Mench M.M. Fuel Cell Engines

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420 Other Fuel Cells

3 MWe direct carbonate fuel cell on LEG.

PROBLEMS

Solid Oxide Fuel Cell

7.1 Describe some markets where the SOFC is appropri-

ate and discuss the potential advantages of this technology

compared to the competing technologies. What are the hur-

dles that still must be overcome before the SOFC can enter

this market?

7.2 Describe the advantages and disadvantages of the

solid-phase electrolyte used in the SOFC.

7.3 There is great potential for increasing overall system

efﬁciency of the SOFC by use of cogeneration or a bot-

toming cycle to utilize the exhaust gas and waste heat.

If the high-temperature hydrogen-rich exhaust gas from

the SOFC anode were burned in a combustor and used to

power a steam turbine, additional electricity could be gen-

erated. Write an expression for the maximum theoretical

cogeneration efﬁciency of a fuel cell–heat engine combina-

tion. For maximum system efﬁciency, what conditions are

optimal?

7.4 Compare the oxygen diffusivity in air at 353 and 1273

K. How dilute would an oxygen stream at 1273 K have to

be to have the same diffusion ﬂux as one at 373 K? Assume

the 373 K stream has a mole fraction of oxygen of 0.21 and

diffusion is to the cathode at limiting current conditions.

What would the mole fraction of the equivalent stream at

373 K have to be?

7.5 What are the practical consequences of a leaky (e.g.,

hydrogen leaks) SOFC fuel cell stack? Think in terms of

performance, efﬁciency, and safety. How could you resolve

the leaky SOFC stack issue if the high-temperature seals in

a planar stack cannot be made to perfectly seal the system.

Molten Carbonate Fuel Cell

7.6 Describe some markets where the MCFC is appropri-

ate and discuss the potential advantages of this technology

compared to the competing technologies. What are the hur-

dles that still must be overcome before the MCFC can enter

this market?

7.7 Use the MCFC model of Eqs. (7.5)–(7.8) to investigate

the performance of a MCFC as a function of temperature.

At 600, 650, and 700

◦

C, what fraction of the polarization

comes from ohmic, anodic, and cathodic activation losses.

Which loss is most sensitive to temperature?

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References 421

7.8 Use the MCFC model of Eqs. (7.5)–(7.8) to investigate

the performance of a MCFC as a function of gas mole frac-

tion. At 650

◦

C, what is the effect of doubling the oxygen

partial pressure on the anode? What is the effect of doubling

the hydrogen partial pressure on the anode? What role does

play on the anode and cathode?

7.9 The Nernst equation for the MCFC shows that the ra-

tio of cathode to anode CO

partial pressure can have an

effect on voltage. Given that you cannot have a differential

pressure between the anode and cathode, what would be the

effect of doubling the CO

mole fraction in the cathode? Is

there a drawback to this. (Hint: What happens to the oxygen

mole fraction?)

7.10 What is the Nernst Open-Circuit Voltage for a MCFC

operating at 650

◦

C with equal mole fraction of CO

on the

anode and cathode and dry air and hydrogen at 1 atm are

used as the reactants.

Phosphoric Acid Fuel Cell

7.11 Describe some markets where the PAFC is appropri-

ate and discuss the potential advantages of this technology

compared to the competing technologies. What are the hur-

dles that still must be overcome before the PAFC can enter

this market?

7.12 Given a 0.5-m

-active-area PAFC 250-cell stack op-

erating at 150 mA/cm

, a cathode stoichiometry of 2.0, and

an anode stoichiometry of 1.3, (a) plot the amount of elec-

trolyte lost to the anode and cathode ﬂow over 40,000 h

operation as a function of H

ppm vapor pressure and

(b) use another resource to ﬁnd the vapor pressure of H

as a function of temperature from 160 to 200

◦

C and discuss

the difference in onboard stored H

that can be achieved

from reducing operating temperature from 200 to 175

◦

7.13 Methanol can be steam reformed at a relatively low

temperature of reformation of 200

◦

C. Can methanol be in-

ternally reformed in the PAFC? What would have to be

done to the anode ﬂow to allow this? Discuss the potential

problems with this approach.

7.14 The SiC matrix that holds the PAFC electrolyte has

very small, uniform pores. Describe why the pore size is

smallest in the matrix compared to the porous ﬂow ﬁeld or

catalyst layer, and what is the consequence of a nonunifor-

mity in the SiC particle size?

7.15 In all electrolytes, there is some solubility and diffu-

sion of reactant gas into the material that results in crossover

losses. Is some limited crossover due to this effect as major

an issue for the PAFC as the PEFC? Why or why not?

7.16 What is the difference in the Nernst Open-Circuit

Voltage for a PEFC operating at 80

◦

C, a PAFC operating

at 200

◦

C, and a SOFC operating at 1000

◦

C. Assume all

systems operate on air and hydrogen at 1 atm. How do the

high-temperature systems make up for this difference?

Alkaline Fuel Cell

7.17 Describe some markets where the AFC is appropri-

ate and discuss the potential advantages of this technology

compared to the competing technologies. What are the hur-

dles that still must be overcome before the AFC can enter

this market?

7.18 Do some reading and discuss the major approaches

available for CO

scrubbing from atmospheric air. Is the

major impediment to application of this to AFCs efﬁciency

of conversion, cost of the approach, bulk, or something else?

7.19 If a given AFC operates at 0.8V, 1 A/cm

, compared

to the PEFC, at 1 A/cm

, 0.65 V, what is the savings in

terms of waste energy dissipated as heat for a 100-kWe

AFC system compared to a PEFC system.

Biological and Other Fuel Cells

7.20 Describe some markets where a microbial fuel cell

is appropriate and discuss the potential advantages of this

technology compared to the competing technologies. What

are the hurdles that still must be overcome before this tech-

nology can enter this market?

7.21 Write a one-page summary of microbial fuel cells.

Describe the anodic and cathodic reactions and electrolyte

used.

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Cells—Fundamentals, Technology and Applications, Vol. 1, W. Vielstich, A. Lamm, and H.

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Lamm, and H. A. Gasteiger, Eds., Wiley, New York, 2003, pp. 832–843.

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40. D. A. Landsman and F. J. Luczak, “Catalyst Studies and Coating Technologies,” in Handbook

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H. A. Gasteiger, Eds., Wiley, New York, 2003, pp. 811–831.

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44. M. Neergat and A. K. Shukla, “A High-Performance Phosphoric Acid Fuel Cell,” J. P ower

Sources, Vol. 102, No. 1/2, pp. 317–321, 2001.

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Hydrogen Storage, Generation,

and Delivery

People say that hydrogen cars would be pollution-free. Lightbulbs

are pollution-free, but power plants are not.

—University of Calgary engineering professor David Keith

The politics of the so-called hydrogen economy are fascinating, and it will be interesting

to see how the future of energy generation, storage, and delivery develops. Some people

see the hydrogen economy as an economic or practical impossibility and instead advocate

an electron-based economy [1], where electrons are transmitted from distributed power

plants to generate hydrogen or recharge battery devices locally, eliminating some of the

economic and engineering difﬁculties inherent with hydrogen. Others advocate the use of

clean-burning coal or natural gas, energy sources that are available in the near term in

sufﬁcient quantities to help offset crude-oil usage. The purpose of this chapter is not to

debate the particular economic or political merits or limitations of the myriad possibilities

but to present the reader with a basic background on the methods of hydrogen storage,

generation, and delivery. Regarding future energy supply, this much is clear; over time,

worldwide fossil fuel reserves will eventually peak, causing oil prices to rise to the point

where other energy sources become economically viable. This, coupled with environmental

and world-security concerns, will eventually force a change in the way energy is stored,

delivered, and generated. It is likely that hydrogen derived from noncarbon or carbon-neutral

sources will eventually play an important role in this future society.

8.1 MODES OF STORAGE

In fuel cell applications where the power source is mobile, such as automotive or portable

usage, a hydrogen carrying fuel reservoir is needed. For stationary applications, where the

fuel cell can be attached to a natural gas or hydrogen pipeline, storage is not as critical.

Consider a portable electronic application. Even if the fuel cell is optimized to the point

where the stack itself is quite small, there is a need for a highly dense fuel storage reservoir

or the total system will be unacceptably large. Similarly, in automotive applications, there

needs to be a hydrogen fuel tank capable of achieving approximately the same total driving

range as the gasoline tank it replaces with at least comparable volumetric and gravimetric

426

Fuel Cell Engines

Matthew M. Mench

c08 JWPR067-Mench December 15, 2007 21:19 Char Count=

8.1 Modes of Storage 427

Table 8.1 Selected Thermodynamic Properties of Fuels

Property/Fuel Hydrogen H

Gasoline, C

∗

Ethanol C

OH Methanol CH

Boiling Point (K) 20 398 352 338

Heat of combustion

(MJ/kg) based on

LHV

120.97 44.43 28.87 19.94

Density (at STP)

(kg/m

)

0.09 703 789 792

Flammability limits

in air (%)

4–75 1–6.5 3.3–19 6–36

∗

Note that gasoline is actually a very complex blends of fuels, detergents, and other agents that vary among

region, season, and manufacturer. It is common, however, to estimate the thermodynamic properties of gasoline

with octane.

energy densities. This turns out to be a difﬁcult challenge. Gasoline is a high-energy-

content, high-density liquid that can be stored at atmospheric pressure. Hydrogen also has

a molar high-energy content but a very low density as a gas phase. Some of the properties

of hydrogen compared to gasoline and some other alcohol fuels are shown in Table 8.1 for

comparative purposes. The U.S. Department of Energy (DOE) goals for hydrogen storage

are as follows [2]:

Ĺ By 2010, develop and verify on-board hydrogen storage systems achieving 2 kWh/kg

(6 wt. %), 1.5 kWh/L, and $4/kWh.

Ĺ By 2015, develop and verify on-board hydrogen storage systems achieving 3 kWh/kg

(9 wt. %), 2.7 kWh/L, and $2/kWh.

Example 8.1 How Much Hydrogen Is Needed? Consider that a gasoline tank on a

common automobile is about 15 gal. Estimating the gasoline as octane (C

), determine

approximately how many kilograms of hydrogen are needed to replace the gasoline in the

conventional automobile on an energy basis. You can consider the hydrogen fuel cell to be

approximately twice as efﬁcient at energy conversion compared to the gasoline engine.

SOLUTION We ﬁrst convert the 15 gal of gasoline into kilograms and look up the values

of octane density from online sources or reference books:

(15 gal)(0.003785411784 m

/gal)(917 kg/m

) = 52 kg

The heat of combustion of octane can be solved from the balanced chemical reaction, found

in thermodynamic reference books, or from an energy balance of the chemical reaction:

+ 12.5

(

+ 3.76N

)

−→ 8CO

+ 9H

O + 47N







−







=−5512 kJ/mol

This value is negative because it is an exothermic reaction. This is equivalent to 48.3 MJ/kg

(MW

octane

= 114 kg/kmol). Therefore, the typical gasoline tank contains

(52 kg) (48.3MJ/kg) = 2.5135 × 10

c08 JWPR067-Mench December 15, 2007 21:19 Char Count=

428 Hydrogen Storage, Generation, and Delivery

Assuming the average fuel cell efﬁciency is about double that of gasoline, we only need

about half of the initial chemical energy, or 1.25 × 10

MJ. The heat of combustion of

hydrogen is 120.97 MJ/kg. Therefore, we would need

1.257 × 10

120.97 MJ/kg

= 10.4kgH

This seems like a very favorable comparison to gasoline, in that the mass of gasoline was

52 kg. However, when you consider the density difference, the hydrogen storage problem

is obvious. At room temperature and pressure, the density of hydrogen is 0.08988 kg/m

over 10,000 times less than gasoline.

COMMENTS: Assuming a typical driving range of 350 miles on a tank of gasoline, a

15-gal hydrogen tank at room temperature and pressure would only have a driving range

of about 0.05 miles! Obviously, the challenge is ﬁnding a way to store enough hydrogen so

the tank is not prohibitively large or heavy.

From Example 8.1, we learned that, depending on the fuel cell efﬁciency, about 5–10 kg

of hydrogen storage is needed to replace gasoline in conventional automotive applications.

A similar calculation could be made for portable electronic applications. The required

hydrogen storage can be technically achieved by several methods:

1. Compressed gas

2. Cryogenic liquid

3. Hydride storage

4. Carbon storage

5. Liquid fuel storage

The advantages and difﬁculties of each approach will be discussed in the following sections.

Compressed Gas Compressed hydrogen is perhaps the most well-developed approach

to providing hydrogen storage. Pressurized gas storage vessels are common in everyday

life (e.g., medical oxygen, helium balloons, welding equipment). The storage pressures

of these vessels are normally below 17 MPa. While the technology and safety for these

storage vessels are well developed, vehicular applications require much higher gas pressures

up to 71 MPa (700 bars, or ∼10,000 psig), while maintaining a low storage vessel mass

and high inherent safety.

With a 71-MPa storage vessel, 1.3 kWh/kg mass storage density and 0.8 kWh/L

volumetric storage density have been achieved [3]. This compares to 2010 DOE goals of

2.0 kWh/kg and 1.5 kWh/L, respectively More advances can be made, although relative to

other competing technologies, compressed storage is fairly well developed. Figure 8.1 is a

picture of a wound-ﬁber pressure vessel developed for this purpose. Modern high-pressure

gas-phase storage vessels are constructed of plastic or thin metallic inner liners wrapped

in high-strength load-bearing woven ﬁber embedded in a composite matrix material [4].

These so-called composite vessels are signiﬁcantly lighter and more robust than their fully

metallic predecesors.

The desired proﬁle of a gasoline tank in an automotive application is typically ﬂattened,

and although a purely cylindrical proﬁle is ideal to hold pressurized gas, the shape is

c08 JWPR067-Mench December 15, 2007 21:19 Char Count=

8.1 Modes of Storage 429

Figure 8.1 Photograph of 10,000-psig hydrogen storage tank. (Reproduced with permission from

Ref. [4].)

inefﬁcient to ﬁt in a normal automobile. For this reason, noncylindrical and conformed

shape vessels are also desirable. Some relevant design data for composite high-pressure

hydrogen storage vessels are given in Table 8.2. It can be seen that a 1-m-long, 30-cm-

diameter vessel at 700 bars (∼71 MPa) pressure stores only 2 kg of hydrogen, so that three

to four of these would be needed in a typical automobile to achieve a normal cruising range.

At such high pressures, non–ideal gas effects are present, as discussed in Chapter 3. Thus,

the scaling relationships between storage pressures are nonlinear, and the weight speciﬁc

energy storage of the highest pressure vessels actually decreases.

Another major drawback of the high-pressure storage concept is the work required to

compress the gas and time to deliver the compressed gas into the fuel tank. The minimum

thermodynamic work of compression of gas for an internally reversible compressor (gas)

or pump (liquid) per unit mass is [5]

int.rev.

=−



(8.1)

Table 8.2 Selected Properties of Composite Compressed Hydrogen Vessels of 200, 400, and 700

bars Pressure

Property 200-Bar Vessel 400-Bar Vessel 700-Bar Vessel

Internal volume (L) 50 50 50

Vessel diameter (m) 0.3 0.3 0.3

Vessel length (m) 1.0 1.0 1.0

Vessel weight (kg) 25 45 85

Stored energy (kWh) 24 43 66

Stored H

(kg) 0.7 1.3 2.0

Vessel-to-hydrogen weight ratio 35.7 34.7 42.5

Weight speciﬁc energy storage (kWh/kg) 0.96 0.96 0.78

Volume speciﬁc storage (kWh/L) 0.48 0.86 1.32

Source: Adapted from [4].