Hydroelectric dams create vast hydrostatic potential energy stores, move large amounts of water through turbines, and spin generators that produce electricity. The largest hydroelectric dam in the world―the Three Gorges Dam in China's Yangtze River―can generate up to 22,500 MW of electricity. That means the dam can produce enough electricity to power over 18 million homes (based on average enery consumption rates in the US). Hydroelectric power plants generate nearly 6.7% of the world's electricity.
How do hydroelectric dams generate electricity?
How do these massive hydroelectric dams work? Let's explore the fundamental components of a hydroelectric dam and the process of creating sustainable, productive power.
Step 1: Harnessing potential energy
A roaring river releases potential energy in the form of kinetic energy and sound―gravity causes water to flow through a specific path. Dams halt this kinetic energy release and effectively store a river's potential energy in a lake-like reservoir. Hydroelectric dams work by "reserving" a river's water and potential energy. The hydraulic head, created by the depth of the water and the water's velocity as it travels through the dam's penstock, enables the turbine in the hydroelectric plant to spin.
Step 2: Turning the hydroelectric turbine
Mechanical fans vary in shape and size. Generally speaking, the larger the fan, the harder it is to move it. Large fans also require higher voltages and bigger motors to spin them. Large turbines, such as the Francis turbines in Three Gorges and many other hydroelectric dams, also require more energy to accelerate their movement.
The Francis turbine relies on the hydraulic head to spin. The hydrostatic pressure created by the dam's potential energy creates this hydraulic head, which increases as the fluid's velocity grows. In short, a taller dam produces more hydraulic head and can spin larger turbines. As the turbine spins, the hydraulic head converts into kinetic energy. This conversion effectively eliminates the water's velocity and hydrostatic pressure, allowing it to flow out slowly from the base of the dam.
Step 3: The hydroelectric turbine generator
The turbine's rotational motion generates electrical energy. Turbines are coupled with electrical generators either directly or via a gearbox or transmission that spins the shaft and the generator's armature. The commutator and brush assembly capture the flow of electricity, which is created by the generator's spinning armature relative to its stator.
Massive generators, such as those at work in hydroelectric dams, create a significant amount of reluctance―resistance to mechanical spinning―which can only be overcome by large torque forces as a result of the spinning turbine. Using a transmission system like the one in a car, the turbine's rotational motion converts into varying torque and speed ratios.
Step 4: Capturing and transmitting hydroelectric power
Generated electricity quickly transforms into grid-level voltages, which the local power company then transmits via power lines. The step-up transformers in hydroelectric dams create utility-scale voltages from relatively lower generator output voltages. The higher voltages that exit a hydroelectric power plant are ideal for long-distance power transmission, given the low electrical current that makes them more efficient. For example, the Glen Canyon Dam in Paige, Arizona, provides electricity to homes nearly 900 miles away in northern Nebraska, likely utilizing 500kV transmission as a part of the electricity journey.
Hydroelectric renewable energy advancements
Hydroelectric power plants are the largest source of renewable energy in the world. As new hydroelectric dams and tidal power plants are created and system efficiencies improve, hydroelectric power's impact grows. Fluid dynamics modeling is also improving, and manufacturing tolerances are shrinking, bringing turbine efficiency closer to 100%. Developments in gearbox manufacturing, lubricant material sciences, and manufacturing tolerances continue to increase efficiency, as will stronger magnets, more efficient conductors, and better control systems. As long as flowing water and elevation are available, hydropower will be a valuable option for sustainable energy.
