WIND ENERGY – TECHNOLOGY IN WIND TURBINES

WIND ENERGY – TECHNOLOGY IN WIND TURBINES

           Over 5,000 years ago, the ancient Egyptians used wind power to sail their ships on the Nile River. Later, people built windmills to grind their grain and pumping water. In 1891, the first electrical output wind machine was developed incorporating the aerodynamic design principles. Nowadays, wind energy is mainly used for generating electricity. The future looks bright for wind energy because technology is becoming more advanced and windmills are becoming more efficient.

TYPES OF TURBINES

Wind turbines can be classified into two types:-

 1.      VAWT (Vertical Axis Wind Turbine) :-    

            Drag is the main force utilized for this type of turbine. Nacelle is placed at the bottom but yaw mechanism is not required. This turbine suffers with the problem of difficulty in mounting the turbine, unwanted fluctuations in the power output and low or insignificant starting torque.

             In the case of Darrieus devices, the rotor must be brought up to speed either by using the generator as a motor or by means of a small secondary rotor, such as a Savonius, mounted on the Darrieus main shaft.

 2.      HAWT (Horizontal Axis Wind Turbine) :-

             Lift is the main force acting in this type of turbine. 95% of the existing turbines are HAWTs. Nacelle is placed at the top of the tower and Yaw mechanism is required. It is affected by much lower cyclic stresses. Upwind turbines are used mostly. Because wind velocity increases at higher altitudes, the backward force and torque on a horizontal axis wind turbine (HAWT) blade peaks as it turns through the highest point in its circle.

             The tower hinders the airflow at the lowest point in the circle, which produces a local dip in force and torque. These two effects combine to produce a cyclic twist on the main bearings of a HAWT. The combined twist is worst in machines with an even number of blades, where one is straight up when another is straight down. There are two types of HAWTs:-

       a)      Downwind Turbine

       b)      Upwind Turbine

                  A wind turbine can be either "upwind" (where the rotor faces into the wind) or "downwind" (where the rotor faces away from the wind). A downwind design offers some engineering advantages, but when a rotor blade passes the "wind shadow" of the tower as the rotor revolves, it tends to produce an "impulsive" or thumping sound that can be annoying. Today, almost all of the commercial wind machines on the market are upwind designs, and the few that are downwind have incorporated design features aimed at reducing impulsive noise.

TURBINE DESIGN AND CONSTRUCTION

                  The wind speed is higher in offshore. Offshore turbines are having less noise and less visual impact but difficult to install and maintain. Most of the countries are following design criteria for turbines using IEC (International Electro technical Commission) standards. ISO (International Organization for standardization) standards are also used for designing gears and bearings. The primary objective of designing a wind turbine is to get the maximum efficiency and long term working. The factors involved in achieving this target are as follows:-

 a)      Turbine Blades:-

                                In the past, older turbines with metal blades caused television interference in areas near the turbine. This problem has been solved by making components of composites. The blades used in turbine are the most important factor in achieving the objectives. The main factors taken into consideration while designing or constructing turbine blades are:-

   Ø    Material :-

              One of the best construction materials available is graphite-fiber in epoxy. Graphite composites can be used to build turbines of 60m radius, enough to tap a few megawatts of power. Smaller household turbines can be made of lightweight fiberglass, aluminum or laminated wood. Wood and canvas sails, used on early windmills were superseded with solid airfoils due to higher maintenance and low aerodynamic efficiency.

   Ø    Typical length :-

               The maximum blade-length of a turbine is limited by both the strength and stiffness of its material.

Wind power intercepted by the turbine  α  (blade length)²

   Ø    Number of blades :-

               Turbines can be built with any number of blades. But there are many constraints such as vibration modes that increase in peak intensity as the number of blades decreases. Thus, noise and wear considerations point to larger numbers of blades (at least 3). Multiple blade turbines are generally used for water pumping purposes.

              Many small scale wind turbines use 2 blades because they are easy to construct, avoid the need for using a hub with linkages to individual blades, and the blade can be shipped easily in one long package. Three-bladed turbines, which are much more efficient, and quieter, require more complicated onsite assembly.

 b)     Tower height:-

               The wind blows faster at higher altitudes because of the drag of the surface (sea or land) and the viscosity of the air. The variation in velocity with altitude, called wind shear is most dramatic near the surface. For HAWT, tower heights approximately twice the blade length have been found economical.

                                 Wind speed  α  (altitude)^1/7

               Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%. Doubling the tower height generally requires doubling the diameter as well, increasing the amount of material by a factor of eight.

 c)      Rotational control:-

  The speed at which wind turbines rotate must be controlled for several reasons:-

  Ø  Maintenance :-

        It is dangerous to have people working on a wind turbine while it is active. It is sometimes necessary to   bring a turbine to a full stop.

Ø  Noise reduction :-

        The noise from a wind turbine increases with the fifth power of the relative wind speed. In noise-              sensitive environments, noise limits the tip speed to approximately 60 m/s. High efficiency turbines may          have tip speed ratios of 5-6, which, for onshore turbines, limits high efficiency operation to winds of just 10    m/s. Noise can be reduced by streamlining nacelles, sound-dampening buffer pads, special gearboxes.      

Ø  Centripetal force reduction :- 

         As the rotational speed increases, so does the centripetal force working on the central hub or axis.           When it exceeds safe limits blades could snap off and the turbine would fail gradually.                                         On a pitch controlled wind turbine, electronic controller checks the power output of the turbine                several times per second. When the power output becomes too high, it sends an order to the blade pitch        mechanism, which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the          blades are turned back into the wind whenever the wind drops again. 

Ø  Mechanisms :-        

        The mechanisms involved in running turbine blades are mainly stalling and furling. Passive stall controlled     wind turbines have the rotor blades bolted onto the hub at a fixed angle. The geometry of the rotor blade       profile however has been aerodynamically designed to ensure that the moment the wind speed becomes         too high. This stall prevents the lifting force of the rotor blade from acting on the rotor.

           An increasing number of larger wind turbines (1 MW and up) are being developed with an active stall      power control mechanism. To attain a large torque at low wind speeds, the active stall machines will             usually be programmed to pitch (turn) their blades much like a pitch controlled machine at low wind               speeds.   The pitch mechanism is usually operated using hydraulics or electric stepper motors. 

           If the generator is about to be overloaded, the machine will pitch its blades in the opposite direction in     active stall whereas it will increase the angle of attack of the rotor blades in order to make the blades go         into a deeper stall in passive stall control, thus wasting excess energy in the wind.

 d)     Yaw Mechanism :-

               The part of the rotor which is closest to the source direction of the wind, however, will be subject to a larger bending torque than the rest of the rotor. It means that the rotor will have a tendency to yaw against the wind automatically. On the other hand, it means that the blades will be bending back and forth in a flap wise direction for each turn of the rotor. Wind turbines which are running with a yaw error are therefore subject to larger fatigue loads than wind turbines which are yawed in a perpendicular direction against the wind. It is being rectified by using a combination of electric motors and gear boxes.

 e)      Wind turbine safety :-

              Apart from the target of attaining maximum efficiency, it is also required to consider the aspect of safety of the wind turbines so that it can be used for a long time. Some of the technological improvements which have been done are as follows:-

Ø  Sensors :-

                     Sensor consists of a ball resting on a ring. The ball is connected to a switch through a chain. If the turbine starts shaking, the ball will fall off the ring and switch the turbine off. There are many other sensors in the nacelle, e.g. electronic thermometers which check the oil temperature in the gearbox and the temperature of the generator.

Ø  Over speed protection :-

              It is essential that wind turbines stop automatically in case of malfunction of a critical component. There are basically two types of braking systems:-

§  Aerodynamic Braking System

Aerodynamic braking system is provided for turning rotor blades about 90° along their longitudinal axis (in case of a pitch controlled turbine or an active stall controlled turbine), or turning the rotor blade tips 90° (in case of a stall controlled turbine). Aerodynamic braking systems are extremely safe.

§  Mechanical Braking System

 The mechanical brake is used as a backup for the aerodynamic braking system.

FUTURE WIND TURBINES         

                        

1.      Counter Rotating HAWT :-

           Counter rotating turbines can be used to increase the rotation speed of the electrical generator. When the counter rotating turbines are on the same side of the tower, the blades on the one in front are angled forwards slightly so as to never hit the rear ones. They are either both geared to the same generator (which suffers additional gearing loss) or one is connected to the rotor and the other to the field windings (mechanically simpler but wastes some electric and mechanical power due to slip rings for field windings).

            Counter rotating turbines can be on opposite sides of the tower. In this case it is best that the rear one to be smaller than the front and set to stall at a higher wind speed. This way, at low wind speeds, both turn and the generator taps the maximum proportion of the wind's power. At intermediate speeds, the front turbine stalls; but, the rear one keeps turning, so the wind generator has a smaller wind resistance and the tower can still support the generator. At high wind speeds both turbines stall, the wind resistance is at a minimum and the tower can still support the generator. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower.

             To reduce vibrations, the two turbines should turn with certain speed ratios. Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds.

2.      Wind Amplified Rotor Platform (WARP) :-

               Wind Amplified Rotor Platform system amplifies the ambient wind speed, through its multi-tasking aerodynamic modules or wind frames, to simple, standardized commodity horizontal axis (propeller-type) wind turbines.

           Each modular wind frame provides highly amplified wind flow fields from 50% to 80% over free air wind speed to each conventional, small diameter wind turbine of no more than 1 meter to 3 meters in diameter. Each module also serves as a support for the wind turbines, a yaw assembly and protective housing for the core support tower and other internal subsystems.

3.      DISC TYPE WIND TURBINE :-

               Rotatable shutters mounted on a circular disk automatically open when directed into the wind, regardless of the wind's direction.  Pairs of upper and lower shutters are geared together. The bottom shutter opens in the downward direction and its weight helps to lift the upper shutter in the upward direction, as the wind applies an opening force against both shutters. When the shutters reach the vertical position, the force of the wind is transferred from the open shutters to the circular disk.  And the circular disk is attached to the vertical axis for power output. The circular disk, shutters, and outer vertical axis rotate together. The outer vertical axis is mounted via bearings over an inner vertical axis that is stationery.

            The shutters work in the opposite direction also as they reverse direction during their rotation and move into the wind on the opposite side of the wind turbine.  When the wind is not blowing, the shutters open by gravity because the lower shutter is weighted to be slightly heavier than the upper shutter. Wind blows against the open shutters and the open shutters with stops apply a force against the disk, but the open shutters with no stops merely close due to the force of the wind and the wind turbine begins spinning no matter what direction the wind comes from. Operation of the turbine is remarkably quiet.

                  Hunt's vertical axis creates leverage by increasing its width instead of height.  This allows the vertical turbine to be used in many applications, in which horizontal axis turbines cannot be used, such as the rooftop of a house or building, as a sailboat wind turbine over a cabin area etc. This turbine is much more efficient than HAWT.

4.       CONCENTRATORS :-

           The turbines described above are quite feasible in open areas such as farmlands and villages. The construction of wind turbine is very difficult in developed areas such as towns and cities. The flow of the wind is being restricted by the buildings in cities due to which wind turbines find it hard to accumulate maximum wind energy. 

                            Wind turbines were unable to provide the desired efficiency in developed cities around the globe. To overcome this issue, concentrators are being designed. These concentrators are coupled in between buildings or sometimes kept alone. They are additional features provided to wind turbines for accumulation of maximum amount of wind energy. The wind flow is being concentrated towards the wind turbine due to the streamlining features in the concentrators. Nowadays, research is going on coupling two or more concentrators for multiple turbines so as to obtain the maximum power output.   

CONSTRAINTS & ADVANTAGES

CONSTRAINTS:-

           Some of the constraints involved in determining the economics of the wind energy are:-

Ø  Costs depend very much on the wind speed at that site since the power varies as cube of the wind speed

Ø  More than 60% of total costs are contributed on the design and construction of turbines

Ø  Larger wind farms are known to be more economical than small wind farms due to rated capacity of the turbine

Ø  Exact location and orientation of the turbine greatly affects the economy of the wind energy

Ø  Improvements to be made on turbine design such as use of light weight material

Ø  Wind energy is a capital intensive source of energy

ADVANTAGES:-

           The advantages of the wind turbines are as follows:-

Ø  Greater fuel diversity

Ø  Wind turbines are easy to construct and does not require long gestation periods

Ø  Maintenance costs are very less compared to installation costs

Ø  Except for wind speeds greater than 30 mph, once installed, wind equipment last for more than 25 years

Ø  Farmers earn additional income by leasing their land for wind turbine

Ø  Wind industry produces more jobs per unit energy produced than other forms of energy

Ø  No hidden costs, which greatly reduces the environmental impacts