Comparison of unidirectional and bidirectional airfoils in a tidal stream turbine

Tidal stream turbines offer an attractive method for stable renewable energy generation. Due to the periodicity of the tidal stream, a tidal stream turbine can be designed to operate in a bidirectional manner, thereby avoiding a yaw control system. This article compares a unidirectional design with a bidirectional design to estimate the expected power loss for the bidirectional design. First, a blade-element momentum theory approach is used to find optimum pitch angles for the blades and give a low-cost estimate of the power production. Next, fully-resolved computational fluid dynamics simulations are performed to validate the BEMT approach and gain insight into the flow patterns. The two approaches estimate that the power output of the bidirectional design is approximately 15-20 % lower than for the unidirectional design. This suggests that although a bidirectional design will have some power loss compared to a unidirectional design it is an interesting alternative as it can yield the same power output for both the ebb and the tide. The study also serves as a starting point for further optimization of the bidirectional design.

The Effect of Using Different Wake Models on Wind Farm Layout Optimization: A Comparative Study

The accuracy of analytical wake models applied in wind farm layout optimization (WFLO) problems is of great significance as the high-fidelity methods such as large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) are still not able to handle an optimization problem for large wind farms. Based on a variety of analytical wake models developed in the past decades, Flow Redirection and Induction in Steady State (FLORIS) have been published as a tool that integrated several widely used wake models and their expansions. This paper compares four wake models selected from FLORIS by applying three classical WFLO scenarios. The results illustrate that the Jensen wake model is the fastest, but the issue of underestimating the velocity deficit is obvious. The Multi-zone model needs additional tuning on the parameters inside the model to fit specific wind turbines. The Gaussian-curl hybrid (GCH) wake model, as an advanced expansion of the Gaussian wake model, does not perform an observable improvement in the current study, where the yaw control is not included. The Gaussian wake model is recommended for the WFLO projects implemented under the FLROIS framework and has similar wind conditions with the present work.