Energy-efficiency of flying cars
Flying cars have been the epitome of high technological advancements and utopia for several decades. The vision for urban air transportation is to eliminate the runway for take off and land to minimize space usage, a scarce resource in cities. Along with this vision, there is hope for sustainability, enabled by electrification, which is currently permeating through road transport. In 2018, we analyzed the performance requirements needed of batteries for enabling electric vertical take-off and landing (EVTOL) aircraft, addressing the large gap between the energy content inside jet fuel and modern Li-ion batteries.
Jet fuel contains within it about 12,000 Watt-hours for every kilogram while the best battery packs today are around 200 Wh/kg. The last decade has seen massive progress in battery performance and cost, in large part to the electric vehicle revolution. There are prototype cells hitting well over 300 Wh/kg-cell today with aggressive goals to reach the 500 Wh/kg mark.
Now, the possibility of designing an EVTOL aircraft with over 100 miles of range is looking technically feasible. EVTOL aircraft also promise a two-to-six-fold faster means of point-to-point mobility compared to terrestrial alternatives. Large amounts of money, amounting to 10s of billions of dollars have been invested in making this sector a reality and there are over 100 companies working on this idea currently.
So how energy efficient can these aircraft be in relation to road transport? Are current and near-term available batteries capable of supplying the unique demands of EVTOL aircraft? Or are we putting the horse before the cart here? We looked at exactly these questions in our latest study in the Proceedings of the National Academy of Sciences. To think about energy efficiency and compare EVTOLs to ground-based vehicles, we have to make sure we’re comparing apples to apples–EVTOLs fly from point to point while ground vehicles travel on roads which are never a straight line. The other important factor to consider is occupancy; thus, Watt-hours per passenger-mile is the appropriate metric to compare efficiencies of different modes of mobility.
Aircraft with fixed wings can cruise very efficiently and the longer the cruise segment, the lower the overall energy consumption is. For certain aircraft designs, these cruise segments are so efficient that a fully occupied EVTOL would have a Wh/passenger-mile rating equivalent to or lower than an EV with the expected occupancy of on-road passenger vehicles. The key takeaway, contrary to popular belief, is that EVTOLs are not going to be the typical luxury vehicles that consume several fold more energy than equivalent consumer vehicles. They could fit into the larger vision of sustainable mobility.
Now, comes the question of whether batteries are ready to supply the requisite demands for EVTOL aircraft. One unique challenge of batteries for EVTOL aircraft compared to ground-based vehicles is the need to supply high discharge power for take-off and landing. Supplying high-power especially for landing, or reserve segment, is currently the limiting factor for next-generation batteries. Batteries suffer from what we like to call the “AND problem”. It is easy to design a battery to satisfy one metric, but the challenge comes when those batteries have to satisfy them simultaneously. The “AND problem” for automotive batteries is to deliver requisite cycle life, fast charging times. In addition to these two constraints, EVTOL batteries need to deliver high discharge power at low state-of-charge. EVTOL batteries have to provide this discharge power even under scenarios of partial failure.
[This article was co-written with Shashank Sripad.]