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October 16, 2020
Hydropower has been used by mankind since ancient times. The energy of falling water was used by the Greeks to turn waterwheels that transferred their mechanical energy to a grinding stone to turn wheat into flour more than 2000 years ago. In the 1700s,mechanical hydropower was used extensively for milling and pumping.
The modern era of hydropower development began in 1870 when the first hydroelectric power plant was installed in Cragside, England. The commercial use of hydropower started in 1880 in Grand Rapids, Michigan, where a dynamo driven by a water turbine was used to provide theatre and store front lighting (IPCC, 2011). These early hydropower plants had small capacities by today’s standards but pioneered the development of the modern hydropower industry. Hydropower schemes range in size from just a few watts for pico-hydro to several GW or more for large-scale projects. Larger projects will usually contain a number of turbines, but smaller projects may rely on just one turbine. The two largest hydropower projects in the world are the 14 GW Itaipu project in Brazil and the Three Gorges project in China with 22.4 GW. These two projects alone produce 80 to 100 TWh/year (IPCC, 2011).
Large hydropower systems tend to be connected to centralised grids in order to ensure that there is enough demand to meet their generation capacity. Small hydropower plants can be, and often are, used in isolated areas off-grid or in mini-grids. In isolated grid systems, if large reservoirs are not possible, natural seasonal flow variations might require that hydropower plants be combined with other generation sources in order to ensure continuous supply during dry periods.
Hydropower transforms the potential energy of a mass of water flowing in a river or stream with a certain vertical fall (termed the “head”10). The potential annual power generation of a hydropower project is proportional to the head and flow of water. Hydropower plants use a relatively simple concept to convert the energy potential of the flowing water to turn a turbine, which, in turn, provides the mechanical energy required to drive a generator and produce electricity (Figure 2.1). The main components of a conventional hydropower plant are:
» Dam: Most hydropower plants rely on a dam that holds back water, creating a large water reservoir that can be used as storage. There may also be a de-silter to cope with sediment build-up behind the dam.
» Intake, penstock and surge chamber: Gates on the dam open and gravity conducts the water through the penstock (a cavity or pipeline) to the turbine. There is sometimes a head race before the penstock. A surge chamber or tank is used to reduce surges in water pressure that could potentially damage or lead to increased stresses on the turbine.
» Turbine: The water strikes the turbine blades and turns the turbine, which is attached to a generator by a shaft. There is a range of configurations possible with the generator above or next to the turbine. The most common type of turbine for hydropower plants in use today is the Francis Turbine, which allows a side-by-side configuration with the generator.
» Generators: As the turbine blades turn, the rotor inside the generator also turns and electric current is produced as magnets rotate inside the fixed-coil generator to produce alternating current (AC).
Transformer: The transformer inside the powerhouse takes the AC voltage and converts it into higher-voltage current for more efficient (lower losses) long-distance transport.
» Transmission lines: Send the electricity generated to a grid-connection point, or to a large industrial consumer directly, where the electricity is converted back to a lowervoltage current and fed into the distribution network. In remote areas, new transmission lines can represent a considerable planning hurdle and expense.
» Outflow: Finally, the used water is carried out through pipelines, called tailraces, and re-enters the river downstream. The outflow system may also include “spillways” which allow the water to bypass the generation system and be “spilled” in times of flood or very high inflows and reservoir levels. Hydropower plants usually have very long lifetimes and, depending on the particular component, are in the range 30 to 80 years. There are many examples of hydropower plants that have been in operation for more than 100 years with regular upgrading of electrical and mechanical systems but no major upgrades of the most expensive civil structures (dams, tunnels) (IPCC, 2011).
The water used to drive hydropower turbines is not consumed” but is returned to the river system. This may not be immediately in front of the dam and can be several kilometres or further downstream, with a not insignificant impact on the river system in that area. However, in many cases, a hydropower system can facilitate the use of the water for other purposes or provide other services such as irrigation, flood control and/or more stable drinking water supplies. It can also improve conditions for navigation, fishing, tourism or leisure activities.
The components of a hydropower project that require the most time and construction effort are the dam, water intake, head race, surge chamber, penstock, tailrace and powerhouse. The penstock conveys water under pressure to the turbine and can be made of, or lined with, steel, iron, plastics, concrete or wood. The penstock is sometimes created by tunnelling through rock, where it may be lined or unlined.
The powerhouse contains most of the mechanical and electrical equipment and is made of conventional building materials although in some cases this maybe underground. The primary mechanical and electrical components of a small hydropower plant are the turbines and generators.
Turbines are devices that convert the energy from falling water into rotating shaft power. There are two main turbine categories: “reactionary” and “impulse”. Impulse turbines extract the energy from the momentum of the flowing water, as opposed to the weight of the water. Reaction turbines extract energy from the pressure of the water head.
The most suitable and efficient turbine for a hydropower project will depend on the site and hydropower scheme design, with the key considerations being the head and flow rate (Figure 2.2). The Francis turbine is a reactionary turbine and is the most widely used hydropower turbine in existence. Francis turbines are highly efficient and can be used for a wide range of head and flow rates. The Kaplan reactionary turbine was derived from the Francis turbine but allows efficient hydropower production at heads between 10 and 70 metres, much lower than for a Francis turbine. Impulse turbines such as Pelton, Turgo and cross-flow (sometimes referred to as Banki-Michell or Ossberger) are also available. The Pelton turbine is the
most commonly used turbine with high heads. Banki Michell or Ossberger turbines have lower efficiencies but are less dependent on discharge and have lower maintenance requirements.
There are two types of generators that can be used in small hydropower plants: asynchronous (induction) and synchronous machines (NHA and HRF, 2010).
Asynchronous generators are generally used for microhydro projects.
Small hydropower, where a suitable site exists, is often a very cost-effective electric energy generation option. It will generally need to be located close to loads or existing transmission lines to make its exploitation economic. Small hydropower schemes typically take less time to construct than large-scale ones although planning and approval processes are often similar (Egre and Milewski, 2002).
Large-scale hydropower plants with storage can largely de-couple the timing of hydropower generation from variable river flows. Large storage reservoirs may be sufficient to buffer seasonal or multi-seasonal changes in river flows, whereas smaller reservoirs may be able to buffer river flows on a daily or weekly basis.
With a very large reservoir relative to the size of then hydropower plant (or very consistent river flows), hydropower plants can generate power at a nearconstant level throughout the year (i.e. operate as a base-load plant). Alternatively, if the scheme is designed to have hydropower capacity that far exceeds the amount of reservoir storage, the hydropower plant is sometimes referred to as a peaking plant and is designed to be able to generate large quantities of electricity to meet peak electricity system demand. Where the site allows, these are design choices that will depend on the costs and likely revenue streams from different configurations.