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Hydropower is a renewable energy source that utilizes the natural flow of falling or fast-moving water. To generate hydroelectricity, a hydroelectric power station needs to be set up to transform the movement of water into electricity. In this article, we look at some of the key machines required in hydroelectric power stations to generate electricity.
In reality, there are many different types of hydroelectric power stations, and some do not require all the machines mentioned here. The main difference between different hydroelectric power stations is whether they use a dam or not, as some waterways can be diverted through a passageway away from the waterway, which negates the need for the whole waterway passing through a dam. However, hydroelectric power stations that utilize a dam are the most common, so the machines used in these power stations will be the focus of this article. Overall, the machines used can be categorized into three distinct classes: hydraulic structures, water turbines and electrical equipment.
The hydraulic structures are not technically machines, rather solid structures that divert water. Nevertheless, they are still important if the power station is to function as it should. The dam is perhaps the most well-known hydraulic structure and can be made of a number of materials including concrete, stone and even earth, with the material type being dependent on the size of the dam. In some cases, more than one dam is required.
In addition to the dams, the hydraulic structures also consist of spillways, headworks, surge tank and penstocks. The spillways are in place for when there is too much water in the waterway and the volume of water exceeds the capacity of the reservoir. In most cases, these situations arise when there has been heavy rainfall and the spillways discharge the excess water away from the dam. The headworks are a series of booms and racks that prevent debris from entering the turbine (which could cause damage) and valves are used in these areas to control the flow of the water to the turbines.
Surge tanks are a small reservoirs in place to change the water volumes around a turbine so that the pressure on the turbine changes. They are there to increase or reduce the amount of water in the system when the load on the turbine decreases or increases, respectively. Finally, penstocks are conduits that carry the water turbines, but the passageways are fitted with a series of valves to control the water flow and prevent ruptures arising from over-pressurizing the passageway. The surge tanks are also connected to the penstocks.
The turbines are responsible for converting the flow of the water into electricity. As the water flows over the turbine it causes it to spin, generating electricity. However, there are different types of turbines, all of which have slightly different ways of working. There are two main types of turbine: impulse and reaction.
Impulse turbines are used for high head applications, with the most common being the Pelton wheel. The ‘head’, also known as hydraulic head difference, refers to how much mechanical energy can be turned into electricity; the higher the head, the greater the electricity production. In impulse turbines, the entire water pressure is converted into kinetic energy via a nozzle, and the velocity of the jet from the nozzle drives the wheel of the turbine. The amount of water falling on to the turbine is controlled by a needle in the tip of the nozzle. As they can produce the most electricity, impulse turbines are widely used in hydroelectric power stations.
Reaction turbines, on the other hand, are low to medium head turbines. There are two main types: Francis and Kaplan turbines. The Francis turbine has two rings of guiding blades. The outside ring is stationary, whereas the inside ring rotates. These two rings form a runner, and the water is guided down the runner (first radially, then downward) where the pressure and velocity of the water are reduced when passing over the runner. This causes a reaction force, and it is this force that drives the turbine. The Kaplan turbine is similar to the Francis turbine, but the runner receives the water axially rather than radially. The water flows into the runner radially from all directions, which changes the flow of the water to an axial flow, and a reaction force again drives the turbine.
As with any energy conversion machine, electrical components are required to help facilitate a smooth transfer of energy. Hydroelectric dams use alternators to convert mechanical energy (from the spinning turbine) into an alternating electrical current. Additionally, hydroelectric power stations also utilize transformers, so that the electrical current produced can be changed and transferred to the electrical grid. Hydroelectric power stations also employ various circuit breakers, and other circuit protection devices, to prevent the electrical systems within the power station from become damaged from high currents.