Fuel Cells, Part 1

Airplanes and rotorcraft have traditionally been propelled by internal combustion (IC) engines or turbines/jets burning hydrocarbon-based fuels. This dependence on combustible fuels is principally due to the high energy content of gasoline and diesel fuel.
Although we have recently seen the reemergence of electric automobiles, their range is limited by the total energy density of today’s battery technology, which is significantly lower than the equivalent energy content of gasoline. As an interim solution, auto manufacturers have combined the best properties of electric drive with small, highly optimized internal combustion engines to produce hybrid electric vehicles that offer good performance with high gasoline mileage (typically 50-70 mpg).
The current optimum efficiency of electric motors and controllers exceeds 90%, and they have proven useful on a number of unmanned solar-powered electric aircraft such as the AeroVironment Pathfinder, Centurian, and the recent 14-motor Helios. Unfortunately, the weight of sufficient batteries for piloted aircraft takeoff and any reasonable flight duration has, to date, been prohibitive for use in GA aircraft and too costly for the average homebuilder.

However, recent developments in advanced performance batteries, particularly with rechargeable Silver-Zinc, NiMH, Li-Ion and LiPolymer batteries, as well as advancements in hydrogen proton exchange membrane (PEM) and solid-oxide fuel cells, could begin to make electrically powered flight commercially feasible. The cost of advanced batteries and fuel cells is still a major issue, although the volume usage of advanced batteries (and fuel cells) in emerging electric vehicles should ultimately bring prices down to affordable levels.
To become commercially viable, it is essential that critical components such as motors, control electronics, batteries, and fuel cells become available at reasonable prices from production for other high-volume industry applications. Aviation, by itself, has insufficient volume to reduce the cost of such critical components to an acceptable level.

The Energy Challenge

Most rechargeable batteries offer less than 2% of the energy per pound of gasoline. (*Figure 1 compares specific energy and specific power of various advanced battery technologies.) Even after considering the poor conversion efficiency of internal combustion engines of less than 25% (versus over 90% for electric motors), gasoline still has a 20:1 advantage of specific energy and energy density over rechargeable batteries.
The energy density of batteries has improved dramatically over the last 10 years and continues to improve at a rapid rate due to the significant funding of the U.S. Advanced Battery Consortium (ABC) and the U.S. Council for Automotive Research (USCAR) initiatives. But it needs further improvement in performance and cost to become commercially viable and competitive with gasoline for aviation use. However, by combining breakthroughs in energy storage devices and key aircraft components, and using composite materials and advanced propeller design, the possibility of practical manned electric flight can be approached.

Pioneering Efforts

Over the last 20-30 years, there have been a variety of electric aircraft developments and activities, most notably those of Paul MacCready, starting with his man-powered flight records at MIT. This was followed by his development of the photovoltaic-powered Solar Challenger, which made a record-breaking 163-mile flight from Paris to England in 1981 powered solely by solar electric power.
MacCready’s company, AeroVironment, and several others, have pursued the development of a variety of unmanned solar-powered aircraft including the Pathfinder, Centurion, and Helios, the high-altitude solar-powered unmanned aerial vehicle (UAV) that recently flew to more than 96,000 feet using 14 small electric motors mounted on a wing spanning nearly 250-foot. MacCready has demonstrated that high-altitude unmanned solar-powered aircraft can be flown successfully for up to 13 hours.
The company now plans to demonstrate “perpetual flight” using regenerative hydrogen-based fuel cells to power an airplane at night, using hydrogen obtained by electrolyzing water from excess daylight solar power. The water produced by the fuel cells will be stored and then reconverted into hydrogen the following day from excess solar power in a cycle repeated every 24 hours.

Several electrically assisted, self-launching single-place gliders that can climb for up to 9 minutes with onboard battery power have also been developed in Europe over the last five years. The most significant of these is the Air Energy model AE-1 Silent electric glider. This unique self-launched glider uses a 13-KW conventional brush-type electric motor with a NiCd battery pack. (Eric Raymond at Solar Flight in California currently holds most of the manned electric flight records including distance of 247 miles and altitude of 16,600 feet.)

Electric Propulsion and Hydrogen Fuel Cells

By combining hydrogen-based fuel cells, which have a significantly higher net system energy density than batteries, to produce electricity to power high-efficiency electric motors for propulsion, a new era of lower cost, quiet, safe, emission-free aviation should be achievable.
There are, however, numerous issues to resolve, even for basic hydrogen fuel cell use. Source and storage of hydrogen, as well as the cost and complexity of the balance of plant equipment required to properly operate and cool fuel cells are but a few. But over time the cost and complexity of fuel cells will be reduced, making them more attractive for GA aircraft.

Benefits of Electric Aircraft

Improved reliability: With only one moving part (motor armature plus propeller) electrically powered aircraft will be less susceptible to failure. Gasoline engines are subject to problems in starting and running properly plus a range of potential operating problems including fouled spark plugs, alternator failure, carburetor icing, vapor lock, or jet clogging. Electric motors have a significantly higher TBO, perhaps as much as 10 times the life of traditional IC engines.
Advanced electric motors also offer serviceability at a much lower cost, with rapid replacement and greater diagnostic simplicity. This translates to improved reliability and less chance of catastrophic failure. Also, most potential electric motor problems such as bearing failure are predictable with simple monitoring equipment. Related items with higher risk of failure, like the control electronics (motor controllers), could be made redundant with automatic switching to the backup controller if the primary unit failed.
Improved safety: Although most GA accidents involve pilot error, many such errors relate to fuel quality, fuel exhaustion, or improper engine operation, all of which can be eliminated with electric propulsion systems. For example, when a gasoline engine runs out of fuel, it may be difficult to get the engine primed and restarted in the air, even if a reserve tank is available. Switching to a backup battery would be instantaneous. Many accidents are due to loss of power or sudden engine failure. Most of these emergencies and accidents would be eliminated with electric drive systems due to the simpler operation and overall improvement in reliability. And electric aircraft will be much less likely to catch on fire in the event of a crash.
Additional features for pilot safety will include an emergency reserve battery capable of operating the motor for 6-10 minutes, backup control electronics, and an electronic diagnostic monitoring system.
Environmental compatibility: This is one of the biggest benefits of electric propulsion, as electric motors produce virtually no vibration or noise except for propeller noise, which can be reduced significantly with proper design. No fuel is consumed with electric propulsion, so there will be no emission of exhaust gases, fuel leakage or fumes in the cabin. (The use of auxiliary fuel cells could add the requirement for storing hydrogen or methanol, or generating hydrogen on board, which poses potential environmental and safety concerns. However, the only byproduct of fuel cell operation would be water vapor.)
Thus, electric-powered aircraft should be appealing to environmentalists and people living close to airports. They are potentially a true “green” type of transportation, particularly if the batteries are recharged by solar panels.
Ease of operation and passenger comfort: Operation of electrically powered aircraft will be simple. Because there is no fuel, there would be no need to check drains, vents, tank selector valves, fuel pumps and fuel lines, or filters. Warm ups, engine runups, and proper mixture and manifold pressure settings would be unnecessary. Although electric motors have bearings that may require occasional lubrication, there would be no need to check or change crankcase oil. There will also be no mixing oil with gas for two-stroke engines, or worries about plug fouling and resultant loss of power. Gone will be the carburetor de-icer and concerns about carburetor icing or engine damage from descending too quickly at low power settings.
With the reduction in noise, passengers would find it easier to hear and converse. Passenger comfort would be further enhanced due to the lack of fumes or emissions. The only comfort issue might be cabin heating: auxiliary heaters may be required because electric motors are so efficient that there may be insufficient waste heat to effectively heat the cabin.

Recharging electrically would take longer than ordinary refueling even though access to electric power is often greater than availability of gasoline in many locations. But with rising fuel costs and concerns over the future availability of crude oil, alternate propulsion systems are important.
Acquisition and life-cycle costs: Aside from the fuel and lubricants consumed, most of the life-cycle costs of traditional aircraft engines comprise annual inspections, maintenance of the engine, and replacement of worn components like tires and spark plugs.
Electric motors have a significantly longer useful life than gasoline engines, typically by a factor of 10, as well as virtually no periodic maintenance, so the life-cycle cost of the electric system will be considerably less. The cost of electricity required for recharging the batteries is estimated to be less than one-third of the equivalent cost of gasoline, particularly if charging occurs during off-peak hours.
Over the course of the typical 20- to 35-year life of an aircraft, battery replacement would be required at least every 6-8 years, as the life of most rechargeable batteries is limited to 1000 (or fewer) recharge cycles. Batteries may require replacement after approximately 2000 hours of use, hopefully with an improved pack providing more energy at lower cost and weight.
However, this replacement battery cost should still be less than the cost of rebuilding or replacing an engine, which is required with gasoline-powered aircraft about every 1500-2000 hours of use. Based on preliminary estimates, total life-cycle costs of small one- or two-place electric aircraft could ultimately be 10-25% less than traditionally powered aircraft.

Technical Objectives

There are, however, numerous issues to resolve, even for basic hydrogen fuel cell use. Source and storage of hydrogen, as well as the cost and complexity of the balance of plant equipment required to properly operate and cool fuel cells are but a few. But over time the cost and complexity of fuel cells will be reduced, making them more attractive for GA aircraft.
The initial Phase I goal of the program is to determine if state-of-the-art technology of electric motors, fuel cells, batteries, capacitors and related components can be optimized to design a safe, practical single-place piloted electric airplane powered only by DC electricity. The plane will be able to take off and fly at least 100 miles (or 2 hours) on a single energy charge, with sufficient electrical energy left to land safely.
A secondary Phase I goal is to analyze various fuel cell systems and determine if a lightweight, high-power-density system could be added to the basic electric airplane to extend its range to at least 250 miles. To facilitate the effective analysis of the various components and optimize the design of an electric airplane, a custom, multi-variable computer model will be created. This model will be used to validate and optimize the performance of the two electric aircraft designs that will be built and flight tested, with at least one of them employing a hydrogen-based fuel cell that could provide enough additional power for long-range cruising augmented by batteries for takeoff and climb.
To validate the viability of electric propulsion, a proven GA aircraft will be converted, replacing its IC engine and gas tank with the optimum electric motor, controller, and propeller system, and appropriate fuel cell, battery packs and instrumentation.
Technical questions must be answered to ultimately determine the feasibility and viability of electric aircraft. These include:

• What are the total equivalent electrical energy requirements necessary to safely take off and fly typical one- to two-place GA aircraft, such as the Liberty, Diamond Katana or Eagle 150, as well as smaller Experimental aircraft such as the Pulsar and American Ghiles Lafayette III?
  
• Can suitable off-the-shelf electric motors and controllers provide the performance and durability required for sustained aircraft use at reasonable cost?
  
• Can affordable advanced high-energy-density battery packs be developed with sufficient peak power and total energy capacity, and environmental (temperature range) suitability for use onboard the aircraft, providing sufficient energy for flight duration of at least 2 hours per charge without overburdening the aircraft or severely limiting its range?
  
• Can a reliable energy storage method like ultracapacitors or special high-current batteries be employed to provide the huge surge in motor current required for “over-powering” the electric motor for takeoff, climbout, and emergency go-arounds?
  
• How much energy can be delivered by the rechargeable batteries without damaging the batteries or reducing their life?
  
• Are there reliable, cost-effective methods of producing electric power aloft – such as photovoltaic panels, hydrogen-based fuel cell systems, or even compact high-efficiency motor-generators using new compact diesel engines – that could augment the range of the aircraft?
  
• Do new solid oxide or direct methanol fuel cells offer higher power output with lower weight for total fuel cell system (stack, fuel storage, and support equipment), and are they viable for aircraft use?
  
• Is the net level of energy density and specific energy that can be achieved with fuel cell systems significantly better than the best rechargeable battery technologies?
  
• Are fuel cells mature enough to be reliable and safe for such applications?
  
• How can hydrogen be stored (or created onboard) with lowest total system weight per unit of hydrogen?
  
• Can advanced hybrid-powered multi-module electric energy storage and electric drive systems be cost effectively integrated to provide adequate power for GA applications?
  
• Can a low-cost demo aircraft be built that will validate these concepts and overall system design, and demonstrate the overall viability of electric aircraft?

There are currently numerous limitations to the emergence of electrically propelled airplanes, such as long recharge time for cross-country travel, high battery acquisition costs, reduced battery capacity in high and low temperatures, and limited range due to the weight of sufficient batteries and capacitors.
In addition to battery/weight tradeoffs, electric airplanes have a disadvantage in that they don’t get lighter as they use up their charge like gas-powered planes do. Some batteries, like ZnAir, actually get heavier when discharged, as the anodes oxidize.
Electric airplanes also have less total available peak power as the batteries get weaker, so if a go-around was required after a missed approach with the batteries nearly depleted, the motor might not achieve full power without an emergency boost battery.
Despite these limitations, a combination of advancements in energy storage, materials, electric motors and propeller design may be optimally integrated to produce a reliable, manned test aircraft. This will be an important precursor to a new era of aviation.