Aircraft Charging Unit
Emergence of Electric Aircraft
Over the past decade, we’ve seen exponential growth in the number of eVTOL (electric vertical takeoff and landing aircraft) and eCTOL (electric conventional takeoff and landing aircraft) development efforts. Motivated by the opportunities to introduce new modes of passenger transportation and logistics, start-up companies have attracted more than $14B in private and public capital. The Vertical Flight Society, tracking the emergence of new development activity, created the World eVTOL Aircraft Directory. By their current count, 698 unique eVTOL concepts are on the drawing boards:
The number of eCTOL projects is not tracked as carefully, though the National Renewable Energy Laboratory estimates the current total as 170.
In June 2020, the Pipistrel Velis Electro became the world’s first electric aircraft to receive a Type Certificate, under European EASA regulations. While there are no electric aircraft currently certified under US FAA regulations, the number is expected to blossom over the next decade, with multiple aircraft manufacturers forecasting certification as soon as 2023.
The Unique Challenges of Charging Electric Aircraft
Many of us grew up watching the Jetsons cartoon series, imagining a future of flying cars, whisking us from home to work without a concern for wasted time sitting in traffic on the ground. In 2016, Uber published their landmark whitepaper, “Fast-Forwarding to a Future of On-Demand Urban Air Transportation,” which provided a detailed description of how the Jetsons lifestyle could happen sooner rather than later. The Uber roadmap, and those subsequently described by others, requires a complete ecosystem of aircraft and infrastructure to realize the dream of ubiquitous eVTOLs. However, the vast majority of capital investment has gone toward the development of aircraft, with only modest investment toward companies like Skyports, developing modern vertiports. That means we are about to see a flood of eVTOLs and eCTOLs coming to market, having far too few Jetsons-like locations where they can take-off, land, and recharge.
Thus, in the early period of electric aviation, electric aircraft will mostly operate from the existing public and private airports. Worldwide, the CIA estimates there are 41,700 public and private airports. Within the United States, there are 5,082 public airports, though 64 large and medium hub airports handle the vast majority of commercial passenger air travel. The balance of the US airports is largely underutilized, addressing a General Aviation market with about 200,000 aircraft, mostly older single-engine piston-powered airplanes. The smaller airports, underutilized and begging for a new purpose in life, will serve as the ideal starting locations for electric aircraft, as the cost of operating from these locations is low, and the airports are spread around the world in locations closer to suburban and rural homes.
Smaller airports typically have hangars to store aircraft based there, though transient and other aircraft are parked on the apron near Fixed Based Operators (FBOs). Much of the revenue for FBOs is derived from refueling operations. An aircraft in transit is parked at an available location, and the FBO typically brings a fueling truck into proximity of the aircraft, then pumping gasoline or JetA fuel into the tanks on the aircraft.
Emulating existing small airport operations for electric aircraft requires a mobile charging platform. Unlike ground-based electric vehicles, which are recharged by bringing the vehicle into proximity of the charger, an electric aircraft requires that the charger be brought into proximity of the aircraft. Early experience with aircraft charging has demonstrated the impracticality of having long charging cables, as they are invariably too short and waste valuable power through resistive losses.
Small airports around the US, often including a tower, FBOs, hangars, and perhaps a terminal, generally have access to commercial-grade three-phase power. The amount of power that a subscriber can access would generally be limited to 75kVA (480/277VAC at 100A). While a 75kVA circuit would address most commercial and small industrial applications, it falls short of the power required to fast-charge most eVTOLs currently under development. With a typical 200kWhr capacity, eVTOLs capable of carrying 5 passengers require a charging rate of at least 200kW (1C charge rate). Thus, the electric aircraft charger must offer a “boost charge” capability, whereby the rate at which it conveys power to the aircraft is much higher than the rate at which it draws energy from the grid.
Another distinction of charging electric aircraft, in contrast with ground-based vehicles, is that electric aircraft are never AC charged, as in Level 1 and Level 2 charging. Because weight is a key consideration in electric aircraft design, an onboard charge controller is impractical. Hence, the aircraft charger need not support Level 1 and Level 2 charging, offering only DC fast charging. However, on the input side, the aircraft charger must certainly include the AC-to-DC charging block, to allow the transfer of power from a commercial 3-phase power outlet to the charger battery storage.
Existing Aircraft Charging Solutions
Many developers of electric aircraft utilize DC fast chargers designed for ground-based electric vehicles, including those of ChargePoint. Some developers have created their own custom DC fast chargers, having long cords to connect the chargers between the AC power, to a charger on wheels, to the electric aircraft. The current use case is charging within a hanger, away from weather conditions, during the development phase of new electric aircraft. One company in the world is currently offering a standardized DC charger specifically designed for electric aircraft, Electro.aero, based in Perth, Australia. Electro.aero offers 30kW and 80kW models, though both models remain tethered throughout charging, do not offer boost charging capability, and lack weatherproofing for outdoor operation. Hence, there is a clear need to address the technical requirements of charging electric aircraft that is not being met.
Aircraft Charging Unit
In 2019, I filed a patent describing a solution to the problem of charging electric aircraft at small airports. The solution is a mobile supercharger, which I call an Aircraft Charging Unit (ACU). A first commercial version will have these characteristics:
• A mobile cart-based solution that can be towed on the apron of an airport, pulled behind a standard airport tug, to a location in proximity of an electric aircraft.
• Cabinet designed for IP 55 rating, allowing continuous outdoor operation.
• The storage within the ACU will consist of an array of Lithium-Iron-Phosphate batteries, including a Battery Management System, having a nominal voltage of 800V, and a capacity of 240kWhr. The ACU will be capable of at least 5,000 charge/discharge cycles, maintaining at least 80% of its initial energy capacity.
• The input charging block for the ACU will allow power from a three-phase 480/277VAC circuit to be conveyed into the storage batteries at a rate of at least 50kW.
• The output DC-to-DC charger will communicate with the aircraft-under-charge using a standardized interface, such as that specified by SAE AE-7D (SAE AS6968 and GB/T 20234).
• The ACU will support a charging rate of up to 240kW, limited by the 1C continuous discharge rating of the 240kWhr storage battery array.
• The ACU will include a 4G/5G cellular modem, allowing a connection to cloud-based services.
• A cloud-based reservation system will be developed, allowing aircraft operators to calendar their use of an ACU at a particular airport, ensuring its availability when required.
I’m pleased to report that I received a US Patent #11,433,775 for the Aircraft Charging Unit on September 6, 2022. It’s now time to move the project toward commercial reality, recognizing the growing need to charge the thousands of electric aircraft that will soon take to the skies.
