Understanding the benefits of carbon dioxide conversion technology
11 November 2020
Last update: 11/11/20 14:49
The greatest challenge that we face as a species for our planet’s continued habitability is, ultimately, a rate problem. The rate at which we are pulling carbon-equivalents out of the ground (as fossil fuels) and putting them into the atmosphere (as CO2) is higher than it ever has been in human history. Photosynthetic organisms have been on Earth for nearly two billion years and have historically brought balance to this rate equation with a useful byproduct – O2 that enables aerobic-respiring life. However, the increasing concentration of CO2 in the Earth’s atmosphere is evidence that humanity has thrown off this rate equation by emitting more CO2 than nature can capture.
As we deploy technologies to capture CO2 and bring ourselves back to equilibrium, it’s important to know where we stand in comparison to nature’s existing methods so that we can make the best decisions for the planet with the data available to us. That is what our recent study in MRS Energy & Sustainability’s special issue on Solar Energy and the Circular Economy aims to do with an example of carbon capture, utilization, and storage (CCUS) technology and reforestation. We calculated the average rate of trees sequestering CO2 to be around 24 grams per tree per day using tree growth data from the US Forestry Service. This number is not high compared to our ability to burn fossil fuels – in comparison, a typical US passenger vehicle emits around 525 times that per day, on average.
Part of the challenge is how little of the sun’s energy gets converted to chemical energy in trees. Nature was never trying to build efficient carbon capture machines; its goal is adaptation and reproduction. As such, we found a CO2-to-alcohols 0.5% system-level thermal efficiency as the ‘break-even’ point where an artificial photosynthetic system would have the same CO2 sequestration ability as a well-curated forest. We can do better than this; building more efficient CO2 electrolysis, hydrogenation, solar thermal, and biocatalytic systems is where we, as scientists, can develop and deploy technologies that use renewable energy to effectively make use of CO2.
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