(i). Wind energy is the kinetic energy associated with movement of large masses of air resulting from uneven heating of atmosphere by the sun which create temperature, density and pressure differences.
(ii). It is estimated that 1 per cent of all solar radiation falling on the face of the earth is converted into kinetic energy of the atmosphere, 30 per cent of which occur in the lowest 1000 m of elevation. It is thus an indirect form of solar energy.
(iii). In contrast to diurnal availability of direct solar radiation, wind energy can be available continuously throughout 24-hour day for much longer periods, though it can vary to a great extent including no-wind periods.
(iv). It is a clean, cheap, and eco-friendly renewable source.
(v). Wind energy is harnessed as mechanical energy with the help of wind turbine. The mechanical energy thus obtained can either be used as such to operate farm appliances, water pumping, etc., or converted to electric power and used locally or fed to a grid. A generator coupled to wind turbine is known as aerogenerator.
(vi). Very slow winds are useless, having no possibility of power generation. On the other hand, very strong stormy winds cannot be utilized due to safety of turbine. Moderate to high-speed winds, typically from 5 m/s to about 25 m/s are considered favourable for most wind turbines.
(vii). Main disadvantages: are a dispersed, erratic and location-specific source.
(viii). Favourable winds for small-scale applications such as wind pumps, battery chargers, heaters, etc., are estimated to be available on about 50 per cent of earth’s surface that means small-scale wind turbines can be practical in many parts of the world.
Globally, wind energy has become a mainstream energy source and an important player in the world’s energy markets, and it now contributes to the energy mix in more than 70 countries across the globe.
Major factors that have led to the accelerated development of wind power are as follows:
(i). Availability of high strength fiber composites for constructing large low-cost rotor blades.
(ii). Falling prices of power electronics
(iii). Variable speed operation of electrical generators to capture maximum energy
(iv). Improved plant operation, pushing the availability up to 95 per cent
(v). Economy of scale, as the turbines and plants are getting larger in size.
(vi). Accumulated field experience (the learning curve effect) improving the capacity factor.
(vii). Short energy payback (or energy recovery) period of about one year.
NATURE OF WINDS
To be able to understand and predict performance of wind turbines it is essential to have some knowledge of the behaviour and structure of wind which varies from site to site depending on the general climate of the region, the physical geometry of the locality, the surface condition of the terrain around the site and various other factors.
Characteristics of the flow in the region near ground. Main conclusions may be drawn as:
(i). Wind speed increases with height
(ii). Wind speed is fluctuating with time, i.e., turbulences are present at the site.
(iii). The turbulence is spread over a broad range of frequencies.
Variation of Wind Speed with Height
(i). At the earth surface wind speed is always zero.
(ii). Wind speed increases with height above the ground. The rate of change of wind speed with height is called wind shear which results from the retardation of wind near the earth surface due to surface roughness.
(iii). The lower layers of the air retard those above them, resulting in change in mean wind speed with height. The height, for which the shear forces are reduced to zero, is called the gradient height and is typically of about 2000 m.
(iv). Above the gradient height, known as free atmosphere, changes in wind speed are not affected by ground conditions.
(v). The layer of air from ground to gradient height is known as planetary boundary layer. The planetary boundary layer mainly consists of:
(a). surface layer, which extends from the height of local obstructions to a height approximately 100 m.
(b). Ekman layer, which starts from 100 m and extends up to gradient height as shown in figure below. In the surface layer the variation of shear stress can be neglected and mean wind speed with height can be represented by Prandtl logarithmic law model:
Where, V is characteristic speed, d is zero plane displacement, its magnitude is a little less than the height of local obstructions, z0 is roughness length, (z0 + d) is the height of local obstructions.
Wind Speed Variation with height
As seen in the diagram, near the line of local obstructions the average wind speed does not follow Eq. (1), (i.e. the dotted line) but derivates from it and becomes highly erratic. It is very important then to place the wind turbine well above the height of local obstructions so that the turbine disk receives a strong uniform wind flux across its area without erratic fluctuations.
The standard wind speed measurement is often taken at a height of 10 m from ground, but wind turbines often operate at a height above this. A simple empirical power law model can be used to estimate wind speed uz at a height z relative to that available at standard reference height H.
Where uH is mean wind speed at reference height H (usually 10 m), α depends on surface roughness and the range of height being covered. Great care should be taken in using this formula, especially for z > 50 m.
Note: Good sites should have low values of α.
Power in Wind:
If u0 is the speed of free wind in unperturbed state.
Volume (V) of air column passing through an area A per unit time:
V = Au0 ………………... (1)
If ρ is the density of air, the air mass flow rate, through area A, is given by:
Power (P0) available in wind, is equal to kinetic energy rate associated with the mass of moving air, i.e.:
Power available in wind per unit area:
This indicates that power available in wind is proportional to the cube of wind speed. The air density ρ varies in direct proportion with air pressure and inverse proportion with temperature as:
Where P is air pressure in Pa.
T is air temperature in kelvin.
R is the gas constant, (= 287 J/kg K).
At the standard value of air pressure, 1.0132 × 105 Pa (i.e. 1 atmosphere), and at 15°C, the value of air density:
Assuming the above value of wind density, ρ at 15°C and at sea level, the power available in moderate wind of 10 m/s is 613 W/m2.
Every wind turbine-generator has a specific cut-in speed, where it starts generating power, a rated speed, and a furling speed where it stops generation, as shown in Figure below.
MAJOR APPLICATIONS OF WIND POWER
Wind turbines have been built in a power output range from a kilowatt to a few MW to suit a wide range of applications. Major applications may be grouped into three categories.
- Applications Requiring Mechanical Power
- As an Off-Grid Electrical Power source
- As Grid Connected Electrical Power source
For the detailed schedule of the ESE Mechanical Engineering(ME) 2021 Achievers Study Plan, click here
If you aiming to crack GATE & ESE, Other PSU Exams then you must try Gradeup Super to get unlimited access to all the live structured courses and unlimited mock tests from the following links:
Click on the Links Below to Avail Green Card:
Sahi Prep Hai Toh Life Set Hai !!!