Researchers use regenerative braking to put direct air capture onto railway tracks

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Global rail systems could help mitigate climate change and remove CO2 by capturing the sustainable energy generated when trains brake and decelerate.

CO2Rail Company

This is the claim of CO2Rail Company, a Texas start-up that has worked with several researchers, including engineers from Sheffield University, to design Direct Air Capture (DAC) technology that can be used within special rail cars placed on trains in regular service.

The DAC rail cars work by using large intakes of air that extend up into the slipstream of the moving train to move ambient air into a large cylindrical CO2 collection chamber, eliminating the need for energy-intensive fan systems currently necessary with stationary DAC operations. 

The air then moves through a chemical process that separates the CO2 from the air and the carbon dioxide free air then travels out of the back or underside of the car and returns to the atmosphere.

After enough has been captured, the chamber is closed and the harvested CO2 is collected, concentrated, and stored in a liquid reservoir until it can be emptied from the train. It can then be used as value-added feedstock for CO2 utilisation, or sequestered into suitable geological sites nearby. 

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Each of these processes are powered by on-board generated, sustainable energy sources that require no external energy input or off-duty charging cycles.

According to the team’s paper published in Joule, the system ‘uniquely exploits the substantial sustainable energy generated on-board the train through regenerative braking’. 

In a statement, Professor Peter Styring, director of the UK Centre for Carbon Dioxide Utilization at Sheffield University and co-author of the research, said: “Currently the enormous amount of sustainable energy created when a train brakes or decelerates is simply lost. This innovative technology will not only use the sustainable energy created by the braking manoeuvre to harvest significant quantities of CO2, but it will also take advantage of many synergies that integration within the global rail network would provide.

“The technology will harvest meaningful quantities of CO2 at far lower costs and has the potential to reach annual productivity of 0.45 gigatons by 2030, 2.9 gigatons by 2050, and 7.8 gigatons by 2075 with each car having an annual capacity of 3,000 tonnes of CO2 in the near term.”

Stationary DAC operations require large areas of land to build equipment and to construct renewable sources of energy to power them, whereas CO2Rail would be transient and generally be unseen by the public.

Eric Bachman of CO2Rail Company, said: “On average, each complete braking manoeuvre generates enough energy to power 20 average homes for an entire day so it is not a trivial amount of energy. 

“Multiply this by every stop or deceleration for nearly every train in the world and you have about 105 times more energy than the Hoover Dam produces within that same period, and that was a hydro-electric construction project that took six years and cost $760 million in today’s dollars.”

The team found each DAC car can harvest about 6,000 metric tons of carbon dioxide from the air per year and more as the technology develops. Moreover, since trains are capable of hosting multiple CO2Rail cars, each train will harvest a corresponding multiple of CO2 tonnage. 

With its sustainable power requirements supplied by train-generated sources that are without incremental cost, savings of 30 – 40 per cent per tonne of harvested CO2 can be realised from energy inputs alone, the team claims. 

This, along with other significant savings such as land, brings projected cost at scale down to less than $50 per tonne, making the technology commercially viable.

The research team included experts from the University of Toronto, MIT, Princeton, business, and industry.