APPLICATIONS – TRANSPORTATION | Aviation: Fuel Cells

APPLICATIONS – TRANSPORTATION | Aviation: Fuel Cells

Aviation: Fuel Cells TH Bradley, Colorado State University, Fort Collins, CO, USA BA Moffitt, D Mavris, and DE Parekh, Georgia Institute of Technology...

224KB Sizes 0 Downloads 72 Views

Aviation: Fuel Cells TH Bradley, Colorado State University, Fort Collins, CO, USA BA Moffitt, D Mavris, and DE Parekh, Georgia Institute of Technology, Atlanta, GA, USA & 2009 Elsevier B.V. All rights reserved.

Introduction Fuel cell-powered aircraft have been of long-term interest to the aviation community because of their potential for improved performance and environmental compatibility. Only recently have improvements in the technological readiness of fuel cell power plants enabled the first aviation applications. Based on the results of conceptual design studies and a few technology demonstration projects, a widespread understanding of the importance of fuel cell power plants for near-term and future aviation applications has emerged. In the near term, fuel cells can exhibit compelling performance advantages in applications such as auxiliary power generation and propulsive power generation for small-scale aircraft and long-endurance aircraft.

General Design Metrics for Aviation Fuel Cell Power Plants To date, the designers and developers of fuel cell power plants have been primarily responsive to the needs of the automotive and stationary power generation applications. To maximize the performance of fuel cell-powered aircraft, fuel cell power plants will have to be designed and built to meet aviation-specific design criteria. These may involve significant departures from conventional automotive or stationary fuel cell design. For aviation, some primary aircraft-level performance metrics are range, endurance, rate of climb, and maximum speed. These aircraft design requirements can be translated into first-order requirements for the fuel cell system by analyzing Newton’s laws for an aircraft in steady level flight. A simplified ranges equation for unconventional power plants can be derived where aircraft weight is constant: ds ¼

Z

dE T

½1

where s is the range, E the propulsive energy, and T the thrust. During steady level flight at small angles of attack, T ¼ D and L ¼ mg (where D is the drag and L the lift force),         E L E E CL s¼ ¼ ¼ D D L m gCD

186

½2

with m the aircraft mass, g the acceleration due to gravity, CL, CD the coefficient of lift and drag, respectively. A similar approach can be followed to derive a simplified endurance equation for unconventional power plants: T¼

CD W CL

½3

L ¼ 12rv2 Sw CL

½4

where W is the weight force, r the air density, v the air speed, and Sw the wing area. Rearranging eqn [4] with W ¼ L,  v¼

W ð1=2ÞrSw CL

1=2 ½5

The propulsive output energy is the integral of the propulsive output power. Under the assumption that the weight of the aircraft changes negligibly over the course of the flight, E¼

Zt 0

 1=2  3=2 CD W CD t W W dt ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CL ð1=2ÞrSw CL ð1=2ÞrSw CL

Solving for the aircraft endurance (t),  t¼

E m3=2



3=2

ðð1=2ÞrSw Þ1=2 CL g 3=2 CD

½6

! ½7

Under similar assumptions, the aircraft flight path angle, g, can be expressed as g¼

vL vT CL ¼ ¼ ¼ mg mg CD

  E˙ CL

m gCD

½8

and the aircraft airspeed as v¼

      E˙ E˙ E˙ CL ¼ ¼ T D m gCD

½9

Based on these analyses, the design metrics to be maximized can be characterized so as to maximize fuel cell aircraft performance. To maximize the range and endurance performance of the fuel cell-powered aircraft, one can maximize (E/m), which is directly proportional to aircraft range, (E/m3/2), which is directly proportional to aircraft endurance, and (E˙/m), which is proportional to flight path angle, maximum speed, and many other aircraft performance metrics. For the remainder of this chapter, the performance of various fuel cell power plants for aircraft will be expressed in terms of specific power

187

Applications – Transportation | Aviation: Fuel Cells

Modern aircraft require integrated design processes that can allow trade-offs between the design characteristics of aircraft subsystems. Power plants for fuel cell aircraft must incorporate compromises between the design challenges associated with aviation operating conditions, the characteristics of the environment, and the fuel cell systems themselves. Reactant storage is one of the primary design challenges for fuel cell aircraft of all types, but especially for those that consume hydrogen. Whereas hydrocarbon fuels can be stored in liquid form in irregular containers, distributed throughout the airframe, gaseous or liquid hydrogen must generally be stored in centralized spherical or cylindrical vessels. This necessitates an increase in aircraft frontal area and an increase in wing structure, both of which increase aircraft power consumption. Although all fuel cell aircraft that have incorporated chemical hydrogen storage media have used a centralized hydrogen storage unit, this is not necessarily an intrinsic feature of chemical hydrogen storage. The development of high-specific-energy hydrogen storage systems such as low-pressure composite cylinders and chemical hydrogen storage systems will improve the performance of fuel cell-powered aircraft. In general, there exists a trade-off between the energy conversion efficiency of the power plant and the weight of the power plant system. The more efficient the power plant, the less hydrogen must be carried and the lower the weight of the hydrogen storage subsystem, for a constant duration mission. The design characteristics of higher efficiency fuel cells, such as humidification systems, higher fuel cell active area, and improved controls, can lead to higher weight of the fuel cell subsystem. To illustrate this trade-off, assume that (1) the hydrogen tank is 5% hydrogen by weight, (2) the fuel cell stack overpotential is proportional to current density, (3) the fuel cell stack weight is proportional to its active area, and (4) the output power of the stack is 100 W. The set of equations that define this example are m ¼ mairframe þ mpower ¼ mairframe þ mfuel cell þ mH2 tank mfuel cell ¼ kA   1 ncells I MWH2 t mH2 tank ¼ 5% 2F   I 0 E ¼ tVI Zpower train ¼ t V  R ncells I Zpower train A

½10

710

5

700

4.5 ⎛⎜ E ⎞⎟ ⎝mpower⎠ 4

690 680

3.5

670

mH

2tank

3

660

2.5

650

2 V0 = 1.23V −2 1.5 k = 0.1 kg cm R = 0.032 Ω cm2 cell−1 1 power plant = 80% 0.5 ncells= 62 0 13 14 15 16 17 18 19 20 2 Fuel cell active area, A (cm ) mfuel cell

640 630 620 610 11

12

Subsystem weight (kg)

Design Considerations for Aviation Fuel Cell Power Plants

where the various m’s are the respective masses, k the ratio of fuel cell stack active area to fuel cell stack mass, A the fuel cell active area, ncells the number of cells in the fuel cell stack, F the Faraday number, MWH2 the molecular weight of hydrogen, V the potential of oxygen reduction reaction, Z the efficiency, and V0 the standard cell voltage. R is the equivalent ohmic resistance of the cell and I the fuel cell stack current. Assume that the goal of the designer is to optimize the power plant specific energy for a fuel cell-powered unmanned aerial vehicle (UAV) of long endurance (for this example t ¼ 33 h). Constants and results for this analysis are shown in Figure 1. As the active area of the fuel cell stack goes up, the mass of the fuel cell stack goes up, but the fuel cell stack operates at lower current density, higher voltage, and higher efficiency. By operating at higher efficiencies, the fuel cell stacks at higher active area require less hydrogen, thereby reducing the mass of the hydrogen tank. In this example, a strong maximum in power plant specific energy exists near an active area of 15 cm2. At higher active areas, the fuel cell stack mass dominates. At lower active areas, the hydrogen tank mass grows with the decreasing efficiency of the stack. Aircraft must be able to operate efficiently at high altitude to improve the high-speed airframe efficiency and fly above atmospheric disturbances. The oxygen source for most fuel cell–powered aircraft conceived to date has been the atmosphere so as to avoid carrying the weight and bulk of stored oxygen. At a cruising altitude of 10 km, the atmospheric pressure is only 0.26 atm and the oxygen partial pressure is 0.05 atm. For air-breathing fuel cells, this decrease in ambient oxygen partial pressure can cause the fuel cell activation and mass transport overpotentials to increase. A majority of fuel cell aircraft designs have incorporated compressors to maintain the cathode pressure at a fixed absolute pressure, but this solution has costs in terms of efficiency, weight, and power output. For example, for a fuel cell aircraft that

Power plant specific energy (Whkg−1)

(E˙/m) and specific energy (E/m), with the understanding that these metrics are directly or nearly proportional to many aircraft performance metrics.

Figure 1 Results of a power plant design example showing the trade-off between fuel cell and hydrogen storage systems design.

188

Applications – Transportation | Aviation: Fuel Cells

operates at a 10 kW cruise, 0.7 V per cell, 10 km altitude with a cathode stoichiometry of 2.0, an 80% efficient compressor will consume 3.1 kW of power to maintain a constant 2 bar of cathode pressure. As the altitude increases, the power required to maintain the required cathode pressure will increase. This problem might be overcome by designing aviation-specific fuel cells with compressor–expander modules, increased catalyst loadings, or higher active area. The water and thermal management of the fuel cell system is also complicated by the aircraft altitude. Ambient humidity and temperature are very low at altitude. At 10 km altitude, the standard atmospheric temperature is  50 1C and humidity is on the order of 0.2 g (kg dry air)1, B30 times less water content than summer desert air (42 1C, 10% relative humidity). Aviation-specific water and thermal management strategies must be incorporated into fuel cell power plant design. These could include input air heating, exhaust water recirculation, or radiative heat rejection. In automotive or portable fuel cell power plants, hybridization of the power plant with a battery or supercapacitor bank can improve the efficiency and the performance of the system. These improvements are caused by isolating the low-power, high-energy fuel cell system from high-power, low-energy transients. In most aircraft applications, the majority of power transients are high-power, high-energy transients such as takeoff, landing, and acceleration. Because of this mismatch, only a few researchers have considered or constructed hybrid electric aircraft power plants, and in the hybrid fuel cell aircraft that have been constructed, the battery is indeed primarily used for high power takeoff. A highly generalized comparison of automotive and aviation fuel cell systems and conditions of use is shown in Figure 2. Note the dramatic difference in the characteristics of the propulsive power requirements. Further study of the

conditions of use of UAVs will define the role that hybridization can play in improving power plant efficiency.

Near-Term Applications Small-Scale Unmanned Aerial Vehicles One of the primary drawbacks of conventional turbine and reciprocating combustion engines is that their efficiency cannot be preserved at very small scales because of issues such as combustion quenching, high surface area to volume ratios, and low reactant residence times. As highly modular direct energy conversion devices, fuel cell power plants have no such limitations and they are able to maintain high thermodynamic efficiency and therefore high specific energy even at the subkilowatt scales. This scale of power plant is of interest to the aviation community for applications such as long-endurance small- and micro-scale UAVs. For example, a conventional hydrocarbon-fueled internal combustion power plant that can propel an aircraft at 375 W for 10 h endurance has a power plant specific rotational energy of B178 Wh kg1. Proton-exchange membrane (PEM) fuel cells with compressed gaseous hydrogen or liquid hydrogen storage can exhibit specific mechanical energy of >900 Wh kg1 or >10 000 Wh kg1 respectively. Solid oxide fuel cells (SOFCs) fueled by propane can exhibit specific energies of >1600 DC Wh kg1. A comparison of potential small-scale UAV power plants is shown in Table 1. Table 1 assumes that the airframe mass (mairframe) is 5.1 kg, the electric motor mass (where appropriate) is 283 g, electric motor efficiency is 71%, propeller efficiency is 69%, and each power plant can produce 1560 Wh of propulsional energy at 70 W. Range and endurance for the internal combustion power plant are calculated using the Breguet

Batteries

Fuel cell stack

Fuel cell stack

Hydrogen tank Hydrogen tank

Normalized propulsive power (P/Pmax) (%)

Normalized propulsive power (P/Pmax) (%)

High-power, low-energy transients

+100

+100

0

−100

High-power, high-energy transients

−100

Time (h)

(a)

0

Time (h)

(b)

Figure 2 Generalized comparison of fuel cell systems and propulsive power requirements for (a) automotive and (b) aviation applications.

Applications – Transportation | Aviation: Fuel Cells

Primary power plant comparison for small-scale aircraft   Power plant type Power plant E specification from m literature (Wh kg1)

189

Table 1

Compressed hydrogen PEM fuel cell Propane-fueled solid Oxide fuel cell Zinc–air battery Lithium polymer battery Small internal combustion engine



 E m 3=2 (Wh kg3/2)

Calculated range (km)

Calculated endurance (h)

1000 DC Wh kg  1

186.4

64.4

1642

44.0

660 DC Wh kg1

157.2

49.9

1384

34.1

350 DC Wh kg1 166 DC Wh kg1 0.3 kg of gasoline h1 at 105 W

108.0 62.9 125.5

28.4 12.6 35.6

951 554 1509

19.4 8.6 38.6

PEM, proton-exchange membrane.

18 ° Aero Vironment (http://www.avinc.com/) ° Wiesbaden ° Naval Research Lab ° Adaptive Materials Inc. ° California State Univ. ° Georgia Institute of Technology ° Global Observer

16 14

Wingspan (m)

12

11

10 8 6

8

4 2 2

1

6

4 3

10 9

7

5

0 0.1

1

10

100

Gross takeoff mass (kg)

Figure 3 Comparison of small-scale fuel cell-powered unmanned aerial vehicles (UAVs) constructed to date. Top photo: Reproduced with permission from Journal of Power Sources. Bottom photo: Reproduced with permission from Shigeru Matsuyama.

range equation 

   CL Wi Isp ln s ¼ vt ¼ v CD Wf

½11

where Isp is the thrust-specific fuel consumption, Wi the aircraft weight with fuel, and Wf without the fuel. Based on this comparison, it can be concluded that the fuel cell power plants have the ability to outperform other electrochemical storage media as well as conventional internal combustion power plants at this scale. Because of these performance advantages, fuel cells have found their first aviation applications as power plants for small-scale UAVs. In 2003, AeroVironment Inc., a vehicle design and manufacturing company in Monrovia, California, built and flew the first fuel cellpowered aircraft. Its monopolar fuel cell system consumed hydrogen from a sodium borohydride reaction vessel. Between those first flights and the present,

a number of researchers and commercial entities have developed fuel cell-powered UAVs with increasing size and scale, as shown in Figure 3. To date, most smallscale UAV power plant systems have been designed with PEM fuel cell systems, which are self-humidified or passively humidified, unhybridized, and with compressed or chemical hydride hydrogen storage. Demonstration of an SOFC UAV fueled by propane has also been accomplished by Adaptive Materials Inc. In all these demonstrations, the fuel cell power plants were designed for high specific energy, so as to maximize endurance and range, and for high specific power, so as to allow for easy handling and controllability. It is anticipated that smallscale UAVs with endurances of >24 h and ranges of >2000 km will be developed in the near future. These aircraft will have significant value as low-altitude, lowcost, autonomous reconnaissance, and remote sensing platforms for both commercial and military applications.

190

Applications – Transportation | Aviation: Fuel Cells

Commercial Jet Auxiliary Power Unit NASA (National Aeronautics and Space Administration) and various aerospace companies have performed research on the development of a solid oxide-based fuel cell auxiliary power unit (APU) for passenger aircraft. Aircraft APUs are gas turbine generators that generate electric power during ground operations to power aircraft electrical loads such as lighting, cabin environmental conditioning, and main engine startup. Conventional APUs are fueled by the onboard jet fuel. The systems that are proposed to replace these APUs consist of a hybrid SOFC and gas turbine system with onboard jet fuel reformation. Whereas a conventional APU achieves B15% electrical energy generation efficiency, the hybrid SOFC system should achieve between 41% and 60% efficiency. In addition to these fuel savings, the SOFC APU would offer lower nitric oxide emissions, longer service intervals, and power conversion at cruise. Disadvantages might include higher upfront costs and a longer startup time. Fuel cell APUs will also be an enabling technology for the More Electric Airplane architecture. The More Electric Airplane Architecture is an aviation industrywide development concept wherein the hydraulic and pneumatic systems of conventional aircraft are replaced with electrical servo-actuated systems of higher reliability and lower cost. The More Electric Airplane Architecture would enable functions that would be powered by the APU including motor-powered ground taxi, electrically redundant controls, and high bandwidth control surface optimization. Solar Regenerative Aircraft Regenerative PEM fuel cell systems have been proposed as an enabling technology for a new class of aircraft with unlimited endurance. Using electricity generated from solar cells and composite pressure vessels to store reactants, a rechargeable, high-efficiency, high-specific energy aircraft power plant can be constructed. This power plant could be a component in an airship or gossamer aircraft. To compare rechargeable systems for a long-endurance application, one can again compare their energy density. For rechargeable systems though, the energy of interest must be the electrical charging energy, so that the figure includes both charging and discharging efficiencies. Advanced batteries can reach an electrical discharging specific energy of 200 Wh kg1 at the module level and have a charge efficiency of nearly 100% at low current. A rechargeable fuel cell/electrolyzer energy storage system with compressed reactant storage can have a discharging specific electrical energy of >800 Wh kg1, and a charging efficiency of 80%. This results in a round-trip, specific electrical energy of >640 Wh kg1. So, in comparison with advanced battery

technologies, compressed hydrogen regenerative fuel cells can exhibit significantly higher specific energy. Most research on regenerative fuel cell systems for very long-endurance aircraft has concentrated on conceptual aircraft and power plant system design. The NASA ERAST project and its Pathfinder test aircraft are notable exceptions. A planned fuel cell-powered flight by the Pathfinder aircraft was halted only by the catastrophic failure of the aircraft in 2003. The NASA Glenn Research Center has developed and tested a laboratory version of a regenerative fuel cell for aviation applications. No functional regenerative fuel cell power plants have been demonstrated in aviation applications to date. General Aviation General aviation is a subset of aviation consisting of chartered passenger aircraft, private aircraft, and other components of civil aviation that are not regularly scheduled airline flights. The purpose of developing fuel cell power general aviation power plants is to demonstrate fuel cell technology in a manned application and to mitigate the noise and air pollution of general aviation. This would allow for 24 h operations from urban airports with noise and/or pollution abatement regulations. A number of groups have proposed these projects and completed feasibility studies, battery-powered test flights, and laboratory tests. These projects have generally used fuel cell systems and components derived from automotive applications. Boeing constructed and flew the first manned fuel cell-powered aircraft in 2008.

Long-Term Applications In the longer term, many envision fuel cells as a primary power plant for advanced aviation concepts. These might include SOFC-powered liquid hydrogen aircraft with B20 days endurance, distributed onboard accessory power generation, and multifunctional fuel cells that generate power and make up the skin of the aircraft. Some researchers have proposed fuel cell power plants for large-scale passenger and commercial aviation applications. Because commercial aircraft are very highpower, high-energy applications, it is estimated that a fuel/propulsion system specific power of 2 kW kg1 would be required, B15 times the performance of currently available technology.

Commercialization Considerations There are several aspects of the aviation application that will have an effect on the success of adoption of fuel cell power plants.

Applications – Transportation | Aviation: Fuel Cells

First, the environmental effect of aviation is low compared to other applications such as automotive or stationary power generation, which suggests that adoption of fuel cells by the aviation industry may not be justified only by a desire for improved environmental compatibility. For this discussion, environmental compatibility for aviation is broken down into metrics of pollution and energy sustainability, as these are where fuel cell technologies may have a beneficial effect. For the foreseeable future, aviation will be a contributor to local and global atmospheric pollution in the forms of carbon monoxide, nitric oxides, hydrocarbons, particulate matter, sulfur oxides, hydrogen sulfide, carbon dioxide, and water. Aviation is responsible for B0.4% of the national nitric oxide inventory. Locally, the effect of aviation emissions can be larger. For the urban area of Dallas-Fort Worth, Texas, aviation is responsible for 6.1% of the local nitric oxide inventory in 1996. Aviation also has a significant effect on local noise pollution in the form of engine noise emissions during taxi and flight operations. Carbon dioxide, nitric oxides, and upper atmospheric water emissions are the primary globally active pollutants from aviation. By 2015, aviation is predicted to be responsible for roughly 5% of all anthropogenic radiative forcing – a measure of climate change. Aviation is also a minor contributor to global petroleum depletion. For the United States in 2006, aviation consumed 4.8% of the nation’s energy flow and 8.4% of its petroleum. In sum, aviation is a small but consequential component of local and global environmental degradation, and transitioning aviation to lower emission fuel cell power plants may have environmental benefits. Despite this, in some aviation applications, fuel cell power plants exhibit performance benefits that can justify further development and commercialization outside of any environmental benefits. Second, aviation continues to be an avenue for the commercialization of high-performance power plant technologies, for example, gasoline fuel injection and turbojet engines. There are a number of factors that have allowed advanced technologies to gain initial adoption in the aviation industry. In many aviation applications, the metrics of per• formance (i.e., speed, endurance, and payload) are





more valued than the purchase cost. This is especially true of military and private transportation applications. The scale of aviation component production is several orders of magnitude below that of mass-production industries such as the automobile industry. Technologies that do not scale well to very large production numbers (e.g., structural thermoset composites) can find early and enthusiastic adoption in the aviation industry. Aviation is highly infrastructure based. Aircraft fly to and from dedicated airfields whose infrastructure is



191

geographically, structurally, and operationally adapted to meet the needs of the aircraft that are present. Military micro-UAVs are an exception to this rule. Aircraft are purchased, used, and serviced by specialists instead of by the general public. Problematic characteristics of many advanced technologies such as low consumer acceptability, strict inspection intervals, and finite component lifetimes are much less important in aviation than in other potential applications.

If fuel cell systems can establish their performance benefits over conventional systems in the aviation industry, aviation may prove to be an early adopter of fuel cell technologies.

Conclusions Fuel cells for use in aviation applications is a rapidly developing field. This chapter reviews the present understanding of fuel cell power plant design considerations and near-term applications. It is anticipated that fuel cell power plants for small-scale UAVs will be the first commercially available fuel cell aviation application. These power plants will replace advanced batteries to allow for long-endurance and long-range missions. Fuel cell power plants may have a larger effect on the entire aviation industry in the far future as the development of fuel cells and hydrogen storage media advance.

Nomenclature Symbols and Units A CD CL D E˙ F g I Isp k L m mairframe

mfuel cell mH2 tank mpower MWH2

fuel cell active area (cm2) aircraft coefficient of drag aircraft coefficient of lift drag force (N) propulsive energy (J) Faraday number acceleration due to gravity (m s  2) fuel cell stack current (A) thrust specific fuel consumption (N/N) ratio of fuel cell stack active area to fuel cell stack mass (cm2 kg  1) lift force (N) aircraft mass (kg) mass of the aircraft airframe, which consists of all components of the aircraft excepting power plant (kg) mass of the fuel cell stack (kg) mass of the hydrogen storage tank (kg) power plant mass including power train, fuel, and tankage (kg) molecular weight of hydrogen (g mol  1)

192

Applications – Transportation | Aviation: Fuel Cells

ncells P Pmax R s Sw t T n V0 W Wf Wi c gpower q

train

number of fuel cells in the fuel cell stack propulsive power (W) maximum available propulsive power (W) equivalent ohmic resistance of a fuel cell (O) range (m) wing area (m2) endurance (h) thrust (N) airspeed (m s  1) standard cell voltage reduction reaction (1.229 V) weight force (N) aircraft weight without fuel (N) aircraft weight with fuel (N) aircraft flight path angle, climbing is positive (rad) power train efficiency air density (kg m  3)

Abbreviations and Acronyms APU DC NASA PEM SOFC UAV

auxiliary power unit direct current National Aeronautics and Space Administration proton-exchange membrane solid oxide fuel cell unmanned aerial vehicle

See also: Applications – Transportation: Electric Vehicles: Fuel Cells; Fuel Cells – Proton-Exchange Membrane Fuel Cells: Cells; Fuel Cells – Solid Oxide Fuel Cells: Overview.

Further Reading Anderman M (2003) Brief assessment of improvements in EV battery technology since the BTAP June 2000 report. California Air Resources Board. Baldock N and Mokhtarzadeh-Dehghan MR (2006) A study of solarpowered, high-altitude unmanned aerial vehicles. Aircraft

Engineering and Aerospace Technology: An International Journal 78: 187--193. Bradley TH, Moffitt BA, Mavris DN, and Parekh DE (2007) Development and experimental characterization of a fuel cell powered aircraft. Journal of Power Sources 171: 793--801. Burke K (1999) High energy density regenerative fuel cell systems for terrestrial applications. NASA/TM-1999-208429. Burke KA (2003) Unitized regenerative fuel cell development. NASA/ TM-2003-212739. Contreras A, Yigit S, Ozay K, and Veziroglu TN (1997) Hydrogen as aviation fuel: A comparison with hydrocarbon fuels. International Journal of Hydrogen Energy 22: 1053--1060. Crumm A (2006) Solid oxide fuel cell systems. In: Proceedings of the Fuel Cell Seminar. 13–17 November, Honolulu, Hawaii. Freeh J, Pratt JW, and Brouwer J (2004) Development of a solid-oxide fuel cell/gas turbine hybrid system model for aerospace applications. NASA/TM-2004-213054. Himansu A, Freeh JE, Steffen CJ, Tornabene RT, and Wang X-YJ (2006) Hybrid solid oxide fuel cell/gas turbine system design for high altitude long endurance aerospace missions. NASA/TM-2006214328. Kohutt LL and Schmitz PC (2003) Fuel cell propulsion systems for an all-electric personal air vehicle. NASA TM-2003-212354. Menon S, Moulton N, and Cadou C (2007) Development of a dynamometer for measuring small internal-combustion engine performance. Journal of Propulsion and Power 23(1): 192--202. Moffitt B, Bradley TH, Mavris D, and Parekh DE (2006) Design space exploration of small-scale PEM fuel cell long endurance aircraft. In: 6th AIAA Aviation Technology, Integration and Operations Conference, 25–27 September. Witchita, Kansas. AIAA-2006-7701. Penner JE, Lister D, Griggs DJ, Dokken DJ, and McFarland M (1999) Aviation and Global Atmosphere. New York: Cambridge University Press. Pratt JW, Brouwer J, and Samuelsen GS (2007) Theoretical and experimental performance of a proton exchange membrane fuel cell at high-altitude conditions. Journal of Propulsion and Power 23: 437--444. Putt R, Naimer N, and Atwater T (2004) Fourth generation zinc–air batteries. Proceedings of the 41st Power Sources Conference. June 14–17, Philadelphia, PA. Royal Commission on Environmental Pollution (2002) The Environmental Effects of Civil Aircraft in Flight – Special Report. ISBN 0-9544186-0-3. Wentz WH and Mohamed AS (2004) Preliminary design considerations for zero greenhouse gas emission airplanes. SAE Transactions Journal of Aerospace 113: 1--16. Wickheiser TJ, Sehra AK, Seng GT, Freeh JE, and Berton JJ (2003) Emissionless aircraft: Requirements and challenges. In: Proceedings of the AIAA International Air and Space Symposium and Exposition: The Next 100 Years. 14–17 July. Dayton, OH. AIAA-2003-2810. Youngblood JW, Talay TA, and Pegg RJ (1984) Design of longendurance unmanned airplanes incorporating solar and fuel cell propulsion. In: Proceedings of the 20th Joint AIAA/SAE/ASME Propulsion Conference. 11–13 June. Cincinnati, OH. AIAA-19841430.