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Gezhou HidroPower Dam
Hydropower (from hydro meaning water) is energy that comes from the force of moving water. The fall and movement of water is part of a continuous natural cycle called the water cycle. Energy from the sun evaporates water in the earth’s oceans and rivers and draws it upward as water vapor.
When the water vapor reaches the cooler air in the atmosphere, it condenses and forms clouds. The moisture eventually falls to the earth as rain or snow, replenishing the water in the oceans and rivers. Gravity drives the water, moving it from high ground to low ground. The force of moving water can be extremely powerful.
Hydropower is called a renewable energy source because the water on the earth is continuously replenished by precipitation. As long as the water cycle continues, we won’t run out of this energy source.
History of Hydropower
Hydropower has been used for centuries. The Greeks used water wheels to grind wheat into flour more than 2,000 years ago. In the early 1800s, American and European factories used the water wheel to power machines. The water wheel is a simple machine. The water wheel is located below a source of flowing water.
It captures the water in buckets attached to the wheel and the weight of the water causes the wheel to turn. Water wheels convert the potential energy (gravitational energy) of the water into motion. That energy can then be used to grind grain, drive sawmills, or pump water.
In the late 19th century, the force of falling water was used to generate electricity. The first hydroelectric power plant was built at Niagara Falls in 1879. In the following decades, many more hydroelectric plants were built. At its height in the early 1940s, hydropower provided 33 percent of this country’s electricity.
By the late 1940s, the best sites for big dams had been developed. Inexpensive fossil fuel plants also entered the picture. At that time, plants burning coal or oil could make electricity more cheaply than hydro plants. Soon they began to underprice the smaller hydroelectric plants. It wasn’t until the oil shocks of the 1970s that people showed a renewed interest in hydropower.
Hydro Dams
It’s easier to build a hydro plant where there is a natural waterfall. That’s why the first hydro plant was built at Niagara Falls. Dams, which are artificial waterfalls, are the next best way.
Dams are built on rivers where the terrain will produce an artificial lake or reservoir above the dam. Today there are about 80,000 dams in the United States, but only three percent (2,000) have power-generating hydro plants. Most dams are built for flood control and irrigation, not electric power generation. A dam serves two purposes at a hydro plant. First, a dam increases the head or height of the water. Second, it controls the flow of water. Dams release water when it is needed for electricity production. Special gates called spillway gates release excess water from the reservoir during heavy rainfalls.
Hydropower Plants
Hydropower Plant Parts
As people discovered centuries ago, the flow of water represents a huge supply of kinetic energy that can be put to work. Water wheels are useful for generating mechanical energy to grind grain or saw wood, but they are not practical for generating electricity. Water wheels are too bulky and slow.
Hydroelectric plants are different. They use modern turbine generators to produce electricity, just as thermal (coal, oil, nuclear) power plants do, except that they do not produce heat to spin the turbines.
How a Hydro Plant Works
A typical hydro plant is a system with three parts:
- a power plant where the electricity is produced.
- a dam that can be opened or closed to control water flow
- a reservoir (artificial lake) where water can be stored
To generate electricity, a dam opens its gates to allow water from the reservoir above to flow down through large tubes called penstocks. At the bottom of the penstocks, the fast-moving water spins the blades of turbines. The turbines are connected to generators to produce electricity. The electricity is then transported via huge transmission lines to a local utility company.
Head and Flow
The amount of electricity that can be generated at a hydro plant is determined by two factors: head and flow. Head is how far the water drops. It is the distance from the highest level of the dammed water to the point where it goes through the power-producing turbine.
Flow is how much water moves through the system––the more water that moves through a system, the higher the flow. Generally, a high-head plant needs less water flow than a low-head plant to produce the same amount of electricity.
Storing Energy
One of the biggest advantages of a hydropower plant is its ability to store energy. The water in a reservoir is, after all, stored energy. Water can be stored in a reservoir and released when needed for electricity production.
During the day when people use more electricity, water can flow through a plant to generate electricity. Then, during the night when people use less electricity, water can be held back in the reservoir. Storage also makes it possible to save water from winter rains for summer generating power, or to save water from wet years for generating electricity during dry years.
Pumped Storage Systems
Some hydro plants use pumped storage systems. A pumped storage system operates much as a public fountain does. The same water is used again and again. At a pumped storage hydro plant, flowing water is used to make electricity and then stored in a lower pool. Depending on how much electricity is needed, the water may be pumped back to an upper pool. Pumping water to the upper pool requires electricity so hydro plants usually use pumped storage systems only when there is peak demand for electricity.
Pumped hydro is the most reliable energy storage system used by American electric utilities. Coal and nuclear power plants have no energy storage systems. They must turn to gas and oil-fired generators when people demand lots of electricity. They also have no way to store any extra energy they might produce during normal generating periods.
Hydropower Production
How much electricity do we get from hydropower today? Depending on the amount of rainfall, hydro plants produce from five to ten percent of the electricity produced in this country (10 percent in 1997, 5.9 percent in 2008 due to the droughts in the west. In Oregon, Washington, and Idaho, hydropower supplies over 73 percent of the electricity each year. Today, there are about 78 million kilowatts of hydro generating capacity in the United States. That’s equivalent to the generating capacity of 80 large nuclear power plants. The biggest hydro plant in the U.S. is located at the Grand Coulee Dam on the Columbia River in northern Washington State. The United States also gets some hydropower generated electricity from Canada.
Some New England utilities buy this imported electricity. What does the future look like for hydropower? The most economical sites for hydropower dams have already been developed so the development of big hydro plants is unlikely. Existing plants could be enlarged to provide additional generating capacity. Plus, many flood-control dams not equipped for electricity production could be retrofitted with generating equipment. The Federal Energy Regulatory Commission estimates 30 thousand megawatts of additional generating capacity could be developed in the United States.
Hydropower for Baseload Power
Demand for electricity is not steady; it goes up and down. People use more electricity during the day when they are awake and using electrical appliances, and less at night when they are asleep.
People also use more electricity when the weather is very cold or very hot. Electric utility companies have to produce electricity to meet these changing demands. Baseload power is the electricity that utilities have to generate all the time. For that reason, baseload power should be cheap and reliable. Hydropower meets both these requirements.
Generating electricity with hydropower is the cheapest way to generate electricity in the U.S., and the fuel supply flowing water is always available. Hydro plants are more energy efficient than most thermal power plants, too. That means they waste less energy to produce electricity. In thermal power plants, a lot of energy is lost as heat. Hydro plants are about 95 percent efficient at converting the kinetic energy of the moving water into electricity.
Economics of Hydropower
Hydropower is the cheapest way to generate electricity today. No other energy source, renewable or nonrenewable, can match it.
Today, it costs about one cent per kWh (kilowatt-hour) to produce electricity at a typical hydro plant. In comparison, it costs coal plants about three cents per kWh and nuclear plants about two cents per kWh to generate electricity. Producing electricity from hydropower is cheap because, once a dam has been built and the equipment installed, the energy source—flowing water—is free.
Hydropower plants also produce power cheaply due to their sturdy structures and simple equipment. Hydro plants are
dependable and long-lived, and their maintenance costs are low compared to coal or nuclear plants. One requirement may increase hydropower’s costs in the future. The procedure for licensing and relicensing dams has become a lengthy
and expensive process. Many environmental impact studies must be undertaken and as many as 13 state and federal agencies must be consulted. It takes anywhere from three to seven years to get a license to build a hydroelectric dam or a relicense to continue operations.
Hydropower and the Environment
Hydropower can potentially harm fish
Hydropower dams can cause several environmental problems, even though they burn no fuel. Damming rivers may permanently alter river systems and wildlife habitats.
Fish, for one, may no longer be able to swim upstream. Hydro plant operations may also affect water quality by churning up dissolved metals that may have been deposited by industry long ago. Hydropower operations may increase silting, change water temperatures, and lower the levels of dissolved oxygen.
Some of these problems can be managed by constructing fish ladders, dredging the silt, and carefully regulating plant
operations. Hydropower has advantages, too. Hydropower’s fuel supply (flowing water) is clean and is renewed yearly by snow and rainfall. Furthermore, hydro plants do not emit pollutants into the air because they burn no fuel. With growing concern over greenhouse gas emissions and increased demand for electricity, hydropower may become more important in the future.
Hydropower facilities offer a range of additional benefits. Many dams are used to control flooding and regulate water supply, and reservoirs provide lakes for recreational purposes, such as boating and fishing.
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Hydropower - Systems Overview
Hydropower is based on simple concepts. Moving water turns a turbine, the turbine spins a generator, and electricity is produced. Many other components may be in a system, but it all begins with the energy already within the moving water.
What Makes Water Power
Water power is the combination of head and flow. Both must be present to produce electricity. Consider a typical hydro system. Water is diverted from a stream into a pipeline, where it is directed downhill and through the turbine (flow). The vertical drop (head) creates pressure at the bottom end of the pipeline. The pressurized water emerging from the end of the pipe creates the force that drives the turbine. More flow or more head produces more electricity. Electrical power output will always be slightly less than water power input due to turbine and system inefficiencies.
Head is water pressure, which is created by the difference in elevation between the water intake and the turbine. Head can be expressed as vertical distance (feet or meters), or as pressure, such as pounds per square inch (psi). Net head is the pressure available at the turbine when water is flowing, which will always be less than the pressure when the water is turned off (static head), due to the friction between the water and the pipe. Pipeline diameter has an effect on net head.
Flow is water quantity, and is expressed as “volume per time,” such as gallons per minute (gpm), cubic feet per second (cfs), or liters per minute. Design flow is the maximum flow for which your hydro system is designed. It will likely be less than the maximum flow of your stream (especially during the rainy season), more than your minimum flow, and a compromise between potential electrical output and system cost.
Head and flow are the two most important things you need to know about your site. You must have these measurements before you can seriously discuss your project, how much electricity it will generate, or the cost of components. Every aspect of a hydro system revolves around head and flow. In Part 2 of this series, we will discuss how to measure them.
Power Conversion & Efficiency
The generation of electricity is simply the conversion of one form of energy to another. The turbine converts the energy in the moving water into rotational energy at its shaft, which is then converted to electrical energy by the generator. Energy is never created; it can only be converted from one form to another. Some of the energy will be lost through friction at every point of conversion. Efficiency is the measure of how much energy is actually converted. The simple formula for this is:
Net Energy = Gross Energy x Efficiency
While some losses are inevitable as the energy in moving water gets converted to electricity, they can be minimized with good design. Each aspect of your hydro system—from water intake to turbine-generator alignment to transmission wire size—affects efficiency. Turbine design is especially important, and must be matched to your specific head and flow for best efficiency.
A hydro system is a series of interconnected components. Water flows in at one end of the system, and electricity comes out the other. Here is an overview of these components, from the water source to the electrical controls.
Water Diversion (Intake)
This variable-flow, crossflow turbine uses a belt-drive coupling to a 40 KW synchronous generator.
The intake is typically the highest point of your hydro system, where water is diverted from the stream into the pipeline that feeds your turbine. A diversion can be as simple as a screened pipe dropped into a pool of water, or as big and complex as a dam across an entire creek or river. A water diversion system serves two primary purposes.
The first is to provide a deep enough pool of water to create a smooth, air-free inlet to your pipeline. (Air reduces horsepower and can damage your turbine.) The second is to remove dirt and debris.
Trash racks and rough screens can help stop larger debris, such as leaves and limbs, while an area of quiet water will allow dirt and other sediment to settle to the bottom before entering your pipeline. This helps reduce abrasive wear on your turbine.
Another approach is to use a fine, self-cleaning screen that filters both large debris and small particles.
Pipeline (Penstock)
Elements of a Hydroelectric System
The pipeline, or penstock, not only moves the water to your turbine, but is also the enclosure that creates head pressure as the vertical drop increases. In effect, the pipeline focuses all the water power at the bottom of the pipe, where the turbine is. In contrast, an open stream dissipates the energy as the water travels downhill.
Pipeline diameter, length, material, and routing all affect efficiency. Guidelines are available for matching the size of your pipeline to the design flow of your system. As you’ll see in the next article in this series, a small-diameter pipeline can considerably reduce your available horsepower, even though it can carry all available water.Larger diameter pipelines have less friction as the water travels through.
Powerhouse
The powerhouse is simply a building or box that houses your turbine, generator, and controls. Its main function is to provide a place for the system components to be mounted, and to protect them from the elements. Its design can affect system efficiency, especially with regard to how the water enters and exits your turbine. For example, too many elbows leading to the turbine can create turbulence and head loss.
Likewise, any restrictions to water exiting the turbine may increase resistance against the turbine’s moving parts.
Turbine
An in-stream screen keeps debris and silt out of the penstock at the small-stream intake for a microhydro system in Washington.
The turbine is the heart of the hydro system, where water power is converted into the rotational force that drives the generator. For maximum efficiency, the turbine should be designed to match your specific head and flow. There are many different types of turbines, and proper selection requires considerable expertise. A Pelton design, for example, works best with medium to high heads. A crossflow design works better with lower head but higher flow. Other turbine types, such as Francis, turgo, and propeller, each have optimum applications.
Turbines can be divided into two major types. Reaction turbines use runners (the rotating portion that receives the water) that operate fully immersed in water, and are typically used in low to moderate head systems with high flow. Examples include Francis, propeller, and Kaplan.
Impulse turbines use runners that operate without being immersed, driven by one or more high-velocity jets of water. Examples include Pelton and turgo. Impulse turbines are typically used with moderate-to-high head systems, and use nozzles to produce the high-velocity jets. Some impulse turbines can operate efficiently with as little as 5 feet (1.5 m) of head.
The crossflow turbine is a special case. Although technically classified as an impulse turbine because the runner is not entirely immersed in water, this “squirrel cage” type of runner is used in applications with low to moderate head and high flow. The water passes through a large, rectangular opening to drive the turbine blades, in contrast to the small, high-pressure jets used for Pelton and turgo turbines.
Regardless of the turbine type, efficiency is in the details. Each turbine type can be designed to meet vastly different requirements. The turbine system is designed around net head and design flow. These criteria not only influence which type of turbine to use, but are critical to the design of the entire turbine system.
Minor differences in specifications can significantly impact energy transfer efficiency. The diameter of the runner, front and back curvatures of its buckets or blades, casting materials, nozzle (if used), turbine housing, and quality of components all affect efficiency and reliability.
Drive System
The drive system couples the turbine to the generator. At one end, it allows the turbine to spin at the rpm that delivers best efficiency. At the other, it drives the generator at the rpm that produces correct voltage and frequency— frequency applies to alternating current (AC) systems only. The most efficient and reliable drive system is a direct, 1:1 coupling between the turbine and generator.
This is possible for many sites, but not for all head and flow combinations. In many situations, especially with AC systems, it is necessary to adjust the transfer ratio so that both turbine and generator run at their optimum (but different) speeds. These types of drive systems can use either gears, chains, or belts, each of which introduces additional efficiency losses into the system. Belt systems tend to be more popular because of their lower cost.
Hydro Terms
| Flow | Francis Turbine | Friction Loss | Head | |||
| Refers to the quantity of water supplied from a water source or exiting a nozzle per unit of time. Commonly measured in gallons per minute (gpm). | A type of reaction hydro-turbine used in low to medium heads. It consists of fixed vanes on a shaft. Water flows down through the vanes, driving the shaft. | Lost energy due to pipe friction. In hydro systems, pipe sized too small can lead to serious friction losses. | The difference in elevation between a source of water and the location at which the water from that source may be used (synonym: vertical drop). Expressed in vertical distance or pressure. | |||
| Headrace | Hydroelectricity | Impulse Turbine | Intake | |||
| A flume or channel that feeds water into a hydro turbine. | Any electricity that is generated by the flow of water. | Turbines with runners that operate in air, driven by one or more high-velocity jets of water from nozzles. Typically used with moderate- to high- head systems. Examples include Pelton and turgo. | The structure that receives the water and feeds it into the penstock (pipeline). Usually incorporates screening or filtering to keep debris and aquatic life out of the system. | |||
| Pelton Wheel | Penstock | |||||
| A common impulse turbine runner (named after inventor Lester Pelton) made with a series of cups or “buckets” attached to a hub. | The pipe in a hydro system that carries the water from the intake to the turbine. | |||||
To be continued soon in next article: Hydropower – Systems Overview (2)
SOURCE: Dan New, homepower.com
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