Even though wind energy is a deep-rooted technology, but not yet mature and hence there are bounteous scopes for improvement to reduce the cost of wind energy. An experimental investigation has been carried out on 1:25 scaled S809 aerofoil blade featuring boundary layer fence at various span wise location. Quantifying electrical power obtained by rotation of wind turbine rotor coupled with dynamic testing system. A baseline model with no flow control and an upgraded model with detachable boundary layer fence have been studied in the wind tunnel. For upgraded model, fences were placed along the location of 40% to 90% of the blade span. The rotor blades are then tested dynamically in wind tunnel at open terrain condition for 7 m/s, 9 m/s and 11 m/s velocities. In order to study the effect of boundary layer fence test has been carried out in the low speed wind tunnel having test section of size 0.9 m × 1.2 m × 2 m. Scope corder DL 750 is used to measure time varying voltage and proximity sensor with its compatible display unit is used to measure the rotor RPM. The flow behaviour was found to be considerably favourable from conventional rotor blades. Installation of fence has been found promising for increased energy extraction from air column by controlling the three dimensional span wise flow. Results demonstrate the potential of the proposed model which can obtain a maximum of about 11.8% increase in the power. In addition, the significance of the location of wing fence and blade pitch angle has been analysed.
In recent years, wind energy has emerged as one of the most reliable means of producing large quantities of power in the field of renewable energy technology. In order to improve the economic viability of the wind turbines, wind energy projects should concentrate on efficient energy capture out of the rotors. As most of the wind turbines have restrictions on rotor diameter in some form or another, the only way the power production can be optimized at any specific wind velocity is through maximizing the Cp of wind turbine (Johansen et al. [
Taehwan Cho [
Flow control is one of the leading areas of research of many scientists and engineers in Fluid mechanics. Flow control involves passive or active devices that have a beneficial change in the flow field. A passive flow control (PFC) is a method in which a flow is modified without external energy expenditure. Passive techniques include geometric shaping to manipulate the pressure gradient, the use of fixed mechanical vortex generators for separation control, and placement of longitudinal grooves or riblets on a surface to reduce drag. In Contrast, Active Flow Control ( AFC ) is a method in which energy, or auxiliary power, is introduced into the flow. Many researches had been carried out to change the flow behaviour over the blade surface from past years. Shane Merchant [
In the present study, models were designed with the goal of supporting experimental observations in wind turbine aerodynamics. The need to support these diverse applications dictated a number of specific design requirements which includes;
・ A realistic energy conversion process enabled by good aerodynamic performance at the aerofoil and the blade level translating in to reasonable power production
・ Dimensions of the models as large as possible, to avoid excessive miniaturization which would complicate the realization of flow control capabilities.
・ On the other hand, the rotor dimension should not cause excessive blockage effects due to interference with the wind tunnel walls.
・ A comprehensive and dynamic testing system to measure the electrical output power produced as a result of the wind turbine rotation due to wind flow. It should be capable of measuring the voltage values with respect to time for all the measurements.
The experimental model was designed based on the NREL phase VI rotor model having a full scale rotor diameter 10.058 m. In order to keep blockage below 5% of test section area, the model rotor diameter was taken as 40 cm, leading to a geometric scaling factor of 1:25. The height of the turbine blades was set to 20 cm, the detailed dimensions were given in [
Experiments are conducted in the low speed open circuit wind tunnel with Turbulence Intensity of 0.1%, in which the fan speed can be varied in steps of 1 RPM. The wind tunnel has a rectangular test section with a cross sectional area of 0.9 m ×1.2 m and length of 2 m and a maximum speed in the test section is 40 m/s, the test section is equipped with multi channel pressure scanner which can measure simultaneous pressure measurement with the sampling frequency of 325 Hz. The tunnel is powered by the speed controlled electrical motor that drives Propeller. Inlet guide vanes at the entrance of the contraction, a honeycomb and screens are put upstream of the test section to maintain appropriate flow quality. Additionally, dynamic testing system was developed to measure the electrical output power produced as a result of the wind turbine rotation due to wind flow in the test section of the wind tunnel. The testing system had been instrumented with a 6V D.C Generator, which is fitted in the nacelle; wind turbine rotor model assembled in the generator shaft, a decade resistance box, an ammeter, Rpm measurement sensor with an indicator and Yokogawa scope corder―DL 750. Predescu [
The experiment is conducted to find the effectiveness of the resistances in the circuit by measuring the voltage and amps due to varying the resistance in a wide range of 0 to 4000 ohms using DRB is executed. A wide variation in voltage and amps were found due to the variation of resistance in the range of 100 ohms to 1000 ohms. The tests were conducted by varying 3 different resistance values 100 ohms, 400 ohms and 700 ohms and the corresponding current and voltages are measured. In order to record time varying voltage, Scope corder is connected to the circuit as shown in
A digital multimeter is installed in the circuit to measure the current. The wind turbine rotational speed is measured with a proximity sensor connected to a digital display. The sensor is set to sense a metal needle attached to the hub of the turbine rotor. Once the effect of resistance on power production has been analysed, the scaled down model of the conventional rotor model, i.e. the baseline model is tested at 3 different air speeds (say 7 m/s, 9 m/s and 11 m/s) and the generated electric power is measured utilizing the dynamic testing system. Following that, to analyse the effectiveness of the wing fences and its location on the wind turbine blade the upgraded model is mounted on the test section. Then boundary layer fence of 1 mm thickness had been installed in a different span wise locations r/R = 0.9, 0.8, 0.7, 0.6, 0.5 and 0.4 its effectiveness is studied by testing it under the conditions similar to the previous one.
In all these cases, the power output measurements are performed using the dynamic testing system, which helps understanding the effect of the location of the wing fences on wind turbine blades. The wind turbine rotor model assembled in wind tunnel is shown in
Scope corder, which is used to measure the voltage in time series have an accuracy of ±0.25%. The display unit which is used to measure RPM is accurate to 2 decimal places. To obtain a stable output, the RPM measuring unit needs at least 30 seconds, so far all the test cases, air flow is allowed to stabilize for 30 seconds to 1 minute and then the readings are taken. The blade pitch angle has been changed from 0˚ to 30˚ in steps of 5˚. These angles are marked in turbine hub and another marking is provided in the blade. Marking provided on the blade is made to coincide with the corresponding markings of angles manually for which the experiments are performed. There is a chance for manual error and it can be eradicated by providing an automatic angle changing mechanism in future. Machining of the prototype had been done in 5 axis milling machine and it has been replicated in FRP. Chance for error in this regard is very minimum.
Wind turbine blades fitted with Boundary layer fence were tested in the experimental facility available at Madras Institute of Technology, Chennai. Installation of wing fences produced a promising increase in power performance. This is most likely due to the fact that the wing fence acts as a barrier, preventing the growth of a span wise and separated flow outboard the fence. The effect of the fence is discussed in detail in the following section.
Experimental investigations were performed to find out the effect of resistance on the increase in power for different blade pitch angles. Multiple resistances have been considered for experimental observation and experiments were carried out on the rotor blade fitted with fence having thickness of 1 mm at different span wise locations. It is evident from the result that the 700 ohms resistance stands out among the rest of the resistances contemplated i.e. 400 ohms and 100 ohms. When the greatest resistance is applied, it will create an impact on the generator and forces the generator to rotate at a much higher rpm which will induce a mechanical stress on the generators. The rotor blades have to sweep the atmospheric air at a much greater rate than before to relieve the mechanical stress induced in the generator. This dictates that higher resistance results in peak rpm. However, as expected, the maximum power performance observed is at 700 ohm resistance, followed by decrease in power for further low resistances. The explanation of this effect is based on the load applied to the wind turbine rotor blades. The greater the resistance offered in the electrical system greater will be the load implied on the turbines to produce power output. The relation between the blade rotational velocities relative to the local wind speed also depends upon the resistance change on the wind turbines. The effect of resistance change over the % of power has been studied. At 50% span location, for 7 m/s velocity the peak power is associated with the blade pitch angle of 20 degree which yields 11.4% increase in power for 700 ohm as shown in
production at various velocities has been estimated as 11.6%, 10.9% and 9.8% for 9 m/s, 11.88%, 11.51% and 10.31% for 11 m/s respectively. It gives a clear picture that higher resistances provide a commendable rise in percentage of power. A Similar trend has been observed in other span wise locations also. Further, the results prove that power performance decreases with the least resistance. Even though the trend of power output and resistances is same, the free stream velocity, blade pitch angles and span wise fence locations have its own impact on the power production.
It is evident from the research that the installation of the wing fences increases the power production. In order to deduce the effect of fence location, fences have been placed at various span wise locations, observations were carried out for wind turbine blades with fences at several span locations based on the factor entitled r/S (ratio of radius at which the fence is installed to the overall blade span). The blade with fences positioned at various r/S is shown in
Observations are carried out from r/S = 40% of the span to r/S = 90% of the span and the increase in power production has been calculated using Equation (1).
Wing fences installed at various locations produce a significant increase in power production. The graphs were plotted between the percent of increase in power with respect to the pitch angle for various fence locations. Observations were carried out for wind turbine blades with fences at several span locations based on the factor entitled r/S. The factor r/S shows the resistance along the span wise distribution. In order to deduce the effect of the location of the wing fence the observations are carried out at wide ranges from r/S = 40% of the span to r/S = 90% of the span as shown in
A similar trend had been observed for 9 and 11 m/s velocities in which fence installed at r/S = 0.5 prove better than the other locations is shown in Figures 9-11.
In order to optimise the effect of fence size, fence with 3 different sizes have been selected: L/t = 1, L/t = 2 and L/t = 3 and placed at various span wise locations. Effect of Fence Location (r/S) on Power have been studied and the results for r/S =1 have been reported in section 4.2. The blade set with fences of different sizes, L/t = 2 and L/t = 3 are shown in
Similar to section 4.2, the tests were carried out for blade set with fences L/t = 2 & L/t = 3 in the spanwise locations 40%, 50%, 60%, 70%, 80% and 90%. Though the results are found to be optimum at 50% spanwise location, the comparison for L/t = 1, 2 and 3 were presented in Figures 14-16 for 50% location, 7 m/s free stream velocity and 700 ohms resistance. From the graphs 14, 15 and 16, it is evident that fence with L/t = 1 stands out when compared with L/t = 2 and 3. The maximum increase in power is found in 20 degree pitch angle for all the cases. Also it is inferred that, when the fence size increase beyond L/t = 1, the power decreases. Maximum of 11.88% increase in power is recorded at 20% blade pitch at 11 m/s velocity for configuration with L/t = 1. Configutrations with L/t = 2 and L/t = 3 produce 9.67% and 8.12% increase in power respectively. From the results, it can be concluded that the fence L/t = 1 is the optimum.
The starting wind speed is one of the pertinent key aspects among the wind turbine technologies. The start-up- speed is the minimum wind speed needed for the blades to begin spinning. Lower the start up speed greater will be the power production. Generally, various active and passive devices have been used by researchers to influence the startup speed. However, the fence effect will influence without trading off the performance. In this research influence of fence contributes to the earlier start up speed. At 50% r/S, it is observed that the starting velocity is lower than the conventional rotor blade model as shown in
The wind tunnel experiments carried out on the conventional wind turbine blade and the blade with boundary layer fence revealed the influence of boundary layer fence in power production. Results show that the influence of blade fence and its effect on power production has been discussed in detail. The major conclusions are:
1) Wind turbine rotor with boundary layer fence exhibits better performance than the conventional wind turbine
rotor model. Maximum of about 11.8% increase in the power had been obtained with the use of blade fence.
2) The blades with span wise fence locations show significant increase in power. This is most likely due to the physical flow phenomena, where the 3-Dimensional flow has been effectively controlled, which results in increased pressure difference, in turn increases the RPM and hence the increase in 11.8% (at maximum) of power is obtained.
3) From the results, it is observed that maximum increase in 11.8% of power is obtained at the pitch angle of 20%.
4) Investigation of fence location studied from 40% to 90% of the span wise length endows that the efficiency is predominant at 50% of fence location.
5) It is evident from the results that fence with L/t = 1 stands out when compared with L/t = 2 and 3 to produce the maximum power increase of 11.8%.
6) Moreover, the starting speed of the wind turbine is decreased by 8.65% due to the inclusion of fence in conventional rotor. Concussive, the pressure difference is increased and the lift component and the torque component have been initiated at much lower wind speeds.
7) In addition to increase in percentage of power, the fence L/t = 1 at 50% span wise location records earlier starting speed.
Sundaravadivel Arumugam,Nadaraja Pillai Subramania,Senthilkumar Chidambaram, (2016) Effect of Boundary Layer Fence Location on HAWT Power Performance. Circuits and Systems,07,1177-1189. doi: 10.4236/cs.2016.78101
NREL: National Renewable Energy Laboratory
TSR: Tip Speed Ratio
DRB: Decade Resistance Box
Cp: Power Coefficient
V00: Free Stream Velocity (m/s)
VStarting: Wind velocity at which rotor starts its rotation m/s
AFC: Active Flow Control
PFC: Passive Flow Control
r: distance along the spanwise direction in blade (m)
S: Radius of rotor (m)
L: Length of the Fence projected normal to the Blade (mm)
t: Thickness of the Fence (mm)
PU: Power produced by the upgraded rotor blades (w)
PC: Power produced by the baseline rotor blades (w)