Electric Propulsion: Challenges and Opportunities

This project will design, build, and test a 1 megawatt electric machine and thermal management system that is targeted for aircraft propulsion applications and achieves a specific power density exceeding 14 kilowatt/kilogram (active mass) with 99% efficiency.  The machine will be integrated with its power electronics drive operating at voltages greater than 2000 Volt dc and achieving a 25 power density and 99% efficiency.  The machine will be studied as part of an overall aircraft system which will include an energy management system.

The system integration work is led by Georgia Tech.  After first providing the initial requirements for the mission, they then resized the vehicle based on the preliminary technology goals.  Trade studies were performed and showed not only the benefits of distributed propulsion but also the contribution of the hybridization of the system on a regional aircraft.  A hybrid-distributed propulsion provided at 15% fuel burn reduction compared to a next generation A320 aircraft. With the addition of boundary layer ingestion (yet to be demonstrated) we can achieve a significant reduction of fuel burn and carbon dioxide emission of 23%.

Power electronics work is being performed at Ohio State's Center for High Performance Power Electronics (CHPPE) lab.  The team is developing advanced technology required to achieve high-efficiency high-power-density power electronics converters that achieve reliable operation with 2 kilovolt dc bus voltage in a harsh working environment.

Efforts are in 3 major technology areas:

• Partial discharge study for power electronics and electric machines at low air density;
• Power module and power converter designs to achieve high power density;
• Advanced control strategies and protection schemes for aircraft onboard power

The work completed thus far has shown that the target power densities are achievable and most importantly that partial discharge at low ambient pressure can be controlled.

The focus on energy storage is to integrate the electro-mechanical thermal characterization methods with the performance, safety and durability of the battery cells.  The team at Ohio State's Center for Automotive Research is working with the most advanced cell producers to design and demonstrate novel Energy Storage System based on lithium-ion batteries and is optimizing the integration of these cells for the airplane mission.

The aim of this technical thrust is:

• Characterization and modeling of high energy density state of the art and advanced battery technologies based on experimental testing, including energy density assessment, electro-thermal modeling, aging models, analysis of behavior at extreme conditions;
• Design exploration for high level pack sizing under performance, durability, cost and weight constraints (activity in collaboration with Georgia Tech);
• Battery Management System for pack control and safe operation, including dynamic power limits estimation of the cells considering temperature and aging, estimation of state of charge and state of health, safe operation at high voltage, cell balancing;
• Pack Architecture (modules and submodules) that enhances modularity, capability of reconfiguration and adaptation in case of fault, self-diagnosis, and operation with state-of-the-art charging infrastructures.

A high performing motor was identified early as one of the challenges of the program. A thorough integration of the mechanical and electromagnetic design were required.  The Center of Design and Manufacturing Excellence (CDME) at The Ohio State University was engaged early to work with the University of Wisconsin team to develop the motor.  The teams have worked closely to develop a well-integrated design could be made available first in a 200 kilowatt machine and then in a 1 megawatt motor.  The team plans to run the 200 kilowatt machine in the laboratory and the 1 megawatt machine at the NASA Electric Aircraft Testbed facility.

Thermal cooling for a high megawatt electrical machine is undoubtedly a major challenge.  The thermal management system is being addressed by the University of Maryland and North Carolina A&T State University.  New designs were investigated and the team kept close communication as to what was needed between these Universities as well as the University of Wisconsin and Center of Design and Manufacturing Excellence (CDME) at The Ohio State University as they worked on the development and production of the 200 kilowatt and 1 megawatt reliable design.