By taking advantage of gravity and the water cycle we have tapped into one of nature's engines to create a useful form of energy. In fact, human are capturing the energy of moving water for thousands of years. Today, harnessing the power of moving water to generate electricity, known as hydroelectricity power is the largest source of emissions-free, renewable electricity in the worldwide.
Although the generation of hydropower does not emit air pollution or greenhouse gas emission, it can have negative environmental and social consequences. Blocking rivers with dams can degrade water quality, damage aquatic and riparian habitat, block migratory, fish passage and displace local communities. The benefits and drawbacks of any proposed hydropower development must be weighed it's done right, hydropower can be a sustainable and non polluting source of electricity that can help decrease our dependence on fossil fuels and reduce the threat of global warming.
CONVERTING MOVING WATER TO ELECTRICITY:
In order to generate electricity from the kinetic energy in moving water, the water has to must with sufficient speed and volume to spin a propeller-like device called a turbine, which is turn, rotates a generator to generate electricity. Roughly speaking one gallon of water per second falling one hundred feet can generate one kilowatt of electricity. To increase the volume of moving water, impoundments or dams are used to collect the water. An opening in the dam uses gravity to drop water down a pipe called a penstock. The moving water causes the turbine to spin which causes magnets inside a generator to rotate and create electricity.
There are a variety of types of turbines used at hydropower facilities, and their use depends on the amount of hydraulic heat (vertical distance between the dam and the turbine) at the plant. The most common are Kaplan, Francis, and Polten wheel design. Some of these designs, called reaction and impulse wheels use not just the kinetic force of the moving water but also the water pressure.
The Kaplan turbine is similar to a boat propeller, with a runner (the turning part of a turbine) that has three to six blades, and can provide up to 400MW of power. The Kaplan turbine is differentiated from other kinds of hydropower turbines because its performance can be improved by changing the pitch of the blades. The Francis turbine has a runner with nine or more fixed vanes. In this turbine design, which can be up to 800MW in size, the runner blades direct the water so that it moves in an axial flow. The peloton turbine consists of a set of specially shaped buckets that are mounted on the outside of a circular disc, making it look similar to a water wheel, peloton turbines are typically used in high hydraulic head sites and can be as large as 200MW. Electricity can also be generated without dam, through a process known as run- of-the river. In this case, the volume and speed, of water is not augmented by a dam. Instead, a run -0f –river project spins the turbine blades by capturing the kinetic energy of the moving water in the river. Hydropower projects that have dams can control amount of electricity generated because the dam can control the timing and flow of the water reaching the turbine. Therefore, these projects can be chosen to generate power when it is needed and most valuable to the grid. As run-of –river projects do not store water behind dams, they have much less ability to control the amount and timing of when electricity is generated.
Another type of hydropower technology is called pumped storage. In a pumped storage plant water is pumped from a lower reservoir to a higher reservoir during off peak times when electricity is relatively cheap using electricity generated from other types of energy source. Pumping the water uphill creates the potential to generated hydropower later on. When the hydropower power is needed, it is released back into the lower reservoir through turbines. Inevitably, some power is lost, but pumped storage systems can be up to 80 percent efficient. There is currently more than 90 GW of pumped storage capacity worldwide with about 20 percent of that in the United States. The need to create storage resources to capture and store for later use the generation from high penetrations of variable renewable energy (e.g. wind and solar) could increase interest in building news pumped storage projects.
Prajwal Dhakal
Teacher