ÂÌñÏ×ÆÞ

A recent case study describes how a collaboration with Exel Composites (Vantaa, Finland) supported the development of a biomimetic hydrokinetic system for generating electricity developed by deep tech startup  (Paris, France). 

EEL Energy seeks to redefine how we harness energy from nature. Specializing in hydrokinetic machines, the company’s patented undulating membrane draws inspiration from the movements of fish tails. This biomimetic design captures kinetic energy from water currents, providing a sustainable method for generating electricity that is efficient and nondisruptive to aquatic ecosystems.

The undulating membrane traces its origins to research in the medical field by Jean-Baptiste Drevet in 1996. He leveraged a polymer membrane that mimicked the undulating movement of marine animals to propel fluids, functioning as a pump that creates flow similar to the human heart. This technology has proven effective in medical devices and is adaptable for use in renewable energy generation. The vortices created by turbulent flow in water can be captured by a flexible membrane and the undulating motion used to drive an electrical generator

“Our membrane used to be made up of a semi-rigid structure covered in a rubber layer,” explains Xavier Peroutka, CEO at EEL. “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 membrane.

“Coils of wire placed within the structure moved relative to magnets embedded in the system during deformation,” continues Peroutka. “This movement induces an electrical current through electromagnetic induction, converting the membrane’s mechanical energy into electrical energy.”

However, as EEL embarks on bringing hydrokinetic energy production to market, it faces significant challenges. The underwater environment is particularly demanding, necessitating materials that can withstand extreme stress and pressure.

“Hydrokinetic generation exerts up to 30 times more mechanical stress on equipment than wind energy production,” notes Peroutka. “Despite the challenges, it’s worth persevering. While 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. The predictability of hydrokinetic energy generation is needed by communities across the globe.”

Seeking material expertise

Initially, EEL reinforced its membrane with fiberglass, but this original design revealed a critical flaw: delamination. Under the harsh cyclic loading of the underwater conditions, the membrane underwent large deformations, inducing high strains in the structure. The strain caused the layers of the membrane to separate, enabling water to infiltrate and compromise its functionality.

Recognizing the serious implications of this issue — where a failure of the membrane during operation could jeopardize the entire hydrokinetic system — EEL sought the expertise of materials specialist Exel Composites to develop a solution capable of withstanding the continuous mechanical stress required for effective operation. 

For Exel, specializing in pultruded and pull-wound composite materials, this challenge illustrated the complexity of creating a solution that was both durable, while balancing flexibility and resilience. The quest for a material that could endure the rigors of underwater energy generation became the focal point of the collaboration.

Interestingly, the solution didn’t require inventing something new. Rather, it resided in a proven material already designed to withstand the rigors of renewable energy production. One that can resist bending without breaking under the demands of the environment.

“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, R&D manager at Exel Composites. “These flats provide the stiffness and strength required to withstand harsh wind conditions, making them ideal for this application.”

Exel used multiple layers of discrete carbon flat profiles, the same as those developed for wind turbine applications. These carbon fiber flats are stacked to create beams which were strategically integrated into the membrane. Three bars were placed across it at 50%, 80% and 100% of its length. This arrangement prevents bulging and limits overall deformation, ensuring the membrane maintains structural integrity and operates optimally.

Excessive deformation, particularly uneven bulging outside the intended undulatory motion, can significantly reduce energy capture efficiency. When the membrane deforms, it disrupts the smooth oscillating flow of water over its surface, diminishing the area available for energy conversion and leading to suboptimal mechanical energy transfer to the electromagnetic converters.

How did carbon flats improve the design?

The integration of these carbon fiber flats was crucial for enhancing the membrane’s mechanical performance. By increasing stiffness and resistance to deformation, they help preserve the membrane’s shape during operation, ensuring effective energy capture while adhering to biomimetic design principles.

The high strength and stiffness provided by the carbon fiber flats supplied by Exel Composites resolved the delamination issues previously encountered with glass fiber-reinforced composites (GFRP), which compromised the membrane’s effectiveness.

Notably, the mechanical properties of Exel’s carbon fiber-reinforced polymer (CFRP) composites were essential to the membrane’s success. With an E-modulus of approximately 120 GPa, CFRP exhibits higher stiffness than traditional GFRP, which is only a third of CFRP stiffness. This high stiffness ensures the membrane retains its shape under operational stress, preventing excessive deformation and energy loss.

CFRP’s tensile strength of 2,500 MPa, compared to GFRP’s 1,000 MPa also enables the former to withstand greater forces without failure. Additionally, CFRP’s compressive strength, at 1,500 MPa, is much greater than the 600 MPa typical of GFRP. This increased strength, combined with CFRP’s optimal fatigue resistance, ensures the membrane can handle the cyclic loading conditions experienced during operation, supporting up to 6,000 full reversal cycles per day. This durability is key for maintaining high efficiency in energy capture over the long term.

The future of EEL hydrokinetics

With these challenges resolved, EEL is now shifting its focus to the development of the main hydrokinetic generator, a crucial step toward commercialization after nearly a decade of membrane refinement. This transition also opens new opportunities for Exel Composites’ carbon flats, which could be pivotal in meeting future performance demands.

EEL’s current project includes developing a medium-sized hydrokinetic generator designed to generate 50 kilowatts per hour. This generator targets isolated communities across the Americas, where many areas face significant energy access challenges due to geographic isolation from the main grid. The vast expanse of this region leaves numerous rural populations disconnected from centralized power sources. EEL aims to support local governments by providing consistent, reliable hydrokinetic energy tailored to the communities’ needs, eliminating reliance on expensive and polluting hydrocarbon generators.

In parallel, EEL is developing a larger hydrokinetic generator aimed at energy producers such as EDF and Octopus, targeting an output of 1 megawatt per hour. These installations are envisioned as hydrokinetic farms located in rivers and coastal areas. A single hydrokinetic generator could power 796 North American homes, and generator farms with multiple generators could enhance regional energy capacity.

EEL says its hydrokinetic technology promises predictable, sustainable energy, reducing reliance on fossil fuel-powered generators prevalent in remote areas. As the company prepares for commercialization, ongoing discussions with electricity providers and local governments demonstrate its commitment to supporting communities in the shift toward cleaner energy sources.