Contributed by Adam Rondinone, Senior Scientist, Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences
Motor-vehicle transportation presents a special challenge in the fight against climate change. In all its forms, transportation accounts for about 26 percent of the U.S. CO2 emissions according to the U.S. EPA. But unlike other major sources of CO2 emissions, transportation may be the most difficult to address due to the technical requirements of mobility and the high economic value placed on the activity. Alternative transportation energy carriers (e.g., battery, fuel, etc.) must be small, light, tolerate vibration and temperature excursions, and last for the lifetime of an average consumer automobile with minimal maintenance. Most importantly, transportation energy carriers must refuel quickly and store enormous amounts of energy in as little weight as possible. That is easy for a metal fuel tank, but it is much more difficult for a battery or fuel cell.
So what are the options for CO2-free transportation? Two major pathways have been demonstrated to date: biofuels and electrification via batteries. Biofuels represent an appealing choice to the consumer in the sense that the cars look and act familiar. They can be fueled quickly, they drive the same and the ownership experience is the same. Miles per gallon are lower, but that is a matter of engine design choices and can be addressed. Although the carbon in biofuels originates in the atmosphere, there is controversy regarding the actual carbon balance for North American biofuels due to the need for fertilizers, the transportation of the biomass, and other impacts on the soil and groundwater. But if we assume that the carbon balance is beneficial, we still have to dedicate large areas of arable land to growing the biomass, and that puts a limit on our supply.
Electric automobiles are the second major pathway. I believe that electric cars and light trucks will be the mainstay of commuter and family transportation in the future, as the charge cycles are well-matched to the driving requirements, with cars parked for recharging during the evening and driven limited miles during the day. The energy efficiency of electric automobiles is unmatched by internal combustion, and the total efficiency can exceed 60 percent for the entire drivetrain.[ii] The flexibility for charging off hours makes electric automobiles particularly well suited for renewable electricity generation, which often doesn’t match up with normal electricity demands on the grid.
But as good as electric automobiles have become, it is hard to envision how to electrify all parts of the transportation sector. Commercial transportation — such as airplanes, trains, and heavy trucks — operates on much more aggressive duty cycles, sometimes continuously, with downtime only for repairs, which may preclude the long charging times necessary for today’s batteries. The size and weight of current battery technologies makes them generally inappropriate for heavy trucks and airplanes.
This is where electrochemical fuels can play a role. With electrochemical fuels, we use electrical energy to electrochemically synthesize a liquid fuel (e.g., ethanol) from a carbon source (e.g., carbon dioxide). The ethanol is identical (fungible) to ethanol produced by fermentation – it drops into the fuel distribution and supply infrastructure that we already have and powers internal combustion engines that we already use. Ethanol can be up-converted to diesel and jet fuel; other electrochemical fuels are also a possibility. Depending on the source of the electricity, the electrochemical fuel can be completely carbon neutral. The carbon dioxide can be sourced from the myriad existing point sources, such as power plants and CO2 that gets intercepted and used a second time, preventing the emission of an equivalent amount from a fossil source.
In essence, this process takes the electrochemistry (the battery) out of the vehicle and centralizes it at a factory (electrochemical fuel). Electrical energy, which used to be stored in a battery, instead becomes stored as a liquid readily moved or dispensed.
If successful, this would achieve several things: It would relieve the vehicle manufacturer of the need to make a battery that can tolerate years of heavy use with little or no maintenance, and it would render moot the concerns of battery weight and size, as the electrochemical part of the system becomes stationary. By moving the capital expense and risk out of the vehicle and centralizing it, one might realize significant improvements in economy of scale and maintainability. It would also render moot the need to have time dedicated to charging, as the electrochemical process that generates the fuel operates continuously and independently of the vehicle.
The technologies to achieve this goal are all still very early stage. Research groups all over the world, including ours, are working on the problem of electrochemical CO2 conversion to useful products. Several technical hurdles must still be overcome, including the activity and selectivity of the catalysts and improved energy efficiency. But if this endpoint can be achieved, it offers another possible strategy to minimize CO2 emissions without disruptions to economic activities.
Adam Rondinone is a senior scientist at the Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences. The opinions expressed here do not necessarily represent the opinions of the Oak Ridge National Laboratory or UT-Battelle, LLC. Copyright 2017, UT-Battelle, LLC.
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