Core tips：French start-up Eel Energy has turned to Finnish pultruded and tensile-wound composite manufacturer Exel Composites to reinforce the undulating membrane of its hydrokinetic generator. The membrane is inspired by the movements of fish tails to produce ener
Specialising in hydrokinetic machines, French start-up Eel Energy is developing a biomimetic technology that harnesses the kinetic energy of water currents to generate electricity. This method, which does not disrupt aquatic ecosystems, is based on a patented undulating membrane inspired by the movements of fish tails.
“Our membrane used to be made up of a semi-rigid structure covered in a rubber layer
” explains Xavier Peroutka, Eel’s CEO. “
The rubber acted as a sail, capturing the pressure from the water’s current and transmitting it to the membrane’s skeleton. As the membrane deformed,strain energy was generated and transformed into electricity through electromagnetic converters positioned on the membran
e,” he continues.
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The subsea environment is particularly demanding, requiring materials capable of withstanding extreme stress and pressure. “
Hydrokinetic generation exerts up to 30 times more mechanical stress on equipment than wind energy production
,” explains Peroutka. “
Despite the challenges, it’s worth persevering. Whilst a wave power generator relies on intermittent wave action and solar and wind depend on favorable weather conditions Hydrokinetic generators can produce energy constantly throughout their life by leveraging the water currents in rivers or tidal flow,
” he continues.
Composites as reinforcements
To meet this challenge, Eel called on Exel Composites, a specialist in pultruded and wound composite materials. The two companies first met at the JEC World trade show in 2021. At the time, Eel was looking for materials capable of withstanding the extreme conditions of the underwater environment, and initially reinforced its membrane with fibreglass. However, the membrane underwent significant deformation, leading to high stresses in the structure. These deformations caused the layers of the membrane to separate, allowing water to seep through and then delaminate. The aim of the collaboration with Exel was therefore to improve the resilience of the Eel membrane and fine-tune its performance in difficult underwater conditions.
Exel has used several layers of discrete flat carbon profiles, the same as those developed for wind turbine applications. These flat carbon fibre profiles are stacked to create beams integrated into the membrane. Three bars have been placed across the membrane at 50, 80 and 100% of its length. This arrangement avoids bulging, which significantly reduces the efficiency of energy capture, and limits overall deformation, ensuring the structural integrity of the membrane and its optimum performance.
“The key to our successful collaboration was recognizing that our carbon fiber flats, originally designed for wind turbine blades, could be repurposed for EEL’s hydrokinetic membranes
,” explains Neil Dykes, research and development manager at Exel Composites. “
These flats provide the stiffness and strength required to withstand harsh wind conditions, making them ideal for this application
“he continues.
The carbon fibre laminates have solved the delamination problems previously encountered with glass fibre composites. The tensile strength of glass fibre composites (1000 MPa) is lower than that of carbon fibre-reinforced composites (2500 MPa), as is the compressive strength (600 MPa vs 1500 MPa). This increased strength means that the membrane can withstand up to 6,000 complete inversion cycles per day.
Eel is now concentrating on developing the main hydrokinetic generator, a crucial step towards commercialisation after almost a decade of perfecting the membranes. Exel Composites’ carbon plates could play a part in this.
The French start-up’s current project includes the development of a hydrokinetic generator to produce 50 kWh, aimed at isolated communities in the Americas. In parallel, Eel is developing a larger hydrokinetic generator for energy producers such as EDF and Octopus, aiming to produce 1 MW per hour.
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