Zero-Emissions Electric Aircraft: Theory vs Reality
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.
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!
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.
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
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.
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.
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)
- 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
- 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
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.
Composites, Paint, & Solar Cells Composite materials degrade in UV light. The intensity of UV radiation increases with altitude. Aircraft built using composite materials exposed to UV must be painted to extend the life of the airframe.
Paint increases the weight of an aircraft. According to Boeing, to fully paint a B-747, aircraft weight is increased by 555 pounds (252 kg). Full paint includes upper and lower half of fuselage and tail plus customer markings.
Presently solar cells provide in full sunshine 300 watts per kg. Suppose a paint-on solar cell that provides both electrical power and UV protection were developed. Would this new invention offer any benefit to an electrically powered aircraft? The approximate solar power for a B-747 painted in solar cells would be
(0.3)(252 kg)(300W/kg) = 23 kW
The factor of 0.3 accounts for the cosine factor and surfaces in the shade. At cruise, the B-747 power is about 20 MW. Hence, the solar cells provide 0.12% of cruise power under favorable conditions. If the solar cell output per unit mass could be increased by a factor of 10, then painted-on solar cells become attractive. In this case, a hybrid electrical aircraft becomes a Tribrid. Tribrid is fuel cells, batteries, and solar cells.
Special Features of Fuel Cell Electrical Aircraft
Figure 5 is a block diagram for a hybrid fuel cell-battery electric aircraft. For every kilogram of hydrogen consumed, 36 kg of air is needed. This assumes 100% usage of the oxygen; lower percentage usage means more air. Also, for every kilogram of hydrogen consumed, 9 kg of water is produced.
Inputs to the fuel cell are air and hydrogen. The C? is a symbol to ask the question if an air compressor is needed so that the fuel cell can operate at a fixed pressure. Pressure varies widely with flight speed and altitude. Quite likely the compressor is needed. Besides DC electricity, two other outputs from the fuel cell are water and unused nitrogen. The air inlet causes ram drag. The nitrogen nozzle reduces some of the ram drag penalty by creating thrust.
Not shown is an inverter between the fuel cell and the superconducting, SC, motor/generator (M/G). In motor mode, the M/G drives the ducted fan. During aircraft descent in generator mode, the M/G charges the battery while the fan acts as a wind turbine.
Combustion yields steam; elimination of steam to avoid contrails is an intractable problem. Fuel cells produce water; disposal of water is an imminently tractable problem. Contrails are to be shunned because of warming effects. The water produced by the fuel cell cannot be accumulated, and it cannot be simply dumped overboard thereby creating a contrail.
The Hail Stone Machine operates similar to the ice cube dispenser on your refrigerator. The ice cubes fall from the aircraft and melt before hitting the ground. Continuously functioning software predicts ice cube trajectory and melting based on real time meteorological information. Based on the meteorological data, the size of ice cubes can be adjusted. Also the ice cube flow is monitored to avoid hitting other aircraft; awareness of aircraft traffic avoids ice cube collision.
In the event ice cubes cannot be safely ejected, a backup exists. Simply eject liquid water which unfortunately creates a contrail. To sooth the environmentalists, this situation is anticipated to be infrequent.
With the correct vertical atmospheric temperature and humidity profiles, an electric aircraft flying at 30,000 feet can make a contrail at perhaps 12,000 feet. The cumulus cloud type contrail is caused by the melting ice cubes and seems to originate due to an invisible aircraft. The contrail has water droplets. Cumulus clouds reflect sunlight and have a net cooling effect.
Think energy and energy conversion. Apply the aviation filter. Options narrow quickly to combustion and electrochemistry. The heat of combustion moves objects but is also the source of uncontrolled emissions. A combustion-electric aircraft can never be emission free. On the other hand, electrochemistry is largely controlled. Specifically, the success of the totally emission free electrical aircraft depends on:
- Greenhouse gases eliminated by the choice of fuel - hydrogen.
- Oxides of nitrogen eliminated by staying cool with electrochemistry.
- Contrails eliminated by freezing excess water.
The overwhelming key technologies to be conquered are hydrogen infrastructure and hydrogen storage on board aircraft.
This article was written by Dr. Allen E. Fuhs, SAE Fellow (Carmel, CA). For more information, Click Here .