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Many different electrically propelled aircraft have been or are soon to be flying. Recently a zero-emissions electric aircraft, Solar Impulse, flew around the world with zero fuel. However, an electric aircraft with the payload and range of a B-777 and having zero-emissions is a few more decades in the future.

Figure 1. Boeing Demonstrator Hybrid Fuel Cell-Battery Electric Aircraft. (Image courtesy Thomas Koehler)

The electrically propelled aircraft, E-747, discussed in this article will have zero-emissions. E-747 was selected as identification for this study; obviously E is for electrical and the 747 is for the reference aircraft B-747. E-747 will have the same payload and range as a B-747 but with zero emissions. The schedule for such an aircraft to enter service would be about 2050.

Examples of Electric Aircraft

From an array of electric aircraft flying today, two examples have been selected for subsequent discussion.

In 2008, the world’s first cruising flight of an aircraft powered only with a fuel cell occurred with the Boeing Demonstrator Hybrid Fuel Cell-Battery Electric Aircraft (Figure 1). To have the power for take-off and climb, the combined output of a battery with the fuel cell is needed. Cruise power, which is significantly lower, is supplied by the fuel cell only. Now just scale the aircraft by a factor of 25, and passengers and cargo can be flown across continents and/or oceans. It is not that easy!

Figure 2. Antares H2 electric glider, developed by German Aerospace Center DLR, is powered by hydrogen fuel cell.

Antares H2 (Figure 2) is emission free except for contrails. It’s first flight was in 2009. The DC motor is directly connected to the fuel cell with maximum voltage of 400 V. Cruise power is about 10 kW giving a range of 750 km. The Antares H2 is a flying laboratory with hydrogen stored in a pressurized tank.

Emissions-Free Flight: Nuclear

In an ideal world, nuclear power offers zero-emissions transportation. The concept of using nuclear power to have emissions free aircraft propulsion is very alluring. In the 1950’s and 1960’s, the allure of aircraft nuclear propulsion, ANP, was not emissions but unlimited range. In this time era, the USAF conducted R & D on ANP and concluded the idea was not feasible. Even if the idea yielded a tempting design, public opinion hardened; the public would not allow nuclear reactors flying overhead.

Emissions-Free Flight: Electrochemistry

Combustion, along with steam, launched the industrial revolution and has sustained the revolution for more than 200 years. Combustion yields the quantity being sought – namely heat – yet sets off an uncontrolled scrambling of molecules. Combustion may be uncontrolled, but is not random for the details of physical chemistry are precisely followed. Although engineers can fine tune combustion, output molecules cannot fully be controlled.

On the other hand, fuel cells offer superior control of the output molecules and avoid the apparent random scrambling of molecules. In contrast to combustion, electrochemistry is really cool. Really cool is an expression used by the vocabulary-limited to express enthusiastic support. Really cool also is physically correct so that nitrogen from the air passes through the fuel cell unchanged. Further, with electrochemistry, the Carnot efficiency is sidestepped.

The NASA N3 + X can be more accurately described from the emissions viewpoint as a combustion-electrical aircraft. Combustion precludes zero emissions. The E-747 discussed here is an electrochemistry- electrical aircraft enabling zero emissions.

Emissions-Free Flight: Electrical

A major driver for R & D in electric propulsion is greatly reduced or zero emissions. Implementation has already begun with numerous small electricpowered aircraft, and the Solar Image mentioned earlier. For large commercial and business aircraft, much lengthy R & D is needed. Begin by sorting today’s technology to find possible avenues to success. Superconducting motor/generators (M/G) almost assuredly will play a major role. Superconducting M/G are soon to drive ships.

Table 1. Power Source Comparison

Additional electrical technology to be sorted includes solar cells, ultracapacitors, batteries and fuel cells. For the latter three, electrochemistry is the dominant science. The critical design issue is aircraft range. Devices which store energy are unlikely to have the specific energy (Joule/kg) necessary to give the desired range. That fact reduces the choice to one technology – the fuel cell – which has a fuel, namely hydrogen.

Table 1 provides information concerning weight and volume for two fuels and a Li-Ion battery. A particular aircraft fueled by jet fuel has a 10,000 km range. The aircraft has a certain weight fraction devoted to fuel; the other weight fractions are airframe and payload. If hydrogen were substituted for the fuel using the same weight fraction, the range jumps to 30,900 km. However, the substitution ignores fuel tank volume. Likewise, if batteries are substituted for jet fuel using the same weight fraction, the range is only 200 km. By 2025 the range may jump to 400 km. Stored electrical energy will fail to yield the required range capability.

Hydrogen as a Fuel

Figure 3. Contrails at Sunrise (USAF photograph)

Use of hydrogen in an aircraft requires complete redesign of the fuel systems including tanks, pumps, fire protection, monitoring, etc. Hydrogen has a high specific energy (Joule/kg); this fact is favorable. Compared to jet fuel, for the same energy, the mass of hydrogen fuel is about 32%. Hydrogen has a very low energy per volume, which is energy density. The value depends on the method of storage. If highly compressed gaseous hydrogen is used, the fuel tanks must be about 5 times larger than the jet fuel tanks. If liquid hydrogen is used, for identical energy content, the hydrogen fuel tanks must be about 3.2 times larger than tanks filled with jet fuel. Depending on the hydrocarbons, the mass density of jet fuel is about 0.8 kg/L. At the critical point, the mass density of liquid hydrogen is 0.081 kg/L.

Think Super Guppy aircraft. That might be the shape of a large electrically propelled aircraft using hydrogen with fuel cells. Cruising at Mach 0.85 is not possible. The need for another design approach to accommodate hydrogen is obvious.

The B-747 Series 300 has fuel capacity of 52,410 gallons which is 356,000 lbs. The hydrogen fuel necessary to yield the same energy would be only 132,000 lbs. If compressed hydrogen gas is used, the volume for the fuel jumps to 262,000 gallons. If liquid hydrogen is used, the volume jumps to about 168,000 gallons.

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