In this article we will discuss about:- 1. Introduction to Wind Power Generation 2. Wind Energy Generators (WEG) 3. Combined Wind-Diesel Power Plants 4. Combined Solar-Wind Power Plant 5. Wind Potential 6. Advantages and Disadvantages of Wind-Generated Electricity.
Introduction to Wind Power Generation:
Winds are essentially caused by the solar heating of the atmosphere. They carry enormous quantity of energy. Before the development of electric power on large scale, wind powers has served many countries as source of power in early days and were called as windmills. Wind power has been used for centuries to sail vessels, pump water, and grind wheat and corn.
At the turn of this century, around 30,000 house windmills capable of generating a total of 100 MW and 3,000 industrial windmills generating another 100 MW were in operation in Denmark. Every year the winds provide 26,000 trillion kWh of which one-third is recoverable.
Wind as a source of power is very attractive because it is plentiful, inexhaustible, renewable, and non-polluting. There is no depletion of scarce resources. In large portion of the world, wind blows for 320 days in a year and this gives them an advantage over sunlight in direct conversion programmes. Operating cost of a windmill is negligible. Further, it does not impose extra heat burden on the environment.
Unfortunately this source of power is non-steady and unreliable. There are wide variations in the speed and direction of winds. Also it does not possess the basic requirements of any energy source i.e., readily availability and long lasting supply. It makes necessary to store wind energy in some other forms during periods of high winds for use later on during calm-such as in the forms of chemical energy in rechargeable batteries, potential energy in high level reservoirs, mechanical energy in flywheels etc.
Another drawback of wind power is the unwieldy size, high structural area and quite large finance requirement. Also the varying wind speed creates problem in the case of windmills employed for electric power generation (ac output). Due to this the speed of the windmill will vary resulting in fluctuating voltage and frequency.
However, methods have been devised to generate constant frequency power with varying wind speeds and consequently varying speeds of windmill propellers. In one of the mechanism, the pitch of the wind blades is controlled. Any change in wind speed is sensed by a governor and the blade pitch is automatically adjusted to regulate the output.
The kinetic energy per unit volume is given as 0.5 ρv2 where ρ is the density of air particles in kg/m3 and v is the velocity of wind in m/s. For a surface area A, the kinetic energy comes out to be 0.5 ρv2A per unit distance in the direction of wind. The wind energy is convected along at velocity v.
This means that kinetic energy available per second i.e., power available is 0.5 ρv3. All this power cannot be extracted because for this the wind velocity would have to be reduced to zero which means that the wind mill would accumulate static air around it which would prevent the windmill operation.
It is clear that the power output of a windmill varies as the cube of the wind velocity, so the windmill produces maximum power at high wind velocity. The modern windmills are usually 50 m above the ground. However, the real power available is between 0.5 and 5 km above the ground level.
The ideal maximum efficiency using Froud momentum theory is equal to 59% but an overall efficiency of 30% could be had due to aerodynamic and other mechanical losses. This gives a power of about 0.3 kW/m2 for a wind velocity of 10 m/s.
Worldwide, five nations—Germany, USA, Denmark, Spain and India—account for 80% of the world’s installed wind energy capacity. Total worldwide wind power installed capacity is 79,300 MW. Kinetic energy available in the wind is converted into electrical energy by using rotor, gearbox and generator. The wind flows over the airfoil-shaped blades causing lift, like the effect on airplane wings, causing them to turn.
The blades are connected to a drive shaft that power a pump or turns an electric generator, producing electricity. Larger blades capture more wind. As the diameter of the circle formed by the blades doubles, the power increases four times. Wind turbines are being installed on offshore oil and gas platforms in many areas to develop power for operating the equipment.
For wind machines to be economical there must be winds that blow consistently above 16-24 km per hour. Many offshore areas have ideal wind conditions for wind machines. Denmark and UK have installed large offshore wind parks to take benefit of the consistent winds. Several offshore parks are planned for the United States in the near future.
Wind Energy Generators (WEG):
Wind energy conversion devices are commonly known as wind turbines because they convert the energy of the wind stream into energy of rotation; the component which rotates is called the rotor. The terms turbine and rotor are however regards as being synchronous. The combination of the wind turbine and generator is called the aero generator.
The fraction of free flow wind power that can be extracted by a rotor is known as power coefficient i.e.,
The maximum theoretical power coefficient is 0.593.
The amount of air passing per unit time,
Q = Area through which air passes, A × Velocity of air, v
Mass of air travelling through area A swept by the rotating blades of windmill type generator
M = ρQ = ρAv
where ρ is density of air
Kinetic energy of moving air,
KE = 1/2 Mv2 = 1/2 ρAv x v2 = 1/2 ρAv3
Available Wind Power = KE 1/2 ρAv3 ….(7.11)
The physical conditions in a wind turbine are such that only a fraction of available wind power can be converted into useful work. The power available in the wind increases rapidly with the speed and therefore wind energy conversion (WEC) machines should be installed preferably in areas where winds are strong and persistent.
Wind turbine generators have been built having capacity in the range of a kW or so to a few thousands kWs. Wind power has been successfully used for cooling of homes, space heating, for operating irrigation pumps, navigational signals and for offshore drilling operations.
Basics Components of Wind Energy Conversion System:
Various parts of a wind electric generator power plant are shown in Fig. 7.43.
Basic Components of a Wind Energy Conversion System (WECS):
Aero turbines convert the wind energy into rotary mechanical energy. A mechanical interface consisting of a step up gear and a coupling transmits the rotary mechanical energy to an electric generator. The generator output is connected either to load or power grid. The purpose of controller is to sense wind speed, wind direction, shaft speed and torques, output power and generator temperature.
Combined Wind-Diesel Power Plants:
The wind power plant can be integrated with diesel generator power plant. Generation of electrical energy takes place in wind/diesel power plant. If the wind is not favourable for a prolonged period or the machine is under maintenance, the diesel generator set supplies the required electrical power. A storage battery bank, inverter and power distribution electric grid make the power supply system complete. This combined power plant is useful for remote areas where wind is favourable and diesel supply is ensured.
Combined Solar-Wind Power Plant:
The PV system can be integrated with wind power plant. Generation of Electrical energy takes place in a solar and wind power plant. The block diagram of combined wind- solar power plant is shown in Fig. 7.46.
During favourable wind periods, the wind turbine produces electrical energy. The electrical energy is stored in the battery system. During day time solar PV plant produces electrical energy. The system is connected to the grid or to the consumer.
In order for a wind energy system to be feasible there must be an adequate wind supply. A wind energy system usually requires an average annual wind speed of at least 15 kmph.
A wind generator develops lesser power in summer than in winter at the same wind speed as air has lower density in summer than in winter. Similarly, a wind generator produces lesser power in higher altitude—as air pressure as well as air density is lower than that at lower altitudes.
The wind speed is the most important factor influencing the amount of energy a wind turbine can develop. Increase in wind speed causes increase in amount of air passing the rotor and thereby increase in output of the wind system.
In order for a wind system to be effective, a relatively consistent wind flow is required. Obstructions like trees or hills can interfere with the wind supply to rotors. To avoid this, rotors are placed on top of towers to take advantage of the strong winds available high above the ground. The towers are usually placed 100 m away from the nearest obstacle. The middle of rotor is placed 10 metres above any obstacle that is within 100 m.
An important consideration in wind power generation is the economic flexibility of the wind power. The windmill designed to generate 1 MW at wind speeds above 48 kmph was erected in Vermount (USA) in 1941 had to be abandoned in 1945 for financial reasons as electricity could be made available at half the price from conventional sources.
Another 9 MW windmill was proposed for the same place but the project was not found to be economically viable. However, wind power project may prove feasible for small power needs in isolated and remote areas. A local windmill generator may offer a cheaper alternative in case long transmission lines are required to bring power from grids to these remote and isolated areas.
Advantages and Disadvantages of Wind-Generated Electricity:
Wind energy is free, renewable resource, therefore, no matter how much is used today, there will still be the same supply in the future. Wind energy is also a source of clean, non-polluting electricity. Unlike conventional power plants, wind plants produce no air pollutants or greenhouse.
Disadvantages of Wind-Generated Electricity:
i. Environmental Concerns:
Although wind power plants have relatively little impact on the environment compared to fossil fuel power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts and birds and bats having been killed (avian/bat mortality) by flying into the rotors. Most of these problems have been resolved or largely reduced through technological development or by properly siting wind plants.
ii. Cost Issues:
Even the cost of wind power has decreased dramatically in the last decade; the technology needs a higher initial investment than fossil-fueled generators. Roughly 80% of the cost is the machinery, with the balance being site preparation and installation. If wind generating systems are compared with fossil-fueled systems on a “life cycle” cost basis (counting fuel and operating expenses for the life of the generator), however, wind costs are much more competitive with other generating technologies because there is no fuel to purchase and minimal operating expenses.
iii. Supply and Transport Issues:
The major challenge to using wind as a source of power is that it is intermittent and does not always blow when electricity is required. Wind cannot be stored (although wind- generated electrical energy can be stored with the use of storage batteries) and not all winds can be harnessed to meet the timing of electricity demands.
Further, good wind sites are located in remote locations far from load centers such as cities. Finally, wind resource development may compete with other uses for the land and those alternatives use may be more highly valued than electrical energy generation. However, wind turbines can be located on land that is also used for grazing of even farming. Transportation of windmill components particularly blades is difficult due to their large.