Hydroelectric Power Generation

From Thermal-FluidsPedia

Currently there are about 680,000 MW of hydroelectric capacity installed throughout the world, which fulfills about a quarter of the world’s electricity demand and supplies more than one billion people with power. Worldwide, the amount of hydroelectric power that can be developed is about five times greater than current capacity. With 332 billion kWh and 12.5% of the world’s production, Canada is the leading hydropower producer; it satisfies 60% of its electricity needs by hydropower. After Canada, Brazil, and China, the United States is the fourth largest producer of hydroelectricity in the world. (1) Norway relies on its vast hydropower resources for 99% of its electricity, making it the cleanest country in the world in terms of energy use. (2)

Hydroelectric plants range in scale from large falling water plants used in developed nations to small river runoff plants with no dams or water storages used for rural electrification in less-developed countries. China has the most hydropower resources and has installed more than 40% of the entire world’s small hydro (50 kW_e-10 MW_e) capacity.( a ) For now, the largest hydroelectric plant in the world is located at Itaipú along the Parana River between Paraguay and Brazil. It has a power output of 12,600 MW and supplies 80% of the electricity consumed in Paraguay and a quarter of Brazil’s total supply (Figure 1). The Itaipu plant will, however, concede its ranking to the Three Gorges plant being built over the Yangtze River in China. Slated for completion by 2010, it will produce 18,200 MW. The tallest waterfall in the world is Angel Falls in Venezuela, with a total drop of 980 m. The largest hydroelectric plant in the United States (and the third largest in the world after Itaipu in Brazil and Guri in Venezuela) is the Grand Coulee on the Columbia River in Washington State. Its three power plants can collectively produce 6,800 MWe. The power plant in Niagara, at 1,950 MWe capacity, is only of modest size.

Although the majority of large hydroelectric plants (>10 MWe) have already been constructed where it made economic sense, small-scale hydro plants have been largely ignored. Figure 2 shows the exploited and potential hydroelectric resources for various continents. (3) As this data indicates, most of the potential hydro sources for industrial nations have been used up. However, a huge hydro potential remains to be utilized by developing countries.

Figure 1: The hydroelectric generating station in Itaipu with a capacity of 12,600 MWe boasts the largest facility of this kind in the world (a). The tallest waterfall in the world is the Angel Falls in Venezuela (b).

Figure 2: Exploited hydro potential by continent.

1 Plant Types
2 Plant Design
3 References
4 Additional Comments
5 Further Reading
6 External Links

Plant Types

There are three kinds of hydropower plants: impounded plants, pumped storage plants, and run-of-the-river plants.

Impounded plants, in which water is impounded in a reservoir behind a dam, are the most common. The water storage and release cycles can be relatively short (storing water at night for daytime power generation), or long (storing spring runoff for power generation in the summer). In these plants, water always flows downward from a storage reservoir behind a dam to run a turbine. Turbines are devices that convert the energy of a moving fluid (usually water, steam, or air) into the rotational energy of a shaft. During peak demands, where sufficient electricity cannot be generated by conventional means, enough water is released from the reservoir to meet additional power requirements. The primary issue with these plants is that the water flow rate downstream from the dam can change greatly, causing a sudden power surge. This often involves dramatic environmental consequences including soil erosion, degrading shorelines, crop damage, disruption of fisheries and other wildlife, and even flooding or droughts.

Pumped storage plants (PSP) reuse water after its initial use to generate electricity. This is accomplished by pumping water back into a storage tank at a higher elevation during off-peak hours when the need for electric power is low. During peak demands or when there is an unexpected spike in the electrical load, water is allowed to flow back into the lower reservoir to produce more electricity. An important advantage of PSPs is the quick delivery of power during emergencies and power surges. In comparison, a typical coal- or natural gas-fired power plant takes many hours to start. In the United States, about one quarter of all hydropower generated is from pumped storage plants.

Instead of water being pumped to the storage reservoir at a higher elevation, during the peak demand, water can be allowed to fall in underground reservoirs dug out in hard rocks. When demand falls, the base load can pump the water back to the reservoir. Since there is smaller fluctuation in the level of water in the storage reservoir, the system is more stable.

In modern pumped storage plants, the same turbine-generator that generates electricity from falling water can also be used to pump the water back into the storage tank. In this case, the generator changes the direction of the electric field, forcing the turbine to rotate in the reverse direction and act as a motor which runs the pump.

Run-of-River Plants are typically low dams where the amount of water running through the turbine varies with the flow rate of water in the river. The flow rate of water and the amount of electricity that is generated in the run-of-river plants is generally smaller than in pumped storage plants, and changes continuously with seasons and weather conditions. Since these plants do not block water in a reservoir, their environmental impact is minimal.

Plant Design

Water used by a hydroelectric plant is stored behind a dam at a certain elevation (head) above a turbine. The water flows through a penstock and through the blades of the turbine, causing the turbine to rotate. Depending on amount of electric energy required, the flow through the turbine is adjusted by series of gates and valves. The turbine shaft is normally coupled to the generator shaft to produce electricity.

In a typical small hydro scheme, a portion of the water is diverted from a river or stream through an intake valve. It is then passed through a metal screen into a settling chamber where stones, timbers, and other debris are removed and suspended particles of dirt settle before water enters the turbine. Since no reservoir is blocking the flow of water, the impact on the river and its habitat is minimized.

Depending on its application, either an impulse or a reaction turbine is used. In an impulse turbine, the blades are fixed to a rotating wheel and available head is converted into kinetic energy by a contracting nozzle. The high velocity jet then impinges on the blades and turns the turbine. The most common impulse turbines are of the Pelton type, where a series of cupped buckets are set around its rim. A high-speed jet of water enters the wheel tangentially (Figure 3a) and is deflected 180 degrees by the cups. Nearly the entire momentum of the water is used to impart an impulse that forces the wheel to turn. Impulse turbines are used most often with falling mountain streams with large heads and relatively small flow rates.

A reaction turbine uses the pressure difference across the blades for producing a hydrodynamic lift that propels the blades. To produce the highest lift, blades must be highly aerodynamic and shaped like airfoils. Unlike the impulse turbine, the rotors (also called runners) of reaction turbines are fully immersed in water. Two common reaction turbines are the Kaplan and Francis turbines. In Kaplan (propeller) turbines, water impinges on a propeller -- similar to a ship’s propeller-- fitted inside the penstock (Figure 3b); in Francis turbines, water flows radially inward to the runner housed in a spiral casing and is exited through a diffuser (Figure 3c). Kaplan turbines are common for the run-of-river projects with low head and large flow rates. Francis turbines are suitable in dams where high flow rates and relatively large heads are available.

The main advantage of reaction turbines over impulse turbines is that the blades can be adjusted to match the rotor speed closely to the generator speed. Recent advances in power electronics allow turbines and generators to run at varying speeds. This allows simpler and cheaper propeller turbines like Pelton to be used instead of the more advanced and expensive Kaplan and Francis.

Figure 3: a) Pelton; b) Kaplan; and c) Francis Turbines.

References

(1) EIA DOE- 2002 data can be found at http://www.eia.doe.gov/pub/international/iealf/table15.xls.

(2) Energy Information Agency (http://www.eia.doe.gov)

(3) Paish, O., “Small hydro power: technology and current status”, Renewable & Sustainable Energy Reviews, 6 (2002) 537-556.

Additional Comments

(a) Please note the difference between MWe denoting the electrical power output and MW, which refers to thermal power output. As we will see later, only 30-35% of the thermal energy can be converted to electricity. For hydroelectric plants we can use kW and kW_e interchangeably.