Thinking beyond batteries

The age in which we rely on fossil fuels for nearly all our energy needs appears to be coming to an end, albeit more slowly than scientists say we need to to avert the most severe impacts of climate change. The signs are all around us. Electric cars continue to grow in market share, and some countries are even close to phasing out new fossil fuel-based car sales already. Renewable energy from solar and wind continues to get cheaper, gradually replacing coal in the electricity generation mix. And there’s even growing trepidation around the future of natural gas as a “bridge fuel,” both because it still packs a heavy carbon footprint itself and because leaking infrastructure can emit methane — a greenhouse gas that’s 25 times more potent than CO2. 

For sure, we’re still a ways from our lives being powered exclusively by carbon-free fuels, and the world is quickly realizing there are many problems without straightforward solutions. The transportation sector, which accounts for the largest share of greenhouse gas emissions in the U.S., poses some of the stiffest challenges, according to Assistant Professor of Mechanical Engineering Doohyun Kim. And this is especially the case when you start looking beyond the passenger vehicle fleet, which does indeed show a lot of potential in pivoting from gas and diesel to electricity. The core issue is that with more energy-intensive transportation, like long-haul trucks, freight trains, container ships and airplanes, the electric-motor-plus-battery recipe doesn’t really work. Compared with fossil fuels, batteries simply aren't very "energy dense." For example, the onboard lithium-ion battery pack powering the 259-mile range Chevy Bolt spans the full width of the car and weighs 960 pounds. On the other hand, the gas-powered Chevy Cruze can travel nearly 550 miles on a single fill-up of its 13.7-gallon gas tank. An airplane carrying enough onboard batteries to fulfill its energy needs, Kim says, just can’t exist, at least not with existing technologies. Given that mass-based energy density is more than 50 times higher for liquid fuels, a battery-powered plane would simply be  “unrealistically heavy.” 

Against the backdrop of the climate crisis, the solution can’t simply be to electrify our passenger vehicle fleet with renewable energy-based electricity and then continue to use fossil fuels for everything else. Trains, planes and ships will also need to be powered by renewable sources, but to do that, Kim says we likely need to expand our ideas about storage. Typically, we think of storage and we think batteries. But fundamentally, storage is just a way of holding onto energy for future use, and in the case of transportation, we also have to be able to carry it around. Batteries in electric vehicles fulfill this mobile storage function, but so does the gas in your gas tank. So Kim says, if you can find some way to store renewable-based electricity not in a battery but in the form of an energy-dense liquid fuel, you’d be on your way to a real solution for the heaviest sectors of our transportation network.

Turning something ethereal like electricity into a physical liquid fuel might sound like magic, but Kim says researchers have been making big strides in this kind of energy conversion. Through a U.S. Department of Defense grant, Kim’s own work in this area has thus far focused on simulation and modeling techniques for converting other fossil fuels like coal and natural gas into a liquid that behaves a lot like jet fuel. But he says the same concepts can be applied to using renewable-based electricity to make synthetic gasoline or diesel. In one such process, electricity is used to separate the hydrogen from plain water through electrolysis; and then that hydrogen is combined with CO2, which could either be directly captured from the atmosphere or from the emissions of existing coal or natural gas-fired power plants. The process takes some energy, high pressures and a catalyst, but the resulting hydrocarbon molecules form the basis of a liquid fuel that acts a lot like gasoline, diesel or jet fuel. Further, if the whole process uses only renewable electricity, you’re suddenly talking about a high-density, liquid energy source that can power the parts of our transportation sector that are hardest to get off fossil fuels. 

Before you start thinking this is a world-saving technology all on its own, Kim says there are some important caveats. First, the real-world efficiency of burning electricity-derived fuels, or efuels, pales in comparison to using the electricity to directly power motors. This is why the “tank-to-wheel” efficiency of electric cars is so great compared to their gas counterparts, Kim says. Battery-powered vehicles can send close to 80 percent of the energy they pull from the grid to their wheels. For a small gasoline engine, that number is closer to 30 percent (though large diesel engines have much higher efficiency). So widespread use of efuels may only make sense once we are harvesting so much solar and wind energy, we can actually afford this lower efficiency. (A future scenario like this is far from impossible, however, given that the amount of solar energy hitting the Earth in a single hour exceeds humanity’s yearly energy usage.) Second, even when you use carbon directly captured from the atmosphere, efuels are at best carbon neutral, because they rerelease that carbon when they’re burned.

Even so, Kim sees potential in efuels occupying energy niches for which there aren’t any other solutions, and in the next phase of his research, he’s excited to apply some of his earlier work to the efuels format. “My take is that to truly have our lives powered by renewable energy, we have to have some diversity,” Kim says. “For small passenger cars, electric motors and batteries may be the best solution right now. But for other transportation modes, a different form of energy will probably be better.” Without a doubt, in tackling a problem as big as the climate crisis, the more tools we have, the better.

###

Story by Lou Blouin. If you’re a member of the media and would like to interview Assistant Professor of Mechanical Engineering Doohyun Kim about this topic, drop us a line at [email protected] and we'll put in you touch.