The push to deploy electric vertical takeoff and landing (eVTOL) aircraft in a variety of roles – from unmanned delivery drones to Urban Air Mobility (UAM) air taxis – is governed by the limits of available onboard electric power. Because of their flight profiles, eVTOLs require substantial levels of power during peak performance phases of flight during takeoff, landing, and flying into headwinds.
In theory, any power requirement can be met by carrying a sufficient number of rechargeable batteries, however, this solution can consume an eVTOL’s payload capacity by adding excessive weight to the airframe. Any realistic battery solution has to thus balance the provision of sufficient power against the weight and size of the batteries carried onboard.
There is no doubt that considerable progress has been made in developing efficient and powerful battery solutions for eVTOLs. What is open to debate is whether these solutions will be sufficient to power eVTOLs through all phases of flight, or whether limitations in the current state-of-the-art require a hybrid power generation solution, akin to the gasoline/battery approach used in hybrid cars.
Many Battery Options
There are many options for building eVTOL batteries, ranging from proven technology to the repurposing of radioactive waste.
BAE Systems is harnessing 20 years of experience in high power/high voltage electric propulsion systems for transit buses to provide lithium-ion powering solutions for eVTOLs.
“Our focus right now is purely on electric batteries,” Robert Hess, systems engineering manager for BAE Systems’ Controls & Avionics Solutions, told Avionics International. “We are bringing forward battery products based on what we call ‘Generation 2 Cell Chemistry’, which use commercially available in-production cells with moderately good power and energy performance.”
This lithium-ion power envelope includes what Hess describes as the ‘Uber Elevate AAM/UAM use case’. It encompasses electric batteries powering a four-person AAM/UAM across a 25-60 mile range, cruising at 150 knots for 25 minutes or so.
“We feel very comfortable to bring solutions forward for this kind of vertical takeoff/vertical landing applications while meeting the weight, performance, and battery cycle life requirements for those platforms,” he said.
BAE Systems is currently developing this kind of eVTOL battery solution for Jaunt Air Mobility’s ROSA (Reduced rotor Operating Speed Aircraft), plus some other clients that the firm is not able to discuss publicly.
Worth noting: Although known for its use of lithium-ion cells, the company is not wedded to them. In fact, BAE Systems is testing new formats of current Li-NMC, Li-Ti, and Li-Air batteries, plus new ‘future state technology’ like metal anodes, Li-S (lithium-sulfur), and other non-metal-based options such as solid-state cells and self-heating solutions — all to discover which options work best for eVTOL power applications.
OXIS Energy’s Dr. Mark Crittenden, the company’s head of battery development and integration, definitely believes that – based on their power characteristics — Li-ion batteries are not light enough for eVTOL deployments. This is why OXIS Energy has developed Li-S batteries for eVTOL flight, with its Li-S cells currently being tested on manned aircraft made by Bye Aerospace. Li-S battery technology “is extremely light, at around half the weight of lithium-ion,” Dr. Crittenden said in an Opinion article published on www.evtol.com. “Or looking at this another way, (Li-S) is able to store double the amount of energy for a given weight.”
Nano Diamond Batteries (NDB) is developing a completely different approach to eVTOL (and all forms of battery-based) powering. They are using extremely minute amounts of Carbon-14 nuclear waste encased in layered industrial diamonds to create self-charging batteries. Known as Diamond Nuclear Voltaic (DNV) technology, the computer component-sized NDB battery can provide up to 28,000 years of operational life.
“With the NDB battery, we have achieved a massive, groundbreaking, proprietary technological breakthrough of a battery that is emission-free, lasts thousands of years, and only requires access to natural air in order to power devices,” said Nima Golsharifi, CEO and co-founder of NDB, said in a press statement.
So far, the NDB battery has passed two proofs of concept tests at Lawrence Livermore National Laboratory and the Cavendish Laboratory at Cambridge University. A commercial NDB battery prototype is also under development.
Whatever battery solutions prove to be practical for powering eVTOL flight, all of them will have to pass the strict certification requirements imposed by the FAA and other aviation regulators. These requirements will compel battery manufacturers to invest considerable time and money to deploy their products in civil aviation, at a time when the eVTOL sales market for batteries is new, unproven, and relatively small.
The Case for Hybrid Solutions
eVTOL batteries must be capable of providing peak charges both at liftoff and landing, the latter occurring after the batteries have been drawn down during flight in a variety of atmospheric conditions. This is why some industry experts are questioning the wisdom of launching the eVTOL industry by relying solely on batteries. They are asking if an approach that combines batteries with a second power source would be more practical and prudent.
This question was considered in the research paper, ‘Hydrogen Fuel Cells and Batteries for Electric-Vertical Takeoff and Landing Aircraft’ (Wanyi Ng and Anubhav Datta), which examined pairing onboard hydrogen cells and batteries on eVTOLs.
“For any mission beyond 50 miles, fuel cells appear to be a compelling candidate,” said the paper’s abstract. “In the combined powerplant, the fuel cell is sized to the low-power cruise mode, and the battery supplements during higher power.”
This notion of using a non-battery source to power eVTOLs, with batteries serving to provide extra power as needed, has also been examined by Embry-Riddle Aeronautical University’s Eagle Flight Research Center (EFRC). It recently simulated the in-flight performance of an onboard 100kW hybrid generator powering a 950 lb quadcopter through all phases of flight, using actual equipment connected to programmable DC loads to generate real-world results. Like Ng and Datta, EFRC’s model used the hybrid generator as the eVTOL’s main power source, with the batteries being reserved for peak demand times including periods of severe turbulence.
It may seem counterintuitive to primarily power eVTOLs with non-battery electrical sources. But EFRC Director Dr. Richard Pat Anderson defends the idea on the basis that today’s batteries share the same problems as 1980s electric car batteries: They don’t have enough power to do the job on their own.
“There are a lot of people that would love to be fully electric and there are some really short-range aircraft that may be able to do that,” Anderson told Avionics International. “As to whether they are commercially viable right now? It's questionable. This is why we believe the first commercially viable eVTOLs will be hybrid in nature.”
In line with this logic, LaunchPoint Electric Propulsion Solutions has unveiled an onboard gas-powered generator for eVTOLs called the 40kW LaunchPoint HPS400 GenSet. It consists of a lightweight aircraft engine, an air-cooled alternator, software/controls, and a battery management system that allows the generator to charge an eVTOL’s battery system while in flight.
“All-electric battery-powered flight is not going to work for the markets until we have much better batteries, which I believe is on the order of four, five, or even probably 10 years out,” Michael Ricci, LaunchPoint Electric Propulsion Solutions’ CTO told Avionics International. “Batteries have very good power density, while liquid fuel has good energy density. So you use the liquid fuel to handle your continuous load during eVTOL flight, and the batteries to handle all the high-power peaks during takeoff and landing.”
It remains to be seen which power option(s) win the day for the world’s first eVTOL aircraft. This said, the hybrid option seems more likely to achieve certification from safety-minded regulators, for the same reasons that they have historically preferred multi-engine aircraft for passenger service.