Unit – 4
Wind Energy
Wind has been utilized as a source of power for thousands of years for such tasks as propelling sailing ships, grinding grain, pumping water, and powering factory machinery. The world’s first wind turbine used to generate electricity was built by a Dane, Poul la Cour, in 1891.
The developments in wind systems declined as the utility grid expanded and became more reliable and electricity prices declined.
The oil shocks of the 1970s, which heightened awareness of our energy problems, coupled with substantial financial and regulatory incentives for alternative energy systems, stimulated a renewal of interest in wind power.
The wind power systems were performing below expectations and this was compensated by giving the subsidies and tax incentives to manufacturer.
Meanwhile the wind turbine technology development continued—especially in Denmark, Germany, and Spain—and those countries were ready when sales began to boom in the mid-1990s.
Total installed capacity (MW) in 2002, by country (AWEA Data) is shown in below chart.
Introduction:
A windmill is a machine for wind energy conversion. A wind turbine coverts the kinetic energy of the wind’s motion to mechanical energy transmitted by the shaft. A generator further coverts it to electrical energy, thereby generating electricity.
Wind aero generators or wind turbine generators of WECs are generally classified as,
i) Horizontal axis type and
Ii) Vertical axis type
Depending on their axis of rotation, relative to the wind system.
Horizontal axis wind machines are further sub-classified as single blades, multi bladed and by-cycle multi blades type. Soil, wind, multi bladed are example of horizontal axis wind machines, savonius and Darrieus rotor are example of vertical axis machines.
1. Horizontal – Axis machines:
The common wind turbine with a horizontal axis is simple in principle, but the design of a complete system, especially a large one that will produce electric power economically, is complex.
Some of the horizontal axis wind machines are briefly described below.
1. Horizontal axis using two aerodynamic blades:
In this type of design rotor drives a generator through a step up gear box.
2. The blade rotor is usually designed to be oriented downwind of the tower.
3. The components are mounted on a bed plate which attached on pinttle at the top of the tower.
This arrangement is shown schematically in fig.
a. Horizontal axis – two bladed mils:
i) In this arrangement rotor drives generator through a step-up gear box.
Ii) The components are mounted on a bed plate which is mounted on a pinttle at the top of the tower.
Iii) The two blade rotor is usually designed to be oriented downwind of the tower.
Iv) When the machine is operating its rotor blades are continuously flexed by unsteadily aerodynamic, gravitational and inertial loads.
v) If the blades are metal, flexing reduces their fatigue life.
b. Horizontal Axis – Multi bladed wind mills:
(i) This type of wheel has narrow rims and wire spokes.
(ii) The wire spokes support lightweight aluminum blades.
(iii) The rotor of this design have high strength to weight ratios and have been known to survive hours of free wheeling operation in 100kmph winds.
Tower
● Towers are used to support all the mechanism of WECS.
● Their height may vary from 10m to 80m.
● They are broadly classified based on their construction concrete, pole shell tube and truss.
2. Vertical axis type-
The only vertical axis machine that has had any commercial success is the Darrieus rotor, named after its inventor the French engineer G. M. Darrieus, who first developed the turbines in the 1920s.
The shape of the blades is that which would result from holding a rope at both ends and spinning it around a vertical axis, giving it a look that is not unlike a giant eggbeater.
Advantages-
- Darrieus rotor doesn’t need any kind of yaw control to keep them facing into the wind.
- The heavy machinery contained in the nacelle (the housing around the generator, gear box, and other mechanical components) can be located down on the ground, where it can be serviced easily.
- Since the heavy equipment is not perched on top of a tower, the tower itself need not be structurally as strong as that for a HAWT.
- The tower can be lightened even further when guy wires are used, which is fine for towers located on land but not for offshore installations.
- The blades on a Darrieus rotor, as they spin around, are almost always in pure tension, which means that they can be relatively lightweight and inexpensive since they don’t have to handle the constant flexing associated with blades on horizontal axis machines.
Disadvantages-
- The blades are relatively close to the ground where wind speeds are lower. As the power in the wind increases as the cube of velocity so there is considerable incentive to get the blades up into the faster wind speeds that exist higher up.
- Winds near the surface of the earth are not only slower but also more turbulent, which increases stresses on VAWTs.
- In low-speed winds, Darrieus rotors have very little starting torque; in higher winds, when output power must be controlled to protect the generator.
1. The wind resources, how fast it blows how often and when a significant role plays in its power generation cost.
● The power output from wind turbine rises as a cube of wind speed.
● Therefore higher power is achieved at higher wind speed.
● The wind speed at surface of ground is zero because of frictional effect and increases rapidly with increase in height.
● The increase in velocity with altitude rises proportionally to the seventh root of altitude i.e., . This variation is shown in Fig.
Variation of wind power density and velocity with height.
Wind Class | At 10m Height | At 50m Height | ||
Wind Power | Speed | Wind Power | Speed | |
1 2 3 4 5 6 7 | 100 150 200 250 300 400 1000 | 4.4 5.1 5.6 6.0 6,4 7.0 9.4 | 200 300 400 500 600 800 2000 | 5.6 6,4 7.0 7.5 8.0 8.8 11.9 |
● As we go higher, the wind blows faster variation of wind velocity with altitude is called wind shear.
Consider the packet of air having mass ‘m’ and velocity.
The kinetic energy can be written as,
mass of air, velocity of air (m/s)
But we have the power contain m wind or air given as
Power
Where, Energy
Time
Hence the power represented by air of pocket moving at velocity v with mass m through area A is given by
Power
If is the mass low rate through area (A) which is given as
Where Air density
Cross-sectional area
Equation -2 becomes,
From above equation, we observe that
It means doubling the wind speed, power increases 8 times.
Also
Area is given by then
Hence we can say that wind power is proportional to the square of blade diameter therefore, doubling the diameter, power increases by 4 times.
Limitations of wind energy:
The wind energy is available enormously in the world but has certain limitations for extracting. This is because of
● Wind turbines work safely between wind speed to . The lower wind speed below needs large rotor and higher wind above develops large stress on blade and system.
● The election of wind turbine is costly because the higher velocity winds are available above from ground level and need tall tower.
● Wind farms are to be located in the areas away from tall buildings, tower, cities etc.
Environment Impact:
● Wind energy is a nuisance because of mechanical (gear box) and aerodynamic noise.
● The wind turbine produces electromagnetic interference when placed between radio, television etc. stations as it reflects some electromagnetics radiations.
● It is also hazardous for civil aviation.
● It produces visual glaring because of reflection and refraction which depends on turbine size, number of turbines in wind form, colour, design etc.,
● Fatal collisions of birds caused by rotating turbine blades.
● Safety consideration for life because accidental braking of blades.
● emissions indirectly during construction of wind mills.
● Noise pollution.
● Injury to man due to mechanical failure of mills.
● Increased soil moisture.
● Global warming and windiness.
The upwind velocity of the undisturbed wind is V, the velocity of the wind through the plane of the rotor blades is , and the downwind velocity is . The mass flow rate of air within the stream tube is everywhere the same, call it . The power extracted by the blades Pb is equal to the difference in kinetic energy between the upwind and downwind air flows,
The mass flow rate is thus,
Let us assume that, the velocity of the wind through the plane of the rotor is just the average of the upwind and downwind speeds,
Let us define the ratio of downstream to upstream wind speed to be .
Power in the wind=
Fraction Extracted=
Thus the rotor efficiency, is given as,
To find the maximum possible rotor efficiency, we simply take the derivative of ,
Maximum Rotor Efficiency
Maximum Rotor Efficiency,
Blade efficiency versus wind speed with three discrete steps in rotor rpm as a parameter is shown in figure.
Unless the rotor speed can be adjusted, blade efficiency changes as wind speed changes.
It is interesting to note, however, that Cp is relatively flat near its peaks so that continuous adjustment of rpm is only modestly better than having just a few discrete rpm steps available.
Thus we know that blade efficiency is improved if its rotation speed changes with changing wind speed. In this figure, three discrete speeds are shown for a hypothetical rotor.
The figure shows the impact of varying rotor speed from 20 to 30 to 40 rpm for a 30-m rotor with an assumed gear and generator efficiency of 70%.
The figure shows the impact that a three-step rotational speed adjustment has on delivered power. For winds below 7.5 m/s, 20 rpm is best; between 7.5 and 11 m/s, 30 rpm is best; and above 11 m/s, 40 rpm is best.
The generator may need to spin at a fixed rate in order to deliver current and voltage in phase with the grid that it is feeding. So, for grid-connected turbines, the challenge is to design machines that can somehow accommodate variable rotor speed and somewhat fixed generator speed—or at least attempt to do so.
If the wind turbine is not grid-connected, the generator electrical output can be allowed to vary in frequency (usually it is converted to dc), so this dilemma isn’t a problem.
Methods of speed control-
- Pole changing induction generator
- Multiple gearboxes
- Variable slip induction generator
- Indirect grid connection system
Numerical 1- Using the data given in following table and graph, find the average wind speed and the average power in the wind (W/m2). Assume the standard air density of 1.225 kg/m3. Compare the result with that which would be obtained if the average power were miscalculated using just the average wind speed.
V (m/s) | Hrs/Year | V (m/s) | Hrs/Year |
0 | 24 | 13 | 243 |
1 | 276 | 14 | 170 |
2 | 527 | 15 | 114 |
3 | 729 | 16 | 74 |
4 | 869 | 17 | 46 |
5 | 941 | 18 | 28 |
6 | 946 | 19 | 16 |
7 | 896 | 20 | 9 |
8 | 805 | 21 | 5 |
9 | 690 | 22 | 3 |
10 | 565 | 23 | 1 |
11 | 444 | 24 | 1 |
12 | 335 | 25 | 0 |
|
| Total Hours | 8047 |
Solution-
Here prepare a spreadsheet to determine average wind speed v and the average value of .
V (m/s) | Hrs at Vi/Year | Fraction of Hours at Vi | Vi*Fraction of Hours at Vi | (Vi)3 | (Vi)3*Fraction of Hours at Vi |
0 | 24 | 0.002740 | 0.000 | 0 | 0.0000 |
1 | 276 | 0.031507 | 0.032 | 1 | 0.0315 |
2 | 527 | 0.060160 | 0.120 | 8 | 0.4813 |
3 | 729 | 0.083219 | 0.250 | 27 | 2.2469 |
4 | 869 | 0.099201 | 0.397 | 64 | 6.3489 |
5 | 941 | 0.107420 | 0.537 | 125 | 13.4275 |
6 | 946 | 0.107991 | 0.648 | 216 | 23.3260 |
7 | 896 | 0.102283 | 0.716 | 343 | 35.0831 |
8 | 805 | 0.091895 | 0.735 | 512 | 47.0502 |
9 | 690 | 0.078767 | 0.709 | 729 | 57.4212 |
10 | 565 | 0.064498 | 0.645 | 1000 | 64.4977 |
11 | 444 | 0.050685 | 0.558 | 1331 | 67.4616 |
12 | 335 | 0.038242 | 0.459 | 1728 | 66.0822 |
13 | 243 | 0.027740 | 0.361 | 2197 | 60.9442 |
14 | 170 | 0.019406 | 0.272 | 2744 | 53.2511 |
15 | 114 | 0.013014 | 0.195 | 3375 | 43.9212 |
16 | 74 | 0.008447 | 0.135 | 4096 | 34.6009 |
17 | 46 | 0.005251 | 0.089 | 4913 | 25.7989 |
18 | 28 | 0.003196 | 0.058 | 5832 | 18.6411 |
19 | 16 | 0.001826 | 0.035 | 6859 | 12.5279 |
20 | 9 | 0.001027 | 0.021 | 8000 | 8.2192 |
21 | 5 | 0.000571 | 0.012 | 9261 | 5.2860 |
22 | 3 | 0.000342 | 0.008 | 10648 | 3.6466 |
23 | 1 | 0.000114 | 0.003 | 12167 | 1.3889 |
24 | 1 | 0.000114 | 0.003 | 13824 | 1.5781 |
25 | 0 | 0.000000 | 0.000 | 15625 | 0.0000 |
| 8757 | 1.000 | 6.994 | 105625.000 |
|
The average wind speed is,
The average value of is,
The average power in the wind is,
If we had miscalculated average power in the wind using the 7 m/s average wind speed, we would have found,
The correct answer is nearly twice as large as the power found when average wind speed is substituted into the fundamental wind power equation
The function of the blades is to convert kinetic energy in the wind into rotating shaft power to spin a generator that produces electric power. Generators consist of a rotor that spins inside of a stationary housing called a stator. Electricity is created when conductors move through a magnetic field, cutting lines of flux and generating voltage and current. While small, battery-charging wind turbines use dc generators, grid-connected machines use ac generators.
- Synchronous Generators-
Synchronous generators are forced to spin at a precise rotational speed determined by the number of poles and the frequency needed for the power lines. Their magnetic fields are created on their rotors.
While very small synchronous generators can create the needed magnetic field with a permanent magnet rotor, almost all wind turbines that use synchronous generators create the field by running direct current through windings around the rotor core.
b. The Asynchronous Induction Generator-
An induction machine can act as a motor or generator, depending on whether shaft power is being put into the machine (generator) or taken out (motor). Both modes of operation, as a motor during start-up and as a generator when the wind picks up take place in wind turbines with induction generators.
As a motor, the rotor spins a little slower than the synchronous speed established by its field windings, and in its attempts to “catch up” it delivers power to its rotating shaft.
As a generator, the turbine blades spin the rotor a little faster than the synchronous speed and energy is delivered into its stationary field windings.
Due to larger and more efficient machines located at selected wind sites the wind turbine economics changes rapidly. More efficient machines located in better sites with higher hub heights have doubled the average energy productivity.
- Capital cost and annual cost-
The rated power of new machines has increased year by year, the corresponding capital cost per kW dropped. The labor required to build a larger machine is not that much higher than for a smaller one; the cost of electronics are only moderately different; the cost of a rotor is roughly proportional to diameter while power delivered is proportional to diameter squared; taller towers increase energy faster than costs increase; and so forth.
In general, O&M costs depend not only on how much the machine is used in a given year, but also on the age of the turbine.
b. Annualized cost of electricity from wind turbine-
To find a levelized cost estimate for energy delivered by a wind turbine, we need to divide annual costs by annual energy delivered. To find annual costs, we must spread the capital cost out over the projected lifetime using an appropriate factor and then add in an estimate of annual O&M.
The amount of energy in the wind that can be captured and converted into electricity depends on a number of factors, including the characteristics of the machine (rotor, gearbox, generator, tower, controls), the terrain (topography, 350 WIND POWER SYSTEMS surface roughness, obstructions), and, of course, the wind regime (velocity, timing, predictability).
Annual Energy Using Average Wind Turbine Efficiency
The wind power density is evaluated for a selected site. After assuming the overall conversion efficiency into electricity by the wind turbine, the final estimation of the annual energy delivered can be calculated. The highest efficiency possible for the rotor is 59.3%.
In optimum conditions, a modern rotor will deliver about three-fourths of that potential. To keep from overpowering the generator, however, the rotor must spill some of the most energetic high-speed winds, and low-speed winds are also neglected when they are too slow to overcome friction and generator losses.
Wind systems have negative as well as positive impacts on the environment. The negative ones relate to bird kills, noise, construction disturbances, aesthetic impacts, and pollution associated with manufacturing and installing the turbine. The positive impacts result from wind displacing other, more polluting energy systems.
Birds do collide with wind turbines, just as they collide with cars, cell-phone towers, glass windows, and high-voltage power lines. Early wind farms had small turbines with fast-spinning blades and bird kills were more common but modern large turbines spin so slowly that birds now more easily avoid them.
Noise from a wind turbine or a wind farm is another potentially objectionable phenomenon, and modern turbines have been designed specifically to control that noise.
The air quality advantages of wind are pretty obvious. Other than the very modest imbedded energy, wind systems emit none of the SOx, NOx, CO, VOCs, or particulate matter associated with fuel-fired energy systems. And, of course, since there are virtually no greenhouse gas emissions, wind economics will get a boost if and when carbon emitting sources begin to be taxed.
Winds are caused because of two factors:
● The absorption of solar energy on the earth’s surface and in the atmosphere.
● The rotation of the earth about its axis much around the sun.
Because of these factors, alternate heating and cooling cycle occur, differences in pressure are obtained and the air is caused to move. The potential of wind energy as a source of power is large.
The nature of the wind
● Circulation of air in the atmosphere is called the non-uniform bearing of the earth’s surface by the sun.
● The air immediately above a warm area expand it is forced upwards by coal, denser air which flows from surrounding area causing a wind.
● The nature of the terrain the degree of cloud cover and angle of the sun in the sky are all factors with influence this process.
The wind pattern affects power generation according to following points:
a) Dependence of Tip speed Ratio (TSR):
● The ratio of the speed of rotor blade tips to the speed of wind is called ‘tip speed ratio (TSR)
At which its maximum efficiency is achieved.
TSR (Tip Speed /ratio)
Where - Speed of rotor tip
V - Speed of free wind.
● The faster a rotor runs with respect to the wind speed, the less ‘solidity’ is required to intercept the entire stream of wind passing through rotor disc.
● The rotor with high TSRR need less material and can have relatively slender blades.
● As TSR increases the number of blades decreases see table below:
Tip speed ratio | No. Of Blades |
1 2 3 4 5-8 8-15 | 6-20 4-12 3-8 3-5 2-4 1-2 |
Blade number Vs TSR
b) Dependence of Power Coefficient:
● The efficiency is usually expressed as the (Power coefficient or performance coefficient).
● This is the fraction of wind energy passing rotor disc that is constructed rotor shaft power
c) Dependence of Solidity (S):
● Solidity is normally defined as the fraction of the total circumferential.
Where N – Number of blades
C – Average breadth of blade
D – Diameter of circle described by blade
● Rotor with low tip speed ratio has high value of solidity and they turn at low speed.
● The torque generated by propeller and Darrieus type or rotors is low due to which these are suitable for electric power generation. The solidity and TSR for important type of wind is given in table.
Sr. No | Type of rotor | Solidity | TSR | Torque |
1.
2. 3. 4. | Propeller (1 to 3 blades) Multibladed Savonius Darrieus | 0.01 - 0.1
0.7 1 0.1 - 0.2 | 4-16
1 1 5-6 | Low
High High Low |
● As power available from wind is directly proportional to the cube of wind speed therefore, doubling of wind speed results into eight times that of power output from turbine
i.e
● So turbines are designed for wind speed (upto rated value), after that they should not operate to avoid damage to entire wind turbine generator system.
● It means that rotor should be controlled to reduce driving forces on the rotor blades as well as load on WTG system.
There are three ways to control power output.
(i) 1. Pitch control:
The power output of the turbine is constantly checked by an electronic measuring unit. When power output becomes too high, it actuates the blade pitch mechanism, which turns the rotor blades out of the wind. This reduces the power output.
● In this control system, the blade tips are adjusted automatically to provide feathering action.
● This reduces the speed and power of the turbine to match with the generator speed.
● The pitch angle has wide control between 0-30.
(ii) 2. Yaw control:
The yaw control is provided to position the nacelle automatically in the direction of wind with the help of hydraulic mechanism and continuously orient the rotor in the direction of wind.
The axis is oriented in such a way that rotor swept area is perpendicular to the wind flowing either in upward or downward direction.
(iii) 3. Teething Control:
It is provided with mono and two blade type horizontal axis turbine to prevent failure because of vibration (fatigue) during orientation of nacelle.
The axis of the turbine gets positioned in such a way that the propeller blades revolve on slanting plane at higher speed.
This type of control is not required with 3 blade rotor.
Wind Energy Conversion System (WECS)-
The main components of a WECS are shown in fig. Aero turbines convert energy in moving air to rotary mechanical energy.
Most of the wind turbines have the basic parts:
Blades, shaft, gears, a generator and a cable.
(Some turbines don’t have gear boxes). These parts work together and covert the wind’s energy electricity.
- Wind turbine, including rotor and generator.
- Control systems
- Transmission system; gearbox
- Cable collector system
- Brakes
- Sensors
- Hydraulics
- Yawing system
- Wind turbine
a) Rotors:
● The rotors are only one of the important components.
● For an effective utilization all the components need to be properly designed and matched with the rest of components.
2. Windmill head:
It supports the rotor, its bearing control systems, etc.,
3. Transmission:
● The rotation of rotor blades varies from very low speeds to high speeds.
● The generator operates at approximately constant speed.
● Fixed gear mechanism or similar methods are used to tune the rotation of blades.
4. Generator:
● The generator can be fixed or varying speed generator
● The foxed generators are synchro generator or permanent magnet type
5. Control system:
Controls are very important part of WECS. They are used to control the output of the generator, direction of wind turbine, etc.,
Advantages of WECS:
● The wind energy is free, inexhaustible and does need transportation.
● It is renewable source of energy
● Like all forms of solar energy wind power system are non-polluting, so it has no adverse influence on the environment.
● Wind energy system avoid fuel provision and transparent
● Wind mills will be highly desirable and economical to the rural areas which are for from existing grids.
● Wind power can be used in continuation with hydroelectric plants.
Disadvantages of WECS:
● Wind power is not consistent and steady, which makes the complications designing the whole plant.
● Wind energy available in dilute and fluctuating in nature.
● Unlike of water energy, wind energy needs of capacity because of irregularity.
● The wind is a very hazard are specially a costly designs and controls are always required.
● Wind energy systems are noisy m operation a large unit can be heard many kilometers away.
● It has low power coefficient.
● Careful survey is necessary for plant location.
● Large area is needed.
Turbine is located in this zone of turbulence; result will be poor energy production and increased wear and tear on the turbine.
1) Distance to roads or railways:
This is another factor the system engineers must consider for heavy machinery, structures, materials, blades and other apparatus will have to be moved into any chosen WECS site.
2) Nearness of site to local centers/users:
This obvious criterion minimizes transmission length of line and hence losses and costs.
3) Nature of ground:
Ground condition should be such that the foundation for a WECS is secured. Grand surface should be stable. Erosion problem should not be there, as it could possibly later wash out the foundations of WECS destroying the whole system.
4) Favorable land cost.
5) Hill effect
6) Roughness or the amount of friction that earth surface exerts on wind.
7) Tunnel effect.
8) Turbulence
9) Variations in wind speed.
10) Wind obstacles/wind shear.
● In general during the day the air above the land tends to heatup more rapidly than the air over water.
● In coastal regions this manifests itself in strength onshore wind. At night process is reversed because the air coats dawn more rapidly over the land and the breeze blows off shore.
● Average wind speeds are greater in hilly and coastal areas than they are will in land.
● The wind also tends to blow more consistently and with greater strength over the surface at the cover where there is a less surface drag.
Site Selection for Wind Energy Conversion System-
The selection of a site for WECS is based on various factors. Some of these factors are:
- High annual average wind speed
● For a WECS system to operate properly, adequate supply of wind is necessary. The wind speed is therefore a very important factor.
● The power in wind is proportional to the cube of wind velocity. The wind velocity should be high throughout the year.
● The historical data about the wind velocity at the site should be taken into consideration.
2. Availability of wind V(t) curve at the proposed site:
● This curve determines the energy which can be produced by the WECS. The payback period of the system can be calculated through this curve.
● The plant factor at the site can be predicted by periods of availability and non-availability of wind.
3. Availability of anemometry dates:
● The anemometry height above grand, accuracy, linearity, location on the support tower, shadowing an inaccurate reading there from icing inertial of rotor.
● The principal object is to measure the wind speed which basically determines the WECS output power.
4. Wind structure at the proposed site
Wind especially near the ground is turbulent and gusty, and changes rapidly in direction and in velocity.
5. Terrain and its aerodynamic
It may be possible to make use of hills or mountain which channel the prevailing winds into a pass region, therefore obtaining higher wind power.
6. Local ecology or turbine height
In general wind turbines should be steel well above trees, buildings or a tree, the wind is slow down and turbulent is crated.