Throughout the world, energy is the most predominating factor for the development of industrial and economic growth of any nation [1]. Numerous sources are used to deliver energy for humankind on the globe. Most common sources of energy are fossil fuels because they are simple to exploit although expensive [2]. But the combustion of these fuels raises the atmospheric concentration of carbon dioxide and other greenhouse gases [3–4]. To reduce emissions of greenhouse gases and to offer sustainable energy options, non-traditional sources of energy are widely promoted and employed to generate cleaner energy production [5–6]. Renewable energy includes solar power, wind, tidal, geothermal and hydro energy etc, are employed to meet out the energy demands. In terms of contribution to electricity production hydropower generation is the most substantial renewable energy source, as rivers or canals have steady water currents almost all year long. The drawbacks encountered from hydropower are selection of site, construction of dams, greater capital cost and high maintenance cost. Due to these drawbacks hydrokinetic power were taken into consideration. The power extracted from the flowing water is termed as hydrokinetic energy and turbines which utilize energy from flowing water are known as Hydrokinetic turbines. In India, however, there hasn't been much advancement in the harnessing of kinetic energy from river currents. This type of energy generation could rank among the finest renewable energy sources since it is more reliable than wind or solar power. This technology may deliver unavoidable and reliable power derived from flowing rivers.
There are different designs of hydrokinetic turbines available, such as horizontal axis (HAHT) and vertical axis-flow hydrokinetic turbines (VAHT). Further, VAHT are mainly categorised into two kinds based on their design: Darrieus turbines and Savonius turbines. Darrieus hydrokinetic turbines are generally lift based turbines whereas Savonius turbine are drag based turbine. The Savonius turbine is a strong contender to harness hydrokinetic energy due to its ease of construction and installation and superior starting torque [7–8]. It is easy to install and fabricate less environmental effects and very low noise [9]. Savonius hydrokinetic turbines (SHT), in contrast to traditional hydraulic turbines, can operate even at 0.5 m/s [10]. Despite these benefits, the fact that it's a drag-type turbine which implies that it has poor efficiency [11]. Consequently, its poor efficiency is a serious issue that has aroused the interest of various scientists and researchers across the world [12]. The Savonius turbine’s efficiency can be enhanced by changing its designing parameters like gap ratio, aspect ratio, overlap ratio, blade profile.
Patel et al. [13] experimentally investigated the impact of overlap ratio, aspect ratio, and end plate on the performance of SHT. End plates with an aspect ratio less than 0.6 and an overlap ratio of 0.11 were reported to have higher power coefficients for narrow open channels. For a large channel, coefficient of power (Cp) increases as the aspect ratio increases for a fixed overlap ratio, but after aspect ratio 1.8, the Cp value becomes constant. Mosbahi et al. [14] performed experimental and numerical study to determine the performance of the twist-bladed SHT by implementing three different deflector designs. Numerically, it was found that turbine with deflector having diameter of 204 mm produces the best result. It was discovered that the Cp was 17.47% higher when compared to a traditional turbine without a deflector plate. Shashikumar et al. [15] used Ansys (v14.5) to carry out a numerical study on tapered and conventional turbine blades for the low velocity hydrokinetic turbine. The results suggested that for a conventional Savonius turbine, at a tip speed ratio (TSR) of 0.9, the turbine has a maximum Cp of 0.21. The main cause of poor performance in tapered turbine blades is energy loss in exit part of the advancing blade. Alipour et al. [16] proposed a parabolic blade shape for SHT based on CFD simulation. As a result, modified Savonius turbine yielded higher Cp compared to new conventional SHT and Savonius style hydrokinetic turbine, which are about 7.7% and 12% greater than conventional and new conventional profiles, respectively.
Thévenin et al. [17] optimized the shape and position of deflector plate using CFD, which resulted in higher value (by 15%) of maximum power coefficient. Talukdar et al. [18] performed an experimental study on 2-blade semi-circular profile and a 3-blade semi-circular profile to find out ideal number of blades. Later on, 2-bladed elliptical was also tested experimentally. Experimentally it was found that the 2-blade semi-circular profile outperforms the other two profiles. The maximum Cp was 28.6% higher than 2-bladed elliptical profile and 39.2% 3-bladed semi-circular profile. Numerical investigation was performed to know more about the hydrodynamic performance. Also, effect of immersion level has been investigated experimentally and it was observed that 2-bladed semi-circular turbine gives the best results compared to other two turbines at 60% and 80% immersion level. Nag et al. [19] investigated the performance of the helical Savonius hydrokinetic turbine (HSHKT), Savonius hydrokinetic turbine (SHKT), and modified Savonius hydrokinetic turbine (MSHKT) without and with a converging-diverging duct. Experimentally it was found that HSHKT and MSHKT yields 9.04% and 2.74% higher power in comparison to SHKT for 1\(\pm\)0.2 m/s, respectively. Jeeva et al. [20] proposed the installation of SHT at 15\(^\circ\) inclined from vertical. Authors reported better Cp of turbine by 60% compared to vertical SHT. However, it needs more investigation to optimize the inclination angle to minimize the effect of loading force on the shaft.
Hashem and Zhu [21] performed numerical analysis on proposed bio inspired SHT similar to that of a "Koi Carp" fish type blade profile. It was found that for the given TSR of 1.1, the proposed bio inspired turbine yielded the Cp as 0.2521 which was 17.6% more than the conventional SHT. Sarma et al. [22] conducted an experimental and numerical investigation on SHT for low velocities (0.3 m/s to 0.9 m/s) and further compared the performance of the turbine with Savonius wind turbine. The maximum for of 0.39 corresponds to TSR of 0.77 was obtained which is significantly better than output of wind turbine under same input values. Kumar and Saini [23] numerically investigated twisted profile of blade for SHT. The results showed that for twist angle of 12.5\(^\circ\), the Cp of 0.39 was achieved at a TSR of 0.9. Further, the influence of blade shape factor and blade arc angle on the performance of twisted SHT was also studied [24]. As a conclusion, optimum blade shape factor and blade arc angle are found as 0.6 and 150° respectively. Kumar et al. [25] evaluated the influence of multi-staging of twisted type SHT. The highest Cp of 0.44 at TSR of 0.9 was obtained for a double stage rotor at water velocity of 2 m/s.
It is clear from above mentioned studies that several investigations have been carried out to enhance the performance of SHT. However, most of the work has been carried out using different blade profiles - twist blade profile, a Koi Carp fish profile, a parabolic profile, an elliptical profile, Benesh profile, to name but a few. Apart from this, a very few studies [26, 27, 28, 29], have also been reported the use of slot in Savonius wind turbine to enhance the performance. But there is no information available on the use of slot for Savonius rotor in water channel. This has resulted in a possible scope to study the slotted blade profile for SHT. Furthermore, the effect of different slot parameters, like slot shapes, slot positions, slot gaps and slot shape factor (ε) on the performance of SHT is still undetermined. Keeping these gaps in mind, the main aim of this paper is to evaluate the influence of slot parameters on the performance of SHT. A commercially available software ANSYS FLUENT has been used to perform 2-D transient CFD simulations to achieve the objective of this paper. An attempt has also been made to explore the flow contours across turbine blades which help to understand the performance of turbine under different slot parameters considered. Figure 1 shows the methodology adopted for the present study.