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Applications of Fuel Cell Technology in Micro Air Vehicles – A Senior Design Project

Ryan Daines, Scott Longshore and Jennell, Sall: Senior Students Embry-Riddle Aeronautical University, Daytona Beach, FL 32114

Dr. Mankbadi, Distinguished Professor Embry-Riddle Aeronautical University, Daytona Beach, FL 32114

There are many innovations that can greatly increase the use of the Micro Air Vehicle (MAV). These improvements will advance many aerospace industries particularly the intelligence our military can gain about the enemy. This research will provide information to prove the advantages of using fuel cells over the generally used battery. The final findings show that a PEM fuel cell fueled by hydrogen is plausible at an MAV level; a fuel cell weighing 4.71 ounces producing over 42 Watts. While this weight is a little over an ounce too much for an MAV, it shows the opportunities that lie ahead for the use of PEM fuel cell in an MAV. Final discussions will include how the fuel cell designed can be scaled down to meet weight restrictions while still producing the required power.

Nomenclature Acell = Active Cell Area ER = Reversible Cell Potential E = Cell Potential Difference i = Current Density io= Reference Current Density I = Current Wel = Power Output for Cell ΔW = Work Done ΔV = Voltage Rise Q = Heat Produced NH2 = Hydrogen Consumed (Moles/sec) N02 = Oxygen Consumed (Moles/sec) NH20 = Water Consumed (Moles/sec) mH2 = Hydrogen mass flow rate (g/sec) m02 = Oxygen mass flow rate (g/sec) mH2O = Water mass flow rate (g/sec) S – Stoichiometric Ratio NactH2 = Actual amount of Hydrogen consumed (mol/sec) NactO2 = Actual amount of Oxygen consumed (mol/sec)

ηfuel utilized = Efficiency of the fuel utilized VEnd Plate = Volume of the end plate WFuel Cell = Weight of the Fuel Cell H2Needed = Weight of Hydrogen Needed O2Needed = Weight of Oxygen Needed H2OGenerated = Weight of water generated VH2 = Volume of Hydrogen VO2 = Volume of Oxygen h = height of fuel tank r = radius of the fuel tank d = diameter of the fuel tank Vcompressed = Volume of compressed gas Wtank = Weight of the tank Wsystem = Weight of the system ηcell = Efficiency of the fuel cell R = Range (feet) ηpr = Propeller Efficiency L/D = Lift to Drag Ratio W0 = Initial Weight of the system

I. Introduction The power system of an MAV needs to look

to new technology to get the full benefit of its miniscule size. Powering the propulsion system is the first concern in enhancing the performance of the

MAV. With the previous attempts at a fuel cell powered MAV not fully investigated it is believed that there is much opportunity for the use of a fuel cell as a power source. The first step in the process will be to examine two differently fueled cells and make a decision for which type should be further

44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit21 - 23 July 2008, Hartford, CT

AIAA 2008-4909

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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Figure 1. Black Widow (Grasmeyer, 2001)

investigated. A hydrogen fuel cell will be analyzed versus a direct methanol fuel cell. Once a fuel cell is determined to be further examined it will be designed at an MAV size level. Finally it will be compared to a battery powered system on a number of characteristics to determine the most efficient power source available.

With the fuel cell being the new power source of the future, a fuel cell would be a perfect fit to power a MAV. One of the leading fuel cells is the Polymer Electrolyte Membrane fuel cell, or PEM fuel cell. The common name for the core membrane in this fuel cell is the Nafion, which is located between the anode and cathode. The fuel for this PEM fuel cell is going to be either pure hydrogen or hydrogen rich fuels. As the hydrogen is pulled through the anode the platinum catalyst extracts the electrons from the hydrogen and the free electrons flow away from the fuel cell generating electricity. The important issues to analyze with a fuel cell are ratios such as power density and endurance-to-weight, as well as the lifespan of the fuel cell. There are opportunities for optimization with this PEM fuel cell such as examining how fuel flow rates affect the fuel cell, the performance parameters for this specific fuel cell and attempting to decrease the size and weight. With the fuel cell previously discussed using either hydrogen or hydrogen rich fuels, it would be beneficial to investigate the advantages to alternate fuels. Methanol has a much larger energy density than highly compressed hydrogen making it appear to be a perfect fit in a fuel cell. There are many other advantages to using methanol over hydrogen such as methanol can be stored safer than hydrogen making the use of Methanol a much safer choice. Methanol does not need to be stored at high pressures or at low temperatures allowing it to use the current petroleum network for distribution, causing it to be a more practical fuel. Currently methanol fuel cells are highly inefficient, to the point of making them unusable for propulsion applications. This does however open a large opportunity for optimizing the methanol fuel cell. With the by-product being carbon dioxide there are waste issues, as well as the generic fuel cell needs of decreasing the size and weight. Maximizing the power output and efficiency will be a major concern with a Direct Methanol Fuel Cell.

II. Survey of Current Systems DARPA (Defense Advanced Research Projects Agency) defines a micro aerial vehicle, MAV, as being no larger than six inches in any dimension, weighing no more than four ounces, and having a cost of less than $1,000. (Ashley, 1998) The

price restriction is so that the vehicle has an affordable or disposable cost. With these strenuous constraints, there are many obstacles to overcome. The power and propulsion systems must be very efficient and have a high thrust-to-weight ratio. DARPA is excited about the military importance of an MAV, but there are also many other advantages to generating a successful MAV. (Ashley, 1998) The major areas of focus are; maximizing the thrust-to-weight, decreasing the size, and decreasing the noise to aid the vehicle in better blending into its surroundings.

Currently the most popular power source is a battery, which is used with a variety of different propulsion systems. The most famous MAV is the

Black Widow created by AeroVironment, a company from the Los Angeles area, which has a six-inch wing span and weighs only around 2 ounces. The Black Widow uses a battery and a direct-driven propeller for its power and propulsion system. It has been perfected to achieve a 30 minute flight duration with cruising speeds of 30 mph. (Grasmeyer, 2001) This MAV was a success and gives a good comparison/starting point for the MAV this documentation intends to design. AeroVironment also produced another MAV powered solely by a fuel cell, named the Hornet. The fuel cell was hydrogen powered and expected to have three times the endurance of the Black Widow. But, after the fuel cell kept drying up in testing, DARPA decided to back the battery system and so the fuel cell powered system was never perfected. (Office of the Secretary of Defense, 2005) The third MAV which shows potential is the Honeywell Micro Air Vehicle. This vehicle uses a ducted fan with a somewhat helicopter type configuration. This vehicle weights 12.5 lbs and has a diameter of 13.” (Honeywell, 2004) This design does not meet DARPA’s specifications for a MAV and stability concerns will arise if the vehicle is shrunk to an MAV size.

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Figure 2. Honeywell Micro Air Vehicle (Honeywell, 2004)

III. Fuel Cells

Fuel cells have been experimented for use with Micro Air Vehicles, but have failed in past attempts. A fuel cell is an electrochemical device that converts chemical energy into electrical. This offers many benefits over conventional power systems, such that they have high operating efficiencies, numerous fuel types, and few if any moving parts. These benefits are highly desirable in a primarily military driven market. The two different fuel cells that were investigated in the beginning were the Direct Methanol Fuel Cell (DMFC) and the Polymer Electrolyte Membrane Fuel Cell (PEM). These fuel cells were compared head to head under the constraints that a MAV require; the overall best choice was chosen and investigated further. This paper was written to demonstrate the possibilities that fuel cells possess for MAV applications. The first option investigated was the DMFC, which uses liquid methanol and water as the fuels. The oxidant, or air, is fed in through the cathode side while the fuel is fed in through the anode side. This option was first explored because of the small scale which it can be built and because liquid methanol can be easily stored and obtained. The reactants flow through the catalyst and membrane where the electrons are carried over an external circuit load. This type of cell offers one main advantage, the use of methanol. Liquid methanol can be used and it does not have to be compressed or stored at high pressures. Methanol also offers four times the energy density per volume than that of hydrogen. However, the disadvantages far outweigh the benefits for transportation applications. The DMFC is plagued by problems such as low power output, methanol crossover and, carbon dioxide poisoning. The other option was the PEMFC, which uses hydrogen as the fuel and oxygen as the oxidant. The hydrogen enters through the anode side and the protons are transported from the anode to the cathode through the membrane. The electrons are then carried over an external circuit. Like the DMFC there are many advantages and disadvantages. The PEMFC has a very low operating temperature, which allows it

to have a quick start up time. This quality allows it to be a likely candidate for use in transportation. The overall cost of the system and fuel are relatively low allowing it to meet DARPA’s criteria (under $1000) in order for the vehicle to be disposable. Fuel cells and batteries have been in competition since Micro Air Vehicles were devised. At this time, fuel cells have yet to prove their supremacy over a battery. By looking at a head to head comparison of the two systems it can be seen that fuel cell theoretically offer more than a battery. Fuel cells can offer 40,000 hours of actual life time use, while batteries have a 6-48 month life span. Because a battery can never be fully shut off, its life span slowly depreciates. Only about 30 percent of all batteries reach that 48 month life span. The environment also plays a part in the life of a battery. If the battery is store in extreme hot or cold conditions, its life span can be cut in half.

The initial cost of a fuel cell can be expensive, around $3000 for of all the equipment needed to generate 1 kilowatt of power. Roughly 333 battery cells would be needed to produce the same amount of power and only cost $0.50 each or $166.50 total. But, over time the fuel cell price averages only about $0.30-0.60 per kilowatt hour. This price range includes fuel, maintenance and any equipment replacement. Since batteries have one time use, the price is $166 per kilowatt hour. (Buchmann, 2003)

A. Design The initial thoughts on the design of a fuel

cell for MAV’s was to use a PEM fuel cell because of the benefits mentioned above as well as their simple design and construction. When it came to creating or storing the fuel the choice was to use a ground-based electrolyzer to split distilled water into hydrogen and oxygen. Knowing this, the final system was determined to include a PEM fuel cell and a ground based electrolyzer.

The first step in the design was to examine the constraints which were given out by the project manager. The three constraints that were to be initially design around were as follows: First, the operating temperature was set to 80C. This temperature is in the middle of the normal temperature operating envelope for a PEM fuel cell. Second, the system weight was to be less than three ounces. In this context the system is referencing the weight of the PEM fuel cell, the weight of the fuel, and the weight of the tanks. The constraint of three ounces came from the DARPA MAV requirement, which states that “a MAV will weigh less than four ounces.” (Ashley, 1998) The weight of the vehicle and the onboard systems were estimated to have a combined weight of one ounce. Third, no dimension

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can exceed four inches. Again, this came form the DARPA requirements that state, “No vehicle dimension can exceed six inches.” (Ashley, 1998) Based on that requirement, four inches was chosen as the upper limit.

The next step was to set the initial parameters for the fuel cell. The values were set to acceptable ranges. First, the active cell area, or the area of the cell that is used (Acell), was set to 50cm2. Second, the reference exchange current density io was set to 0.001 mA/cm2. Third, the internal resistance was set to 0.18Ω/cm2. Finally, the cell voltage was set to 0.75V.

With all of these known values, the process of designing the fuel cell could then begin. The first calculation is the reversible cell potential or ER. According to Li the definition of reversible cell potential is as follows.

“The maximum possible electrical energy output and the corresponding electrical potential difference between the cathode and the anode are achieved when the fuel cell is operating under thermodynamically reversible condition. This maximum possible cell potential is called the reversible cell potential, one of the significantly important parameters for fuel cells.” (Li, 2006)

The calculation of ER is as follows:

( ) ( ) ( ) ( )( )

,80 ,1 25 ,1

80 ,1 1.172

refR R ref

R

s T PE C atm E C atm T T

nFE C atm V

° °

°

Δ= + −

=

(1)

The next calculation was to determine the

current density or i, which simply means current in terms of area. The calculation of i is a follows:

2

0

/15306.1

ln

cmmAi

iRii

FRTEE iR

=

−⎟⎟⎠

⎞⎜⎜⎝

⎛−=α (2)

With these values, a graph showing voltage versus current density with varying temperatures was created.

Voltage vs Current Density

0.68

0.7

0.72

0.74

0.76

0.78

0.8

0.82

0.8 1 1.2 1.4 1.6

Current Density (mA/cm2)

Volta

ge (V

) 65 C

70 C

75 C

80 C

Figure 2. Voltage verses Current Density

Current, or I, is stated as a function of current density and area. Knowing the current density and area, current can be found.

( )( ) 57.653cellI i A mA= = (3) The power output for the cell or Wel is expressed as a function of voltage, current density, and area. This will be found next. ( )( )( ) WAiEW cellel 2398.42== (4) With all of these values two more graphs can be created. The first graph generated is power output versus varying temperature and the second graph is power output versus varying active area.

Power Output with Varying Temperatures

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Temperature (C)

Pow

er (W

atts

)

Figure 3. Power Output with Varying

Temperature

5

Power Output with Varying Active Area

0102030405060708090

0 20 40 60 80 100 120

Area (cm2)

Pow

er (W

atts

)

Figure 4. Power Output with Varying Active Area

B. Fuel Production Since a Micro Air Vehicle is intended for

use by the military, many questions arose about the transportation of the fuel needed to power the cell. The initial thought was to just have multiple compressed hydrogen tanks on hand to refuel the system. The questions of safety lead to the research of other ideas. An electrolyzer “breaks water into hydrogen and oxygen.” (Spiegel, 2007) This is a great option to look at because it eliminates extra compressed hydrogen tanks. Also when using an electrolyzer “many of the typical problems of the PEM fuel cell” are no longer applicable. (Spiegel, 2007) Water management issues are simplified because the positive electrode must be flooded with water. The water supplied to the cathode can be easily used to cool the cell, eliminating thermal management problems. Electrolyzers are highly recommended for portable use since one can “plug-in an electrolyzer to generate a sufficient amount of hydrogen used on a trip.” (Spiegel, 2007) The hydrogen that is produced by the electrolyzer is “very pure, it does not have to be stored and it is much cheaper then gas supplied in high-pressure cylinders.” (Spiegel, 2007)

When breaking apart the water into hydrogen and oxygen gas, the gases can be directly released into high pressure cylindrical fuel containers. The work done during this process can be show by:

2

1

*ln 140.45PW nRT kJP

⎛ ⎞Δ = =⎜ ⎟

⎝ ⎠ (5)

When the hydrogen is pressurized there is a

voltage rise that occurs. This is described by:

2

1

ln 0.3639012

PRTV VF P

Δ = = (6)

The total heat that is produce when using an

electrolyzer is:

KJVWQ 948.385=ΔΔ

= (7)

C. Component Design and Size Fuel consumption into the active area is

based on Faraday’ Law. Knowing that faraday’s constant is 96485 C/mol and that the current is in amps, the consumption rate can be determined for H2 and O2.

2

72.98767 10 / sec2HIN molsF

−= = × (8)

2

71.49383 10 / sec4OIN molsF

−= = × (9)

The rate at which water is produced in the

system can also be found. Knowing this information will aid in determining whether or not the system is at risk for flooding.

2

72.98767 10 / sec2H OIN molsF

−= = × (10)

The mass flow rates of the reactants in

consumption are found by knowing the current, faraday’s constant and the molecular weight of hydrogen, oxygen and water. These reactants “may be supplied in excess of consumption.” (Spiegel, 2007)

2 2

75.97533 10 / sec2

H HIm M gF

•−= = × (11)

2 2

64.78026 10 / sec4

O OIm M gF

•−= = × (12)

2 2

51.07559 10 / sec2

H O H OIm M gF

•−= = × (13)

“The stoichiometric ratio is the rate of actual

flow rate of the reactant at the inlet divided by the consumption rate of the reactant.” (Li, 2006) The stoichiometric ratio is different for hydrogen and oxygen due to the fact that the cathode reaction is much slower then the anode reaction. Values for hydrogen range between 1.1-1.2, and for oxygen 1.2-1.5. As seen by these stoichiometric values the ratios

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for oxygen are high in order to make up for the slower reaction rate. By knowing the range of the stoichiometric ratio, fuel consumption rates versus endurance can be graphed for a low to high stoichiometric ratio as seen below. (Note that the operating point of the fuel cell being designed is shown in bold on each graph)

act act

consumedconsumed

N mSN m

•

•= = (14)

How Area Effects Fuel Consumption(H2 Minimum Consumption Ratio)

0.001.002.003.004.005.006.007.008.009.00

10.00

0 50 100 150

Endurance (mins)

H2

(mili

gram

s)

20 cm2 Area

30 cm2 Area

40 cm2 Area

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 5. H2 Consumption versus Endurance for Minimum Consumption Ratio

How Area Effects Fuel Consumption(H2 Max Consumption Ratio)

0.00

2.00

4.00

6.00

8.00

10.00

0 50 100 150

Endurance (mins)

H2

(mili

gram

s)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 5. H2 Consumption versus Endurance for Maximum Consumption Ratio

How Area Effects Oxidant Consumption(O2 Min Consumption Ratio)

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

0 50 100 150

Endurance (mins)

Oxi

dant

(milig

ram

s)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 6. O2 Consumption versus Endurance for Minimum Consumption Ratio

How Area Effects Oxidant Consumption(O2 Max Consumption Ratio)

0.00

50.00

100.00

150.00

200.00

0 50 100 150

Endurance (mins)

Oxi

dant

(m

iligra

ms)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 7. O2 Consumption versus Endurance for Minimum Consumption Ratio

The previous graphs can give a designer a

good idea on how much fuel will be consumed based off the target endurance, the stoichiometric ratio that the fuel cell is operating at and the active area designed for the fuel cell.

Nact is the theoretical amount of fuel the cell will need to operate and Nconsumed is the amount of fuel that is consumed. Nconsumed was previously calculated, in this documentation, as NH2. The theoretical amount needed for the designed fuel cell is listed below assuming a minimal consumption ratio for both H2 and O2, the values being 1.1 and 1.2. From the stoichiometric ratio equation, the actual consumption rate, Nact, is found for O2 and H2.

72 10573094.6 −

•

•

×=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

consumed

actconsumedactH

m

mNN (15)

7

602 10736323.5 −

•

•

×=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛=

consumed

actconsumedact

m

mNN (16)

The fuel utilization efficiency is found

through the stoichiometric ratio of the cell. For the operating conditions a ratio of 1.1 is used, meaning the efficiency is around 91% for H2 and 83% for O2.

1 90.9%fuel utilized S

η = = (17)

This value varies with different

stoichiometric values for both hydrogen and oxygen. The next two graphs show exactly how they vary.

Fuel Utilization Efficiency for Varying S RatioH2

0.82

0.84

0.86

0.88

0.9

0.92

1.05 1.1 1.15 1.2 1.25

Stoichiometric Ratio

Effi

cien

cy

Figure 8. Fuel Utilization Efficiency with Varying Stoichiometric Ratios for H2

Fuel Utilization Efficiency for Varying S RatioO2

0.6

0.65

0.7

0.75

0.8

0.85

1.1 1.2 1.3 1.4 1.5 1.6Stoichiometric Ratio

Effic

ienc

y

Figure 9. Oxidant Utilization Efficiency with Varying Stoichiometric Ratios for O2

The fuel cell will be composed of four main

components: endplates, electrode backings, Nafion membrane and platinum catalyst. The endplates will be constructed of graphite and will also serve as the current collectors and gas distributors. The electrode

backing layers will be constructed of Teflon and will serve as seals to prevent gas from crossing over. The membrane will be Nafion 115, a common PEM membrane created by DuPont in the late 1960’s. The catalyst will be 80% Platinum on Vulcan XC-72 1g. A readily available catalyst component comprised of Pt on a carbon filter paper.

In order to find the overall weight of the fuel the volume of the endplates must first be calculated.

3* * 5EndPlateV Height Width Thickness cm= = (18)

Once the volume is calculated the weight of the endplates can be determined based of the density of graphite. After researching the weight of typical backings and Nafion membranes, an overall weight is found by:

45.15 1.592Fuel Cell Plates Membrane Teflon Platinum

Fuel Cell

W W W W W

W g oz

= + + +

= = (19)

With the weight of the fuel cell calculated, the design process can now move on to calculating the weight of the fuel tanks. Before this can be done a parameter for flight time must be determined so the amount of fuel can be calculated. A 45 minute (2700 second) flight would be adequate for a vehicle of this size. The following equations show the amount of H2 and O2 that would be needed for a 2700 second flight. ( )2

52 0.001613 5.689 10

NEEDEDHH m T g oz

•−⎛ ⎞= = = ×⎜ ⎟

⎝ ⎠ (20)

( )2

42 0.012908 4.5531 10

NEEDEDoO m T g oz

•−⎛ ⎞= = = ×⎜ ⎟

⎝ ⎠ (21)

With this information multiple graphs can be

created to show the size of the tanks versus endurance for the minimum and maximum hydrogen and oxygen consumptions.

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How Area Effects Fuel Consumption(H2 Minimum Consumption Ratio)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0 50 100

Endurance (mins)

H2

(milig

ram

s)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 10. Tank size versus endurance for H2

minimum consumption

How Area Effects Oxidant Consumption(O2 Min Consumption Ratio)

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

160.00

0 50 100

Endurance (mins)

Oxi

dant

(milig

ram

s)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 11. Tank size versus endurance for O2

minimum consumption

How Area Effects Fuel Consumption(H2 Max Consumption Ratio)

0.00

2.00

4.00

6.00

8.00

10.00

0 50 100

Endurance (mins)

H2

(mili

gram

s)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 12. Tank size versus endurance for H2

maximum consumption

How Area Effects Oxidant Consumption(O2 Max Consumption Ratio)

0.00

50.00

100.00

150.00

200.00

0 50 100

Endurance (mins)

Oxi

dant

(m

iligr

ams)

20 cm2 Area

30 cm2

40 cm2

50 cm2

60 cm2

70 cm2

80 cm2

90 cm2

100 cm2

Figure 13. Tank size versus endurance for O2

maximum consumption

The byproduct of this fuel cell is water vapor. During operation, water vapor is continuously being produced. In this design the vapor will be used to cool the cell will then be wicked off the cell into the air during flight. Due to this fact the weight of the water created is not a factor in the considering the total weight of the cell. For reference the calculation for the amount of water vapor produced is as follows:

( )2

32 0.043562 1.5366 10

GENERATEDH OH O m T g oz

•−⎛ ⎞= = = ×⎜ ⎟

⎝ ⎠ (22)

9

To establish a volume for the fuel tanks of O2 and H2, basic chemistry equations apply:

2 2

2 2

2

12.016 0.001316

0.0008

mol H x mol Hg H g H

x mol H

=

= (23)

( )( )2

22.4 0.017992HV x L= = (24)

2 2

2 2

2

132.0 0.012908

0.000403

mol O x mol Og O g O

x mol O

=

= (25)

( )( )2

22.4 0.009036OV x L= = (26) For the ease of production and calculations a cylindrically shaped tank will be used. Based off the previous calculations of H2 and O2 volume, the largest volume (H2) will be used to calculate the size of the tanks. The size needed to store the gas at atmospheric pressure is as followed:

( ) ( )( )2

8 , 26.70 , 53.40

V r h

h cm r cm d cm

π=

= = =

(27)

As can be seen above storing the gas at

atmospheric pressure will not allow the initial constraints to be achieved. The other option is to store the gas at a higher pressure. Typically 3000psi is considered a normal pressure vessel where 6000 psi is considered a high pressure vessel. After the electrolyzer separates the gases the tanks will then be pressurized in order to meet the constraints of a Micro Air Vehicle. The first step was to choose a pressure for the tanks. A pressure of 3000psi was chosen because the weight of a normal pressure tank would be less than that of a high pressure tank. The relation used to compress gas is Boyle’s Law. The volume of the gas in a compressed tank can be expressed as follows:

0.000088atmospheric atmospheric compressed compressed

compressed

P V P V

V L

=

= (28)

Knowing that the volume of the compressed gas is 0.000088L the new tank size can be determined. As seen below the dimensions of each

tank are within the acceptable range for the given constraints. ( )( )( )2

8 , 1 .87 , 3 .74

V r h

h cm r cm d cm

π=

= = = (29)

Knowing this the weight of the tanks can be

determined; the tanks would be constructed out of a mandrel wound carbon fiber epoxy with a combined density of 2.995 g/cm3 the calculations for the weight of the tanks are as follows. (Note the weight shown below is the weight of each tank.)

[ ] /* *

35.37 1.559Tank carbon epoxy

Tank

W Height Width Thickness

W grams oz

ρ=

= =(30)

With all of the information that has been

computed the weight of the fuel cell system (fuel cell and tanks) can be determined. ( )( )( )2 4.71system Fuel Cell TanksW W W oz= + = (31)

What this shows is that the system is

overweight by 1.71 ounces; the next section will discuss options to reduce the weight to fit within the constraints.

The first most obvious solution would be to reduce the endurance or flight time of the vehicle. (Note that the fuel tanks account for 66% of the current systems weight.) This would reduce the amount of fuel needed and thus the weight of the system. A second option would be to explore compressing the gases to a higher pressure such as 6000 psi. This would allow for smaller tank sizes, without taking the penalty in endurance. Yet another option includes scaling down the fuel cell. The power requirements call for roughly 10 Watts of power, if a linear relation existed the entire fuel cell could be scaled down to a quarter of the size. Even more solutions such as changing the shape or material the tanks are constructed should be looked into before production.

To calculate the overall efficiency of the fuel cell the product that is produce needs to be determined. If the product is liquid then the fuel cell is operating at high heating values, or HHV. If the product is water vapor then the fuel cell is operating at low heating values, or LHV. The efficiency equation is defined as:

cellE

LHVη =

(32)

10

Where E is the fuel cell’s operating voltage and LHV is the low heating value, 1.25V. The design fuel cell has water vapor as a product therefore the low heating value is used. If the fuel cell were operating at high heating values then the value in the denominator would be 1.48V.

60.0%

cell

cell

ELHV

η

η

=

= (33)

The overall efficiency of the fuel cell is

60%.Because the maximum efficiency of a fuel cell is thermodynamically limited to 83%. The calculated efficiency of 60% seems acceptable.

Another key requirement in MAV construction is range or the distance that the vehicle will be able to travel. All initial range calculations are based off of a propeller powered vehicle. AeroViromnent has already proven that propeller efficiencies of up to 80% can be achieved on a micro scale. For calculations, a chosen propeller efficiency of 70% was used. Seen below is the range equation used to compute the range for a propeller powered aircraft.

0

1

lnpr WLRc D Wη

= (34)

In the above equation ηpr is the propeller

efficiency, c is the specific fuel consumption, W0 is the initial weight of the vehicle and W1 is the final weight of the vehicle. L/D is the lift to drag ratio the ratio would be determined from experimental wind tunnel test that could be run on a scale model. For a since of scale a lift to drag ratio of 4 represents that of a house sparrow while a lift to drag ratio of 60 represents that of a modern glider. Most current MAV’s have a lift to drag ratio of around 1.2 to 1.5. The graph that has been created shows an expected range based of the lift to drag ratio.

Range vs. L/D

0

5

10

15

20

0 50000 100000 150000

Range (feet)

Lift

to D

rag

Rat

io

Figure 14. Range versus Lift to Drag

C. Cost Analysis According to DARPA a MAV should cost under $1000 to be disposable the following section will examine the cost of ‘off the shelf components’ which could be used to construct the fuel cell and system. Keep in mind that if this system went into mass production the cost of each system would drop by a sizeable amount. Prices below were taken from www.fuelcellstore.com

A. Fuel Cell Costs Graphite Plates - $69.90 Teflon Masks - $15.00 Nafion 115 Membrane - $169.10 Platinum Catalyst - $112.12 Screws, Tubing, and Accessories - $15.00 Total Fuel Cell Cost - $381.12

The cost above represents the cost of just the fuel cell. Estimates were taken at the cost of the components that would be needed to create the system and can be seen below in the system total. The cost of the system is still well below the allotted $1000 this would allow nearly $300 for the vehicle, onboard systems and the propulsion system.

B. System Costs Fuel Cell - $381.12 Tanks - $200.00 Ground Based Electrolyzer - $100.00 Tubing, Screws, and Components - $25.00 System Total - $706.12

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IV. Conclusion

A complete design of a PEM fuel cell using hydrogen as its fuel has been accomplished. It has been established that a fuel cell is not only attainable at an MAV level, but it is more efficient than the current system of using a battery. The revolutionary concept of using an electrolyzer to separate the hydrogen and oxygen required to run the fuel cell will quite many skeptics on the issues of hydrogen transportation. The fuel cell was able to produce over 42 watts, more then what is required to power the Micro Air Vehicle, and still be within a the weight restrictions. The fuel tanks designed for the cell is able to provide flight time of 45 minutes. This allows the military time to perform most reconnaissance missions. Most importantly the design is able to be altered to fit the needs of any size vehicle or any style of mission. There is now a better alternative to using a battery that only requires the transportation of water. The design that was been completed shows the advancements available to the military for the MAV field. Using the graphs and relationships provided, it is now possible, knowing the power and weight restrictions, to design a fuel cell and tank size to provide the largest endurance. The MAV community will gain a large contribution from the introduction of fuel cells as the power source.

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Reference

Anderson, John D. Aircraft Performance and Design. New York: McGraw Hill, 1999.

Ashley, Steven. "Palm-Size Spy Plane." The American Society of Mechanical Engineers. 1998.

Buchmann, Isidor. "The Cost of Battery Power." Battery University. 2003. 02 Apr. 2008.

Grasmeyer, Joel M., and Matthew T. Keennon. "Development of the Black Widow Micro Air

Vehicle." AIAA 0127 (2001).

Li, Xianguo. Princioles of Fuel Cells. New York: Taylor & Francis, 2006.

"Micro Air Vehicle." Honeywell. 15 Apr. 2008.

Spiegel, Colleen S. Designing and Building Fuel Cells. New York: McGraw Hill, 2007.

"Unmanned Aircraft Systems Roadmap 2005-2030." Office of the Secretary of Defense. 2005.