This video segment adapted from NOVA scienceNOW explains how geophysicist Klaus Lackner and two engineers, Allen and Burton Wright, teamed up to develop a technology to capture an important greenhouse gas, carbon dioxide (CO2), in the air. Modeling their design after a tree—and one of Lackner's daughter's science experiments—the team tested different materials, configurations, and coatings that together would act as leaves do to remove CO2 from the air. However, instead of mimicking photosynthesis, their process uses a manufactured fabric that attracts CO2 and pulls it out of the air for subsequent storage.
Certain gases in the atmosphere trap heat. These gases are called greenhouse gases because they effectively form a blanket over Earth that prevents heat from escaping to space. Greenhouse gases occur naturally. However, since the industrial revolution, human activities have generated more and more of them—especially carbon dioxide (CO2), which is released as fossil fuels are consumed. As CO2 becomes more concentrated in the atmosphere, it absorbs more heat. As a result, global temperature rises.
Scientists and engineers are working on ways to slow the accumulation of man-made CO2. One such device, called a "scrubber," is used to capture exhaust gases from power plants. While effective, smokestack scrubbers are an expensive solution that must be installed at the source of emissions. The innovative technology featured in this video offers a different approach, one that would remove CO2 already in the atmosphere. Synthetic trees are designed to mimic the function of natural trees, using artificial leaves to pull CO2 out of the air. Unlike smokestack scrubbers, these "wind scrubbers" could be installed almost anywhere—presumably at a lower cost.
To effectively capture CO2, the scientist and engineering team had to conceive of a "leaf" surface that would remove the most CO2 from the air, and they had to determine a leaf coating that would react with the CO2. Because all leaves need to allow sunlight, air, and water to contact their surface for photosynthesis to occur, the location of the leaves on a plant determines how much sunlight they absorb and how air flows around them. Since synthetic leaves do not require sunlight, the team determined that their leaves could be packed much more tightly together—but not so tightly as to produce too much resistance to air as it passed through them. Tests confirmed the optimal leaf configuration to contain long, flat, vertically oriented sheets.
Lackner's daughter's science project demonstrated that a hydroxide solution would remove CO2 from the air. When CO2 comes into contact with sodium hydroxide, it binds with it, producing a liquid solution of sodium carbonate. However, because sodium hydroxide is highly corrosive, the team decided against using it to capture CO2. Rather, they developed a proprietary fabric material whose surface coating could attract CO2 just like sodium hydroxide—without the downside. Once captured, the CO2 is rinsed from the fabric for storage.
If realized, the synthetic tree would purportedly remove 90,000 tons of CO2 each year—significantly more than a natural tree captures. While the impact of this cannot yet be known, 90,000 tons is the equivalent to the emissions of 20,000 cars in a year. However, the process of removing, isolating, and compressing CO2 for storage is itself energy-intensive. So, unless the design team can develop a way to either reduce the amount of energy used in the process or use "cleaner" renewable energy, such as solar or wind power, to drive it, they could end up contributing to, rather than helping to resolve, the growing CO2 pollution problem.
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