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The purpose of this subproject is to expand the propulsion configurational options for transport class aircraft by the development of electric drive technologies. Today I am going to talk you through our approach to this technical challenge by reviewing how hybrid propulsion can impact aircraft design, what technologies are needed to enable hybrid propulsion, and explain how we are using the combination of top down architecture analysis and bottoms up technology development to cover this exciting area The key to the interest in hybrid propulsion for aviation is the way electric drive systems can influence the vehicle design trade space. For illustrative purposes, I'm going to use the notion of a 3 ‐ dimensional space with a state of the art single aisle aircraft at the origin of this cube.
I'm showing it this way because I want you to embrace the idea that we have multiple option for changing the aircraft propulsion system. Again at our origin or baseline is all propulsive power coming from the turbine engine. There already is electricity generated at the turbine shaft, but that electricity is only used for hotel power. But just as we can adjust the bypass ratio in a traditional turbine, we can adjust how the fuel is converted in to propulsive power by going all the way to a fully turbo electric system where the turbines only produce electricity or have an intermediate state where the turbines produce both mechanical thrust and electricity for propulsion. We could envision other configurations including fuel cell energy conversion.
Along the other axis I want you to envision varying degrees of energy storage with the extreme being all propulsive power coming from entry storage charged on the ground and no energy conversion in the air. And of course between turbo and all electric we have various ways to implement hybrid electric.
Now, the reason we are interested in hybrid propulsion was not shown in that notional x ‐ z plane of the trade space at all. For motivation we have to introduce propulsion airframe integration. In this notional x axis I would like you to picture bins of configurations options that improve aerodynamics. Strictly speaking, wing tip propulsors, varying degrees of boundary layer ingestion and distributed propulsion do not require hybrid propulsion.
However the advancements in electric drive & distribution systems open the possibility for these PAI concepts to be implemented in new, efficient vehicle configurations.
In this illustrative space, I am showing aircraft studies. The SUGAR Volt was a 737 ‐ like aircraft that used HEP to replace fuel with energy storage. This reduced fuel been and in flight emissions, but did not improve the overall energy efficiency of the aircraft because the configurations changes were limited. However hybridizing on wing is an attractive option for easy entry into service if an economic benefit can be identified. More advanced analysis of this sort of turbine hybridization is currently being performed under contract by UTRC & RR to see if overall mission efficiency can be gained by not only adding energy storage but by resizing other elements. Expect phase II recommendations in Oct 2016.
The N3 ‐ X was another concept vehicle but in this case it was a highly advanced, but low technology readiness, 300 passenger class craft that used hybrid wing body as well as highly efficient, fully turbo electric distributed propulsion based on superconducting technology. After exploring this advanced corner of the tradespace, NASA AATT has embarked on a series of turboelectric vehicle studies looking at potential aerodynamic ‐ driven vehicle benefits for varying degrees of propulsion distribution.
Recent effort has been applied to assessing the feasibility of a minimal turbo electric distributed propulsion application at 150 PAX. This studied resulted in a nominal 10% fuel burn reduction compared to a N+3 baseline, with modest technology advancement assumptions. It is important to note that this a preliminary study, but it is an important preliminary study because it suggests at least one configuration with notable energy improvements with technology assumption that appear to be feasible in the 10 ‐ 20 year time frame To recap let me summarize NASA's investment in large aircraft hybrid propulsion. We have a cooperative agreement with academia to develop a top level understanding of the hybrid prop trade space for a range of vehicle sizes. Again, we have ongoing contacts with industry to do carefully analysis of hybridization of turbine engines in order to learn if any propulsion efficiency can be obtained without changing the vehicle design. And we are investing in point design analysis of some specific distributed propulsion vehicle concepts to understand the benefits & penalties of these configurations.
Now I'm switching gears from top down analysis to bottoms up tech development. The subsystems supporting hybrid propulsion can be binned in a number of ways. Here I'm showing bins of e drive, etc. For a number of reasons that I won't go into now, we have chosen to focus first on the electrical drive system When I say e drive, I am including Electric machines Power management and control Power architecture Power distribution Electric machines were identified as a true long pole components since existing high power machines are far from the specific powers required for flight systems. To understand the relationship of key performance parameters of electric drive systems for turbo electric aircraft, Breguet range equation analysis was used to bound electric drive system weight versus fuel burn benefits using generic assumptions of aerodynamic benefit. Here the black solid and dotted lines represent a break even line with points above the line providing net benefit. It is interesting to not that the single motor driven fan configuration appears to have a lower break even line than the nominal ones calculated for a more highly distributed system.
Our strategy for investing in bottoms up technology development includes both farther term investment in fundamental materials and superconducting technology as well as investment in nearer term exploration of advanced machine topologies, power systems and integrated testing of electric drive subsystems Enabling System Testing • Use system-level simulation capability to inform requirements.
• Demonstrate technology at appropriate scale for best research value.
• Integrate power, controls, and thermal management into system testing.
• Validated tools and data that industry and future government projects can use for further development.
Non traditional propulsion architecture …..
Long term is 3 ‐ 4X, in the next 5 year, 2X at TRL3 Achieve 3 ‐ 4X increase in the specific power of high efficiency electric components and alternate power conversion systems, and 3 ‐ 4X increase in stored energy density for storage devices to make 10 mega ‐ watt onboard power generation and/or utilization feasible for propulsion Key Challenges for Next 5 Years: • Low ac ‐ loss superconducting stator • High conductivity coil , integration of motor design, structural concepts, and thermal management, for non ‐ cryogenic, high power density motor High Level Technical Challenge for 5 years: • 2X increase in power density of electrical motor How is it done today ? What are the limits of current practice ?
• SOA superconducting motors with power density 6hp/lb with use of superconducting rotor and Cu stator, non ‐ superconducting stator limits power density • Power density for SOA ambient temperature motor is 3 hp/lb , experimental small motor have 5 hp/lb power density; limited by conductivity of Cu, temperature limits of constituent materials, thermal management and structural integrity • SOA cryocoolers have 30 lb/hp power density, limited by heavy heat exchanger What is new in our approach ?
• Superconducting electric motor with superconducting rotor and low ac ‐ loss stator • Adv. non ‐ cryogenic motor with high conductivity coil (e.g., Cu ‐ CNT composite, CNT), improved thermal management concepts, and new structural concepts.
• High power density cryocooler with advanced lightweight recuperator design • Soft magnetic materials are a critical element in power generation, conditioning, and conversion technologies.
• Improvements in the efficiency of these materials is essential for achieving the performance levels that are required to enable hybrid electric systems.
• Soft magnetic materials enable low loss inductive • switching, which is useful in inductor, transformer • and filter applications.
• Laminated electrical steels traditionally used for power cores become inefficient at high switching frequencies.
• Nanocrystalline soft magnetic alloys have the greatest potential for achieving these energy efficiencies.
Increasing the thermal conductivity of electrical insulators in order to reduce electrical losses due to temperature-induced increases in resistivity Decreasing diameter of winding wires by increasing dielectric breakdown of insulators (allowing thinner insulating coatings) Wang Adv Fnc Mat 24(21) 2014 The current ‐ carrying capacity (CCC), or ampacity, of highly ‐ conductive, light, and strong carbon nanotube (CNT) fibers is characterized by measuring their failure current density (FCD) and continuous current rating (CCR) values. It is shown, both experimentally and theoretically, that the CCC of these fibers is determined by the balance between current ‐ induced Joule heating and heat exchange with the surroundings. The measured FCD values 7 9 − 2 of the fibers range from 10 to 10 A m and are generally higher than the previously reported values for aligned buckypapers, carbon fibers, and CNT fibers. To the authors’ knowledge, this is the first time the CCR for a CNT fiber has been reported. The specific CCC value (i.e., normalized by the linear mass density) of these CNT fibers are demonstrated to be higher than those of copper