While planning for this year’s activities my manager asked, “Why have so many urban air mobility startups emphasized battery-only solutions?” This question motivated me to study recent developments happening in energy supply for Vertical Take Off and Landing (VTOL) vehicles.
Uber’s Elevate white paper (2016) served as a catalyst for the development of electric VTOL (eVTOL) vehicles to address worsening congestion, pollution, and noise in cities. As developments in the evolving Advanced Air Mobility (AAM) space gradually continue, vehicle developers rely on improvements in battery characteristics, which define time-to-market of both fixed- and multi-rotor VTOL electric aircraft. These vehicles are going to require high levels of power to get off the ground and maintain control in ‘helicopter’ mode.
While significant advancements continue to be made in lithium-ion battery power, storage density and cycle life, battery cells are ideal for electric cars; however, they do not seem to be sufficient for electric vertical takeoff and landing (eVTOL) aircraft. Lithium-ion cells — the dominant cell chemistry used in consumer electronics, electric cars and now for most electric aircraft projects — are plateauing, with power density no longer seeing a steady five percent annual improvement.
Battery cell chemistry involves tradeoffs between specific power — the amount of power the cell can exert at once — and energy storage density, as well as cycle life. eVTOL vehicles are going to be very power hungry on takeoff and landing. The ideal chemistry for eVTOLs is therefore very different from that of passenger cars. A key characteristic of any battery that is going to be flying such aircraft is the ability to have range and quick recharging capability; fast charge is one of those things that can degrade a battery pack if not done properly.
A few startups and some small retrofitted aircraft have successfully demonstrated current battery technology, with around 250-270 Wh/kg of specific energy which may face cycle life and range limitation. Many industry leaders and the Elevate program suggest that an energy density around 350 or 400 Wh/kg is necessary for the industry to really emerge. Tesla and SpaceX founder Elon Musk has opined that 400 Wh/kg of energy density with high cycle life, produced in volume (not just in labs), is necessary for electric aviation applications. Researchers suspect that Tesla is working on silicon anode technology, which is a potential way to get there.
Joby Aviation founder JoeBen Bevirt claimed in a recent interview that the company has been able to design a five-seat air taxi that can travel 150 miles powered by lithium-ion battery cells from electric cars, which is < 300 Wh/kg in specific energy and well short of 400 Wh/kg. Joby expects its aircraft to have a maximum allowable gross weight of 4800 lb/2177 kg; looking at current technology, that would mean a battery pack weighing 2200 lb/ 997 kg. This ultimately suggest that they might have designed an airframe of around 1600 lb/725 kg. That is about 35% lower than any certified production airplane in history.
There are a few other battery chemistries being researched as alternatives to Li-Ion that will allow electric vehicles to move beyond their plateauing capacity. Cuberg, founded in 2015, pursues research and commercialization of lithium metal batteries. One independent analysis on Cuberg’s lithium metal pouch cells conducted by the Idaho National Laboratory indicated a specific energy of 369 Wh/kg, specific power of 2,000 W/kg, and a 370-cycle lifespan, calculated as when the battery drops to 80 percent of its original capacity.
Uber is looking for 1500-plus effective cycles of the battery and a battery pack cost that repays over use to a firm limit of $100 per flight hour. Therefore, the new chemistries have to at least achieve a cycle life of closer to this figure to make their product attractive.
Oxis Energy and Texas Aircraft recently announced a partnership to electrify the light sport Colt S-LSA using the former’s lithium-sulfur (Li-S) batteries, expecting an energy density of 400 Wh/kg, but at the price of between 200-300 cycle life performances. They claim that the use of Sulfur as a non-conductive material provides enhanced safety and is superior to current Lithium-Ion technology. Its 90kWh battery system, which is 40% lighter than current Li-Ion technology, will be powered by its “High Power” cell.
Stringent certification requirements in aerospace create barriers to entry as well, not only for initial product manufacturing approval, but also preventing battery manufacturers from tweaking or upgrading product lines as rapidly as they would for cars or other uses. Agencies may be expecting technology to remain unchanged for 5-7 years after certification, which creates limitation in itself. Currently, the battery demand from eVTOL developers is miniscule so they might be either using B- and C-grade automotive cells — the higher-quality cells typically go to volume customers — or cells procured from Chinese suppliers. If the certification process remains unchanged, to get access to these products in terms of volume and an acceptable price they have to become more vertically integrated.
There are not many VTOL aircraft developers incorporating hydrogen as their power source. The perhaps most well-known VTOL startup Alaka’i, which is developing a hydrogen-powered eVTOL aircraft, is also a member of the Vertical Flight Society’s new eVTOL Hydrogen Council. They unveiled the six-rotor ‘Skai,’ which is expected to have average speeds of 85 mph and a range of 400 miles for four passengers and one pilot.
HyPoint, a venture-funded company in California, has demonstrated an air-cooled hydrogen fuel cell powertrain that produces 800W/kg of specific power with an energy density of 530Wh/kg. The company claims that their future hydrogen fuel cell system can deliver both high specific power > 2000 W/kg and high energy density > 1500 Wh/kg. These figures far exceed the density that lithium-ion batteries with a comparable power output will reach in the next decade. Currently in the prototype phase, HyPoint expects the system will be ready for testing in the beginning of 2022 and commercialized in 2023.
Hydrogen fuel cells may provide improved specific energy over batteries for ranges over 100 miles, but these may not be competitive weight-wise compared to lithium batteries at these shorter ranges of eVTOL aircraft, which are expected to serve transport between 10-60 miles. A previous barrier preventing hydrogen-powered aviation has been its specific power; old fuel cells only had 600 W/kg for the entire system.
Fuel cell (FC) systems are mainly divided into air-cooled and liquid-cooled FC systems. Liquid cooled FC systems make it possible to cool the fuel cell more efficiently and provide greater specific power, but such powertrains will have too much parasitic mass. Air-cooled FC systems can be lighter compared to liquid-cooled, but they achieve relatively low power density due to consuming huge flows of air for cooling.
Particularly in vertical lift operations, fuel cells today can carry limited payloads because of lower power density compared to batteries. Fixed-wing vehicles have far more efficient aerodynamics than multirotor vehicles and use less power so these can be ideal for fuel cells. If HyPoint’s turbo air-cooled fuel cells are able to reach the specific power advertised by the company, they may overcome this drawback and have greater application in vertical lift.
However, there is a much larger barrier to using hydrogen for UAM: infrastructure. This approach has even more supply chain issues than using chargeable batteries. Fuel cells may have longer life and faster recharging/refueling time, but hydrogen just is not readily available anywhere and would require on-site storage at Skyports.
A few startups have pursued a different approach — VTOL aircraft with a hybrid-electric propulsion system. A Hybrid-Electric VTOL aircraft uses a smaller fuel-burning engine as a power generator, thereby supplying electricity to motors that turn the propellers or fans, depending on the design. Batteries add supplemental power during vertical takeoff and landing.
XTI Aircraft, founded in 2012, aims to successfully make and sell hybrid-electric vertical takeoff and landing long-range planes for personal and commercial use. XTI Aircraft unveiled its six-seat fixed-wing TriFan 600 VTOL in August 2015, which has a cruise speed of 345 mph and range of 771 miles.
For all-electric VTOL, battery energy-density limitations will restrict usability to very short segments, especially if regulations require flight time reserves to be greater. For example, if FAA-regulation vehicles maintain a reserve of 30 minute flight time then most e-VTOL flights will be severely restricted. However, a hybrid-electric VTOL aircraft will not suffer from this limitation, as once in wing-borne flight, batteries will be charged back by the generators during flight, so they are fully charged and available for the high-power landing phase.
VErdeGo Aero, founded in 2017, develops hybrid and electric powertrains for AAM to enable aircraft to deliver performance far beyond what battery-electric aircraft are capable of. The company claims their power generation systems with power train that use A piston engines offer 4-8x the equivalent energy density of today’s battery systems. Compared to small turbines, Jet-A piston engines burn up to 40% less fuel, significantly reducing both carbon emissions and direct operating cost.
With additional configurations of turbines/engines, this approach increases the weight of the vehicle and adds controller complexity. Many configurations such as multirotor or fixed-wing were tried in the 1950s and 60s for manned aircraft, but they proved too complex and difficult to fly, with some disastrous results. There are only a handful of hybrid fixed-wing VTOLs currently on the market, but we can expect this to be a much more popular option in the coming years as the technology is perfected.
All three approaches (batteries, hydrogen fuel cell and hybrid) may have economical applications in aerospace, similar to their evolving role in ground-based transportation. But for the moment, the coming generation of VTOL aircraft for urban air mobility will likely be fully-electric and powered by batteries. Uber has analyzed battery, fuel cell, and hybrid-electric eVTOL aircraft, with the clear winner, providing the best performance with the lowest weight and cost, being the battery electric. Capital markets appear to agree with Uber’s conclusion, even in 2021.
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