Design of a novel hydrokinetic turbine for ocean current power generation.

Abstract

In a world with a growing need for energy, but also a growing need to decrease dependence on fossil fuel energy production, new methods of energy generation are required. The energy which exists in flowing rivers, ocean currents and various other artificial water channels is considered a viable option for a source of renewable power. Water energy harnessing systems are referred to as Hydrokinetic conversion systems. These types of systems are still in the infant stages of development as the main focus of renewable energy in recent years has been solar and wind energy harvesting. Research into ocean currents focusing on the Agulhas Current was conducted to be used as a basis for a Subsea Hydrokinetic system to be implemented off the coast of South Africa. The results obtained from Eskom’s Acoustic Doppler Current Profiler (ADCP) readings depict that the Agulhas current is a more than adequate source of renewable energy. The readings indicate that the Agulhas current varies from 0.3 m/s to 1.2 m/s throughout the year. A proposed Vertical-Axis Hydrokinetic turbine was designed to harness power from the Agulhas current. The design focuses on the selection of blade profile to exploit the lift force which is a result of the interaction with the flow medium. The blade’s helical structure is based on the relationship with the initial attack angle of the blade at a 0° azimuthal position based on the mathematical formulae derived by Gorlov. The study compares a toe-out angle which is not modelled by Gorlov. It was assumed that the turbine’s efficiency would be increased as long as the toe-out angle is within a specified range. Analytical and Computational Fluid Dynamic (CFD) simulations have been conducted on the designed hydrokinetic turbine. The analytical simulations implement double multiple stream tube models which have been used for predecessors of the vertical axis helical turbine. The analytical model was performed in QBlade which is software that implements the double multiple streamtube theory. The CFD model was conducted within Star CCM+TM which included complex vortex interaction and interference with the turbine. Results obtained from the analytical model show that the turbine with a toe-out blade pitch had a slightly lower performance coefficient than the turbine with no blade pitch within the range of 1° to 2°. The results from the CFD simulations and the analytical modelling were in good agreement. The results obtained for the performance of the turbine for toe-out blade pitch from the analytical and CFD modelling were 50% and 52%, respectively. The results are in line with that of simulations and testing of similar type turbines conducted by previous researchers. Comparisons from the effect of toe-out angle on the turbine’s performance proved that the turbine does not experience an increase in efficiency; however, the torque fluctuations decreased for increasing blade pitch. The toe out angle reduced the amount of torque fluctuations on the turbine rotor which provides fewer fluctuations to the coupled generator.