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Completed TRL 2 (started at 2, targeting 3)
As the demand for efficient, safe air transportation continues to climb, the demand for fuel increases and the subsequent decrease in supply drives fuel costs higher and higher. To address the challenges of the volatile fuel economy and develop an alternative to conventional aircraft, NASA's Aeronautics Research Mission Directorate has shifted significant focus towards hybrid-electric and all-electric aircraft, reducing or eliminating the reliance on jet fuel, while also reducing emissions and noise. Flow battery technology offers an alternative to traditional battery technology, wherein the energy is stored in liquids and pumped across a membrane, similar in fashion to fuel cell operation. In this instance, the specific energy of the system is limited only by the size of the tanks available to store the electrolyte fuels. This transformative concept eliminates the long recharge time of traditional batteries as the used liquid electrolyte can be drained and replaced with fresh electrolyte, similar to refueling with jet fuel. The flow battery system can also function in a rechargeable fashion, with either an on-board or off-board source to regenerate the liquid fuels for reuse. Flow batteries offer longevity over traditional battery technology, as well as flexibility with sizing to provide higher energy or power capability based on the aircraft requirements. Although the overall system would require a higher degree of mechanical integration and control, such as pumping capability, the concept of flow battery technology shows considerable promise and would be considered an enabling technology to overcome the hurdles of electric aviation. The goal of this research and development effort is to focus on the development of a new chemistry that will enable both high specific energy and high power to satisfy electric aircraft power/energy demands.
Since existing lithium-ion battery technology is limited to less than 200 Wh/kg, a significant shift in battery chemistry or battery type must be made to make the giant leaps required to enable transformative transportation concepts such as UAM. Flow batteries offer flexibility in that they store energy in electrolytes stored in tanks, similar to a fuel cell, and flow these electrolytes through the battery to generate electricity. The electrolyte storage tanks can be sized appropriately for each aircraft, allowing a tailorable system with a custom range based on the fuel carried. It also reduces tarmac down by eliminating the 1-2 hours typically allotted to nominally recharge a standard battery. Standard flow battery applications involve stationary power, such as energy storage for grid applications. However, flow batteries have also been considered for use in electric vehicles, with voltage being a limiting factor. Existing battery technology can achieve respectable range for electric cars, but the same battery built for a UAM concept would simply be too heavy and too voluminous. Flow battery technology requires further investigation as it applies to electric aircraft, including better identifying the benefits and limitations of the technology. A wide variety of chemistries can be employed for flow batteries, and this requires further investigation to determine the best chemical combinations and system sizing to enable both power and energy storage capability. Flow battery technology development could result in small UAM and Thin-Haul aircraft or hybrid-electric aircraft introduced to the market within the next 5 to 10 years, feeding into research to further optimize the technology to enable all-electric thin-haul aircraft with increased passenger and cargo-carrying capability over the next 10 to 20 years.
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