Abstract
The hydrodynamic performance of a dual-rotor horizontal axis marine turbine (HAMCT) is investigated for the power gain in operating the rear rotor without blade-pitch control. This kind of turbine can be advantageous for a rectilinear tidal current of reversing directions, where each rotor blade is optimally fixed-pitched towards its upstream velocity. The blade element momentum (BEM) method is coupled with the Park wake model. A generic three-blade turbine is shown to gain up to 20% in the coefficient of power CP as relative to the front rotor CP when operating the rear rotor at the same tip speed ratio (TSR) as the front one, gaining overall CP up to 0.55. Analytic model is derived to backup the estimate of power gain. Plots for turbine performance variation with TSR and profile hydrodynamic efficiency are given, and analysed for lab and small-medium size turbines.
Highlights
Marine energy has an abundant potential around the world, where the tidal energy industry is emerging as a promising sector
A dual-rotor horizontal axis marine current turbine (HAMCT) was considered for its hydrodynamic performance and power gain obtained by operating the rear rotor
A gain of up to 20% in the overall CP was recorded as relative to the CP of just the front rotor, bringing the overall CP to about 0.55 as long as the rear rotor operated at the same to low speed ratio (TSR) as the front rotor
Summary
Marine energy has an abundant potential around the world, where the tidal energy industry is emerging as a promising sector. This is because the blade profile is at the wrong angle of attack (AOA) during some of the the cycle of the vertical axis turbine (Korakianitis et al, 2015) This can be partly mitigated using a variable pitch for the blades, setting the blade orientation continsouly during the cycle in order to place it in a way of producing high hydrodynamic efficiency (lift to drag ratio) most of the cycle. Other methods that follow the spirit of simplicity as one would wish to find in small to medium low cost marine current turbines is passive control This can be achieved by maintaining the vertical axis turbine at high TSR, but this may cause the turbine to miss the optimal coefficient of power CP. They still cannot mitgate completely the problem that the profile’s AOA keep changing during the rotation of the blade and the expected level of CP is still usually lower than that of the horizontal axis turbine that maintains a steady AOA for the blade profile for a steady incoming stream when it is properly positioned
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