Tidal turbine developers and researchers use small scale testing (i.e. tow tank and flume testing) as a cost effective and low risk way to conduct proof-of-concept studies and evaluate early stage device performance. This paper presents experimental performance data for a three-bladed 1/20 th scale NREL S814 tidal turbine rotor, produced at the 4.6 x 2.5 m and 76 m long Kelvin Hydrodynamics Laboratory tow tank at Strathclyde University. The rotor performance was characterised from very low tip speed ratios to runaway for four carriage speeds. A maximum C P of 0.285 and a maximum C T of 0.452 were recorded at tip speed ratios of 3.53 and 4.45 for a carriage speed of 1m/s. The uncertainty in the instrument calibration and experimental measurements was quantified, allowing accurate representation of the experiments in numerical models. The methodology behind the uncertainty calculations is described in this paper. The uncertainty in the experimental measurements was found to be less than 5% for over 87% of the tests. Reynolds number scaling effects were found to be influential on the rotor performance in the range of velocities tested.
This paper presents a sensitivity analysis on a numerical tidal stream turbine model where a multitude of input parameters' effect on the load output were determined. The statistical procedure used, known as the Morris method, provided insight into the interactions between the parameters as well as showing their comparative influence on the turbine loading. The investigation covered parameters from the operational, geometric design and inflow variable domains where the rotor radius, current shear, blade root pitch, surface velocity and wave height were identified as most influential. The blade pitch was regarded as a surprisingly prominent influence on the loads. The turbine's operating depth and the blade geometry were also found to be of limited influence in the ranges investigated. In terms of load transmission into the internal components of a turbine's drive train, the rotor out-of-plane bending moment, or eccentric bending moment, was found to be a considerable contribution to the off-axis loads on the shaft. Therefore, special attention was paid to the input parameters' relationship to the eccentric load component by performing a detailed study on the load variations caused by the identified primary input parameters. It is concluded that performing a sensitivity analysis on a tidal stream turbine in a specific operating climate can yield insight to the expected load range and that the eccentric loading transmitted to the shaft is significant for most input cases
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
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