Technical noteInvestigation of self-starting and high rotor solidity on the performance of a three S1210 blade H-type Darrieus rotor
Introduction
In recent years, major global concerns posing threat to the mankind are energy crisis due to fast depleting conventional energy reserves and pollution from burning of fossil fuels in the urban areas. Distributed wind energy systems in the built environment could be installed for power generation and other small-scale applications in a suitable manner for satisfying energy needs of people. Vertical Axis Wind Turbines (VAWTs) are in existence for some time now because of simplicity in structure, independent of wind direction, less expensive than propeller type turbine, less prone to structural failures etc. VAWT rotors can be broadly classified into two types: Savonius and Darrieus rotors. Out of these, Darrieus has higher coefficient of power and hence can be used for electricity generation. Darrieus type VAWT has attracted lot of attention in recent time as it eliminates the main problem of yawing associated with horizontal axis wind turbine, and it also has high performance. H-type Darrieus rotor has become more popular due to their straight blade designs and simpler construction features. Such rotor exhibits higher power coefficient compared to the conventional Savonius rotor or even the oval shaped Darrieus rotor; hence this could be used for harnessing wind energy for the performance of various functions ranging from electrical power generation to pumping water, grinding crops, sailing boats etc. especially in remote places where grid-connected electricity is a scarce. However this type of VAWT rotor suffers from poor starting characteristics due to symmetrical blade designs. Thus there is a need for research work to explore alternative blade designs to make the rotor self-starting and also improve their performance. To address the problem of self-starting, many alternative rotor designs and detailed experimentations on them have been performed in the recent past. A combined design of Savonius and oval shaped Darrieus rotor was investigated to make the Darrieus rotor self-starting at any wind speed and also increase its overall power coefficient compared to Savonius rotor [1], [2], [3], [4]. However such designs are too complex to fabricate and erect in the built environment where space could be a limitation. Earlier attempts at improving the self-starting of VAWT were mainly concentrated on optimizing configurations of static geometric parameters which includes turbine solidity, blade camber and thickness, blade offset pitch angle and blade lean forward angle etc [5]. Other approaches that were proposed to improve self-starting includes the use of inclined blades [6], use of flexible sails [7], variable pitch [8] etc. Though these approaches contribute to the increase of the starting torque, but reduction in peak efficiencies and operating range were some of the major problems reported. However, self-acting unsymmetrical cambered airfoil section [9] could be a promising design aspect to obtain good starting characteristics. Barker [6] compared the theoretical performance of a cambered NACA 0012 airfoil with that of a symmetrical NACA 0012 airfoil blade and found that the cambered section would produce more tangential thrust over a wider range of angle of attack and also more energy per cycle. Kirke and Lazauskas [5] predicted that unsymmetrical cambered NACA 4415 blade would have good self-starting characteristics, which can also be achieved with higher rotor solidity. Beri et al. [10] performed two-dimensional unsteady CFD analysis on cambered fixed pitch three bladed H-type Darrieus rotor. The result of simulation showed that the average torque generated could be sufficient to self-start a cambered airfoil. Gupta & Biswas [11] studied the effect of blade twist at chord ends on the self-starting of an unsymmetrical three-bladed H-type Darrieus rotor. It was concluded that self-starting of the rotor could be expected for blade angle greater than 90° with blade twist of 30° when the chord Reynolds number is as low as 6.1 × 104. Another important factor for self-starting is the number of blades of the rotor. Optimising the number of blades will yield good starting characteristics and a good performance. Dominy et al. [12] studied effect of number of blade (NACA 0012 type) on the self-starting of the H-rotor. They concluded that two-bladed turbines have the potential to self-start but that the self-starting capability does not extend to all possible starting positions which could be a problem for commercial turbines. However, they reported that three-bladed rotor is able to self-start under steady wind conditions.
Section snippets
Objective of the present study
From the previous section, it can be understood that replacing VAWT's conventional blades with unsymmetrical blades and increasing rotor solidity could potentially solve the problem of self-starting. However, detailed studies on the effect of increased rotor solidity on the performance of VAWT rotor having unsymmetrical blades are scarce. It is mentioned in the literature [13] that solidity should not be more than 0.5 as close proximity of blades deteriorates the performance of the rotor. But
Design and fabrication of H-type Darrieus rotor
The H-type Darrieus rotor is made from three unsymmetrical S1210 airfoil blades of height (H) 30 cm and chord (c) of 10 cm. The design of the blade is shown in Fig. 1(a). Generally a wide range of materials is used in wind turbine blades. Currently, steel is not much used for blade manufacturing. Only about 5% of the blades are manufactured using it [17], [18]. For selecting the material many factors are considered like strength to weight ratio, fatigue strength, cost per power etc. For
Experimental procedure
The experiments were conducted by placing the rotor (Fig. 2(a)) in the open air at the outlet of the centrifugal blower test rig. A 10 hp electric motor having rated rpm 2880 ran the blower. The wind speed was varied by controlling the flow rate with a hand wheel at the end of circular pipe, which was adjusted according to the requirement. The blower rpm was adjusted so as to maintain turbulent intensity of the test rig less than ±1%. Moreover attaining flow uniformity was also strived for.
Experimental results
In order to determine the self-starting ability of the present rotor, an experimental investigation was conducted to evaluate the static torque coefficients at different rotor azimuthal angles over a complete cycle of rotation for different rotor solidities or rotor aspect ratios (H/D), like 0.8, 0.9, 1.0, 1.1 & 1.2. It has been found that the rotor self-starts irrespective of rotor position for all the solidities considered as the static torque coefficients are positive for all azimuthal
Comparison of self-starting with existing literatures
In comparison with the work of Mohamed [16], the static torque coefficients of the present rotor are much higher at any azimuthal positions; though in both cases, the values are positive. All the static torque coefficients of Fig. 7 correspond to three-bladed H-Darrieus rotors with different blade profiles. And solidity of the present rotor is unity while solidity of the other two rotors is less than unity. As can be seen from Fig. 7, on an average the static torque coefficient for the present
Comparison of dynamic torque and power coefficients with existing literatures
Finally to compare the overall performance of the present design, it is resolved to compare its dynamic torque and power coefficient variations with that of Mohamed [16] for unsymmetrical S1046 blade H-Darrieus rotor and that of Howell et al. [21] for symmetrical NACA 0022 blade H-Darrieus rotor. All these results correspond to three-bladed configuration where solidity of the present rotor and Howell et al. rotor is 1.0 and that of Mohamed, it is 0.25. Fig. 8, Fig. 9 respectively show the
Conclusions
In this paper a three-bladed H-type Darrieus rotor equipped with unsymmetrical S1210 blades was investigated first for determining its self-starting characteristics at various azimuthal positions for different rotor solidities from 0.8 to 1.2. Then the power coefficients (Cp) were evaluated for these solidity conditions at various wind speeds and TSR conditions. The results were compared with some symmetrical/unsymmetrical blade H-Darrieus rotors to obtain some performance insights of the
Acknowledgement
The authors thankfully acknowledge the services of Mr. Samjid Ali and Mr. Sashi M. Roy of Carpentry shop of ME Deptt. for their help in the fabrication of the rotor models.
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