• Energy Research

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.

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.

The non-uniformity and dynamics of the environment tidal stream turbines need to operate within will significantly influence the durability and reliability of tidal energy systems. The loadings on the turbine will increase substantially when the turbine is deployed in high magnitude waves with non-uniform tidal currents. The limitations of numerical solutions will be understood when the outcomes are verified with empirical data from system operations. In this paper, a Blade Element Momentum model is used to predict and compare the performance of a scaled turbine within a flume and a tow tank. Firstly, the numerical and experimental work is analysed for a turbine operating at flow speeds of 0.5m/s amd 1.0 m/s, wave heights of 0.2 m and 0.4 m and wave periods of 1.5 s and 1.7 s. Good agreement between the model and the experimental work was observed. However, in low TSRs the model tends to under predict the thrust, and the variation between the maximum and minimum values obtained within the experiments. Secondly, a turbine operating at flow speeds of 1.0 m/s and 4 different inflow profiles is analysed, where the wave heights for these cases were 0.09 m and 0.19 m and with wave periods of 2 s and 1.43 s. In this evaluation, the model tends to over predict the values of Ct and Cp when compared to those calculated from the experimental data. However, when investigating the values used to calculating both the thrust and torque coefficients, there is better agreement with these, which means the methodology used to determine these coefficients with inflow profiles should be revised.

The non-uniformity and dynamics of the environment tidal stream turbines need to operate within will significantly influence the durability and reliability of tidal energy systems. The loadings on the turbine will increase substantially when the turbine is deployed in high magnitude waves with non-uniform tidal currents. The limitations of numerical solutions will be understood when the outcomes are verified with empirical data from system operations. In this paper, a Blade Element Momentum model is used to predict and compare the performance of a scaled turbine within a flume and a tow tank. Firstly, the numerical and experimental work is analysed for a turbine operating at flow speeds of 0.5m/s amd 1.0 m/s, wave heights of 0.2 m and 0.4 m and wave periods of 1.5 s and 1.7 s. Good agreement between the model and the experimental work was observed. However, in low TSRs the model tends to under predict the thrust, and the variation between the maximum and minimum values obtained within the experiments. Secondly, a turbine operating at flow speeds of 1.0 m/s and 4 different inflow profiles is analysed, where the wave heights for these cases were 0.09 m and 0.19 m and with wave periods of 2 s and 1.43 s. In this evaluation, the model tends to over predict the values of Ct and Cp when compared to those calculated from the experimental data. However, when investigating the values used to calculating both the thrust and torque coefficients, there is better agreement with these, which means the methodology used to determine these coefficients with inflow profiles should be revised.

This Round Robin Test program aims to establish the influence of the combined wave and current effect on the power capture and performance of a generic tidal turbine prototype. Three facilities offering similar range of experimental conditions have been selected on the basis that their dimensions along with the rotor diameter of the turbine translate into low blockage ratio conditions. The performance of the turbine shows differences between the facilities up to 25% in terms of average power coefficient, depending on the wave and current cases. To prevent the flow velocity increasing these differences, the turbine performance coefficients have been systematically normalized using a time-average disc-integrated velocity, accounting for vertical gradients over the turbine swept area. Differences linked to blockage effects and turbulence characteristics between facilities are both responsible for 5 to 10% of the power coefficient gaps. The intrinsic differences between the tanks play a significant role as well. A first attempt is given to show how the wave-current interaction effects can be responsible for differences in the turbine performance. In these tanks, the simultaneous generation of wave and current is a key part often producing disruptions in both of these flow characteristics.