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An Electric Semi Definitely Won’t Work—But Seven in a Row Might

Platooning might make the economics reasonable for short-haul electric trucking.
October 26, 2017
Mark Brake / Stringer

Ever since Elon Musk announced his ambitious plan to roll out the electric Tesla Semi last summer, industry observers and battery experts have openly wondered how the company could make the plan work.

Researchers have long asserted that current lithium-ion batteries are too heavy and expensive to meet the demands of heavy-duty trucking. But a Reuters report in August raised an interesting possibility: correspondence between Tesla and regulators in the Nevada Department of Motor Vehicles suggested the company hoped to test multiple trucks driving in a platoon formation, relying on autonomous-driving technology (see “10-4, Good Computer: Automated System Lets Trucks Convoy as One”).

The basic concept behind platooning is that trucks driving in close alignment significantly reduce aerodynamic drag, boosting the overall energy efficiency of the fleet. Tesla is set to unveil its electric truck at an event next month, which promises to shed more light on the company’s strategy.

But following the Reuters report, battery researchers at Carnegie Mellon University decided to take a close look at this possibility. They concluded that platooning could make heavy-duty trucks more cost competitive, at least with enough semis, according to a study in ACS Energy Letters published Thursday. The sweet spot appears to be seven vehicles driving together on trips shorter than 300 miles, which would reduce aerodynamic drag by 50 percent. Longer hauls, on the other hand, would still face exorbitantly high costs.

Venkat Viswanathan, an assistant professor of mechanical engineering at Carnegie Mellon, undertook the analysis with Shashank Sripad and Matthew Guttenberg, graduate researchers in the department. In the same journal in June, Viswanathan and Sripad cast considerable doubt on the economic feasibility of a solo-operating electric semi, concluding that such a vehicle “would be limited to a driving range well under 600 miles, a small payload capacity, and a prohibitively high cost.”

So how do the economics change when you string together seven electric trucks?

If they only need to travel a range of 300 miles, the required battery pack drops from 1,100 kilowatt-hours to 880 kilowatt-hours, and the cost of the pack falls from about $200,000 to $158,000, “which is quite cost competitive,” Viswanathan says. As a bonus, the potential cargo payload actually increases over the average cargo weight, from 16 tons to 25.5 tons.

By way of comparison, the total cost of an average diesel-powered truck is about $120,000, and the vehicle can run for about 1,000 miles on a single fill-up.

But the economics still don’t seem to work for long-haul electric trucking, even with extended platooning. A semi with 900 miles of range in such a scenario would require a 2,600-kilowatt-hour battery pack that would cost around $420,000.

Why can’t the 300-mile trucks just stop and recharge more often? Charging electric trucks takes a lot more time than refilling diesel tanks, increasing idling periods and undermining delivery economics. Making 300-mile trucks competitive in long-haul scenarios would probably require battery-swapping facilities, which employ robotics to quickly trade fresh batteries for spent ones, the researchers say. Tesla previously explored that option for its cars before settling on Supercharger stations.

The good news here is that short-haul trucking does represent the majority of delivery trips, according to the Bureau of Transportation Statistics, suggesting there could be an addressable market for a lengthy line of electric semis.

On the other hand, stringing together seven trucks would represent “some advanced platooning,” said Bryant Walker Smith, an assistant law professor at the University of South Carolina who focuses on autonomous driving, in an e-mail. That number is well beyond the two or three that most platoon developers are initially aiming for, and more than any single carrier usually has traveling together at the same time, he added.

That suggests the need for cross-carrier collaboration, as well as common standards and coordinated logistics. Lengthy platoons could also raise safety issues, especially if seven trucks all merge onto the highway at once or another driver tries to pass that many vehicles.

Autonomous-driving features are considered a necessary first step in extended platooning, enabling the vehicles to safely and consistently drive in close proximity over long periods.

Tesla, of course, has already been developing and implementing such features in its consumer automobiles. Uber, Otter, and Waymo are all working on autonomous trucking as well (see “10 Breakthrough Technologies 2017: Self-Driving Trucks”). Meanwhile, some vehicle manufacturers like Daimler and startups like Peloton Technology have already begun developing and testing trucks capable of platooning in limited numbers.

The Carnegie Mellon study highlighted another key benefit of platooning electric semis: the lower strains on the battery packs could extend their useful life by another 60,000 to 120,000 miles, essentially adding another year of commercial use on the high end. That plus other factors, like the lower maintenance requirements of electric vehicles, could significantly decrease the lifetime costs of these trucks.

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