Wind power is a great source of energy. Do you know that the energy in the wind is a form of solar energy. Moreover, wind power are turbulent masses of air rushing to even out the differences in atmospheric pressure created when the sun hits the air more in one place than in another. Intuitively, we know that the faster the wind blows, the more power is available – the stronger the wind is. This can be found on high altitude wind power to produce more energy. Though the wind is invisible, its effects are quite apparent. On some days, in a light wind, a kite will barely stay or destroy the power of the wind. For this reason, the wind can be insufficient to support a wind system, but it can also be too powerful for wind equipment to function without damage.
How does the wind renewable energy works?
Today, these present day wind systems consists of a rotor or turbine which often has blades somewhat similar to these of an airplane propeller. An electrical generator, both of which are usually mounted upon a tower. While, the conversion of wind power into electrical or mechanical power, the wind rotates the turbine or rotor which turns the shaft of the electrical generator or mechanical device. If the system produces electricity, the electrical power courses through wires to be used immediately. It can also be stored in battery banks for later use.
The use of inverters, for changing the type of electrical current supplied, and a control panel also make up parts of the typical wind power system. Of all of this components, the characteristics of the rotor and generator and the reaction to varying wind speeds play the most significant roles in determining the power available in the wind especially on high altitude wind power sites.
Wind was another early source of power used to multiply the productive capacity of the human muscle. On the seas, it has been used to propel ships, and on land, it has served a variety of purposes like pumping water, grinding grains and generating electricity.
The theoretical power from wind can be given by the formula:
Theoretical power = (pA V3 ) / 2
Wnere, P = air density, 1.25 kg/m3
A = rotor area, m2
V= wind speed, m/s
However, using the Froude momentum theory, the maximum energy or power recoverable fom wind is only 16/27 0r 0.593 times the kinetic energy. Of this available energy, the amount that is actually utilized depends upon the aerodynamic efficiency of the blades, the friction losses in gearing, power transmission, wind velocity gradient due to ground effects, vibration dampening and other factors. For estimation purposes, the following formulas can be used:
Mechanical energy conservation, e.g. pumping
Available power = .245 V3
Electric energy conversion
Available power = .010 A V3
The power coeffiiceint, Cp, of a windmill is the ratio of the actual power delivered to the theoretical wind power determined. Indeed, the values given are the maximum values which occur at certain tips speed ratios, (blade tip speed/wind speed). In this figure, it shows the power coefficients of various wind rotors.
Classification of wind rotors
|According to rotational speed||According to shaft position||According to blade material|
|1. High speed-four blades or less; more than 30 rpm||1. Horizontal-axis windmills
2. Vertical-axis windmills
|1. Sail type
2. Metal bladed type
|2. Low-speed- multi-bladed; not more than several hundred rpm||2. Inclined-axis windmills/ combination of horizontal and vertical axis windmills||
3. Wooden bladed type
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In Betz’s Formula shows that:
- The power in the wind is proportional to the cube of the wind speed
- Also, the power in the wind is proportional to the area swept by the rotor
- Next, the power in the wind is proportional to the density of the air
- A perfect wind system extracts a maximum of only 59.30% of the total power in the wind.
FACTORS TO CONSIDER IN SITE SELECTION FOR WIND POWER SYSTEM
- First, the average wind speed must be good enough.
- Next, local topography of the area.
- Furthermore, the presence of obstacles and height in the area.
- Is free from risk of accidents, noise, interference with telecommunications?
CATASTROPHIC FAILURES OF WIND ENERGY CONVERTERS
- Blade failure
- Tower failure
- Collision by a low flying aircraft
THE EFFECT OF WIND SPEED
The power in the wind, and therefore its energy is proportional to the cube of the wind velocity. Of course, this means that as the wind speed increases, the power available increases much faster. Next, this extreme variability of the power strongly influences most aspects of the system design. In fact, wind speed can also affect construction, siting, use and economy.
The equation describing the power in the wind is:
Power (W) = ½ x density of air (kg/m3) x Cross-sectional area (m2) x (Velocity )3 in m/s.
THE EFFECT OF AIR DENSITY
What is the effect of air density? So, the density of the air affects the energy available to a very much lesser extent than the wind velocity. Since, the air density is affected by:
- Atmospheric pressure
However, the effects of temperature and atmospheric are very small compared with altitude and may therefore be ignored. Indeed, there should be an allowance made for altitude, however. For example, at an altitude of 1000 meters the energy available from the wind at a given wind speed is reduced by 11%.
Sadly, only part of the energy in the wind is available for use. Furthermore, to extract all energy would require bringing the wind to rest which is impossible. Since, the available energy is extracted by slowing down the wind and using some of its kinetic energy. The maximum amount of energy that can, even in theory, be physically extracted from the wind is 59.3% (Betz limit) of the total available. After all, in practice wind rotors are not perfectly efficient. However, the good ones will be able to extract 25-40% of the total kinetic energy. Finally, most of this energy can be found high altitude wind power.
Tip Speed Ratio (tsr) – is the ratio of the blade tip velocity to that of the wind. Further, high speed windmill have high tip speed ratio (up to 15) while low speed one have (Tsr) from 1 to 2.
Number of Blades – The selection of number of blades affects power coefficient. For this reason, the lower design tip speed ratios in general have higher number of blades for different tip speed ratio.
Tip Speed Ratio
|Number of Blades
Torque of the Wind Renewable System
It is the force produced by the rotor. In fact, it depends on the solidarity and the tips speed ratio of the motor. Identically,high solidarity with low tip speed ratio produced more torque than low solidarity high speed machines, though they both deliver the same power.
- The power of the two wind power systems is the same, but it is delivered at different angular speeds
- The maximum power is delivered at a higher angular speed than at the maximum torque.
- The maximum of the power curves vary with the cube of the angular speed while the corresponding torque values vary with the square of the angular speed.
- The starting torque (torque at zero rpm) is considerably lower for high speed than for the low speed wind power systems.
Pitch or Angle of Attack
All air foils require some angles with the air flow in order to produce a lift. The more lift require, the larger the angle (until a certain extent). This angle is called the pitch or angle of attack.
The rotor is most efficient if this angle “seen” by the blade is large as possible stalling the rotor since the tip of the blade travels faster through points nearer to axis the blade should be twisted so that the angle of attack will be uniform throughout the blade length.
DRAG AND LIFT COEFFEICIENT
To describe the performance of an air foil in dependent of size and air velocity we divide lift L and drag D by
V = slow velocity
A= blade arc = chord x blade length
The results of these are called Lift Coefficient Cl and Drag Coefficient Cd.
Cl = L / (2 p2v2 A) Cd = D / (2 p v2 A)
Even more, it must be emphasized that the amount of Lift and Drag that is produced depends on the angle of attack. Moreover, for optimum rotor design, Cd / Cl ratio must be as small as possible.
Points to Consider:
Solidity – the percentage of the circumference of the rotor blades. Furthermore, the greater is the solidity of the rotor, the slower it needs to turn to intercept the wind and would need lower wind velocity to start.
Betz Coefficient – it is not possible to transform all the wind energy that flows through the cross-sectional area A into mechanical energy. While, the maximum power coefficient was found by Betz to be 16/27 or 0.593. This is known as the Betz Coefficient.
Wind Power Coefficient (Cp) = is the ratio of the actual power converted by the rotor to that of the wind. As stated before, the maximum Cp is 0.593. After all, in actual practice good rotors will have Cp of 0.22-.40.
Wind Energy as a Whole
Energy in the wind is a great source of energy. Moreover, there are many pros and cons in this technology. There are many factors to be considered in having this kind of renewable energy. Lets continue to search for more areas for wind power. High altitude wind power area can be more feasible. Next, there should be a good site study to be conducted to assure the success of the project. In conclusion, renewable energy is good. In fact, nature benefits from these technologies which can easily be replicated. Finally, having varied energy sources especially clean ones will improve the totality of human life.
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