Experimental Investigation on a Speed Controlled Wells Turbine for Wave Energy Conversion

Abstract

Ocean wave energy represents one of the most attractive renewable sources due to its high availability and predictability. Solutions based on the Oscillating Water Column (OWC) principle are one of the most promising for sea-wave energy conversion. The system is composed of two main units, an open chamber that converts the sea wave motion into an alternating airflow, and a turbine driven by this flow. The typical alternating airflow inside the OWC chamber requires a turbine with self-rectifying behavior. The Wells turbine is the simplest and most reliable turbine for this purpose in virtue of its rotor with symmetric blades staggered at 90 degrees relative to the axis of rotation. The non-stationary operating conditions of the Wells turbine strongly affect its performance when working away from its optimal efficiency point. By controlling the turbine rotational speed, the operating conditions can be kept closer to the maximum efficiency point. Recent works, based on dynamic simulations, have proposed control strategies for the turbine rotational speed, to avoid stall occurring under variable wave conditions. The present work investigates a rotational speed control in order to keep the operating conditions closer to the turbine’s maximum efficiency point. The analyses have been conducted in an experimental facility capable to simulate an OWC system with regular (sinusoidal) wave motion. Wells turbine performance has been evaluated for different control laws and it is compared to not-controlled turbine performance in order to evaluate the effectiveness of the control action.

Leading-edge flow prediction in Wells turbine under the influence of tip leakage flows

“…43 Therefore, speed adjustment with accurate control strategies can broaden the stall margin and avoid the degradation of turbine performance. 44,45…”

“…43 Therefore, speed adjustment with accurate control strategies can broaden the stall margin and avoid the degradation of turbine performance. 44,45 The streamline distribution at a 90% span of the incipient stall point (peak torque coefficient point) is displayed in Figure 7. It is worth noting that although the incipient stall point is slightly different under various tip gap and speed combinations, the Reynolds number at the same speed is almost the same.…”

Wells turbine efficiency improvements: experimental application of a speed control strategy

“…The effectiveness of the control action with respect to the stalled conditions is evident, with an increase of about 100% for the turbine overall average efficiency. If the same speed control strategy is applied to the Wells turbine when the operating conditions are not affected by stall, efficiency improvements have been observed to be lower [18], in the order of 2-4%. An open-loop control strategy was chosen and applied iteratively in order to take into account that the pressure drop through the turbine, which was used to predict the control law, varies with the operating conditions.…”

Experimental Analysis of the Three Dimensional Flow in a Wells Turbine Rotor

“…It should be observed that the operating condition selected for the present investigation (the red cross) falls in the stable region, before the stall point, with a safe distance from the latter in order to avoid incipient stall conditions on the blade that are associated with local flow separations which are not investigated in this work. The selected operating condition, i.e., with φ ≈ 0.195, is characterized by a turbine global efficiency of 48.1%, which is near its maximum, as it can be evinced from Licheri et al [21].…”