January, 2011 Issue
Hydropower uses a fuel—water—that’s not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy. Here's a brief rundown:
Hydropower is using water to power machinery or make electricity. Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, then flowing back down to the ocean. The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain. Hydropower uses a fuel—water—that’s not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy.
When flowing water is captured and turned into electricity, it’s called hydroelectric power or hydropower. There are several types of hydroelectric facilities; they’re all powered by the kinetic energy of flowing water as it moves downstream. Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses and by industry.
Types of hydropower plants
There are three types of hydropower facilities: impoundment, diversion and pumped storage. Some hydropower plants use dams and some do not. Many dams were built for other purposes and hydropower was added later. In the United States, there are about 80,000 dams, of which only 2,400 produce power. The other dams are for recreation, stock/farm ponds, flood control, water supply and irrigation.
Hydropower plants range in size from small systems for a home or village to large projects producing electricity for utilities. The sizes of hydropower plants follow.
Impoundment. The most common type of hydroelectric power plant is an impoundment facility. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it. This, in turn, activates a generator to produce electricity. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.
Diversion. A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.
Pumped storage. When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.
Sizes of hydroelectric power plants
Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.
Large hydropower. Although definitions vary, DOE defines large hydropower as facilities that have a capacity of more than 30 megawatts.
Small hydropower. Although definitions vary, DOE defines small hydropower as facilities that have a capacity of 100 kilowatts to 30 megawatts.
Micro hydropower. A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch or village.
This glossary of terms defines the components and concepts that make up hydro turbines and hydropower plants.
Alternating current (AC): Electric current that reverses direction many times per second.
Ancillary services: Operations provided by hydroelectric plants that ensure stable electricity delivery and optimize transmission system efficiency.
Cavitation: Noise or vibration causing damage to the turbine blades as a result of bubbles that form in the water as it goes through the turbine. This causes a loss in capacity, head loss, efficiency loss and the cavity or bubble collapses when they pass into higher regions of pressure.
Direct current (DC): Electric current that flows in one direction.
Draft tube: A water conduit, which can be straight or curved depending upon the turbine installation, that maintains a column of water from the turbine outlet and the downstream water level.
Efficiency: A percentage obtained by dividing the actual power or energy by the theoretical power or energy. It represents how well the hydropower plant converts the energy of the water into electrical energy.
Head: Vertical change in elevation, expressed in either feet or meters, between the headwater level and the tailwater level.
Flow: Volume of water, expressed as cubic feet or cubic meters per second, passing a point in a given amount of time.
Headwater: The water level above the powerhouse.
Low head: Head of 66 feet or less.
Penstock: A closed conduit or pipe for conducting water to the powerhouse.
Runner: The rotating part of the turbine that converts the energy of falling water into mechanical energy.
Scroll case: A spiral-shaped steel intake guiding the flow into the wicket gates located just prior to the turbine.
Small hydro: Projects that produce 30 megawatts or less.
Tailrace: The channel that carries water away from a dam.
Tailwater: The water downstream of the powerhouse.
Ultra low head: Head of 10 feet or less.
Wicket gates: Adjustable elements that control the flow of water to the turbine passage.
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