Using artificial islands to boost the ocean carbon sink

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.

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.

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.

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.

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.

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.

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