Sweetch energy rentabilise (enfin) l’énergie osmotique

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Harnessing osmotic energy to produce electricity has attracted research and industry attention for decades. The cost of membranes and the low efficiency of osmotic systems has long constituted a glass ceiling for imagining large-scale electricity production based on osmotic energy.

The French start-up Sweetch Energy, based in Rennes, has broken this glass ceiling. Indeed, the three creators of Sweetch Energy, Bruno Mottet, Pascal Le Melinaire and Nicolas Heuzé, managed to scale up the phenomenon of nano-osmotic diffusion, highlighted by a CNRS team in 2013. Its exploitation allowed the start-up to produce membranes with increased performance. A technological breakthrough, which allows Sweetch Energy to prepare for the commissioning, in the coming months, of a pilot installation on the banks of the Rhône, for the production of electricity from osmotic energy, on a large scale.

The general director of Sweecht Energy, Nicolas Heuzé, retraced for Engineering Techniques the journey of this entrepreneurial adventure, from the scientific discovery of a physical phenomenon to its industrial exploitation, in the context of the energy transition.

Engineering Techniques: Why is osmotic energy so difficult to exploit? What is its potential?

Nicolas Heuzé : For more than 70 years we have been trying to exploit theenergy osmotic to produce electricity. First of all because it is a massively present source of energy in its natural state, and which makes it possible to set up flexible electricity production facilities.

To capture this energy and transform it, there has been a lot of academic and industrial work around membrane technologies, with a first generation of technologies called PRO: these are semi-permeable membranes which allow water to pass through. water to generate a pression into a compartment and engage a turbine. It’s a very mechanical approach to power generation. The weakness of this type of system is profitability, in particular because the membranes are not efficient enough and are very expensive.

For around thirty years, another process, physicochemical, with a selective membrane, sorts anions and cations, to produce electricity using electrodes, like the processes used in batteries. Here too, we need selective membranes, but as for PRO, the performance of the available membranes was not sufficient: they are selective, but their power is too low, around 0.5 W/m², and a price of several hundred euros per square meter of membrane. The principle works, but the cost of the membranes prevents large-scale deployment.

How did you manage to remove these obstacles?

We have developed a completely new membrane. It all started with an article published in Nature in 2013 by Lyderic Bocquet and his CNRS team, in which they noted that in a boron nitride nanotube, they managed to generate gigantic ionic currents using osmotic pressure. To summarize, they managed to select ions much faster than traditional membranes: this phenomenon is called nano-osmotic diffusion.

The principle of nano-osmotic diffusion is as follows: at the nanometer level, surface phenomena linked to the charge of the materials used and their nature are brought into play. This phenomenon allows the selectivity of ions, but allows much more to pass than a conventional membrane, the mesh of which is of the order of an angstrom.

This publication greatly intrigued us, Bruno Mottet, Pascal Le Melinaire and myself, because it suggested that if we managed to implement this phenomenon of nano-osmotic diffusion in membranes on a larger scale, it was possible to obtain a sufficiently high power density to make the exploitation of osmotic energy profitable. The creation of Sweetch Energy in 2015 originated from the desire to take this phenomenon to an industrial scale, to develop new, more efficient membranes.

How did this scaling up phase take place?

The problem consisted of scaling up this phenomenon of nano-osmotic diffusion, but not only that. This had to be done in inexpensive membranes and using biomaterials. These are the criteria we set for ourselves.

We spent four years developing this new membrane, and developed a proof of concept at the end of 2020, with a membrane that has the following characteristics: a power density twenty times higher than already existing membranes. Made with a biosourced material, it costs ten times less to produce than existing membranes.

These characteristics allowed us to imagine large-scale and competitively priced osmotic electrical energy production, which is our initial objective.

Where are you today?

For almost three years, we have started scaling up the technology, which will lead to the start-up of a first pilot site at the mouth of the Rhône, on the Barcarin site in the commune of Port-Saint- Louis. This is an ideal site to test this technology because there is a clear divide between fresh and salt water. Work to build the installation is currently underway, with commissioning in 2024.

We are in partnership with the national company of the Rhône to develop the demonstrator and more generally to deploy osmotic energy on the Rhône, which is the largest natural source of osmotic energy in France, with nearly 500 megawatts of installation potential.

We also have a partnership with EDF hydro, and projects in mainland France and overseas.

More generally, we have a fairly broad activity today, between the development of osmotic generators, their future industrialization, and the identification and development of osmotic projects.

You are also very widely involved in the future deployment of the industrial osmotic energy production sector.

Indeed. With a view to the deployment of this new industrial sector centered on the production of osmotic energy, we need to work on regulations: we have, in recent months, contributed to bringing osmotic energy into the new European directive on renewable energies RED III.

We are therefore participating in the establishment of a new sector, with our partners in France and Europe.

More broadly, how can osmotic energy fit into the renewable energy landscape in the future?

The next generations of renewable energies, of which osmotic energy is a part, must obviously be completely decarbonized, overcome the limits of traditional renewable energies through their flexibility and permanence, and demonstrate acceptability in terms of landscape. We see today that this acceptability can be an obstacle to the massive deployment of certain renewable technologies.

Are you working on the development of other applications using the membranes you have developed?

Our technology can make it possible to produce hydrogen directly without the need for external electricity. There is also the possibility of exploiting other sources of osmotic energy, on industrial sites for example, via the generation of salinity gradients, to exploit low temperature waste heat. This avenue is the subject of a program financed by Europe, which awarded Sweetch Energy a grant of 2.5 million euros to work on the subject.

Finally, there are also potential applications in everything related to water treatment, and more generally in all applications using membranes.

Comments collected by Pierre Thouverez

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