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Tidal devices are likely to faced with shear flows and subjected to various wave climates. The paper presents an experimental study of the combined impacts of shear profile and irregular waves on the loading of a 1/20th scale device operating at peak power extraction. The experiments presented were conducted at various depths to facilitate analysis of the effects of the shear flow and wave impact on the device at various positions in the water column. The fluid field was measured at three different upstream positions and at three depths (top, middle and bottom of the rotor) for each experiment; in doing so, data from the device were captured three times. The fluid measurements were of a high quality and were analysed to present the structure flow upstream of the device, which contained velocity and turbulence profiles. The upstream measurement was utilised to understand the development of flow structures in the approach to the device, and the impact of the flow structures measured was confirmed via cross-covariance calculations. The long datasets gathered were used to produce full rotational probability density functions for the blade-root-bending moments for three blades. The spectral characteristics were also considered, and showed that rotor loading quantities are less reactive to smaller scale flow structures. [ABSTRACT FROM AUTHOR]

The measurement of power performance is an important procedure in the design verification and ongoing health monitoring of a tidal turbine. Standardised methods state that the performance should be measured relative to two independently located flow sensors, the arrangement of which is often non-trivial and necessitates additional cost. Recent interest in the usage of flow sensors mounted on the turbine has demonstrated their capabilities in profiling the rotor approach flow, but this instrument configuration is not recognised in the performance assessment standard. This study evaluates the merits of the turbine mounted configuration by measuring the performance of a tidal turbine relative to this reference and to a conventional seabed placed instrument. The turbine mounted sensor is found to provide a better reference of the free-stream conditions, evident from an improved agreement with theoretical predictions of device performance and a reduced amount of variation in the results. This new method could reduce both the costs and uncertainty associated with existing performance assessment best practices. • The power curve from a full-scale tidal turbine is measured. • Measurements from conventional seabed instrument inadequately capture turbine flows. • A new method is proposed using measurements obtained from a turbine mounted sensor. • Turbine sensor results show less variation and are in better agreement with theory. • New method reduces costs and uncertainties associated with performance assessment. [ABSTRACT FROM AUTHOR]

To understand the influence of complex hydrodynamic loads on tidal turbines, laboratory testing is necessary as a first approach. Previous laboratory work undertaken gave an indication that the use of speed control strategies may disguise the associated loading range that a turbine may be subjected to when this is operated with a variable speed control strategy. However, the preceding work was undertaken in a highly controlled environment without the influence of turbulent flows. The focus of this paper is directed towards the study of wave-induced loads on tidal turbines when these are controlled using two strategies and the impact that these parameters have on the turbine's performance when this is operated in a recirculating flume. Laboratory tests were undertaken with a 0.9 m diameter horizontal axis tidal turbine subjected to combined wave and current conditions with both regular and irregular waves. Constant speed and constant torque control strategies have been considered, for which rotor thrust, torque and blade root bending moment have been measured. Results show that similar to previous studies, average loads and power capture values remain unchanged between control strategies and the superposition of waves to the current. However, signal fluctuations are 2 to 3 times higher for torque control than for constant speed control strategy. A phase difference between the periodic signals of the turbine thrust and the incoming waves was also identified, in this case, the phase variation was lower when using torque than speed control. This work thus demonstrates the implication of studying strategies to control a marine converter from early stages of development. • Recirculating tank testing demonstrated the loading and performance effects when using two control methodologies. • Thrust, torque and root bending moment fluctuations are higher when using torque control than speed control. • Phase differences between the periodic signals of the turbine thrust and the incoming waves is analysed. • The effect of combined turbulent flows and regular and irregular wave interaction is presented. [ABSTRACT FROM AUTHOR]

The tidal energy industry is progressing rapidly, but there are still barriers to overcome to realise the commercial potential of this sector. Large magnitude and highly variable loads caused by waves acting on the turbine are of particular concern. Composite blades with in-built bend-twist elastic response may reduce these peak loads, by passively feathering with increasing thrust. This could decrease capital costs by lowering the design loads, and improve robustness through the mitigation of pitch mechanisms. In this study, the previous research is extended to examine the performance of bend-twist blades in combined wave–current flow, which will frequently be encountered in the field. A scaled 3 bladed turbine was tested in the flume at IFREMER with bend-twist composite blades and equivalent rigid blades, sequentially under current and co-directional wave–current cases. In agreement with previous research, when the turbine was operating in current alone at higher tip speed ratios the bend-twist blades reduced the mean thrust and power compared to the rigid blades. Under the specific wave–current condition tested the average loads were similar on both blade sets. Nevertheless, the bend-twist blades substantially reduced the magnitudes of the average thrust and torque fluctuations per wave cycle, by up to 10% and 14% respectively. • Laboratory tests demonstrated the bend-twist response of composite blades. • Bend-twist response increases with tip speed ratio. • Average loads on the rotor are similar in current and wave–current conditions. • Composite bend-twist blades damp the wave induced fluctuations in thrust and torque. [ABSTRACT FROM AUTHOR]