• Hydroelectric Energy
• Projects

Is power that is derived from the force or energy of moving water, which may be harnessed for useful purposes.

PRINCIPALS OF HYDROELECTRIC POWER STATIONS

The amount of electrical energy that can be generated from a water source depends primarily on two things: the distance the water has to fall, and how much water is flowing. hydroelectric power stations are therefore situated where they can take advantage of the greatest fall of a large quantity of water- at the bottom of a deep and steep sided valley or gorge, or near the base of a dam.

Water is collected and stored in the dam above the station for use when it is required. some dams create big reservoirs to store water by raising the levels of rivers to increase their capacity. other dams simply arrest the flow of rivers and divert the water down to the power station through pipelines.

While a water turbine is much more sophisticated than the old water wheels, it is similar in operation (see figure 2). in both cases, blades are attached to a shaft and when flowing water presses against the blades, the shaft rotates. the effect is the same as wind pressing against the blades of a windmill. after the water has given up its energy to the turbine, it is discharged through drainage pipes or channels called the "tailrace" of the power station for irrigation or water supply purposes or, in some parts of the world, even into the ocean.

In a conventional coal-fired (thermal) power station each "generating unit" consists of a boiler, a steam turbine, and the generator itself. a hydroelectric generating unit is simpler and consists of a water turbine to convert the energy of flowing water into mechanical energy, and an electric generator to convert mechanical energy into electrical energy. the amount of energy available from water depends on both the quantity of water available and its pressure at the turbine. the pressure is referred to as the head, and is measured as the height that the surface of the water in the dam/river is above the turbine down near the base at the outlet.
The greater the height (or head) of the water above the turbine, the more energy each cubic metre of water can impart to spin a turbine (which in turn drives a generator). the greater the quantity of water, the greater the number and size of turbines that may be spun, and the greater the power output of the generators.

TYPES OF WATER TURBINES

Water for a hydroelectric power station’s turbines can come from a specially constructed dam, set high up in a mountain range, or simply from a river close to ground level. as water sources vary, water turbines have been designed to suit different locations. the design used is determined largely by the head and quantity of water available at the particular site.
The three main types are: pelton wheels, francis turbines, and kaplan or propeller type turbines (named after their inventors). all can be mounted vertically or horizontally. the kaplan or propeller type turbines can be mounted at almost any angle, but this is usually vertical or horizontal.

The Pelton Wheel

Is used where a small flow of water is available with a ‘large head’. it resembles the waterwheels used at water mills in the past. the pelton wheel has small ‘buckets’ all around its rim. water from the dam is fed through nozzles at very high speed hitting the buckets, pushing the wheel around.

The Francis Turbine

Is used where a large flow and a high or medium head of water is involved.
The Francis Turbine is also similar to a waterwheel, as it looks like a spinning wheel with fixed blades in between two rims. this wheel is called a ‘runner’. a circle of guide vanes surround the runner and control the amount of water driving it. water is fed to the runner from all sides by these vanes causing it to spin. propeller type turbines are designed to operate where a small head of water is involved. these turbines resemble ship’s propellers. however, with the kaplan turbines the angle (or pitch) of the blades can be altered to suit the water flow. the variable pitch feature permits the machine to operate efficiently over a range of heads, to allow for the seasonal variation of water levels in a dam.

INDUSTRIAL HYDROELECTRIC PLANTS

while many hydroelectric projects supply public electricity networks, some are created to serve specific industrial enterprises. dedicated hydroelectric projects are often built to provide the substantial amounts of electricity needed for aluminium electrolytic plants, for example.

Small-scale hydro-electric plants

Small hydro is the development of hydroelectric power on a scale serving a small community or industrial plant. the definition of a small hydro project varies but a generating capacity of up to 10 megawatts (mw) is generally accepted as the upper limit of what can be termed small hydro.

Although large hydroelectric installations generate most of the world's hydroelectricity, some situations require small hydro plants. these are defined as plants producing up to 10 megawatts, or projects up to 30 megawatts in north america. a small hydro plant may be connected to a distribution grid or may provide power only to an isolated community or a single home. small hydro projects generally do not require the protracted economic, engineering and environmental studies associated with large projects, and often can be completed much more quickly. a small hydro development may be installed along with a project for flood control, irrigation or other purposes, providing extra revenue for project costs. in areas that formerly used waterwheels for milling and other purposes, often the site can be redeveloped for electric power production, possibly eliminating the new environmental impact of any demolition operation. small hydro can be further divided into mini-hydro, units around 1 mw in size, and micro hydro with units as large as 100 kw down to a couple of kw rating. Small hydro units in the range 1 mw to about 30 mw are often available from multiple manufacturers using standardized "water to wire" packages; a single contractor can provide all the major mechanical and electrical equipment (turbine, generator, controls, switchgear), selecting from several standard designs to fit the site conditions. micro hydro projects use a diverse range of equipment; in the smaller sizes industrial centrifugal pumps can be used as turbines, with comparatively low purchase cost compared to purpose-built turbines.

Installed large scale hydroelectric installations

Large scale hydroelectric power systems have been installed all over the world, with the largest systems having capacities over 10, 000 megawatts (mw) (equivalent to 10 gigawatts (gw)). each of these large scale systems require a very large dam, or series of dams, to store the enormous quantities of water required by the system. the kariba dam in zimbabwe, holds 160 billion m3 of water!

Economics

The major advantage of hydroelectricity is elimination of the cost of fuel. the cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed. Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago.operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation. Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation.