Abstract
As one might expect, tidal currents in terms of ebb and flood tides are approximately bidirectional. A Horizontal Axial Tidal Turbine (HATT) with unidirectional foils has to be able to face the current directions in order to maximize current energy harvesting. There are two regular solutions to keep a HATT always facing the direction of the flow, which are transferred from wind turbine applications. One is to yaw the turbine around the supporting structure with a yaw mechanism. The other is to reverse the blade pitch angle through 180° with a pitch-adjusting mechanism. The above solutions are not cost-effective in marine applications due to the harsh marine environment and high cost of installation and maintenance. In order to avoid the above disadvantages, a turbine with bidirectional foils is presented in this paper. A bare turbine with bidirectional foils is characterized in that it has nearly the same energy conversion capability in both tidal current directions without using the yaw or pitch mechanism. Considering the working conditions of the bidirectional turbine in which the turbine is installed on a mono-pile, the effect of the mono-pile on the turbine’s performance is evaluated in this paper, especially when the turbine is downstream of the mono-pile. The paper was focused on the evaluation of the hydrodynamic performance of the bidirectional turbine. The hydrodynamic performance of the bare bidirectional turbine without any supporting structure was evaluated based on a steady-state computational fluid dynamics (CFD) model and model tests. Performance comparison has been made between the turbine with bidirectional foils and the turbine with NACA foils. The effect of the mono-pile on the performance of the bidirectional turbine was studied by using the steady-state and the transient CFD model. The steady-state CFD model was used to evaluate the effect of the mono-pile clearance, which is the distance between the mono-pile and the turbine on the performance of the turbine. The transient CFD model was used to determine the time-dependent characteristics of the turbine, such as time-dependent power and drag coefficients. The results show that the bare bidirectional turbine has nearly the same energy conversion capability in both tidal current directions. The performance of the bidirectional turbine is inferior to the turbine with NACA foils. At the designed tip speed ratio, the power coefficient of the turbine with NACA foils is 0.4498, which increases by 1.6% compared to the 0.4338 of the bidirectional turbine. The turbine’s performance decreases due to the introduction of the mono-pile, and the closer the turbine is to the mono-pile, the greater effect on the turbine’s performance the mono-pile has. At the designed clearance of 1.5 DS, the presence of a mono-pile decreases the peak Cp value by 1.82% and 3.17% to a value of 0.4156 and 0.4004 for the turbine located in the mono-pile upstream and downstream, respectively. The mono-pile can result in the fluctuation of the turbine’s performance. This fluctuation will detrimentally harm the life of the turbine as it will lead to increased wear and fatigue issues.
Highlights
With the exhaustion of traditional fossil energy sources and their harmful effect on the environment, more and more efforts have been made to search for renewable and environmentally friendly energy resources
Thepurpose purpose of of this this paper evaluate the hydrodynamic performance of the bidirectional turbine in both tidal current directions, to evaluate the hydrodynamic performance of the bidirectional turbine in both tidal current as well as the as effects on turbine performance
The lift drag coefficients are larger than the NACA foils
Summary
With the exhaustion of traditional fossil energy sources and their harmful effect on the environment, more and more efforts have been made to search for renewable and environmentally friendly energy resources. Massive investment has been made in the renewable energy field to exploit sustainable energy resources. The conversion of kinetic energy from tidal currents has drawn a lot of attention as it can provide a regular and predictable energy resource. The seawater is more than 800 times denser than air and that makes marine current turbines more manageable in size than the wind ones [1,2]. The tidal current turbine operates in bidirectional flows. There are two regular solutions which are adopted from the wind turbines. One is to yaw a turbine around the supporting pile to meet the flow direction when it changes direction [3]; the other is to reverse the blade pitch angle through 180◦
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