Jet packs, servant robots and flying cars held promise for the 21st century. Instead, we got mechanized, self-contained vacuums. Today, a team of researchers at Penn State is exploring the requirements for electric vertical take-off and landing (eVTOL) vehicles and are designing and testing potential battery power sources.
“I think flying cars have the potential to eliminate a lot of time and increase productivity and open up the corridors of the sky for transportation,” said Chao-Yang Wang, William E. Diefender Chair of mechanical engineering and director of the Center for Electrochemical Engines. , State of Pennsylvania. “But electric vertical take-off and landing vehicles are a very difficult battery technology.”
Researchers define technical requirements for flying car batteries and report on a prototype battery today (June 7) in Joule.
“Flying car batteries need a very high energy density for you to stay aloft,” Wang said. “And they also need very high power during takeoff and landing. It takes a lot of power to climb and descend vertically.”
Wang notes that the batteries will also need to be recharged quickly so that there can be high income during peak hours. He sees these vehicles making frequent takeoffs and landings and recharging quickly and often.
“Commercially, I would expect these vehicles to make 15 trips, twice a day at peak times to justify the cost of the vehicles,” Wang said. “The first use will likely be from a city to an airport carrying three to four people about 50 miles away.”
Weight is also a factor to consider for these batteries, as the vehicle will need to lift and land the batteries. Once the eVTOL takes off, on short trips the average speed would be 100 miles per hour and long trips would average 200 miles per hour, according to Wang.
Researchers experimentally tested two energy-dense lithium-ion batteries that can recharge with enough power for an eVTOL trip of 80 km in five to ten minutes. These batteries could withstand over 2,000 fast charges over their lifetime.
Wang and his team used the technology they are working on for electric vehicle batteries. The key is to heat the battery to allow rapid charging without the formation of lithium spikes which damage the battery and are dangerous. It turns out that the battery heater also allows rapid discharge of the energy contained in the battery to allow takeoffs and landings.
Researchers heat the batteries by incorporating a nickel foil that quickly brings the battery to 140 degrees Fahrenheit.
“Under normal circumstances, the three attributes needed for an eVTOL farm work against each other,” Wang said. “The high energy density reduces fast charging, and fast charging generally reduces the number of possible recharge cycles. But we are able to do all three with one battery. “
A very unique aspect of flying cars is that the batteries must always hold a certain charge. Unlike cell phone batteries, for example, which perform best if fully discharged and recharged, a flying car battery can never fully discharge in midair because energy is required to stay in the air and land. There should always be a safety margin in a flying car battery.
When a battery is empty, the internal resistance to charge is low, but the higher the remaining charge, the harder it is to inject more energy into the battery. Typically, charging slows down as the battery fills up. However, by heating the battery, recharging can remain within the range of five to ten minutes.
“I hope the work we’ve done in this document gives people a solid idea that we don’t need another 20 years to finally get these vehicles,” Wang said. “I believe we have demonstrated that eVTOL is commercially viable.”
Also working on this project were: Xiao-Guang Yang and Shanhai Ge, both Mechanical Engineering Research Assistant Professors, and Teng Liu, Mechanical Engineering PhD candidate, all at Penn State; and Eric Roundtree, EC Power, State College, Pennsylvania.
The US Department of Energy’s Office of Energy Efficiency and Renewable Energy, the US Air Force’s Small Business Technology Transfer Program, and the William E. Diefenderfer Endowment funded this research.
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Material provided by State of pennsylvania. Original written by A’ndrea Elyse Messer. Note: Content can be changed for style and length.