The utilization of urban wind energy through small wind turbines has become an arising technology to ease the conflicts between rising energy demands in buildings and depletion of traditional energy resources. Many studies have reported that drag type vertical axis wind turbines have superior performance in the unsteady wind because of their attractive features. Several review studies have been conducted on these turbines. They mainly focused on the geometrical design parameters, the flow patterns, the research methodology, and the wind tunnel blockage correction. However, less research has been conducted to classify the Savonius rotor based on the classification criterion of the blade profile and has made a comprehensive performance comparison between different types of Savonius rotors. The blade profile is the essential design issue for a Savonius rotor; therefore, the variation of the blade profile will change the design parameters and affect the rotor's performance significantly. So, the classification based on the blade profile can not only present the effect of key design parameters on rotor's performance for each blade profile easily but also obtain the functional features of different profiles and the comparison among different profiles. Hence, in this article, we aim to summarize the classification of Savonius wind turbines according to the blade profile and present the development of this promising low speed generator.

This paper presents the experimental results of the vertical axis wind turbine (VAWT), especially the Savonius rotors, which are developed for application on top of buildings due to lower wind speed compared with remote areas. The VAWT is regarded as the most suitable elegant candidate to be integrated with buildings for its relatively low operating noise, easy maintenance, and easy access. This study explored the interaction effect of the overlap ratio, phase-shift, and stage on the performance of the Savonius rotors in details. The results show that appropriate choice of the phase-shift angle according to the overlap ratio can not only increase the power coefficient of the Savonius rotors but also eliminate the negative static torque and smooth the variation of the static torque coefficient. Moreover, the performance of the two-stage rotors indicates that the two-stage rotor is the best candidate for this type of wind turbines or as a starting assistant for the Darrieus rotors.