Data set on the experimental investigations of a helical Savonius style VAWT with and without end plates

The performance test on a helical Savonius style VAWT are carried out with end plates and without end plates for low wind velocities from 3 m/s to 6 m/s. The raw data measured using instruments are recorded using digital acquisition system. These data are processed and presented as dimensionless parameters namely, coefficient of power, coefficient of torque and tip speed ratio in order to compare it with other VAWTs.


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The performance test on a helical Savonius style VAWT are carried out with end plates and without end plates for low wind velocities from 3 m/s to 6 m/s. The raw data measured using instruments are recorded using digital acquisition system. These data are processed and presented as dimensionless parameters namely, coefficient of power, coefficient of torque and tip speed ratio in order to compare it with other VAWTs.
& 2018 Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Subject area
Renewable energy More specific subject area Wind engineering Type of data Figures and Tables  How data was acquired Experimental investigations of a helical Savonius style VAWT with and without end plates using cup type anemometer, torque sensor, RPM sensor by mechanically loading it with a dynamometer and recording the data using data acquisition system Data set to provide a benchmark for future simulation studies on this type of VAWT and possible aerodynamic design improvements.

Data
Savonius style vertical axis wind turbine (VAWT) is the simplest among all the modern types of wind energy conversion systems. With its self starting ability, it can operate at relatively low velocity winds and irrespective of the direction of wind, the rotor shaft of VAWT rotate such a way that convex side of the blade heads into the wind. The wind force is larger on the cupped side than the rounded side. This wind which curves around the cupped side exert a reduced pressure and this help to drive the rotation. As can be seen in Fig. 1, the wide gap between the two inner edges of the half cylinder allows the air to whip out the air in forward moving cupped face so that the fresh air is allowed to hit the blade.
Earlier, researchers conducted studies to understand the aerodynamic characteristic behaviors and influence of geometric design parameter on Savonius VAWT [1,2]. Recently, it was established that helical Savonius VAWT had a better performance with positive static torque coefficient at all rotor angles [3][4][5]. Study by researchers also showed that end plates increased the aerodynamic perfor- mance of Savonius VAWT [6]. Therefore, the focus of this present data set is on the effect of end plates on improving the aerodynamic performances of the helical Savonius style rotor system as represented in Tables 2 and 3. The aerodynamics performance of the helical Savonius VAWT is represented in terms of dimensionless parameters namely, coefficient of power (C P ), coefficient of torque (C T ) and tip speed ratio (λ).The data set reported here (refer Tables 2 and 3) is based on the study carried out at low wind velocities ranging from 3 m/s to 6 m/s.

Wind turbine design and experimental setup
Saha and Sukanta Roy [7] reported that an aspect ratio (A R ¼ H/D) of 1.50 gives a good performance characteristic. Hence, this aspect ratio (A R ) of 1.50 is chosen in the design of helical Savonius VAWT. The height of the helical Savonius VAWT (H) is kept as 1 m. Therefore, the swept diameter of this VAWT (D) is 0.666 m. Bhaumik and Gupta [8] proved that the optimum overlap ratio (δ ¼e/d) is 0.147 for the helical Savonius rotor. The schematic dimensional detail of the helical Savonius rotor blade is shown Fig. 2 and the dimensional details are listed in Table 1. Fig. 3 shows the 3D model of the helical Savonius style VAWT with end plates as per the dimensions mentioned in Table 1. The end plates are kept 20% larger than the overall diameter (D)  The helical Savonius blades as well as the end plates are manufactured using fiber reinforced plastic material (FRP). The FRP blades are connected to the rotor shaft by using three pairs of flat semicircular metal ribs. The assembled helical Savonius blade with the rotor shaft is shown in Fig. 4. Taper roller bearing and thrust ball bearing are used at both the ends for mounting the helical Savonius VAWT on the experimental test rig. The taper roller bearing is fixed in the housing and the thrust ball bearing is fixed above to it. The radial load acts on the taper roller bearing and the axial load is taken care by the thrust ball bearing. Fig. 5 shows the helical Savonius VAWT on the experimental test rig used for this present study. An axial fan with a variable frequency drive (ABB TM make) is used to generate free stream of air with its velocity varying from 3 m/s to 6 m/s. Cup type anemometer is used to measure the wind speed (V). A torque sensor (Sushma™ make) is used to measure the torque of this rotating system (T). A non-contact type photo electric sensor is used for measuring the rotational speed of the VAWT (N). All the measured data is recorded in a computer using data acquisition system powered by NI instruments with Labview software. With these data, the tip speed ratio, coefficient of power (C p ) and coefficient of torque (C T ) are calculated. The error of these calculated parameters are estimated to be 72.24% which is based on the standard of error estimation [9]. The performance test is conducted by loading the helical Savonius VAWT using a brake drum type dynamometer. 1 mm thick fishing nylon type thread is wound over the groove of the drum which is fixed to the rotor shaft of the helical Savonius VAWT. One end of this nylon thread is kept fixed and to its other end, a weighing pan is attached. The performance test is carried out by varying the load on the weighing pan from no load to maximum load for different wind velocities.

Performance and loading test data
The performance index of a typical VAWT is expressed in terms of coefficient of power (C p ) and coefficient of torque (C T ).Theoretical available power is expressed as P available ¼ 1 2 ρAv 3 where A is the swept area (m 2 ) and V is the velocity of the wind (m/s).
Power available at the rotor shaft can be expressed as P rotor shaft ¼ 2πNT 60 where T is the brake torque produced (N m) and N is rotational speed of the rotor shaft (rpm)Tip speed ratio (λ) is expressed as TSR ¼ ðω Â RÞ=V ¼ ð 2πN 60 Þ Â RÞ=V where ω is the rotor rotational speed in terms of radians/second and R is the rotor radius in metre.Coefficient of performance (C p ) is given as Tables 2 and 3 lists the performance characteristics of a helical Savonius VAWT at various wind velocity without end plate and with end plates respectively. On comparing the performance parameters namely, C P and C T of both helical Savonius VAWT without end plate and with end plate, the performance of helical Savonius VAWT with end plate is observed to be better. Also, the C P of helical Savonius style wind turbine with end plate is observed to be nearly even.
Polar plots for angle Vs torque for a helical Savonius style VAWT with end plates and without end plates are plotted in order to analyze the distribution of torque at various rotational angles of the VAWT's blade in its 360°of rotation. Fig. 6(a) and (b) shows the variations of torque with respect to angle in its rotational direction for helical Savonius style VAWT without end plates and with end plates respectively for a wind velocity of 4.5 m/s. Highest torque value of 0.310 is observed at 51°for helical Savonius style VAWT without end plates and its values are found to better between 30°to 60°a ngle of rotation. Similarly, the highest torque value of 0.314 is reported at 43°angle for helical Savonius style VAWT with end plates. In general, for a single revolution of VAWT, the torque is initially high (0°to 150°angle) and later it decreases gradually.

Transparency document. Supporting information
Transparency data associated with this article can be found in the online version at http://dx.doi. org/10.1016/j.dib.2018.06.113.