Our dependence on the unabated burning of fossil fuels drives the relentless rise in carbon dioxide (CO2) emissions, worsening the climate crisis. Historic wildfires and heat waves are becoming a regular summer occurrence in the Northwestern US, while unprecedented freezes are shutting down parts of Texas for several weeks. Decarbonizing the economy and transitioning to renewable energy sources are critical to mitigate climate change and meet the goals set in the 2016 Paris Climate Agreement. Unfortunately, the consensus among climate scientists is that this is no longer enough. In their latest assessment, Intergovernmental Panel on Climate Change (IPCC) asserted that we need to start removing CO2, from the atmosphere using a suite of technologies called carbon dioxide removal (CDR) technologies.1 By using CDR technologies, the worst effects of climate change can be decreased by offsetting some of the CO2 that we are still emitting, and eventually decrease the CO2 concentration in the atmosphere to safer levels.

One of the prominent CDR technologies expected to play a role over the next century is called Direct Air Capture (DAC). DAC is highly attractive because unlike other technologies, it does not require large swaths of land to scale up (like biomass carbon removal and storage; BiCRS), and it is not constrained to geographical locations (like afforestation and reforestation). Additionally, since DAC captures CO2 from ambient air, it can be located anywhere, allowing it to be co-located with any end-use partner, whether that be carbon storage or CO2 utilization.

DAC can be defined as a technology which removes CO2 from the atmosphere without using photosynthesis. Current designs are typically large plants which use fans to funnel ambient air near ground level into contact with a CO2-atttractive sorbent or solvent. After the CO2 molecules attach to the sorbent, the rest of the air is released back into the atmosphere. Once the sorbent is saturated with CO2, heat and/or pressure is applied, and the CO2 molecules get released in a process called regeneration, creating a highly concentrated stream. This design, although demonstrated by a few companies at pilot scale, lends itself to unfeasibly high energy and financial costs. By significantly reducing the need for energy, at Arizona State University (ASU), we are working as the research arm of a company which has a creative solution to bring down the cost of DAC.

The Carbon Collect MechanicalTreeTM, based on innovations by ASU’s Dr. Klaus Lackner, is a unique DAC system that is being designed, tested, and manufactured by Carbon Collect Limited. This design is passive and does not require the use of energy-intensive fans to funnel the air into the system. Instead, the MechanicalTree™ relies on the wind to create contact with the sorbent, significantly reducing the cost of the system compared to other contemporary designs. At its full height, the MechanicalTree™ appears as a cylindrical structure which stands a little taller than a two-story building at approximately 10 meters tall and approximately 1.5 meters in diameter. The sorbent, which captures the CO2, is supported on large thin disks stacked vertically with gaps for natural air convection. When the sorbent is saturated with CO2, the disk stack is collapsed and sealed into the base of the MechanicalTree™ which is approximately 2.5 meters in height. In the base, the sorbent is regenerated. After regeneration, the MechanicalTree™ expands to its full height once again and the process starts over. The captured CO2 can be sequestered underground for permanent disposal. It can also be purified and compressed and used for a variety of purposes.

At ASU’s Tempe Campus, a commercial scale MechanicalTree™ DAC system is capturing CO2 from the air beginning in Spring 2022. In addition to being a demonstration unit, it is rigged up with instrumentation for ongoing experimental testing and development of future improvements. Researchers at ASU's Center for Negative Carbon Emissions will be performing exhaustive experiments and testing on this device, determining performance under different weather conditions, and developing a digital twin model. The test facility has also been designed to accommodate a next generation MechanicalTree™. With the commissioning of this first MechanicalTree system, Carbon Collect and ASU have taken a large step toward large scale commercial deployment.

Carbon Collect's MechanicalTree™ embodies a small modular design strategy. As opposed to building large DAC plants which will last for several decades, Carbon Collect plans to rapidly iterate on smaller capacity systems allowing for frequent and adaptable design improvements, significantly shortening the cycle time for next generation systems. Due to the small modular design, Carbon Collect will be independent of scale. They can match the capacity of the large DAC plants by mass producing and deploying their MechanicalTree™ in "farms" for carbon capture and sequestration, and they will also be able to deploy smaller capacity systems to meet more niche target markets like greenhouses, vertical farms, and other commercial use projects. The mass production of MechanicalTree™ will drive down the costs in the medium-to-long term2 and allow Carbon Collect to provide a scalable and affordable tool for climate change mitigation.

References:

1. IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press

2. K. S. Lackner, H. Azarabadi, “Buying Down the Cost of Direct Air Capture”, Industrial & Engineering Chemistry Research, Vol 60, Iss. 22. May 26, 2021.