The effect of tidal flow directionality on tidal turbine performance characteristics

As marine turbine technology verges on the realm of economic viability the question of how long will these devices last is an important one. This paper looks at the axial bending moments experienced from CFD modelling of Cardiff University’s concept tidal turbine in a uniform profile for three different scenarios. The magnitude and direction in which the axial bending moment acts is an important feature in determining likely sources of wear in the drive train, such as bearings. By determining the source and magnitude of these bending moments, possibilities for reducing them and limiting their impact on devices can be made

Simulating marine current turbine wakes with advanced turbulence models

Work is presented which compares the abilities of the Detached Eddy Simulation turbulence model to a Reynolds-Averaged Navier-Stokes turbulence model, for CFD simulations of a horizontal axis tidal turbine under different ambient turbulence conditions. Comparisons are made of the abilities of the respective models to predict both performance characteristics as well as wake length and character. It is demonstrated that whilst Detached Eddy Simulation holds little advantage over ak-! SST model for predicting mean performance characteristics, significant advantages are shown when predicting wake length, as well as allowing the prediction of the magnitude of fluctuations. It is expected that, despite the higher computational expense, hybrid LES-RANS turbulence models such as Detached Eddy Simulation will be of interest to engineers designing arrays of tidal turbines, which are anticipated if tidal energy is to make a significant contribution to the world’s energy resources

An assessment of axial loading on a five-turbine array

A structure that supports five turbines with a power coefficient of 0·40 (efficiency of 68%) has been studied using computational fluid dynamics to assess the power extracted and the flow field in a 3 m/s (6 knot) tidal flow. Peak axial sliding forces were assessed to determine anchorage requirements. While it is recognised that the turbines will most likely be positioned in relatively deep water in areas of steep tidal velocity gradients, this study considers the worst-case scenario for the axial sliding forces – that is, a uniform 3 m/s tidal velocity profile. The analysis shows that the fluid velocity increases around the structure, which could possibly be used advantageously in the placing of multiple turbine arrays. There is minimal interference between the wakes of the individual turbines, but there is interference between the wakes of some turbines and the bracing that forms part of the structure. The axial sliding force was found to be highest when the frame apex is head into the flow, and it is estimated that the coefficient of friction between the seabed and the array frame must be lower than 0·43 for sliding to occur with no additional ballast or anchorage

Performance assessment of a tidal turbine using two flow references

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 influence of solidity on the performance characteristics of a tidal stream turbine

The performance characteristics of a tidal stream turbine are critical when assessing its economical viability. The solidity of the rotor, which is a function of the blade chord length and the number of blades, will affect the performance characteristics, from both a power output and a structural loading viewpoint. This paper investigates the influence of solidity on the performance characteristics of a horizontal axis tidal turbine using experimentally validated CFD models. The solidity was varied by altering the number of blades in the numerical models. Increasing the solidity was found to increase the peak Cθ and peak Cp and reduce the λ at which these occur. Ct was found to be approximately the same at peak Cp which was assumed to be the normal operating condition. At λ above peak Cp, near freewheeling, Ct continued to increase for the 2 bladed turbine, remained approximately constant for the 3 bladed turbine and decreased for the 4 bladed turbine, indicating that higher solidity rotors would have to withstand lower loads in the event of a failure. In addition, the thrust per blade was shown to increase with a reduction in the number of blades