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Evaluating Energy Storage Options for Hybrid Electric Aircraft

The automotive industry has pioneered hybrid electric vehicle development. The aerospace industry has not followed the automotive trend largely due to integration challenges, a long and costly product development cycle that includes airworthiness certification, and the weight of the additional motor, drive, and energy storage systems.

A recently published SAE International technical paper reviewed and compared state-of-the-art energy storage methods as they relate to a commercial single aisle hybrid electric aircraft. Researchers from United Technologies Research Center considered conventional technologies such as batteries, capacitors, and flywheels as well as energy conversion devices such as fuel cells (both proton exchange membrane and solid oxide) and a turbogenerator (turbine directly coupled with an electric generator).

The hybrid architecture used for comparison consisted of twin Geared Turbofan engines assisted by 2500-hp electric motors during takeoff and climb. The motors provide power to the low-speed spools of each engine allowing the core to be downsized. During cruise, the aircraft is powered solely by the turbine engines that are sized for efficient operation during this mission phase. As fuel mass decreases during cruise, excess power can be allocated to recharging energy storage by taking power off the low-spool motor-generator.

At the current state of the art, the researchers found turbogenerators to be the most promising technology for supplementary electricity aboard commercial hybrid electric aircraft. Batteries begin to be competitive with turbogenerators at 1000 W·h/kg on the basis of specific energy, however, non-ideal performance may require even higher specific energy. This analysis of battery weight did not include specific power that may also significantly impact battery weight.

The current state of the art in commercially available lithium-ion batteries is approximately 200 W·h/kg. Batteries also have an inherent drawback in that the weight of batteries remains unchanged throughout the flight envelope whereas jet fuel decreases in weight.

A turbogenerator operating on jet fuel or liquefied natural gas (LNG) is the best option from the point of view of weight for larger energy storage quantities. They have high specific power and in contrast to batteries; the turbogenerators lose their fuel weight early in the flight. In the case of Jet-A fuel, no additional tank weight is required whereas LNG requires an additional tank. The solid oxide fuel cell utilizes Jet-A so no additional tank is required, however, at least an order of magnitude improvement in specific power (mass reduction) is necessary in order to be viable.

Fuel cells were examined due to their high efficiency. The performance metrics of a fuel cell power system are dependent on the system size since efficiency varies with load. Fuel cells are outperformed by batteries and turbogenerators except for applications requiring more than 1300 kW·h where noise and detectability are valued and liquid hydrogen is available. The infrastructure to generate and distribute the hydrogen must also be considered. Similarly, recharging batteries must be taken into account (during descent, on ground, etc).

This article is based on SAE International technical paper 2016-01-2014 by Jonathan M. Rheaume and Charles Lents of United Technologies Research Center. The paper, “Energy Storage for Commercial Hybrid Electric Aircraft,” was presented at the SAE 2016 Aerospace Systems and Technology Conference.