In the month of December, however, there is a wide deviation from this pattern as shown in Fig. There is a rise in the power generated by all three turbines from the second week to the fourth week. The rise in power is due to the change in weather occasioned by the so-called harmattan season. Power 80 60 40 20 1 2 3 4 5 Week 2. Average Power generated per Week in the month of November Av.
Conclusion The suitability of a wind turbine for low wind speed regime is its ability to extract power at that low wind speed. From the performance of the three turbines as explained above, it has been proved thatwhereas the size of the blade positively influence the amount of power produced, the efficiency of the wind turbine increases with decreasing size of the blades.
This evidently is as a result of reduction in aerodynamic loses associated with reduced size of blades. References [1].
Calvert, N. G, Ltd, London, England. Dane Witt up, De Vries, O. Enrich Hau, Wind Turbines. Martin O. Hansen, Mort horst, Paul Erik, Mort horst, P. E; Journal Article- Annual Report, Vol. Prandtl, L. Ragheb, M, The energy extraction is maintained in a flow process through the reduction of kinetic energy and subsequent velocity of the wind.
The magnitude of energy harnessed is a function of the reduction in air speed over the turbine. The zero flow scenario cannot be achieved hence all the winds kinetic energy may not be utilised. This principle is widely accepted [4,5] and indicates that wind turbine efficiency cannot exceed The Betz theory assumes constant linear velocity. Therefore, any rotational forces such as wake rotation, turbulence caused by drag or vortex shedding tip losses will further reduce the maximum efficiency.
Propulsion The method of propulsion critically affects the maximum achievable efficiency of the rotor. Historically, the most commonly utilised method was drag, by utilising a sail faced normal to the wind, relying on the drag factor Cd to produce a force in the direction of the prevailing wind.
This method proved inefficient as the force and rotation of the sail correspond to the wind direction; therefore, the relative velocity of the wind is reduced as rotor speed increases Table 1. Table 1. The two mechanisms of propulsion compared. Unshielded designs rely on curved blade shapes which have a lower drag coefficient when returning into the wind and are advantageous as they work in any wind direction. These differential drag rotors can be seen in use today on cup anemometers and ventilation cowls.
However, they are inefficient power producers as their tip speed ratio cannot exceed one [4]. An alternative method of propulsion is the use of aerodynamic lift Table 1 , which was utilised without precise theoretical explanation for over years in windmills then later in vintage aircraft. Today, due to its difficult mathematical analysis, aerodynamics has become a subject of its own. Multiple theories have emerged of increasing complexity explaining how lift force is generated and predicted.
Aerodynamic force is the integrated effect of the pressure and skin friction caused by the flow of air over the aerofoil surface [7]. Attributed to the resultant force caused by the redirection of air over the aerofoil known as downwash [8]. Most importantly for wind turbine rotors, aerodynamic lift can be generated at a narrow corridor of varying angles normal to the wind direction. This indicates no decrease in relative wind velocity at any rotor speed Table 1.
For a lift driven rotor Table 1 the relative velocity at which air strikes the blade W is a function of the blade velocity at the radius under consideration and approximately two thirds of the wind velocity Betz theory Section 2 [4]. Over the centuries many types of design have emerged, and some of the more distinguishable are listed in Table 2.
The earliest designs, Persian windmills, utilised drag by means of sails made from wood and cloth. These Persian windmills were principally similar to their modern counterpart the Savonius rotor No. Similar in principle is the cup type differential drag rotor No. The American farm windmill No. The Dutch windmill No. Table 2. Modern and historical rotor designs. Ref No. HAWT are very sensitive to changes in blade profile and design.
This section briefly discusses the major parameters that influence the performance of HAWT blades. Energies , 5 5. The efficiency of a turbine can be increased with higher tip speeds [4], although the increase is not significant when considering some penalties such as increased noise, aerodynamic and centrifugal stress Table 3.
Table 3. Tip speed ratio design considerations. This can lead to reduced material usage and lower production costs. Although an increase in centrifugal and aerodynamic forces is associated with higher tip speeds. The increased forces signify that difficulties exist with maintaining structural integrity and preventing blade failure. As the tip speed increases the aerodynamics of the blade design become increasingly critical. A blade which is designed for high relative wind speeds develops minimal torque at lower speeds.
This results in a higher cut in speed [10] and difficulty self-starting. A noise increase is also associated with increasing tip speeds as noise increases approximately proportionately to the sixth power [4,11]. This has been found to produce efficient conversion of the winds kinetic energy into electrical power [1,6]. Several theories exist for calculating the optimum chord length which range in complexity [1,4,10,12], with the simplest theory based on the Betz optimisation [Equation 3 ] [1].
In instances of low tip speeds, high drag aerofoil sections and blade sections around the hub, this method could be considered inaccurate. In such cases, wake and drag losses should be accounted for [4,12]. The Betz method gives the basic shape of the modern wind turbine blade Figure 2. However, in practice more advanced methods of optimization are often used [12—14]. A typical blade plan and region classification. Assuming that a reasonable lift coefficient is maintained, utilising a blade optimisation method produces blade plans principally dependant on design tip speed ratio and number of blades Figure 3.
Low tip speed ratios produce a rotor with a high ratio of solidity, which is the ratio of blade area to the area of the swept rotor. It is useful to reduce the area of solidity as it leads to a decrease in material usage and therefore production costs. However, problems are associated with high tip speeds Section 5. Energies , 5 Figure 3. Optimal blade plan shape for alternate design tip speed ratios and number of blades [1].
Generally, in practice the chord length is simplified to facilitate manufacture and which involves some linearization of the increasing chord length Figure 4. The associated losses signify that simplification can be justified by a significant production cost saving. Figure 4. Efficiency losses as a result of simplification to ideal chord length [15]. A four bladed design offers marginal efficiency increases which do not justify the manufacturing cost of an extra blade.
Tower loading must also be considered when choosing the appropriate blade quantity [6]. Four, three, two and one bladed designs lead to increased dynamic loads, respectively [16]. Energies , 5 The imposing size and location of wind turbines signify that the visual impact must be considered. Three bladed designs are said to appear smoother in rotation therefore more aesthetically pleasing.
Faster one and two bladed designs have an apparent jerky motion [1]. Three bladed rotors are also thought to appear more orderly when in the stationary position [17]. Configuration A favourable reduction in rotor nacelle weight and manufacturing costs occur with the use of fewer blades [16]. However, dynamic structural and balancing difficulties of the polar asymmetrical rotor are apparent [16].
Increased wear, inferior aesthetic qualities and bird conservation problems are also associated with one and two bladed rotors [17,18]. Modern commercially available wind turbines include complex control and safety systems, remote monitoring and maintenance with provision for the survival of lightning strike Table 5. Figure 5. Typical configuration of a modern large scale wind turbine www.
Energies , 5 Table 4. A selection of turbine size and weight configurations. Pitch or Rotor Dia No. A Typical modern 2MW wind turbine specification. This prevents the gearbox from receiving additional loads. Reducing and facilitating its service. Brake Full feathering aerodynamic braking with a secondary hydraulic disc brake for emergency use.
Conductors direct lightening from both sides of the blade tip down to the root joint and from there across the nacelle and tower structure to the grounding system located in the foundations. As a result, the blade and sensitive electrical components are protected. Real time operation and remote control of turbines, meteorological mast and substation is facilitated via satellite-terrestrial network. A predictive maintenance system is in place for the early detection of potential deterioration or malfunctions in the wind turbines main components.
Aerodynamics Aerodynamic performance is fundamental for efficient rotor design [19]. Aerodynamic lift is the force responsible for the power yield generated by the turbine and it is therefore essential to maximise this force using appropriate design. A resistant drag force which opposes the motion of the blade is also generated by friction which must be minimised.
Traditionally aerofoils are tested experimentally with tables correlating lift and drag at given angles of attack and Reynolds numbers [24]. Historically wind turbine aerofoil designs have been borrowed from aircraft technologies with similar Reynolds numbers and section thicknesses suitable for conditions at the blade tip. However, special considerations should be made for the design of wind turbine specific aerofoil profiles due to the differences in operating conditions and mechanical loads.
Energies , 5 The effects of soiling have not been considered by aircraft aerofoils as they generally fly at altitudes where insects and other particulates are negligible.
Turbines operate for long periods at ground level where insect and dust particulate build up is problematic. This build up known as fouling can have detrimental effects on the lift generated. Provision is therefore made for the reduced sensitivity to fouling of wind turbine specific aerofoil designs [25]. The structural requirements of turbine blades signify that aerofoils with a high thickness to chord ratio be used in the root region.
Such aerofoils are rarely used in the aerospace industry. Thick aerofoil sections generally have a lower lift to drag ratio. Special consideration is therefore made for increasing the lift of thick aerofoil sections for use in wind turbine blade designs [25,26]. The angle of attack is the angle of the oncoming flow relative to the chord line, and all figures for CL and CD are quoted relative to this angle.
The use of a single aerofoil for the entire blade length would result in inefficient design [19]. Each section of the blade has a differing relative air velocity and structural requirement and therefore should have its aerofoil section tailored accordingly. At the root, the blade sections have large minimum thickness which is essential for the intensive loads carried resulting in thick profiles. Approaching the tip blades blend into thinner sections with reduced load, higher linear velocity and increasingly critical aerodynamic performance.
The differing aerofoil requirements relative to the blade region are apparent when considering airflow velocities and structural loads Table 6. Table 6. The aerofoil requirements for blade regions [26]. Stall typically occurs at large angles of attack depending on the aerofoil design. The boundary layer separates at the tip rather than further down the aerofoil causing a wake to flow over the upper surface drastically reducing lift and increasing drag forces [6].
This condition is considered dangerous in aviation and is generally avoided. It is therefore preferable that the onset of the stall condition is not instantaneous for wind turbine aerofoils as this would create excessive dynamic forces and vibrations [1].
The sensitivity of blades to soiling, off design conditions including stall and thick cross sections for structural purposes are the main driving forces for the development of wind turbine specific aerofoil profiles [1,26].
The use of modern materials with superior mechanical properties may allow for thinner structural sections with increased lift to drag ratios at root sections. Thinner sections also offer a chance to increase efficiency through reducing drag. Higher lift coefficients of thinner aerofoil sections will in turn lead to reduced chord lengths reducing material usage [Equation 3 ].
Angle of Twist The lift generated by an aerofoil section is a function of the angle of attack to the inflowing air stream Section 5. The inflow angle of the air stream is dependent on the rotational speed and wind speed velocity at a specified radius.
The angle of twist required is dependent upon tip speed ratio and desired aerofoil angle of attack. Generally the aerofoil section at the hub is angled into the wind due to the high ratio of wind speed to blade radial velocity. In contrast the blade tip is likely to be almost normal to the wind. The total angle of twist in a blade maybe reduced simplifying the blade shape to cut manufacturing costs.
However, this may force aerofoils to operate at less than optimum angles of attack where lift to drag ratio is reduced. Such simplifications must be well justified considering the overall loss in turbine performance. Off-Design Conditions and Power Regulation Early wind turbine generator and gearbox technology required that blades rotate at a fixed rotational velocity therefore running at non design tip speed ratios incurring efficiency penalties in all but the rated wind conditions [1].
For larger modern turbines this is no longer applicable and it is suggested that the gearbox maybe obsolete in future turbines [27]. Today the use of fixed speed turbines is limited to smaller designs therefore the associated off-design difficulties are omitted. The design wind speed is used for optimum dimensioning of the wind turbine blade which is dependent upon site wind measurements.
However, the wind conditions are variable for any site and the turbine must operate at off-design conditions, which include wind velocities higher than rated. Hence a method of limiting the rotational speed must be implemented to prevent excessive loading of the blade, hub, gearbox and generator.
The turbine is also required to maintain a reasonably high efficiency at below rated wind speeds. As the oncoming wind velocity directly affects the angle of incidence of the resultant airflow onto the blade, the blade pitch angle must be altered accordingly.
This is known as pitching, which maintains the lift force of the aerofoil section. Generally the full length of the blade is twisted mechanically through the hub to alter the blade angle. This method is effective at increasing lift in lower than rated conditions and is also used to prevent over speed of the rotor which may lead to generator overload or catastrophic failure of the blade under excessive load [1]. Energies , 5 Two methods of blade pitching are used to reduce the lift force and therefore the rotational velocity of the rotor during excessive wind speeds.
Firstly decreasing the pitch angle reduces the angle of attack which therefore reduces the lift generated. This method is known as feathering. The alternative method is to increase the pitch angle which increases the angle of attack to a critical limit inducing the stall condition and reducing lift. Enter the email address you signed up with and we'll email you a reset link. Need an account? Click here to sign up. Download Free PDF. Ijsetr Journal.
A short summary of this paper. The Horizontal axis wind turbines resemble an airplane design of this model wind turbine is to produce electrical power propeller with either two or three blades.
The rotor diameter is 1. Thin and unsymmetrical airfoil must wind generator may be lift or drag systems. This type of wind be selected for low Reynolds number 80, Therefore, GOE turbine is quite efficient capable of producing a useable airfoil is selected.
Blade Element Momentum theory is used output in relatively low winds. It has a high power output at to focus the power and coefficient of performance in theoretical.
This airfoil generally unsuitable for generating electricity. The average All horizontal machines must be free to rotate so that they wind velocity is around 3. Therefore, electrical can be faced into the wind.
Drag system have flat blades and power output range is from 80 to W because wind is rely on the drag force exerted by the wind on the surface of fluctuated and low intensity of wind. At the average of all the blades.
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