Robot housemaids, Jet packs, and flying cars were all promises for the 21st century. Instead, we got fully autonomous vacuum cleaners.
Now a group of scientists at Penn State is studying the requirements for electric vertical takeoff and landing (eVTOL) vehicles and outlining and experimenting with potential battery power sources.
“I believe flying cars have the potential to save a lot of time and improve productivity and open the sky corridors to transportation,” said Chao-Yang Wang, director of the Electrochemical Engine Center and owner of the William E. Defender Chair of Mechanical Engineering, Penn State. “However vertical takeoff and landing of e-vehicles are very challenging technology for the batteries.”
The scientists determined the technical requirements for flying car batteries and file a report on a prototype battery on June 7, 2021, in Joule.
“Batteries for flying cars demand extremely high energy density so that you can stay in the air,” said Wang. “And they also require a lot of power at the time of take-off and landing. It needs very high power to go vertically up and down.”
Wang notes that the batteries will also require to be fast recharged so that there could be high revenue through rush hours. He sees these vehicles having regular take-offs and landings and recharging quickly and often.
“Commercially, I would assume these vehicles to do 15 trips, twice a day during rush hour to justify the expense of the vehicles,” said Wang. “The initial use will plausibly be from a city to an airport taking three to four people about 50 miles.”
As the vehicle goes up and down, the weight is also a concern for these batteries. According to Wang, once the eVTOL goes up, the average speed would be 100 miles per hour small trips and long trips would average 200 miles per hour.
The scientists empirically experimented with two energy-dense lithium-ion batteries that can recharge in five to ten minutes with sufficient energy for a 50-mile eVTOL trip. These batteries could provide more than 2,000 fast charge cycles over their lifetime.
Wang and his group applied the technology they have been operating on for electric vehicle batteries. The key is to increase the temperature to heat the battery to provide rapid charging without the production of lithium spikes that degrade the battery and are dangerous. Though, heating the battery causes a quick outflow of the energy carried in the battery to allow for take-offs and landings.
The Scientists raised the battery temperature rapidly to 140 degrees Fahrenheit by incorporating a nickel foil.
“Under ordinary conditions, the three attributes essential for an eVTOL battery work against each other,” said Wang. “High energy density lessens fast charging, and fast charging normally decreases the number of feasible recharge cycles. But we can do all three in a single battery.”
One novel feature of flying cars is that the batteries must continuously retain some charge. Unlike cellphone batteries, for example, that run fine if fully discharged and recharged, a flying car battery can never be permitted to completely discharge in the air as power is required to stay in the air and to land. There always demands to be a perimeter of safety in a flying car battery.
Internal resistance to charging is when a battery is exhausted, but the higher the residual charge, the more challenging it is to push more energy into the battery. Typically, recharging slows as the battery fills. Yet, by increasing the temperature of the battery, recharging can remain in the five- to the ten-minute range.
“I believe that the work we have done in this paper will provide people a rooted idea that we don’t require another 20 years to eventually get these vehicles,” said Wang. “I think we have explained that the eVTOL is commercially viable.”
“Challenges and key requirements of batteries for electric vertical takeoff and landing aircraft” by Chao-Yang Wang, Teng Liu, Shanhai Ge, Xiao-Guang Yang, and Eric Rountree 7 June 2021, Joule.