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hybrid SOFC-electric power system. An additional multiplier was added to the cruise power level to account for the need to charge the buffer battery in the hybrid system during cruise.

Table 3: Performance estimates for promising demonstrator configurations.

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Cessna 172P w/ SuperHawk STC

Tecnam P2006T w/ single power system

Columbia 300 at Cessna TTx weight

Takeoff gross mass, kg 1159 1230 1636 Typical unmodified empty mass, kg 695 780 1079 Unmodified fuel flow at cruise, kg/hr 25 26 39 Exchange mass, kg -161 -185 -240 Hybrid power system mass, kg 269 294 439 Electric powertrain mass, kg 100 100 60 Fuel mass remaining to gross weight, kg 76 61 119 Estimated cruise fuel flow, kg/hr 13.4 13.6 22.1 Change in cruise fuel flow, % mass -46% -48% -43% Change in cruise fuel flow, % volume -54% -55% -50%

All of these configurations, based on the current estimates for system sizes, are able to show over a 40%

reduction in fuel flow by mass at cruise, and over a 50% reduction in fuel flow on a volumetric basis. This latter difference is due to the use of low-sulfur diesel fuel, rather than lower-density aviation gasoline, as the onboard reactant. As discussed in the companion Boeing paper [23], the low-sulfur fuel is necessary to preserve high- efficiency operation of the SOFC in the power system. This is a formulation that is similar to jet fuel but without the high allowable sulfur content, and is available as diesel fuel at virtually every filling station in the United States. The difference between the volumetric fuel flow is important to note, as fuel is often priced by volume, not by weight. As such, if fuel costs were to be identical per gallon, the concepts described in Table 3 would all meet or exceed the program goals of a demonstrator with a reduction of 50% in fuel costs. In fact, it is likely that diesel sold at the airport would in fact be cheaper per unit volume than aviation gasoline, much as jet fuel is typically cheaper at most airports due to its greater supply. With these differences, even greater operational cost savings could be realized.

In all of these cases, the higher weight of the hybrid power system results in an inability to utilize the full fuel volume available on each aircraft. However, the reduction in fuel flow is significant enough that each of these aircraft could fly for several hours at the cruise power settings prior to fuel exhaustion.

G. Future Work Work is continuing on the conceptual design of a flight demonstrator retrofitted from an existing light aircraft

design. The team is in the process of modeling the power system and powertrain to ensure that the appropriate spatial and mass/balance constraints are met. Additionally, one of the goals of taking this to a flight demonstration vs. ground power demonstration is to identify possible systemic issues with this new technology that could jeopardize the safety of flight. By considering a flight demonstrator, the team is required to think through and defend a system hazard analysis and safety case development for the entire system. This process is ongoing, as it is best to identify hazards early, when the design of the demonstrator is more flexible.

III. Conclusions Electric motors may offer compelling advantages when used for primary propulsion, but the increased weight of

the onboard energy storage system, lack of supporting infrastructure at airports, and lack of operational data all act to stymie adoption of electric propulsion. A better battery will not resolve infrastructure or certification issues, and with slow adoption rates, neither concern will not be rectified in short order. Traditional approaches that convert infrastructure-friendly fuels to electricity (such as large gasoline-powered generators) do so at an efficiency that is not much better than the simple combustion-to-shaft power efficiency seen with today’s aircraft. To overcome the multiple barriers to adoption of electric aircraft, a novel concept is needed that can dramatically increase the efficiency of conversion of traditional fuels to electricity.

This paper proposes a hybrid power system approach that leverages ongoing research into Solid Oxide Fuel Cells as a high-efficiency means to convert hydrogen derived from reformation of traditional hydrocarbon fuels to electricity onboard the aircraft. The use of these traditional fuels at a greatly increased efficiency allows for much longer range and endurance amongst aircraft with electric primary propulsion, and has the potential to offer utility to a much broader sector of the aviation market much sooner. This could lead to an increase in adoption rate of aircraft

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