PERFORMANCE ENHANCEMENT OF A CROSS-FLOW HYBRID HYDROKINETIC TURBINE

Abstract

Hydrokinetic technology is an emerging technology that utilizes the kinetic energy of free stream water flows in rivers and canals, to generate mechanical power. This technology offers non-polluting alternatives to non-renewable energy-based power generation. Depending on the scalability of the project, the advantages of using this emerging technology are; it requires very less or negligible civil work, is eco-friendly, does no harm to the aquatic life, has good predictability, can be installed at a narrow stream of a river and has low initial cost. Axial-flow HKT and cross-flow HKT are two different types of HKTs which are categorized on the basis of the direction of flow relative rotational axis (shaft) of the rotor. Based on earlier studies, it was concluded that the cross-flow HKTs are more suitable for rivers and canals flows. It makes this rotor for application in rivers and canals having shallow depths. Therefore, a cross-flow HKT rotor has been considered in the present study. The Darrieus turbine shows high efficiency compared to the Savonius turbine while the Savonius turbine exhibits better self-starting characteristics. To improve the overall performance of the HKT, the Darrieus HKT rotor can be integrated with the Savonius HKT rotor to make a hybrid HKT. Very few studies have been carried out on the cross-flow hybrid HKT. Therefore, a systematic approach needs to be carried out to enhance the performance of the cross-flow hybrid HKT. Under the present investigation, a cross-flow hybrid HKT consisting of Darrieus and Savonius HKTs is considered. The objectives of this study are; i). To identify the system parameters and develop a 3D model of a cross-flow hybrid hydrokinetic turbine based on the selected system parameters, ii). To carry out numerical simulations for the proposed cross-flow hybrid hydrokinetic turbine model under specific operating conditions for validation, iii). To conduct experimentations for validation of simulation results, iv). To investigate numerically the performance of the model for different system parameters viz. Savonius helical blade angle, radius ratio and attachment angle under different flow velocities and v). To develop a correlation for power coefficient in terms of considered system and operating parameters. A numerical study has been carried out for the considered range of system and operating parameters. The system parameters such as Savonius helical blade angle, radius ratio and attachment angle are considered in the range of 0o to 180o, 0.2 to 0.8 and 30o to 150o, respectively. The operating parameters like water flow velocity and TSR are selected in the range of 0.5 m/s to 2.0 m/s and 0.3 to 1.5, respectively. The data reduction on the basis of numerical results are analyzed in terms of the average power coefficient and static torque coefficient. Based on the considered range of system parameters, different configurations of the cross-flow hybrid HKT along with flow domain were modelled. Afterward, the computational domain was discretized into a small number of cells by selecting unstructured meshing technique. Numerical simulations were carried out by using commercially available ‘ANSYS CFX v.15’ solver. In order to validate the simulation results with experimental results, a model of a crossflow hybrid HKT was designed and fabricated. Experiments were performed at “Hydraulic measurement laboratory, HRED, IIT Roorkee (India)”. The numerical values of the power coefficient were validated with the experimental values which found to be in good agreement. Furthermore, the flow behavior of water across the turbine obtained through experimental analysis has also been validated with contours obtained from the numerical study. Based on the numerical investigations, it was found that the average power coefficient of the hybrid HKT rotor increases as the Savonius helical angle increases and attains the maximum value of 0.209 corresponding to the Savonius helical blade angle of 45o. Further, the starting characteristics of the hybrid rotor with the Savonius helical blade angle is improved in comparison to the hybrid rotor with a conventional Savonius blade. The maximum average power coefficient is found as 0.220 corresponding to the cross-flow hybrid HKT with the optimum value of Savonius helical blade angle, radius ratio and attachment angle of 45o, 0.4 and 90o, respectively. On the basis of numerical results, it was observed that the system parameters such as the Savonius helical blade angle, radius ratio and attachment angle and operating parameters such as water flow velocity and TSR significantly affect the power coefficient of the crossflow hybrid HKT. Therefore, a correlation has been developed for the power coefficient of the cross-flow hybrid HKT to correlate the system and operating parameters. An attempt has also been made to develop nomograms to determine the optimal value of system parameters of a prototype cross-flow hybrid hydrokinetic turbine under different values of water velocity for a given power output within the range of 1.0 kW to 5.0 kW. The predicted model is also validated with the numerical results. The outcomes of this study may be helpful to researchers for further studies and industries for developing the small capacity prototypes of the cross-flow hybrid hydrokinetic turbines.