In 2021, Tompkins Consolidated Area Transit (TCAT) in Ithaca received a grant to procure seven all-electric buses and began a pilot program that didn't go as they'd hoped. In addition to issues with the manufacturers, the buses struggled in Ithaca's hilly terrain and were unreliable, with reduced range, in cold weather.
TCAT had already connected with Cornell researchers to gain insights from the pilot, and now Cornell researchers have analysed the buses' underperformance in the cold - with implications for cities, schools and other groups that are considering the electrification of their fleets, as well as operators, policymakers and manufacturers.
In a study published May 27 in Transportation Research Part D, researchers analyzed two years of TCAT data and quantified the increased energy consumption of the pilot fleet, finding that the batteries on the electric buses consumed 48% more energy in cold weather (between -4 to 0 degrees Celsius, or around 25 to 32 degrees Fahrenheit) and nearly 27% more in a broader temperature range (-12 to 10 degrees Celsius, or 10 to 50 degrees Fahrenheit).
The researchers also identify the operational culprits of the increased consumption and offer recommendations for improving the buses' functioning.
The study is the first to assess and analyze electric buses' performance in the northeastern U.S., with an unprecedented dataset that covers significant distance - more than 80,000 kilometers (nearly 50,000 miles) - at cold temperatures.
"We're benefiting from TCAT being a leader in this region, and it's a real privilege to have access to this data, so we can see the performance in real-time," said senior author Max Zhang, the Irving Porter Church Professor of Engineering in Cornell Engineering and Provost's Fellow for Public Engagement. "One of the lessons we've learned is that these buses should be designed for the whole country, including states with colder climates. We've also found that they're different from conventional diesel buses, with different behaviors, which require different strategies to take advantage of this."
The researchers, including first author and doctoral student Jintao Gu, modeled how the buses would perform at optimal temperatures and compared that to the actual performance across more than 40 complex routes and schedules. They found that half of the increased consumption in cold weather comes from the batteries' need to heat themselves. That's because batteries in electric vehicles operate at an optimal temperature of around 75 degrees Fahrenheit, and the colder the battery is when the bus starts, the more energy it takes to warm it. The other main culprit is the heating of the bus's cabin. With frequent stops, especially on urban routes in which the doors are opened and closed every few minutes, the batteries must work harder to heat the cabins.
"With an all-electric vehicle, the battery is the only onboard energy source," said Zhang, who is also a senior faculty fellow at the Cornell Atkinson Center for Sustainability. "Everything has to come from it."
The researchers also found that regenerative braking, whereby the battery recharges by capturing energy during braking, was also less efficient in cold weather. They said this is likely because the battery, which is about eight times the size of a standard electric vehicle battery, struggles to maintain an even temperature across its cells.
Short-term strategies to improve the batteries' function include storing the buses indoors when not in use, so the ambient temperature is warmer; charging the batteries when they're still warm; and limiting the length of time the bus doors are open at stops.
On a larger scale, Gu said the research points to the need for greater adjustments in, or assessments of, infrastructure to accommodate electric buses.
"You have to try to optimize the schedule of all of the buses and to consider the capability of your infrastructure - how many charging stations you have, and if you have your own garage," he said. "You have to train the drivers, the dispatchers and the service workers. I think from an operational and infrastructure perspective, there are a lot of messages here for future transit system planning."
Gu said the varied urban and rural routes and hilly terrain in Ithaca allowed the researchers to glean even more insights about the buses' performance, finding that the buses experienced a smaller increase in energy consumption on rural routes during cold weather compared to urban routes. That information might help planners strategically choose which routes to assign to electric buses in a mixed fleet.
The drastic increase in energy consumption was unexpected, Zhang said. "But any lessons are good lessons. This helps us learn as a society and do better."
The research is also an example of mutually beneficial collaboration between the Ithaca community and researchers at Cornell. Zhang's team has met repeatedly with TCAT officials as the research has progressed.
"We're learning each other's perspective, and we're sharing our insights," he said. "It's beautiful, right? Cornell researchers and TCAT, we're learning at the same time, we're learning through the data, through the collaboration."
Doctoral student Quanhuan Liao is a co-author.
