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Tempting Detour

On the road to the fuel cell-powered electric aircraft, a tempting detour cannot be ignored. Why not just substitute hydrogen for jet fuel? Almost all emissions problems are solved with only a small fraction of the effort to make the all-electric aircraft. Turbofans can be altered to burn hydrogen with minimal effort. Certainly the carbon dioxide problem in the aircraft exhaust vanishes. The various obnoxious oxides of nitrogen remain but might be reduced by fine tuning the combustion. Water in the contrail may increase; this high-altitude water injection is detrimental to climate change. Contrails at high altitude form thin, ice-crystal, cirrus clouds which allow sunlight to pass inward towards the earth but block outward going infrared radiation from the earth. Hence contrails have a warming effect. Search the internet for The Contrail Effect NOVA for more details.

Figure 4. Tank farm for storing liquid hydrogen. Geometry based on racked billiard balls.

Because of both the oxides of nitrogen and contrails, direct substitution of hydrogen combustion for jet fuel seems to be a bad idea, even if it is relatively easy.

Storage of liquid hydrogen uses spherical tanks which give minimum heat transfer surface area for the volume of the fluid stored. The storage tank dimensions are based on the analysis shown here. The key equation is:

E = WR/(L/D)

where:

  • E = energy required to fly the range (MJ)
  • W = aircraft takeoff weight (kg)
  • R = range (meters)
  • L = lift (Newtons)
  • D = drag (Newtons)

When E is known, the mass of hydrogen is obtained from the specific energy, 142 MJ/kg. Using the density for liquid hydrogen, the volume of liquid hydrogen is found. Knowing the volume, the size of the tanks is determined. The size is shown in Figure 4 which is reasonable for the B-747 size aircraft. In addition, the fuel mass fraction of the overall aircraft weight allows a tankage factor, or gravimetric density, of 4 (kg tank)/(kg hydrogen).

The Fuel Cell Alternative

Fuel cells have a long history of success in critical missions. The electrical power for the Apollo Project to the moon and home again was by fuel cells. On the road, numerous electric vehicles (EV) powered by fuel cells have accumulated millions of miles of reliable service. The main hurdle for the fuel cell powered aircraft is not the fuel cell but the hydrogen storage. Fuel cell powered submarines are operating reliably in several navies today. These are high-power systems indicating the available mature technology for aircraft. One difference is that the submarine fuel cells use pure oxygen and hydrogen. Aircraft will operate with air plus hydrogen.

For the fuel cell electric aircraft, is a hybrid version desirable? The answer is likely to be yes. Consider a fuel cell and battery hybrid. As is done in the automobile world, a non-dimensional hybridness ratio, H, is defined. For the electric aircraft, the ratio is defined as

H = (battery energy)/(battery energy)+(hydrogen energy)

Obviously other definitions for H are possible. Here energy was selected because of the close connection with range. When H = 0, the aircraft is pure fuel cell powered and is not a hybrid. When H = 1.0, the aircraft is pure battery powered and is not a hybrid. For 0 < H < 1, the aircraft is a hybrid. The aircraft can be optimized as a function of H.

Symbols can be introduced for energy in the definition for hybridness, H. The resulting equation can be rearranged to yield:

H = 1/1+SHMH/SBMB

where:

  • S = specific energy from Table 1 (MJ/kg)
  • M = mass (kg)

Subscript H is for hydrogen, and subscript B is for battery. From the equation when MB = MH, H = 0.0125. When MB = 2MH, H = 0.0247. Large battery mass gives tiny values of H. This fact is due to the large difference in SH = 142 and SB = 1.8.

Large battery mass contributes very little to system energy. Stated another way, batteries can never provide significant energy for a hybrid system using fuel cells. However, batteries can provide a surge of power when needed such as during take-off. Energy is needed for range; power is needed for take-off and climb.

The supersonic Concorde needed afterburners to take-off from a runway of reasonable length. View batteries as an afterburner for a hybrid electrical aircraft.

Several different hybrid electric aircraft can be conceived. A Laser-Fuel Cell hybrid may offer advantages and merits study.

Rube Goldberg Contraption, Innovation, or Science Fiction

A Rube Goldberg contraption is an overly-complex device which accomplishes a simple task. Although the search for new approaches to electric propulsion may incubate Rube Goldberg contraptions, the effort may also open the door to a new era. Here are four samples of thinking “outside the box” to use a cliché:

  • Long term storage of photons – not electrons – speculative.
  • Hydrogen production from artificial photosynthesis.
  • Microwave energy beamed from orbit directly to aircraft in-flight.
  • Network of ground-based, globally-distributed, laser beams sending power to individual aircraft.

Synergy for the Big Two A’s: Aviation and Automobiles

Table 2. Correlation between electric automobiles and general aviation.

The NASA SUGAR Volt is a “flying Prius”. Prius shines during the city segment of the EPA Driving Cycle. During the highway segment, Prius is essentially an ordinary car hauling a battery. Now think of the flight profile – or the flight cycle in EPA language - for a passenger aircraft. During most of the flight, the “flying Prius” aircraft is operating in the airway cruise mode, and the hybrid is essentially an ordinary aircraft hauling a battery. The greater the range, the less beneficial the hybrid design becomes. Years ago airlines on both the east & west coasts offered commuter flights. The commuter aircraft, or regional jet, is a niche for a “flying Prius”.

Using the published specifications for a Prius, the hybridness, H, can be calculated. The value is somewhere about 50%. The optimum value of H for a “flying Prius” will be significantly less. Note that the optimum H for Prius depends on the EPA driving cycle.

Extensive Federal funds are being spent to electrify the automobile. Funds include both R & D for supporting technologies and generous subsidies. The pollution from aviation is minuscule compared with the 1,000,000,000 cars on the road globally. Should the electrification of general aviation be included under the existing funding umbrella? Should the large, emissions-free, electric aircraft be included? Table 2 shows the overlap of and differences between electric automobiles and general aviation. The Boeing Demonstrator, shown in Figure 1, is an actual hybrid fuel cell-battery aircraft offering zero emissions.