Staying cool
By Paul Brinkmann|October 2023
Over the years, engineer Matthew Clarke has sought to learn as much as he can about the specific methods electric aircraft companies apply to keep lithium-ion batteries below the well-recognized maximum operating temperature of 35 degrees Celsius.
If batteries get too hot as the aircraft’s motors draw power from them, there is a risk of one cell catching fire and triggering overheating of all the others. Methods for keeping batteries below this danger threshold include drawing in outside air during flight and sometimes chilling the battery with liquid coolant. However, Clarke hasn’t been able to determine which method, or combination thereof, the industry relies on most.
Cooling strategies are “very proprietary,” says Clarke, an assistant professor of aerospace engineering at the University of Illinois Urbana-Champaign. Based on his own inquiries to companies and outside experts, “people say it’s hard to determine how the industry in general is handling the cooling issue.”
Nevertheless, Clarke knows from his research that cooling batteries with ambient air during flight has aerodynamic drawbacks, because the air slows as it’s drawn into the inlets. “As the air exits the aircraft, it’s moving slower than the air slipping around the wing, and that creates a pressure differential, turbulence and a pocket of drag — what we call cooling drag.”
Come January, Clarke and his colleagues will have a venue dedicated to studying potential solutions for cooling drag and related issues: the Laboratory for Electric Aircraft Design and Sustainability. Clarke is in the process of setting up the lab in a vacated space in the Aerospace Engineering department.
When a battery is cooled exclusively or in part by ambient air, Clarke estimates that thrust is reduced up to 15%. To reduce the size of the required inlet and give more design flexibility, some designs may combine the effect of ambient air with a liquid coolant. The coolant absorbs heat energy from the battery and sheds it into the airflow, similar to how conventional and electric cars gather air and direct the air over a radiator (sometimes more than one in electric vehicles) with coolant circulating through it.
Consider California-based Joby Aviation, whose planned air taxis would fly short routes of 10 to 15 minutes: Each aircraft would have “active cooling plates mounted between each battery’s individual cells, which maximize coolant flow over the cells’ external surfaces,” Joby told me.
Photos and videos of Joby prototypes show openings in the airframe that could be air intakes, but Joby would not elaborate.
In July, engineers with Slovenia-based Pipistrel showed me air intake valves on the two-seat Velis Electro they exhibited at the AirVenture show in Oshkosh, Wisconsin.
Pipistrel’s Tine Tomaži says the aircraft has an onboard, in-flight coolant system that requires outside air “to achieve lower cooling drag than we would have with only air cooling. We are able to control the airflow much better, and the placement of the intakes and the ducts that move the air through the aircraft.” (He would not share how much drag the air intakes create for the aircraft.)
Another air taxi developer, Vermont-based Beta Technologies, doesn’t do any in-flight battery cooling on its Alia aircraft, says Sean Donovan, a Beta mechanical engineer. Instead, before takeoff, liquid coolant is circulated through the battery to chill it. This keeps the battery within its safe temperature range for the duration of the planned flight, he says, with no safety compromise.
“The most demand on the battery is during vertical takeoff and landing, right? And because our aircraft is highly efficient while cruising, there’s low power demand during cruise, and the thermal increase in our battery packs is really low.”
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