CO2 from the sea: good for the climate

Enormous amounts of CO₂ are stored in the Earth’s oceans. Energy scientist Paul Straatman has suggested a method for obtaining that carbon dioxide from seawater. Doing so will draw CO₂ out of our atmosphere, which would dramatically reduce the concentration of greenhouse gases. “You can store the CO₂ from the ocean in empty natural gas fields, or you can use it to produce synthetic fuels.” Straatman’s method can be combined with an existing low-cost technology for generating energy from seawater: Oceanic Thermal Energy Conversion (OTEC). “It’s like using one stone to kill three birds, or actually four, because OTEC also produces large quantities of fresh water.”

An OTEC installation generates renewable energy from the temperature differences that naturally occur in the ocean. “It’s actually a form of solar energy”, Straatman explains. “Because you’re using the principle that the sun heats the water in the top layer of the ocean.”

Temperature differences

There are different types of OTEC methods. “In our case, you put warm seawater into a vacuum chamber. Then you put cold water from a kilometre deep into another vacuum chamber.” That creates a difference in vapour pressure between the two chambers. “It’s like filling two pressure cookers with water. Then you heat one and cool the other. The pan with the hot water develops a higher pressure than the other pan.” If you then connect the pans via a turbine, the difference in pressure turns the turbine and generates electricity.

Useful waste flow

The OTEC method releases CO₂ and other gases. But where does the CO₂ come from? “Seawater absorbs CO₂ from the atmosphere. Then photosynthesis converts it into organic material. That organic material eventually dies and sinks to the bottom, where it decays back into CO₂ that is dissolved into the water, and therefore not in the form of gas.” This CO₂-rich deep-sea water evaporates in the vacuum of the OTEC to produce a mixture of steam, CO₂ in gaseous state and other gasses. Those gases are considered to be waste products, but Straatman has calculated how that waste flow can be purified to collect a higher concentration of CO₂. “Then you can store it or use it to produce synthetic fuels. Fuels for applications for which electrical motors are not an alternative, such as in aviation or industrial processes that require extremely high temperatures, such as steel or glass production. In the process, you stop the accumulation of fossil carbon in the atmosphere. And once you condense the steam, you can bring it ashore as fresh water!”

Cheap method

Straatman’s idea has a lot of potential. The oceans store 39,000 gigatons of CO₂, while the atmosphere holds ‘only’ 750 gigatons. The capture, storage and use of oceanic CO₂ will significantly reduce the concentration of greenhouse gas in the atmosphere, while alleviating the problem of ocean acidification. “But only if fossil fuels are phased out”, cautions Straatman. Previous attempts to store CO₂ from the ocean proved to be too expensive or inefficient. But piggybacking on the OTEC principle is expected to eliminate those concerns. “It’s also a much cheaper method than direct CO₂ capture from the atmosphere.”

PhD research

Paul Straatman works on CO₂ reduction technologies at chemical firm Indorama. He will soon begin working on a PhD based on his research into OTEC and CO₂ bonds, under the supervision of Wilfried van Sark. They previously published an article together on an innovative application of ‘solar ponds’ and OTEC.

Publication

Paul J.T. Straatman, Wilfried G.J.H.M. van Sark, 'Indirect Air CO₂ Capture and Refinement based on OTEC seawater outgassing', iScience 24 (2021) 102754, https://doi.org/10.1016/j.isci.2021.102754