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