NASA will pay $6 million dollars over the next three years to support electric aircraft research at the University of Illinois. The NASA-backed program is called CHEETA — the Center for Cryogenic High-Efficiency Electrical Technologies for Aircraft. It will work on the development of fuel cell and cryogenic liquid hydrogen energy storage technologies for completely electric aircraft. Phillip Ansell, a principal investigator for the project, and an assistant professor in the Department of Aerospace Engineering at Urbana-Champaign, answered some questions about the work for CleanTechnica.
Where will CHEETA — the Center for Cryogenic High-Efficiency Electrical Technologies for Aircraft — be located?
The CHEETA Center is configured to feature a consortium of researchers with a comprehensive set of skill sets to make disruptive advancements in cryogenic and superconducting technologies, as well as novel methods for electric propulsion integration, as they apply to aircraft systems. The development of this research group was established through a recent $6M research grant from NASA through the University Research Initiative Program. While not tied to a specific building, the Center is being led by researchers at the University of Illinois at Urbana-Champaign. I have the pleasure of serving as the principal investigator for this effort, while a colleague of mine, Kiruba Haran from our Department of Electrical and Computer Engineering, is serving as the co-principal investigator.
Do you have your staff selected and are they working onsite yet?
We are fortunate enough to have a stellar research team already selected to begin work as a part of the CHEETA Center. These researchers include University faculty, laboratory scientists, and industry engineers across a range of institutions. These institutions include the University of Illinois, Air Force Research Laboratory, Boeing Research and Technology, General Electric Global Research, the Ohio State University, Massachusetts Institute of Technology, the University of Arkansas, the University of Dayton Research Institute, and Rensselaer Polytechnic Institute.
When will the research phase begin?
The technical research will begin immediately following our kickoff meeting, which will be held on our Urbana-Champaign campus in early August.
Why did you choose to work with cryogenic liquid hydrogen for energy storage?
Hydrogen has a specific (gravimetric) energy density that is unparalleled by just about any other energy storage method. In fact, hydrogen has a specific energy over three times that of traditional kerosene-based jet fuels, and has specific energy over 700 times that of modern battery systems. One challenge of this energy storage system, however, is that hydrogen is significantly less energy dense per unit volume, as compared to traditional jet fuels. In order to mitigate this challenge, the hydrogen system can be compressed and stored in a cryogenic liquid form to reduce the large volume requirement. The use of hydrogen also has a direct path to efficient conversion to electricity through fuel cell systems, which have drastically improved over the past decade. Since the hydrogen would be stored at cryogenic temperatures, we also had the idea of doubling the use of this cryogen as a heat sink to enable superconducting electrical transmission and motor systems. These improvements in the drivetrain result in dramatic increases in the overall efficiency, specific power, and rated power capabilities for electric aircraft propulsion.
How would this material be secured and stored onboard an aircraft?
One of the greatest challenges associated with using liquid hydrogen is figuring out how to store it efficiently. Currently we are leaning on the experience of some of our industry partners, who have extensive experience with these systems, to address this challenge. Given that they are applying their own methods, some of which may be proprietary, I cannot directly address this question. However, the many challenges associated with this system is something we are cognizant of.
How many fuel cells would be used in a single aircraft along with it?
The configuration of the fuel cell system is also something that our industry partners are assisting with. Currently the specific configuration of the fuel cell stacks appropriate for the vehicle of the scale we are considering has not yet been defined. However, as one might imagine, this system would feature a fairly extensive array of fuel cell systems in order to become realized. Thankfully, however, the power density of fuel cells have seen substantial improvements in recent years, with even better forecasts on the horizon.
What makes the electric propulsion system ultra energy efficient?
In our case, the electric propulsion system utilizes the principle of superconductivity to enable dramatic increases in overall electrical efficiency. When certain conductive materials become very cold they begin to exhibit zero electrical resistance. In this way, the cryogenic hydrogen doubles as a way to maintain the ultra-low temperatures for this superconductive state. By removing ohmic (resistive) losses from the transmission system, the delivery of electrical power becomes incredibly efficient. In a similar way, larger currents can be routed through these materials, making them ideal for the fabrication of motor windings and other electrical components due to very high current densities.
How much does the cryogenic liquid hydrogen and fuel cell system weigh compared with a conventional plane’s fossil fuels?
When comparing the weight of a conventional fuel storage system to the proposed cryogenic liquid hydrogen system, the difference will strongly depend on what ranges these aircraft are intended to fly. Per unit energy, a liquid hydrogen system is far lighter than Jet A fuel, and the overall propulsive drivetrain is expected to be more efficient as well, reducing the net energy needs of the storage system. However, the weight of the fuel cell systems on the aircraft are fixed and, as a result, the aircraft would likely be heavier for short-range missions. However, the cumulative weight of the liquid hydrogen and fuel cell system is anticipated to actually be lower than conventional Jet A systems for longer ranges, due to the aforementioned high specific energy of hydrogen. That being said, fuel cell technologies have also come a long way in the past decade. 15 years ago, a standard fuel cell had a specific power of around 0.3 kW/kg. Now fuel cells are commercially available with specific power up to 2 kW/kg at the stack level, with additional mid-term future projections featuring even larger increases up to as far as 8-10 kW/kg. Since this aircraft concept is intended as a long-term vision for future aviation, the actual weight difference between the proposed liquid hydrogen energy storage system and conventional fuel storage systems will strongly depend on how these technologies develop over time.
Where is cryogenic liquid hydrogen sourced and what are the costs?
Currently there are numerous ways to produce hydrogen, each with different financial and environmental costs. Indeed, early studies on use of liquid hydrogen as a drop-in fuel replacement for transport aircraft in the early 2000s determined that the concept was technically feasible, highly efficient, safe, and environmentally clean, but not economically competitive at the time. However, scientific studies across the last decade has seen a massive increase in the efficiency of hydrogen production, which have significantly reduced costs. Expansion of renewable resources over the previous decades have also increased the viability and reduced the expense associated with their use in hydrogen production. These improvements are anticipated to drastically reduce the cost and emissions associated with hydrogen extraction, to levels where it is actually forecast to become economically competitive in the near future.
What are the goals of the project? Could it be possible to have a working prototype completed?
The primary goals of the current program are to perform early development of technologies that could be used for the envisioned aircraft platform. While conceptual vehicles that feature electrified propulsion systems are in no shortage these days, the actual electrical motor, power electronics, system management, and propulsion integration practices required do not yet exist to make these vehicles a reality. Our team seeks to address this gap by advancing the capabilities of key enabling technologies for large electrified commercial aircraft of the future.
Image Credits: The University of Illinois
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