Ocean Engineering 217 (2020) 108090 Available online 16 October 2020 0029-8018/© 2020 Elsevier Ltd. All rights reserved. Effect of number of stages on the performance characteristics of modifed Savonius hydrokinetic turbine Anuj Kumar a , R.P. Saini b , Gaurav Saini b, * , Gaurav Dwivedi c a Vellore Institute of Technology, Vellore, 632014, India b Department of Hydro and Renewable Energy, Indian Institute of Technology, Roorkee, 247667, India c Energy Centre, Maulana Azad National Institute of Technology, Bhopal, India A R T I C L E INFO Keywords: Modifed Savonius hydrokinetic turbine Computational fuid dynamics (CFD) Power coeffcient Micro hydropower Multi-stage turbine ABSTRACT Savonius hydrokinetic turbine is considered as an environmentally friendly and cost effective turbine to extract hydrokinetic potential available in fowing streams. However, this turbine has not been fully explored, as the solution for the main problem of low effciency of Savonius turbine is still being investigated around the globe. In order to study the effect of number of stages, this study aims to analyze the performance of modifed Savonius hydrokinetic turbine having twisted blade under different values of number of stages. A commercial unsteady Reynolds-Averaged Navier-Stokes (URANS) solver in conjunction with realizable k-ε turbulence model has been used for the numerical analysis. Pressure and velocity distribution found around the rotor have also been analyzed and discussed. Based on the present investigation, the maximum power coeffcient value of 0.44 is obtained for double stage turbine corresponding to tip speed ratio (TSR) value of 0.9 at Reynolds number of 37.53 × 10 4 . Using numerical data obtained, correlation has been developed for power coeffcient as a function of number of stages Reynolds number and tip speed ratio. 1. Introduction Among all the renewable energy sources, hydrokinetic energy available in river or tidal current has been considered as a great source because of its high energy concentration and predictability. Considering the large energy potential of naturally-occurring high energy fows viz. river stream, canal fow) and the limited availability of non-renewable energy resources, hydrokinetic energy could play a pertinent role (Tampier et al., 2017). Development of hydrokinetic technology can be described in four phases: traditional, empirical, establishment/growth and modern periods (Van Els.and Brasil, 2015). The hydrokinetic tech- nology is capable to meet wide range of energy demands viz. instru- mentation (10 1 W) to small communities (10 4 W) to regional utilities (10 8 W) (Forbush et al., 2016). Although innovative techniques to extract hydrokinetic energy are also being investigated, hydrokinetic energy converters can be broadly categorized as turbine type system (rotational motion) and non-turbine type viz. oscillatory motion (Laws and Epps, 2016). Furthermore, axial-fow (horizontal axis turbine) and cross-fow systems (vertical axis turbine) are considered as two distinct classes of turbine type systems based on rotor axis alignment with water fow (Kumar and Saini, 2016; Lee et al., 2019). The horizontal-axis turbines are predominantly used in tidal energy or marine current energy extraction due to their higher effciency, but these turbines are expensive for small-scale energy conversion. Due to their certain advantages viz. design simplicity, independency of fow direction and ease of installation, the vertical-axis turbine is generally preferred over the horizontal-axis turbine for small-scale power gener- ation (Saini and Saini, 2018). Moreover, these turbines need relatively lesser maintenance as various electrical components can be installed above the free surface of the water. Khan et al. (2009) stated that vertical axis turbines can be installed either as a single unit in small rivers or stacked together in large rivers. These certain advantages of vertical axis turbine have encouraged intensive research for the design and devel- opment of improved turbine (Saini and Saini, 2020a). In recent years, a number of techniques such as cyclic blade system (Hwang et al., 2009), central defector (Amelio et al., 2012), fapping blades (Yang and Lawn, 2011; Yang and Lawn, 2013; Wu et al., 2020), fexible foils (Gundersen, 2015), hybrid rotor (Saini and Saini, 2020b) and variable pitch blade (Jing et al., 2014) have been adopted to improve the effciency of ver- tical axis turbine for river, tidal and ocean applications. One of the most prominent rotor confgurations of hydrokinetic turbines is Savonius rotor turbine which was design and developed by * Corresponding author. E-mail address: gaurav161990@gmail.com (G. Saini). Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng https://doi.org/10.1016/j.oceaneng.2020.108090 Received 31 January 2020; Received in revised form 8 September 2020; Accepted 10 September 2020