Power Plant Engineering

62 POWER PLANT ENGINEERING

The performance of a wind mill rotor stated as coefficient of performance is expressed as:

Cp = A/Pmax

= A/ (1/2 ρV^3 )

where ρ = Density of air

A = Swept area
V = Velocity of the wind
Further the tip speed ratio being the function of speed at the tip of the rotor to the wind speed,
i.e. U/V and in most of the parts of India, the wind velocity being low (through the wind energy average
around 3 kWh/m^2 day) The exploitation of wind mills in India is feasible. Depending upon the survey
of velocity in a region the appropriate value of design parameter may be computed.

2.15.3 Wind Power Systems

You usually stand in an open space to enjoy the wind.

You know how wind originates. Moving air is wind. Since

the wind has velocity it has kinetic energy. This is the en-

ergy of the wind. We shall see how the kinetic energy of the

wind can be used to produce electricity. For that, we can use

windmills. Windmills are devices, which work on wind. How

the kinetic energy of the wind is made use of in windmills

shall be looked into. We shall examine the working of a wind-

mill. The important part of a windmill is a structure with

large leaves, fixed at the top of a high tower.

What will happen when wind blows on these leaves?

You may have seen paper fans available at festival places,

rotating when the wind blows. In a similar manner the speed

of leaves changes with the speed of the wind. What happens

if the rotation of the windmill is given to the rotor of a gen-

erator? Rotor also rotates. Then electricity is obtained from

the generator. What happens if the windmill is connected to

a water pump? As the leaves of the windmill rotate pump

works pumping out water.

Wind machines are just as efficient as coal plants. Wind plants convert 30 percent of the wind's

kinetic energy into electricity. A coal-fired power plant converts about 30-35 percent of the heat energy

in coal into electricity. It is the capacity factor of wind plants that puts them a step behind other power

plants. Capacity factor refers to the capability of a plant to produce energy. A plant with a 100 percent

capacity rating would run all day, every day at full power. There would be no down time for repairs or

refueling, an impossible dream for any plant. Wind plants have about a 25 percent capacity rating

because wind machines only run when the wind is blowing around nine mph or more. In comparison,

coal plants typically have a 75 percent capacity rating since they can run day or night, during any

season of the year.

One wind machine can produce 275-500 thousand kilowatt-hours (kWh) of electricity a year.

That is enough electricity for about 50 homes per year.

In this country, wind machines produce about three billion kWh of energy a year. Wind energy

provides 0.12% of the nation's electricity, a very small amount. Still, that is enough electricity to serve

Fig. 2.6. Windmill farm.