Abstract
Self-starting performance is a key factor in the evaluation of a Darrieus straight-bladed vertical axis turbine. Most traditional studies have analyzed the turbine’s self-starting capability using the experimental and numerical data of the forced rotation. A 2D numerical model based on the computational fluid dynamics (CFD) software ANSYS-Fluent was developed to simulate the self-starting process of the rotor at constant incident water-flow velocities. The vertical-axis turbine (VAT) rotor is driven directly by the resultant torque generated by the water flow and system loads, including the friction and reverse loads of the generator. It is found that the incident flow velocity and the moment of inertia of the rotor have little effect on the averaged values of tip-speed ratios in the equilibrium stage under no-load conditions. In the system load calculations, four modes of the self-starting were found: stable equilibrium mode, unstable equilibrium mode, switch mode and halt mode. The dimensionless power coefficient in the simulations of passive rotation conditions is found to be, on average, 38% higher than those achieved in the simulations of forced rotation conditions.
Highlights
Abundant resources of marine energy make it possible to use the energy in the ocean as an alternative and renewable power source in the future
Besides experimental studies in the wind tunnel, the computational fluid dynamics (CFD) approach was adopted for the investigations on the self-starting of direct-driven turbine
The results indicate that the previous studies that used the methodology of a given rotation underestimated the real operating performance of the vertical-axis turbine (VAT) rotor without any controls
Summary
Abundant resources of marine energy make it possible to use the energy in the ocean as an alternative and renewable power source in the future. The inability of a Darrieus turbine of low solidity to self-start arises from the existence of a band of TSRs below the operating condition during which the net amount of energy collected by each blade in one revolution is negative [5] This negative or reverse torque region is called the “dead band”; this was plotted as Cp (dimensionless power coefficient)—λ (TSR) curves in [6]. Besides experimental studies in the wind tunnel, the computational fluid dynamics (CFD) approach was adopted for the investigations on the self-starting of direct-driven turbine. These studies can reveal the starting characteristics and the related flow fields of a VAT from rest till the operational rotating speed is reached [22,23,24,25].
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