PERFORMANCE EVALUATION OF HYDROKINETIC TURBINES

Abstract

Rivers, tidal currents and other water channels have tremendous potential to generate renewable energy. Hydrokinetic systems are suitable for energy extraction from such sources. Based on research works conducted across the globe, it is believed that hydrokinetic technology will revolutionise both inland and offshore energy sectors. Darrieus type hydrokinetic turbines are better suited for energy extraction compared to savonius type as they function at tip speed ratio greater than one, which allows them to work on the principle of lift force generation rather than on drag force. This enables them to be more efficient and in turn generate more power. But there is one major drawback to Darrieus turbines that inhibits their extensive usage, lack of reliable self starting characteristics. In this study, the main focus was to identify the torque and power characteristics of Darrieus type hydrokinetic turbines during starting conditions. Five NACA profiles were identified for the blades for this study based on literature study. They were NACA 0012, NACA 0015, NACA 0018, NACA 0025 and NACA 63018. These were analysed using CFD simulation software QBlade. The simulation provided the values of starting torque generated by turbines at different blade angle. The number of blades of the turbine was varied from two to four and torque was calculated for all the selected blade profile. After getting preliminary results from simulation, model testing in laboratory was conducted to validate the simulation results. The rotor with blade profile NACA 0018 was selected for fabrication of prototypes. Rotors with two blades, three blades and four blades were fabricated using 3D printers for NACA 0018 profile. The simulation of two blade turbine with NACA 0018 gave a minimum torque of -2Nm. Positive torque at all angles is required for continuous operation of the turbine. During testing the turbine failed to rotate and kept oscillating at the angle at which the negative torque was shown to be generated as per the simulation. The three blade and four blade turbines also performed similar to their simulation results. It was seen that the four blade turbine produced 53% more power than the three blade turbine at similar conditions for tip speed ratio less than one. These findings gave better insight on the low tip speed operation of the turbines that occur during the start-up. From the literature it is found that two blade and three blade turbines give better Power Coefficient than four blade turbine at tip speed ratios above one, but from the tests it is seen that at tip speed ratio less than one, four blade turbines provide a smoother operation and better Power Coefficient.