Vertical Axis Wind Turbine (VAWT)
Wind energy is not a new thing, it was also in vogue in the past. Wind turbines are the heart of the wind energy conversion system, as wind turbines converts wind energy into mechanical power. Wind turbines are of two types- horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT). VAWT’s rotational axis is vertical, and in vertical orientation rotational plane of its blades is horizontal. VAWT is gaining much popularity and getting much attention especially due to possibility of installing in the built environment. Built environment is considered as the defined locations in urban areas, for e.g. building roof top, building front or back sides, and even location between two consecutive buildings. VAWTs are of various types; out of which Savonius rotor configuration has fascinated the research community and wind turbine project implementers due to varied reasons for e.g. ability to start on its own from any direction, simple configuration of two or three semi-circular cup shaped blades mounted on the vertical shaft in ‘S’ fashion (alphabet ‘S’ bifurcated at the middle giving two semi-circular sections of the blades), less noise, low starting torque etc. However, the major problem confronting this configuration is its poor efficiency compared to other types of VAWT or HAWT. To determine the best geometry, S.J. Savonius tested more than 30 different models of the Savonius turbine in wind tunnel and reported very encouraging results. Further, Savonius conducted tests in natural wind also. He reported that the S-rotor would run at a higher speed in natural wind than in the wind tunnel for the same wind velocity, thus ideally suited for the built environment. The best of the rotor models had an efficiency of 31% while the maximum reported efficiency of the prototype was 37% (Savonius, 1931). Inspired by the work of Savonius, Bach (Bach, 1931) made some investigations of the Savonius turbine reporting a maximum efficiency 24%. Between Sixties and early 21st century, many researchers (Macpherson, 1972; Newman, 1974; Khan, 1975; Modi, et al. 1984; Sharma, 2005; Biswas, et al. 2007; Sivasegaram, 1978 and Khan, 1988) had worked on Savonius rotors, and evaluated their performance coefficients. There are various designs and operating attributes for assessing performance of the Savonius wind turbine, for that matter any vertical axis wind turbine. Specific to the Savonius turbine, the design attributes are blade overlap ratio (Savonius, 1931;Bach, 1931; Newman,1974; Khan, 1975; Modi, et al. 1984; Sharma, 2005; Biswas,2007; Sivasegaram,1978; Khan,1988; Mojola, 1985; Nobuyuki, 1992; Gupta et al. 2008; Kamoji et al. (2008) ; Roth, 1982 and Mahamarakkalage, 1980), number of blades (Modi, 1989; Alexander, 1978; Saylers, 1985 and Blackwell, 1978), aspect ratio (i.e. ratio of turbine height to its diameter (Newman,1974; Khan, 1975; Modi, et al. 1984; Sharma, 2005; Biswas,2007; Sivasegaram,1978; Khan,1988; Mojola, 1985; Nobuyuki, 1992; Gupta et al. 2008; Kamoji et al. (2008) ; Roth, 1982 ; Mahamarakkalage, 1980 and Saha, 2008), curtain design near the turbine (Altan & Atılgan, 2008a; Atılgan & Altan, 2008b) etc. The various operating conditions are free-stream wind speed, wind speed relative to the blades, blade tip-speed ratio, and rotational speed, which are variable due to the operating features of the built environment (Savonius, 1931; Biswas, 2007; Sivasegaram, 1978; Khan, 1988; Mojola, 1985; Nobuyuki, 1992; Gupta et al. 2008; Kamoji et al. (2008); Roth, 1982 and Mahamarakkalage, 1980). All these works have led to the determination of the turbine efficiency in the range 0.15-0.42. The performance parameters that depend on these attributes are power coefficient (Cp) and torque coefficient (Ct). Scientists have had attempted to optimize the Savonius turbine performance through parametric investigations so as to understand the contribution of these design parameters for the given operating conditions. Various researchers evaluated the performance of this turbine and obtained power coefficient in the range of 0.15 to 0.50 at various simulated conditions; however none of the above works attempted to optimize the performance of the Savonius turbine.