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Alternative Energy

Artificial photosynthesis uses CO2 drawdown for fuel

Reference: Patterson, B.D., Mo, F., Borgschulte, A., Hillestad, M., Joos, F., Kristiansen, T., Sunde, S. and van Bokhoven, J.A., 2019. Renewable CO2 recycling and synthetic fuel production in a marine environment. Proceedings of the National Academy of Sciences, 116(25), pp.12212-12219.

DOI: 1902335116

How would removing CO2 help?

With global fossil fuel emissions, atmospheric CO2 production is drastically increasing with a current value of ~ 400 ppm. Heightened atmospheric CO2 results in higher global temperatures through the greenhouse effect, essentially trapping more and more heat as CO2 emissions rise. Atmospheric CO2 readily diffuses (or dissolves) into surface oceans, therefore the atmospheric CO2 concentration is very similar to the oceanic CO2. As oceanic CO2 increases, another important carbon species (bicarbonate) separates into a carbonate ion and free hydrogen ion. An increase in free hydrogen ions results in a decrease in pH, also known as Ocean Acidification. Several methods can be used to help reduce this effect by removing some of this atmospheric CO2 from the environment (CO2 drawdown). One method, Carbon Capture and Storage (CCS), captures, compresses and stores CO2 in geological features deep underground (IUPAC). While other methods are used to drawdown CO2 to use for fuel, one such method was evaluated by Patterson et al. (2019) to demonstrate the methods efficacy. If global drawdown and subsequent transformation into usable fuel works well enough, it could have a major effect on global atmospheric CO2 concentration and subsequently global climate change.

An artist’s rendering from Patterson et al. (2019) of the ‘floating islands’ of solar panels (photo-voltaic cells) held together by concentric circles of tubing and ropes with a netted floor. The structure is floating and flexible allowing for durability in rough conditions. Methanol production would occur on a separate island or ship. Image Credit: Patterson, B.D., Mo, F., Borgschulte, A., Hillestad, M., Joos, F., Kristiansen, T., Sunde, S. and van Bokhoven, J.A., 2019. Renewable CO2 recycling and synthetic fuel production in a marine environment. Proceedings of the National Academy of Sciences, 116(25), pp.12212-12219.

How do you take CO2 and turn it into fuel?

Taking CO2 from the environment and efficiently turning it into a usable fuel source requires three main steps, 1) Water desalination, (see surprise impacts of desalination post) 2) CO2 extraction, and 3) Fuel (methanol) production, using solar energy and technologies that currently exist. The authors propose using floating islands of solar panels (photovoltaic cells) to absorb energy to drive these processes (see floating island image).

Water desalination is a critical step when dealing with seawater. Without this step the anode used to generate currents to extract CO2 is in competition with a common salt in seawater (chloride) that produces a corrosive chlorine gas. As a result of competition with this chlorine evolution reaction, efficiency of the anode and therefore the overall extraction process decreases. Desalination of seawater only requires 0.063% more energy than freshwater electrolysis. Another consideration during the desalination process is the build-up of insoluble magnesium chloride (Mg(OH)2) and calcium carbonate (CaCO3) that reduce the efficiency of the cathode. Some solutions include deionization or an acidic solution from the anode.

After desalination extraction of CO2 from the seawater can occur. Because seawater CO2 is, for the most part, in equilibrium with atmospheric CO2, extracting CO2 from the seawater will also drawdown CO2 from nearby air. After desalination, extraction of CO2 from the seawater can occur. CO2 can be removed by heating or increasing the acidity of the water. There are several ways to do this relatively easily by manipulating membranes that only allow hydrogen ions (H+) to pass through. CO2 gas then diffuses and is collected using a vacuum pump. Because seawater CO2 is, for the most part, in equilibrium with atmospheric CO2, extracting CO2 from the seawater will also drawdown CO2 from nearby air.

Once the CO2 is extracted it is converted to methanol, a process that occurs more readily with increased temperature, pressure, and a catalyst. There are multiple technologies that can currently do this, therefore, there are multiple production options.

There are many considerations that must be taken into account when building these floating solar panel islands. Wave action must be low enough that the solar panels aren’t frequently damaged in storms. Other restrictions include a maximum depth of 600 m and a low risk of hurricanes. After applying these limitations, only 1.5% of the world’s oceans are viable for these floating islands (see map of areas where these floats could be deployed). However, if every part of the 1.5% of oceans was used for these floating solar panel islands, with a separation of ~ 300 m, the process would remove 12 gigatons of carbon/year, surpassing the yearly emission of CO2. Even though the task seems daunting, creating even a few thousand of these floating islands would make an impact on global carbon emissions and help reduce global warming.

What remains to be answered

The authors do mention a few problems that still need to be resolved before these islands can be produced, including mass production technical issues, construction techniques for long-term durability in the marine environment, and whether or not methanol fuel is the best choice long term. With continued research some of these questions can be answered, and because most of the technology in this paper is already in use, manufacturing these islands could happen relatively quickly. Hopefully this, and many other climate change mitigation technologies can be implemented soon to help reduce our impact on the environment.

I’m a PhD student in the Rynearson Lab at the University of Rhode Island (URI) Graduate School of Oceanography (GSO). Broadly, my research interests are focused on human impacts on the oceanic ecosystem, particularly effects on the primary producers (phytoplankton) at the base of the food web. Specifically, my interests include phytoplankton ecology and physiology, especially relating to stressors of nutrient limitation, pollutants and human impacts. I am also interested in using molecular analyses for studies of environmental distributions within different phytoplankton functional groups and highlight differences between organisms in culture experiments. Currently, I work with cultures from regions of the ocean that are nutrient limited and will conduct laboratory experiments to help investigate how these phytoplankton survive.

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