Researchers from the National Renewable Energy Laboratory (NREL), in collaboration with two electric utilities, recently examined the causes, costs and lead times of distribution system upgrades anticipated for depot charging of short-haul heavy-duty trucks in the United States.

The team also developed synthetic depot charging load profiles for heavy-duty trucks from real-world operating schedules, and found that charging requirements are met at common light-duty EV charging rates (≤100 kW per vehicle). Applying depot charging load profiles to 36 distribution real-world substations, they then showed that most can accommodate high levels of heavy-duty EV charging without upgrades.

In most cases the existing technology grid can accommodate truck charging at depots. Their paper appears in the journal Nature Energy.

For years, the trucking industry has been skeptical of heavy-duty electric vehicles. That perception is shifting, and our research suggests that certain short-haul operations can be electrified today with relatively low-power depot charging that won’t overwhelm the grid.

—NREL’s Brennan Borlaug, lead author

While most people picture semi-trucks carrying heavy loads over long highways, data collected by the US Census Bureau suggests that nearly 80% of heavy-duty trucks operate primarily within a 200-mile range. These trucks account for around 50% of total heavy-duty vehicle energy use and are typically responsible for distributing goods between warehouses and nearby retail establishments.

As a result, the vehicles are often characterized by short, predictable routes and off-shift periods at central locations such as a vehicle depot, making them prime candidates for electrification. Ideally, fleets could perform all charging at their depots, where it is convenient, inexpensive, and fully controllable.

Commercial heavy-duty trucking operations are motivated to reduce operating costs. A 2020 BloombergNEF report shows that fuel costs for trucks make up more than half their total cost of ownership. The switch to BEV fleets would offer a significant reduction in fuel costs and require less routine maintenance, another key advantage for fleets.

NREL researchers leveraged real-world operating data from NREL’s Fleet DNA clearinghouse to simulate EV charging at fleet depots and made the charging loads results publicly available for other researchers to use. The Fleet DNA tool offers composite data summaries and visualizations for real-world medium- and heavy-duty fleet operations, useful for understanding the operating range of commercial vehicles across vocations and weight classes.

NREL researchers simulated multiple charging strategies, including “smart” charging, where BEVs take full advantage of the time spent parked at the depot to charge at slower rates and reduce peak energy demand. This study shows that charging requirements could be met at power levels in line with current light-duty charging technology (≤100 kW/vehicle) for the fleets studied.

An image of a heavy duty truck with the following statistics: Study of three short-haul heavy-duty fleet operations finds that 78% to 86% of real-world distribution substations studied are capable of supplying a fleet of 100 battery electric trucks with 100 kW/vehicle charging without upgrades. With slower managed charging, that number goes up to 90%.

A final focus of the research was to assess whether today’s electricity distribution grid could adequately support heavy-duty depot charging. NREL collaborated with two utilities—Southern Company and Texas-based Oncor Electric Delivery Company—to perform a load integration study for 36 substations and summarize the costs and timelines required for anticipated grid upgrades. The team found that most (~80%) of the substations studied could supply the time-varying loads of 100 trucks charged at 100 kW/vehicle without any upgrades, and an additional 10% of substations could avoid upgrades if fleets used “smart” charging.

This research is unique in that it paints a more complete picture of what it would look like to electrify these fleets. As technologies that enable heavy-duty fleet electrification become available, studies like this can help anticipate and prepare for the effects of this transition. Customer engagement with utilities to review load profiles, current and future business and operational needs, and deployment timelines will be key in the transition to electrification.

—David Woody, a senior manager in Distribution Planning with Oncor and co-author

Moving forward, NREL researchers are interested in studying how other commercial vehicle operating segments—such as last-mile delivery vans or long-haul trucking—may be electrified and integrated with the evolving grid.

Resources

• Borlaug, B., Muratori, M., Gilleran, M. et al. (2021) “Heavy-duty truck electrification and the impacts of depot charging on electricity distribution systems.” Nat Energy 6, 673–682 doi: 10.1038/s41560-021-00855-0