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Is the ocean the last bastion against climate change?

Using artificial islands to boost the ocean carbon sink

Cédric Tard, CNRS Research Director and Professor at École Polytechnique (IP Paris)
On October 10th, 2023 |
4 min reading time
Cédric Tard
Cédric Tard
CNRS Research Director and Professor at École Polytechnique (IP Paris)
Key takeaways
  • The IPCC considers ocean capture of anthropogenic CO2 necessary in order to limit global warming to 2°C.
  • For the first time, a solution aimed at boosting this absorption will be tested: an artificial island will be placed on the lake of École Polytechnique (IP Paris).
  • This island will be equipped to extract CO2 from the water to increase its capacity to capture atmospheric CO2.
  • At the same time, the model will be capable of producing hydrogen to generate synthetic fuel from seawater.
  • With this carbon-neutral process, which is not yet mature, the researchers hope to be able to produce 1 litre of fuel per day from 4 m3 of seawater.

Phy­to­plank­ton fer­til­i­sa­tion, arti­fi­cial alka­lin­i­sa­tion, etc. Researchers are look­ing at these tech­no­log­i­cal process­es to com­bat glob­al warm­ing. By boost­ing the nat­ur­al CO2 absorp­tion capac­i­ty of the oceans, these solu­tions aim to off­set man-made CO2 emis­sions. Although the Inter­gov­ern­men­tal Pan­el on Cli­mate Change (IPCC) believes that anthro­pogenic CO2 cap­ture is nec­es­sary to lim­it glob­al warm­ing to 2°C1, none of these solu­tions is cur­rent­ly being deployed. As part of the XSeaO2 project, financed by the Ifk­er Cli­mate Fund, Cédric Tard and his col­leagues will be test­ing one of them in the field for the first time.

What approach does your project take?

Our aim is to extract car­bon from the oceans to increase its capac­i­ty to cap­ture atmos­pher­ic CO2. To do this, we are using an exist­ing solu­tion: an elec­tro­chem­i­cal extrac­tion cell based on a bipo­lar mem­brane. In prac­tice, the process involves cap­tur­ing water and arti­fi­cial­ly acid­i­fy­ing it by polar­is­ing elec­trodes. Below pH 5, the dis­solved inor­gan­ic car­bon is trans­formed into a gas (CO2) and released. We recov­er this gas, and water with a slight­ly more alka­line pH is dis­charged. This CO2 extrac­tion process is cur­rent­ly the sub­ject of a great deal of research, and the best yields are around 60% in terms of extract­ed CO2.

There are other solutions for boosting the absorption capacity of the oceans. What is the advantage of the process being tested?

Here, the CO2 extrac­tion mod­ule is com­bined with oth­er tools on an arti­fi­cial island. What’s spe­cial about it? This island pro­duces syn­thet­ic fuel. The water will be pumped through two cir­cuits. In the first, the CO2 is extract­ed from the water. In the sec­ond, the water is first desali­nat­ed and then treat­ed in an elec­trol­yser to pro­duce hydro­gen (H2).  Final­ly, the hydro­gen is com­bined with the CO2 in a reac­tor to form syn­thet­ic fuel. It can then be used in com­bus­tion-pow­ered vehi­cles. Methanol, ethanol, paraf­fin: sev­er­al syn­thet­ic fuels can be pro­duced, and we are cur­rent­ly study­ing the best solu­tion to implement.

No one in the world has ever suc­ceed­ed in test­ing this process – only Google X Lab has tried on a small scale, with­out suc­cess. Our first aim is to demon­strate that this prin­ci­ple can be test­ed at the scale of a lake.

How do you intend to test the viability of this solution?

With­in a year, we are going to build a pro­to­type that will be placed on the École Poly­tech­nique lake. A float­ing demon­stra­tor mea­sur­ing around 20m2 will con­tain all the mod­ules need­ed to pro­duce syn­thet­ic fuel. It will be accom­pa­nied by 300m2 of float­ing pho­to­volta­ic pan­els: the pro­duc­tion of renew­able elec­tric­i­ty is essen­tial for these arti­fi­cial islands to pow­er the fuel extrac­tion and pro­duc­tion mod­ules. Water elec­trol­y­sis is the most ener­gy-inten­sive process. We hope to treat 4m3 of water per day, which should enable us to pro­duce around 1L of fuel per day. At the end of the project, we hope to be able to car­ry out a life-cycle analy­sis and esti­mate the eco­nom­ic prof­itabil­i­ty of the fuel pro­duced, in order to com­pare it with oth­er syn­thet­ic fuel pro­duc­tion processes.

This demon­stra­tor will be a test­ing ground for the entire sci­en­tif­ic com­mu­ni­ty. For exam­ple, we have worked on soci­etal accept­abil­i­ty, and devel­oped a spe­cif­ic design with the help of the Pen­ninghen school of architecture.

What are the obstacles to implementing these artificial islands?

They are main­ly tech­no­log­i­cal. The process of extract­ing CO2 is still in its infan­cy, and com­bin­ing it with desali­na­tion and elec­trol­y­sis mod­ules and a reac­tor rep­re­sents a real chal­lenge. The oth­er major con­straint is the use of float­ing pho­to­volta­ic pan­els. These sys­tems are not yet ful­ly devel­oped either: they need to be made reli­able for use at sea, in a tur­bu­lent and salty envi­ron­ment. We do know, how­ev­er, that their effi­cien­cy will be high­er than that of land-based instal­la­tions thanks to the increase in effi­cien­cy offered by the drop in tem­per­a­ture due to the pres­ence of water and air cur­rents under the pan­els (+0.6% for each degree less).

The sea is a par­tic­u­lar­ly harsh envi­ron­ment: we need to ensure that the pan­els and the chem­istry mod­ule can with­stand storms. Our pro­to­type will not be able to address all these issues, as it will be deployed on a lake. But it is a first step in test­ing the via­bil­i­ty of the process.

If these artificial islands were to be deployed on a large scale, could they present an environmental risk?

The main risk con­cerns the water desali­na­tion process. Sea­wa­ter desali­na­tion plants are a real envi­ron­men­tal dis­as­ter because of the effects of the brine dis­charged into the sea. In our process, how­ev­er, desali­na­tion is only used to extract hydro­gen from the water by elec­trol­y­sis. Less than 1% of the water cap­tured will be used to extract hydro­gen: the vast major­i­ty will be used to extract CO2. Nev­er­the­less, we are plan­ning to test salt elec­trol­y­sers to reduce the envi­ron­men­tal impact. The CO2 extrac­tion process pos­es no prob­lem: the water will be slight­ly more alka­line at the end, which is what we want. As for the rest, we will be work­ing with biol­o­gists and ecol­o­gists to assess the impact of the arti­fi­cial island on the lake’s ecosys­tems, which are cur­rent­ly rel­a­tive­ly unknown.

With this solution, carbon is pumped from the oceans but converted into synthetic fuel. When the fuel is used, the captured CO2 is released back into the atmosphere… What is the benefit in terms of mitigating climate change?

Pro­duc­ing fuel using our process is car­bon neu­tral: no fos­sil fuels are extract­ed. But it’s an inter­me­di­ate stage. Even­tu­al­ly, we would like to extract the CO2 from the water and sequester it. There is not yet a ful­ly devel­oped tech­ni­cal solu­tion for car­ry­ing out this exper­i­ment on the École Poly­tech­nique site, and the process is not wide­ly used around the world, so its ben­e­fits are still being debat­ed.

Anaïs Marechal
1IPCC, 2022: Cli­mate Change 2022: Mit­i­ga­tion of Cli­mate Change. Con­tri­bu­tion of Work­ing Group III to the Sixth Assess­ment Report of the Inter­gov­ern­men­tal Pan­el on Cli­mate Change [P.R. Shuk­la, J. Skea, R. Slade, A. Al Khour­da­jie, R. van Diemen, D. McCol­lum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belka­ce­mi, A. Hasi­ja, G. Lis­boa, S. Luz, J. Mal­ley, (eds.)]. Cam­bridge Uni­ver­si­ty Press, Cam­bridge, UK and New York, NY, USA. doi: 10.1017/9781009157926

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