Temporal and spatial characterisation of tidal blade load variation for structural fatigue testing

To achieve the full potential of tidal stream energy, developers are incentivised to use larger blades on tidal turbines. This requires validation of blade structural designs through full-scale blade fatigue tests to de-risk the engineering process. However, the loading scenarios encountered in testing facilities and those in reality could be significantly different, which induces errors in blade loads and fatigue damage. Here we characterise the unsteady tidal blade load variation through model-scale experiment. It was found that the standard deviations of thrust load range between 200% and 637% of condition without waves. This results in an increase of predicted fatigue damage between 6% and 18%. It was observed that the centre of effort shifts towards the blade root when encountering wave crests of opposing waves, which has not been reported in the literature to date. To reduce errors in fatigue test while the centre of effort is fixed, matching blade shear forces should be sacrificed to match target bending moment at the root. Matching blade shear forces leads to a reduction of predicted fatigue damage ranges from 17% to 25%, which can induce errors in fatigue testing. We anticipate our findings would facilitate the development of fatigue testing of tidal turbine blades

Capturing the Motion of the Free Surface of a Fluid Stored within a Floating Structure

Large floating structures, such as liquefied natural gas (LNG) ships, are subject to both internal and external fluid forces. The internal fluid forces may also be detrimental to a vessel’s stability and cause excessive loading regimes when sloshing occurs. Whilst it is relatively easy to measure the motion of external free surface with conventional measurement techniques, the sloshing of the internal free surface is more difficult to capture. The location of the internal free surface is normally extrapolated from measuring the pressure acting on the internal walls of the vessel. In order to understand better the loading mechanisms of sloshing internal fluids, a method of capturing the transient inner free surface motion with negligible affect on the response of the fluid or structure is required. In this paper two methods will be demonstrated for this purpose. The first approach uses resistive wave gauges made of copper tape to quantify the water run-up height on the walls of the structure. The second approach extends the conventional use of optical motion tracking to report the position of randomly distributed free floating markers on the internal water surface. The methods simultaneously report the position of the internal free surface with good agreement under static conditions, with absolute variation in the measured water level of around 4 mm. This new combined approach provides a map of the free surface elevation under transient conditions. The experimental error is shown to be acceptable (low mm-range), proving that these experimental techniques are robust free surface tracking methods in a range of situations

Environmental & load data: 1:15 Scale tidal turbine subject to a variety of regular wave conditions

Experimental data was obtained in order to investigate the effect of waves on the loads and performance of tidal turbines. An instrumented 1:15 scale tidal turbine was installed in the FloWave Ocean Energy Research Facility, and a wide range of regular wave conditions were generated; systematically varying both wave frequency and height. Waves were generated both following and opposing a fixed mean current velocity of 0.81 m/s. Data are made available of the measured turbine loads and environmental conditions obtained for five repeats of 24 wave conditions via https://doi.org/10.7488/ds/2472. A description of the data collection process, data processing, file structure and naming conventions are provided in this article. The analysis and presentation of the described dataset can be found in Ref. [1]

Experimental optimisation of power for large arrays of cross-flow tidal turbines

As commercial scale tidal energy devices are shortly to be deployed in the first arrays, the knowledge of how different array layouts perform is a key and under-examined field. Here, the Momentum Reversal Lift (MRL) turbine, developed by the University of Exeter, is deployed in five different array layouts utilising up to 15 devices. The use of dynamic turbines allows the inclusion of analysis of the effects of flow direction in the wake. The layouts investigated explore the effect of lateral and stream-wise turbine spacings as well as differences between staggered and in-line layouts on power. The staggered array with decreased streamwise spacing is shown to have the highest total power per ‘footprint’ area among the layouts tested. For the staggered arrays, increased downstream separation had little effect on total power generated, while decreasing the lateral spacing below 2 rotor diameters decreased the power. The in-line arrays showed a lower power per device but similar total power. It was also shown that increased in-flow into a turbine didn't necessarily lead to an increased power extraction. The decrease in power with a decrease in streamwise spacing is in-line with theoretical and CFD predictions