Electrical Energy Storage for Energy Optimized Aircraft

Given the goal of developing energy-optimized aircraft that employ increasingly higher power loads such as electric flight control actuation, directed energy weapon systems and on-demand cooling systems, advances in battery technology and associated integration methodology will be required to achieve a robust electrical power system design. Batteries based on various Lithium-Ion chemistry technologies represent a 50% improvement in both specific energy and specific power over legacy NiCad and Lead-Acid chemistries. However, along with these benefits come challenges in terms of overall safety, cost and availability. Safety considerations primarily include failure modes that result from the battery being subjected to short-circuit conditions and over-charge conditions. Cost and availability challenges arise primarily from one-off point designs and ensuing low production volumes, but also stem from limited marketplace competition. With respect to safety, recent developments in various subsets of Li-Ion chemistry including iron-phosphate cells indicate potential improvements in short-circuit and over-voltage performance. These cells should be extensively tested in effort to verify those claims as well to characterize their performance in general. External to the battery, EPS architectures should employ robust fault coordination and the use of external switches driven by electronics for both short-circuits and over-charge protection. To address cost and availability, as well as safety, it is recommended that air-framers leverage the electric vehicle industry in its pursuit of safe, low-cost batteries. The automotive industry represents greater volume than the aircraft industry as well as broadens the potential supplier base. Further, it is recommended that the air-framers investigate and possess experience in numerous vehicle-battery integration methods and technologies including the unique requirements of more-electric aircraft and DEW systems. Some programs have indeed demonstrated success in floating Lithium-Ion batteries on the bus under more-electric transient as well as emergency operation conditions. Integrating a battery that supports a weapon requires a clear understanding of the expected operation of that weapon including duty cycle and depth of magazine, both of which critically affect discharge and recharge rates of the battery and ultimately it safe operation. This paper addresses in detail various approaches to mitigating the aforementioned challenges of safety, affordability and availability.