Solar4Scholars: Hydroelectric Power
Hydroelectric power is energy that comes from flowing water. Hydroelectric power is the result of two natural principles: gravity, and the hydrologic cycle, which essentially relies on solar energy.
The principle of gravity is simple: water falls.
The hydrologic cycle is the cycle that all water (each drop) goes through. Water in any body of water evaporates at any temperature above freezing. The higher the temperature, the higher is the rate of evaporation. When evaporating water reaches the cool air of the high atmosphere, it condenses into rain and falls on the land. Water collects into streams, rivers, ponds, seas, etc. Drops of water re-evaporate at each stage. The entire earth is a closed system that never loses or gains any of its total water.
In hydropower, machinery is powered by the movement of water. Since the hydrologic cycle never ends, hydroelectricity is an excellent from of renewable energy. Water is used to turn turbines that generate electricity.
Types of hydropower plants:
Impoundment uses water that is stored ("impounded") in a manmade dam. In the United States, there are nearly 2,500 such hydroelectric power plants. Water is released from the reservoir and flows into a turbine, causing it to spin and activate a generator that produces electricity. The water flow is controlled in these systems, and it may be released to meet changing electricity needs or to stabilize the water level in the reservoir.
Diversion, or run-of-river plants divert a portion of the current of a river through a canal or penstock. It may not require the use of a dam. There is a large hydroelectric power plant using diversion, at Niagara Falls, Canada.
Pumped Storage plants store energy by pumping water to a higher reservoir when demand is low. When electricity is needed, the water is released to turn turbines.
The hydrologic cycle is driven by the sun. As many solar purists will tell you, everything is solar energy.