Public discussions about transportation electrification often center on cars, trucks, and buses. Little attention tends to go toward electrifying cargo and tanker ships, which account for about 3% of global carbon dioxide emissions. Northwestern University researchers developed a patent-pending onboard carbon capture technology they say works for long-range vehicles like tanker ships, while also showing potential advantages for short-range vehicles. 

“It might be harder for people to see onboard CO2 capture as climate friendly because it uses conventional, carbon-based fuels,” Northwestern’s Scott A. Barnett, senior author of the study, said in a news release. “People tend to assume hydrogen fuel cells and electric vehicles are more climate friendly. In reality, they often are not. Electricity might come from burning coal, and hydrogen is often produced by natural gas, which generates a lot of CO2 in the process.” 

The innovation

The carbon capture technology consists of a dual-chamber storage tank. One chamber stores a carbon-based fuel, and the other stores the CO2 after the fuel cycles through the fuel cell. The CO2 is pressurized and held in the chamber.

“This technology really doesn’t have any major hurdles to making it work,” Barnett said. “You just have to replace the fuel tank with the double-chamber tank and add CO2 compressors. And, of course, the infrastructure eventually has to be developed to off-load the CO2 and either sequester or use it.” 

The divider between the two chambers can move so the fuel chamber shrinks as fuel is consumed, making more room on the other side to store CO2.

“The solid oxide fuel cell is critical because it burns the fuel with pure oxygen, yielding a concentrated CO2 product that is storable,” said Travis Schmauss, a Ph.D. candidate on the research team. “If we just burned the fuel with air, it would be heavily diluted with nitrogen, yielding too much gas to store. When the concentrated CO2 is compressed, it can be stored in a volume not much larger than that needed for the fuel, which saves space.” 

Comparing methods

The research team performed an analysis comparing onboard carbon capture technology to battery-electric and hydrogen fuel cell-powered transportation. 

• They determined batteries are not viable because of power density constraints. A battery pack on a ship would have to be quite large to provide enough power for the length of time needed to power a ship on long global trips. 
• Hydrogen fuel cells bear similar size constraints. 
• They concluded the best option for making long-range vehicles carbon-neutral is by using a carbon-based fuel combined with onboard CO2 capture technology.

“We calculated that the battery pack for a long-range tanker would take up more room than the storage capacity of the ship,” Barnett said. “A hydrogen fuel tank also would be too large.”

Advancing the concept

The onboard carbon capture method also holds potential for use on shorter-range vehicles, the researchers say. However, battery-electric and hydrogen fuel cell technologies already are being implemented for those types of vehicles.

The researchers say that the onboard CO2 capture method could be taken further to not just make long-range vehicles carbon-neutral, but carbon-negative, by using biofuels like ethanol.