Abstract
The Archimedes screw hydrokinetic turbine is garnering considerable interest because of its potential applicability in harvesting wave and tidal energy. The turbine is well suited to multidirectional flows, as well as low velocity flows, and shallow watercourses. Understanding its performance characteristics and energy conversion mechanisms will be fundamental in determining the optimal geometric properties, which will expedite its use in offshore renewable energy systems. Because the turbine is a fairly new hydro-technology, there is very little literature available on its design and performance optimization. This study experimentally investigates its torque generation mechanism, as well as analyzing its torque output. Laboratory scale models were tested in a water channel at the University of Calgary to measure torque at different flow velocities and varying angles of inclination (β). Maximum coefficient of power (CP) of 0.69 was obtained at β = 30° and maximum torque was obtained when the turbine was stationary. The experimental results showed a time-varying rippling in the torque, with the torque ripple increasing with tip speed ratio (λ) until a critical value of λ above which it did not change significantly. The torque ripple, measured as the difference between the maximum and minimum torques divided by the mean, decreased with an increase in β, from 11.65% at β = 28° to 9.61% at 39°. This could be due to free surface effects. Characterizing and analyzing the torque ripple, and understanding its effects, is important because it has consequences for the power output and the fatigue life of the drive train and generator. Further studies are required to provide more details of the torque ripple’s dependence on the turbine’s geometry as well as its effects on efficiency.