Development and initial application of a blade design methodology for overspeed power-regulated tidal turbines

Abstract

The range and variability of flow velocities in which horizontal axis tidal stream turbines operate introduces the requirement for a power regulation method in the system. Overspeed power regulation (OSPR) has the potential to improve the structural robustness and decrease the complexity associated with active pitch power regulation methods, while removing the difficulties of operating in stalled flow. This paper presents the development of a methodology for the design of blades to be used in such systems. The method requires a site depth, maximum flow velocity and rated power or flow speed as input parameters. The pitch setting, twist and chord distribution were set as input parameters, variable through the use of alteration functions. Rotor performance has been broken down into OSPR performance metrics which consider coefficients of power and thrust, and cavitation inception. Three visual-numerical tools have been developed: the OSPR performance metrics were used in conjunction with a one-at-a-time sensitivity analysis approach to develop a design space; cavitation inception analyses gave plots of converging cavitation and pressure terms for each blade section; the local angle of attack and torque distribution across the blade designs were plotted at key turbine operation states. Alterations to pitch setting and twist distribution are shown to have most impact upon the design requirement of increased gradient in the rotor speed-efficiency relationship in the overspeed region; coupled with such alterations, targeted changes to the chord distribution have been shown to increase the maximum efficiency. The prevention of cavitation has been highlighted as a driver for speed-limiting design alterations. While facilitating blade design, the methodology also produces experiential knowledge which can be stored, and shared in graphical format

Rotor Loading Characteristics of a Full-Scale Tidal Turbine

“…The adoption of this overspeed control strategy removed the requirement for an active pitch system, ensuring that the number of sub-systems and failure modes were minimised for this first-of-kind device. Validation of the design was achieved through detailed numerical modelling [4,16] and subsequent verification from scale-model testing [17], while the overspeed philosophy has also received recent interest elsewhere [18,19]. It is worth noting that power regulation on fixed-pitch rotors is generally achieved through stall control, whereby the rotor is slowed to a lower λ.…”

Assessing extreme loads on a tidal turbine using focused wave groups in energetic currents

“…Tidal turbine control plays an important role in determining the performance of the turbine and the loading of the different parts of the turbine during extreme wave events. The power output of fixed-pitch turbines are often regulated in fast flow conditions [31,32,33,34]. At extreme flow speeds the turbines are sometimes mechanically braked, producing no power.…”

Modelling, control and frequency domain analysis of a tidal current conversion system with onshore converters

“…The research carried out in [31] has reduced loadings, which can be beneficial to blade lifetime, but reduced power generation as well. In [33] the blades of a horizontal-axis tidal current turbine are designed to achieve feathering by over-speeding, optimise efficiency and take into account phenomena such as the cavitation. In addition, authors in [33] describe the potential disadvantages in using the over-speeding method in tidal current turbines:…”