Ifremer puts novel Sabella tidal blade through its paces

French marine research institute Ifremer has performed a series of structural bench tests on compatriot company Sabella’s novel tidal turbine blade to determine the weight loads the blade can bear and validate its mechanical characteristics.

The five-metre-long Sabella blade was evaluated on a bespoke test-bench at one of the research laboratories at Ifremer’s Carnot MERS Institute, which trialed its structural integrity by applying a force of up to 60 tonnes – equivalent to the weight of 60 cars.

Equipped with multiple sensors, the carbon-composite blade underwent various twisting, stretching and compression exercises on a test-bench designed by Ifremer’s Behavior of Structures at Sea (LCSM) research institute.

The optical fibers, installed during manufacture, made it possible to measure the deformations at hundreds of points with high precision, while the acoustic sensors were able to detect the slightest cracking of the material, according to Ifremer.

After about 20 sets of tests, the blade broke under a weight of more than 16 tonnes.

Peter Davies, LCSM researcher at Ifremer, said: “Our testing system has proved its worth. This is the first time that fracture tests have been carried out on this type of blade.

“These full-scale tests are essential to validate numerical calculation and optimisation tools. We can then reduce the dimensioning or modify the materials used, thanks to digital models”.

The trials started in December last year, and ended on 14 January 2021.

The analysis of sensor recordings and video footage provided insight into the damage sequence leading to failure, and now, a second series of tests – with an improved blade design to increase loads – is being planned for the spring.

The tests have been conducted as part of the RealTide research project which aims to significantly improve the performance and reliability of tidal turbines, and reduce the associated manufacturing costs.

Sabella said earlier that the next-generation blade would be installed on two tidal turbines for the PHARES project, a multi-energy project aimed at powering the Ushant Island.

Parallel tests to reduce environmental impacts and optimise blade numbers

In addition to testing the strength of the blade, Ifremer has also contributed with the material aging tests for the turbine.

The blade recycling issue at the end of their lifespan has not been resolved at present, and therefore, the composites with recyclable materials – in polyamide and polypropylene – have been tested to replace those made with epoxy.

Also, the additional tests were carried out on composites reinforced with flax fibers, in order to reduce the environmental impact.

The tests consisted of immersing hundreds of samples of the materials in seawater tanks at different temperatures to accelerate their degradation, that was later followed by mechanical testing.

First results indicate that flax fibers offer good prospects, but will require using thicker layers.

Finally, the Ifremer wave and current basin in Boulogne-sur-Mer was used for RealTide in June 2020 in order to test models of tidal turbines with a number of blades reduced to three instead of five.

The tests made it possible to measure the performance of the tidal turbine but also the forces exerted on the blades. The optimised rotor has proved its worth, while at the same time demonstrating ways to cut manufacturing costs, Ifremer concluded.

This research is also being carried out as part of the RealTide project, which is coordinated by Bureau Veritas, with partners Sabella, Ifremer, University of Edinburgh, EnerOcean, Ingeteam, and 1-Tech.