canada? They are perhaps the most spectacular water falls in the world. If you visit Niagara Falls you will see water rushing over the cliff edge with a thunderous roar. A great amount of energy is released as the water falls. Do you know where this energy comes from?

 Fig. 1   A spectacular view of the Niagara Falls (Photo credit: Parsons, David and NREL/DOE) Fig. 2   As water falls, it gains kinetic energy.

To explain where the energy of the falling water comes from, let's recall the principle of conservation of energy. The principle states that energy can neither be created nor destroyed, but can only be converted from one form to another. Water at the top of a very high waterfall possesses gravitational potential energy. As the water falls, this energy is converted into kinetic energy, resulting in a flow at a high velocity.

As the falling water collides with the bulk of the water at the bottom of the waterfall, water splashes randomly and chaotically in all directions. Part of the kinetic energy gained by the falling water is now converted into the kinetic energy of random motion. As a result, the internal energy of the water increases, and the water temperature rises at the bottom of the falls. It is said that in the 19th century, the famous scientist James Joule first attempted to measure the temperature change of water at a waterfall. His contribution towards the discovery of conservation of energy resulted in the unit of energy joule being named after him.

 Fig. 3   Hydroelectricity is an important renewable energy source.

Is it possible to capture part of the kinetic energy generated by falling water and convert it to a useful form, instead of letting it all dissipate? This is exactly what a hydroelectric power station does. Hydroelectricity is the generation of electricity using the kinetic energy of water. In the case of a waterfall, gravitational potential energy of water first changes into the kinetic energy of water. This kinetic energy is partially converted into electrical energy by a generator.

In Canada, water from Niagara Falls was first diverted for hydroelectricity in 1893. In 1921, the Sir Adam Beck Generating Station No.1 began diverting water from the falls into tunnels to produce electricity. It was once the largest hydroelectric power station in the world.

The Three Gorges Project

In Hong Kong, we do not have a giant waterfall or a large river that provides us with a powerful flow of water to generate electricity. However, on China's largest river the Yangtze River, we can find the world's largest hydroelectric project ever built, the Three Gorges Project.

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

The dam site of the project is situated on part of the Yangtze River in Sandouping, Yichang City, Hubei Province. The 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. On completion, there will be 26 sets of turbine generator units of 700 MW power output each. This will provide a total of 18,200 MW of electricity generating capacity [2]. By comparison, the CLP Power's Castle Peak Power Station in Hong Kong has a generating capacity of 4,110 MW [3].

To understand the operation of a hydroelectric power plant, click on the animation below.

 Flash animation: Hydroelectric power plant

Hydroelectric generation does not consume any fuel or produce air pollution or greenhouse gas emission. It is 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 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 populations 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? The activity below will show you how.

 Activity: Hydroelectric turbine 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 the energy generated by power plants. Electrical energy from power plants is used to pump water to a place of higher altitude and the gravitational potential energy of the water will be used later as the source of energy for electricity generation.

The Guangzhou Pumped Storage Power Station is an example of such an energy storing facility. During off-peak hours, the demand for electricity is low, the station makes use of the electricity generated by power stations to pump water from a lower reservoir to a higher reservoir, converting electrical energy to gravitational potential energy of water. During peak hours when the demand for electricity is high, the facility allows water to flow back to the lower reservoir. The turbine generator reconverts the stored gravitational potential energy of water back to electrical energy to meet the high demand. By design, the water levels of the higher and lower reservoirs have a height difference of 535 m and the facility has a total efficiency of 76 % [5]. The facility has 8 turbine generator units each with a 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 altitude.

As the efficiency of the facility is 76 %, it consumes electricity during operation. The facility is not an energy generating facility, but rather, it is an energy storage facility. You may ask: what is the advantage of using this facility as it uses some energy? Consider this question from an economic point of view. The natural way to meet a high electrical power demand at peak hours is to build and/or operate more generators to provide higher output power. This will increase the cost of power production. Pump storage provides a relatively economical method to store electrical energy during off-peak hours, and retrieve this energy to meet a high power demand during peak hours. This helps lower the overall operation cost of power production.

To find out how the Guangzhou Pumped Storage Power Station works, take a look at this animation.

 Flash animation: How pumped storage works

To know more about Guangzhou Pumped Storage Power Station and to calculate how much electrical energy is used and how much water flows up and down between the high and low reservoirs just by knowing a few facts of the station, try the following activity.

 [1] China Three Gorges Project Corporation web page [2] China Three Gorges Project web page [3] CLP Group Castle Peak Power Station web page [4] Re-energy.ca Renewable Energy Basics web page [5] Chinese National Committee on Large Dams web page [6] [7] For more information on another renewable energy - wind energy in Hong Kong, please refer to the following web pages:Hongkong Electric Company Ltd. Lamma Wind Power Station web pagehttp://www.heh.com/hehWeb/CommunityAndEducation/SmartPowerCampaign/WindTurbine_en.htmCLP Group Commercial Scale Wind Turbine Pilot Demonstration web pagehttp://www.clpgroup.com/clp/SocNEnv/Env/PrjHiLit/WindPrj/?lang=en