Mench M.M. Fuel Cell Engines

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440 Hydrogen Storage, Generation, and Delivery

CO tolerance is one of the primary reasons that higher temperature membranes are sought

for PEFCs. For high-temperature molten carbonate and solid oxide fuel cells, no gas cleanup

is necessary since the CO can be used as a fuel. Alkaline fuel cells are not suitable for

reformed gas since they are intolerant to CO

Despite the environmental drawbacks, the reformation process is generally more cost

effective and compact than other hydrogen generation technologies. Electrolysis is inef-

ﬁcient and consumes excess electrical and freshwater resources. Considering the world

also faces shortages of electrical capacity and freshwater, rapid large-scale conversion to

electrolysis for hydrogen generation is unlikely. Biological production requires very large

hydrogen generation facilities, additional development of the technology, and a biological

waste transportation infrastructure. Therefore, in the near term, fuel reformation remains the

most likely source for large-scale hydrogen generation. Another advantage of reformation

is that the fuel can be transported to the generation location as a liquid, solid, or gas, so that

existing and cost-effective delivery infrastructure can be used. For example, in locations

where natural gas is plentiful and a distribution infrastructure exits, direct reformation of

natural gas can be used to generate hydrogen locally, avoiding the need for a separate

hydrogen infrastructure. There are also new technologies for the generation of liquid fuels

from reformed gas, or even the direct generation of hydrogen from coal resources, which

are plentiful in many parts of the world. There are also many sources of liquid fuel that are

generated from a variety of processes. For example, methanol can be made from nearly any

carbon-containing fuel stock or from biological processes. The methanol is then relatively

easily reformed to hydrogen. There are many different approaches and technologies for

fuel reformation. The three most widely utilized methods are:

1. Steam reformation

2. Partial oxidation

3. Autothermal reformation

Steam Reformation In steam reformation, water and fuel are mixed over a catalyzed bed

to produce hydrogen, carbon dioxide, and other minor species in an endothermic reaction.

Compared to partial oxidation or autothermal reformation, steam reformation produces the

highest hydrogen mole fraction efﬂuent stream, approaching 80% H

on a dry basis:

(

)

O + heat → xCO



2x +



(8.10)

Minor species, such as CO, result from subsequent dissociation reactions governed by

thermodynamics. Of the various fuels involved, methanol is the simplest liquid-based

fuel to reform due to the lack of carbon–carbon bond and low reformation temperature

of only around 200

◦

C [21]. This temperature can easily be coupled to low-temperature

systems. The PAFC is almost perfectly thermally matched to the methanol reformation

temperature, so that waste heat from the fuel cell can be used to drive the process. For PEFCs,

some additional input heat is needed for methanol and other fuel reformation. For high-

temperature fuel cells, methanol (and other fuel) reformation can be accomplished internally

or externally, as discussed in Chapter 7. The reformation temperature of other fuels can be

several hundreds of degrees higher than that of methanol, depending on the complexity and

strength of the chemical bonds as well as the catalyst used to promote the reaction.

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8.2 Modes of Generation 441

The relative ease of methanol reformation and difﬁculty of hydrogen storage and trans-

port have led to substantial development of on-board reformation systems for automotive

applications. However, interest has waned due to the need for rapid transient response

to load demands, CO poising, and the added bulk and cost of equipment needed. Essen-

tially, the automotive fuel cell is already difﬁcult enough to achieve without the reformer

sub system. In portable applications, research has been conducted on ultrasmall methanol

reformers for applications that have shown greater energy density than the lithium ion bat-

teries they would replace [e.g., 22]. Since liquid methanol is ultimately more transportable

than hydrogen, this would be useful for military and other portable applications as long as

the fuel cell size can be reduced to make up for the added bulk of the reformer.

Partial Oxidation Partial oxidation is an exothermic reformation method where air (or

oxygen) is mixed with the fuel stream and the fuel is partially oxidized to produce a

hydrogen-rich stream:

(

+ 3.76N

)

→ xCO +

y H

(

1.88x

)

+ heat (8.11)

The CO fraction from this reaction is obviously much higher than catalyzed steam reforma-

tion. Subsequent or simultaneous reaction in a water gas shift reactor can be used to convert

the CO into CO

. Because this reaction is exothermic, the reaction temperatures are much

higher (>1000

◦

C) and the use of a catalyzed bed is not typically needed. The exothermic

heat release for this reaction can be used for other purposes, such as preheating a cold fuel

cell or providing space heating in residential units. Although the lack of a catalyst bed and

exothermicity of the reaction is an advantage, the use of air introduces nitrogen to the fuel

stream and reduces the ultimate hydrogen fraction available from the process.

Autothemal Reformation The exothermic nature of the partial oxidation reaction, com-

bined with the higher hydrogen mole fraction of the endothermic steam reformation

process, can be combined in an energy neutral conﬁguration, known as an autothermal

reformer (ATR). Typically, a partial oxidation reformer is used to generate heat and a

high-temperature hydrogen, CO, and nitrogen stream (if air and not pure oxygen is used).

Subsequently, the remaining fuel stream enters a catalyzed steam reformer/water gas shift

reactor. The partial oxidation reaction is controlled to release the heat needed to drive the

steam reformer, so that there is no net heat input to the system required. The hydrogen

mole fraction produced by ATR lies somewhere in between the partial oxidation and steam

reformation approaches at around 40–50%.

8.2.2 Electrochemical Hydrogen Production

Electrolysis of water has been used for over a century to produce hydrogen and oxy-

gen. The concept of a reversible fuel cell for space applications is based on a combined

electrolytic–galvanic fuel cell. Like fuel cells, several different types of electrolyzers and

catalysts have been developed, including atmospheric and high-pressure acid-based poly-

mer electrolyzers, alkaline solution systems, and solid oxide–based systems.

A major advantage of electrolysis is that hydrogen generation can be carbon-free,

provided the source of the required electricity is from nuclear or renewable sources, such

as hydraulic, solar, or wind power. Another advantage of electrolysis is that the hydrogen

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442 Hydrogen Storage, Generation, and Delivery

and oxygen produced are extremely high purity, which is especially advantageous for

low-temperature applications where minute impurities can drastically reduce performance.

Although discussed here as a hydrogen generation device, electrolysis also produces high-

purity oxygen, which could be used in a fuel cell or as a valuable product for other

applications.

Another advantage of liquid water electrolysis is that high product pressure up to

20 MPa is possible without mechanical compression, which greatly reduces the energy re-

quired if the produced hydrogen is used to ﬁll a compressed gas tank or charge a pressurized

carbon or metal hydride fuel tank.

Electrolysis is an established method for small-scale local generation of high-purity

hydrogen and oxygen. Small, local electrolysis devices could be used for portable generation

capabilities to circumvent hydrogen infrastracture limitations. For large-scale generation,

electrolysis is less desirable due to several drawbacks:

1. Electrolysis is an inefﬁcient process. Conversion efﬁciencies of only 65–75% based

on the lower heating value are possible [23]. As a result, electrolysis tends to be more

expensive than natural gas reformation unless a source of inexpensive electricity is

available.

2. Electrolysis requires a source of freshwater, which, globally, is also a scarce re-

source.

3. Although production can be carbon free if the electricity is derived from renewables

or a nuclear reaction, it requires an inexpensive source of electricity. Many power

grids in the world do not have the excess capacity to supply the additional power

required, and additional electricity infrastructure would be needed.

8.2.3 Biological Hydrogen Production

Biological hydrogen production is a method to produce hydrogen, or hydrogen-containing

fuels, using biological processes. These processes are generally controlled by fermentive

or photosynthetic organisms and thus utilize natural processes to generate hydrogen.

Because these processes can be used to decompose organic waste products, they offer an

environmentally remediate pathway to simultaneously produce hydrogen and remediate

waste. A wide variety of organic waste can be used as fuel for these processes, including

human waste, farm waste (including animal and fruit/vegetable waste), and miscellaneous

garbage. The main limitation of this approach is the time scale and size associated

with these processes. Compared to hydrogen generation from high-energy-density fuel

sources such as natural gas, the size of the bioplant required would be enormous to

achieve a comparable generation rate. In some cases, however, the hydrogen generated

may be secondary to the economic beneﬁts of the energy-neutral waste remediation that

occurs. According to [24], biological hydrogen production can be grouped into four main

categories:

1. Biophotosynthesis of water using algae and cyanobacteria: In this approach, an

adapted version of photosynthesis is used to generate oxygen and hydrogen from

water.

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8.3 Hydrogen Delivery 443

2. Photodecomposition of organic compounds by photosynthetic bacteria: In this

method, various microbial organisms utilize energy from light to break down organic

material into hydrogen and other products.

3. Fermentative hydrogen production from organic compounds: A fermentative pro-

cess is utilized in this method (as opposed to a photosynthetic process) to break

down the organic matter into hydrogen and other species.

4. Hybrid systems using photosynthetic and fermentative bacteria.

Biological hydrogen production involves carbon, unlike renewables or nuclear energy.

8.2.4 Other Hydrogen Generation Technologies

Besides the technologies discussed for hydrogen generation, many others exist. Not all

liquid hydrogen–rich fuels include carbon. Ammonia (NH

) is a widely distributed chemical

used for fertilizer and in many industrial processes. At low pressure and room temperature,

the liquid storage density of hydrogen in ammonia is 1.7 times that of cryogenic liquid

hydrogen. Over a catalyst bed at around 400

◦

C, ammonia dissociates according to [25]

2NH

+ heat → N

+ 3H

(8.12)

High-temperature thermochemical cycles have also been investigated for hydrogen gener-

ation, and some have shown increased efﬁciency compared to electrolysis [e.g., 26, 27].

These generally require high-temperature catalyst bed ﬂow reactors and are thus suitable

for nuclear or solar power–based hydrogen generation.

Metals can also be combusted in water to generate hydrogen and heat simultaneously.

An example is the aluminum–water reaction:

Al +

O →

+ heat (8.13)

In principle, other energetic metals such as magnesium could also be used in a similar

fashion. Since the reaction is also exothermic, the heat generated could potentially be

used to cogenerate electricity. Many other methods for hydrogen generation exist, but they

are too numerous to give in exhaustive detail here. In particular, methods for generation

of liquid hydrocarbons from a variety of organic stocks exist, and these can be further

reﬁned into hydrogen through conventional reforming or cracking processes. Methods for

coal to hydrogen and synthetic fuel production exist that will become more attractive as

conventional petroleum resources become increasingly scarce and more expensive.

8.3 HYDROGEN DELIVERY

Another vast challenge in the hydrogen infrastructure is the mode of hydrogen delivery,

both to the fueling station and into the vehicle or storage device. For portable applications,

small hydrogen generation devices or replaceable alcohol cartridges for DAFCs are the

most likely sources of fuel. However, for large transportation applications, there must be

a hydrogen-speciﬁc delivery infrastructure. Even if the fuel cell itself is more efﬁcient

than the gasoline engine it replaces, the delivery infrastructure efﬁciency must also be

favorable, or the net well-to-wheel efﬁciency of the fuel cell system may actually be worse

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444 Hydrogen Storage, Generation, and Delivery

than the gasoline engine it is to replace. From a consumer perspective, the delivery in-

efﬁciencies add to the cost, which must also be competitive with the existing petroleum

infrastructure.

Other special concerns are safety and the environment. Hydrogen is an extremely small

molecule with very high diffusivity, and is difﬁcult to contain without leakage. One study

has estimated the hydrogen leakage rate from existing hydrogen transport, generation, and

storage technologies to be as high as 10% [28], or up to 20% for commercial transportation

[29]. Hydrogen leakage is a potential risk for ﬁre or explosion due to its broad ﬂammability

range of 4–77%, especially in enclosed environments, where it can accumulate in high

concentrations if not properly ventilated. Because of this concern, hydrogen fuel cell testing

facilities have extensive recirculation and venting equipment to avoid potential problems.

Besides the ﬂammability concern, hydrogen simply vented to the atmosphere also poses

a potential problem. When released to the atmosphere, hydrogen generates stratospheric

water from oxidation at high altitudes. The effect of this hydrogen emission is predicted

to increase the size and duration of the ozone hole over the Artic and Antarctic polar

regions [30], although this prediction has been disputed as overestimating the leakage

rates.

Gas versus Liquid Truck Delivery For delivery from future pipeline or generation

stations, some hydrogen transport by truck will be needed. The vast majority of pure

hydrogen is presently shipped as cryogenic liquid, and a small fraction is shipped as com-

pressed gas over short distances [23]. The reason for this can be understood by considering

that a compressed hydrogen gas delivery truck carries around 530 kg of hydrogen in the

gas phase (or about 60–70 hydrogen PEFC car fuel tanks), or 3370 kg (about 425 tanks) in

the liquid phase. Gas-bottle hydrogen delivery is limited to small laboratory volumes and

short travel distances.

Pipeline Delivery Where long distances are involved, hydrogen delivery by pipeline can

be the most efﬁcient means for transporting the fuel, similar to the way petroleum or natural

gas is shipped throughout the world. Pipeline delivery of hydrogen can take place by three

methods:

1. Gas-phase hydrogen pipelines

2. Cryogenic liquid hydrogen pipelines

3. Hydrogen carrier pipelines

Interestingly, pure gas-phase hydrogen pipelines have existed for over 75 years, with over

1000 km of total pipeline in operation at various locations all over the world, at near

atmospheric up to 30 MPa pressure [23]. Gas-phase pipelines exist worldwide for natural

gas, so that design principles and experience for optimized design are readily available.

One study estimated the parasitic losses in a 12-MPa, 2000-km pipeline with 480 TWh/year

throughput at 8% of the throughput energy, which is much less than the energy lost shipping

the same hydrogen by truck [23].

Compared to a hydrogen gas pipeline, transport of cryogenic liquid hydrogen has

the advantage of decreased pumping losses for a ﬂuid compared to a gas and greatly

increased energy density, but has the disadvantage of greater initial infrastructure cost to

provide an insulated pipeline to prevent boil-off and the use of expensive cryogenic pump

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8.3 Hydrogen Delivery 445

Heavy

insulation

Superconducting

metal to carry

electrons

Cryogenic

hydrogen

flow (~20 K)

Figure 8.8 Schematic of the cryogenic hydrogen/superconductor annular hydrogen/electron trans-

port system.

equipment. Nevertheless, short subkilometer-long liquid hydrogen pipelines have been

utilized for specialty applications, such as fueling of liquid rocket engines for the Space

Shuttle.

Another interesting concept for delivering hydrogen fuel and electricity demands is

known as the SuperGrid [31]. In a SuperGrid combined electron/hydrogen pipeline (see

Figure 8.8), cryogenic hydrogen would ﬂow around a superconducting metal to deliver

vast amounts of current with high efﬁciency. Because transmission losses through super-

conducting metals are so low (AC resistance in superconductors is about 0.5% of copper

at the same temperature), the distributed energy sources could be located far away from

populated areas.

Another option for hydrogen transport is to ship it by hydrogen carrier species. Chemi-

cal or metal hydrides or carbon–based storage could become feasible if storage densities are

high enough. In terms of pipeline delivery, the transport of gaseous hydrogen is ultimately

less efﬁcient than natural gas, due to the low energy density of hydrogen. Shipping liquid

hydrogen through pipelines is also inherently less efﬁcient than other liquid carbon–based

fuels, due to the low relative energy density and need for complex, highly insulated cryo-

genic pipelines and pumps. For these reasons, in certain situations, it will be more econom-

ical to deliver the fuel in pipelines as a liquid hydrogen–carrying fuel. Examples include

transport via ammonia (NH

), natural gas (CH

), or a liquid fuel such as methanol. The

higher energy density, lower pumping requirements, and room temperature of these fuels

in comparison to pure hydrogen are the main advantages of this alternative, although the

complete elimination of carbon from the fuel cycle would not be possible. For fuel cells

with local or internal reformation, the existing natural gas pipeline distribution can be used

for this purpose. Many stationary fuel cell systems operate directly from reformed natural

gas delivered in this manner.

Fueling Fueling of hydrogen storage tanks is not a trivial concern. Hydrogen refueling

should be rapid, safe, and inexpensive technology. For portable electronic applications,

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446 Hydrogen Storage, Generation, and Delivery

alcohol-based fuel cartridges, similar to replaceable ink cartridges, have been designed for

use in new product designs. For compressed and liquid hydrogen storage reﬁlling, robotic

refueling is probably necessary to eliminate safety concerns with public handling of high

pressure or cryogenic fuels and hydrogen leakage. The potential ﬂammability hazard of

handling of hydrogen is perhaps overemphasized, considering the public deals with a highly

ﬂammable fuel in gasoline. However, given the wide ﬂammability limits of hydrogen,

additional safety concerns must be addressed, including elimination of static potential

accumulation and discharge with grounding cables. For chemical hydride refueling, the

spent fuel residual can be recycled and recharged at a separate location or disposed.

Therefore, a chemical hydride recycling and distribution network for these materials would

need to be established. As discussed, metal hydrides and carbon-based hydrogen storage

are designed to be recharged from a gas-phase hydrogen source in situ in a controlled

temperature and pressure environment.

8.4 OVERALL HYDROGEN INFRASTRUCTURE

DEVELOPMENT

The development of the overall hydrogen infrastructure is a complex topic that will evolve

as a mixture of political, economic, and environmental realities and combines hydrogen

generation, storage, and delivery issues. What is the best possible hydrogen infrastructure?

The answer depends on the location, technological developments, and demand. Right now,

tens of millions of cubic meters of hydrogen is generated per year [32], mostly at the

location of use. The remainder is either shipped by truck as compressed or liqueﬁed fuel or

shipped in small localized pipelines.

In the future, the preferred hydrogen generation, storage, and delivery mode will

depend on the progress of the competing technologies, but a general pathway to a hydrogen

economy is generally considered to occur in several stages, shown as a near- and long-term

infrastructure in Figure 8.9. In the near term, the hydrogen production and availability will

follow from the same infrastructure already in place, with most generation from carbon-

based fuels. As fuel cell vehicles go beyond the prototype and into initial production stages,

localized generation will provide fuel for small centralized ﬂeets of fuel cell vehicles,

such as public transportation buses and taxicabs. The advantages of the initial fuel cell

infrastructure development with ﬂeet vehicles are as follows:

1. Fleet vehicles drive a repeatable drive cycle and return to the same location each day,

so that only a single fuel station is needed to service the vehicles. The additional

capacity from these ﬂeet stations can also be used as the initial public fueling

stations.

2. Only a small initial group of trained mechanics will be needed to maintain the

vehicles.

3. Fleet vehicle purchases can be government subsidized to jump start the demand

required to spur manufacturers to produce the larger numbers needed to achieve

higher manufacturing efﬁciency.

4. The controlled driving cycles allow manufacturers to gain quality performance and

operation feedback that will enable further system improvement.

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8.4 Overall Hydrogen Infrastructure Development 447

Liquid H

truck delivery

Local, small-scale

reformation or

electrolysis to

fuel fleet vehicles

Fleet vehicles

(a)

(b)

pipeline delivery

to fueling stations

Renewable resources

CO, CO

sequestor

Delivery to rural areas

Small-scale local

generation off the

pipeline infrastructure

Gas

pump

Figure 8.9 Schematic of potential hydrogen infrastructure development route over time: (a) near-

term local generation and limited distribution; (b) Long-term distributed generation and pipeline

networks.

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448 Hydrogen Storage, Generation, and Delivery

As the demand for hydrogen evolves into the private sector in the long term, larger dis-

tributed generation networks will grow near population centers and branch out into rural

areas over time. For locations off the grid where network connectivity is not cost effective,

there will continue to be localized generation stations for small quantities, or home-based

electrolysis units.

8.5 SUMMARY

The hydrogen economy is at a very nascent stage, and many competing technologies

exist for hydrogen storage, generation, and delivery. Ultimately, the success or failure

of the hydrogen economy will rest with the cost of competing fuels and the local cost

advantages of different fuel stocks and generation techniques. For portable fuel cells, liquid

alcohol fuels such as methanol are most often used because of the high storage density of

hydrogen. For distributed and high-temperature fuel cells, the existing natural gas or other

fuel infrastructure can be used until a hydrogen infrastructure exists. For mobile fuel cell

applications, however, to compete with conventional gasoline automobiles, around 7.5 kg

of hydrogen storage is needed, which presents signiﬁcant difﬁculties.

For hydrogen storage, six main technologies exist: (1) compressed gas, (2) cryogenic

liquid, (3) metal hydrides, (4) chemical hydrides, (5) carbon-based storage, and (6) liquid

hydrogen carrier fuels (also called liquid hydrides). Each has limitations and advantages,

but none is yet established to fulﬁll all the needs of mobile applications. Compressed gas

is an established technology, but high-pressure (70-MPa) storage is needed to approach

driving range goals. This high pressure is slow to reﬁll and energy intensive to compress in

addition to the safety concerns. Cryogenic liquid hydrogen storage is less energy intensive

for delivery and has relatively higher density but is more energy intensive to produce than

compressed hydrogen and will boil off over time in storage. Metal hydrides hold much

promise but have not yet met the desired storage fraction of 6% by weight at room tem-

perature and pressure. Chemical hydrides are not yet economically competitive and would

require new generation and recovery infrastructure. Hydrogen storage in single-walled

carbon nanotubes have not shown the storage capability initially reported, but variations,

including metal-doped structures, differently organized carbon nanostructures, and carbide-

derived carbons, show some promise. Liquid fuels include hydrogen-carrying liquids such

as methanol, ethanol, or propane. These are used extensively for portable applications where

low efﬁciency is tolerable in light of compact storage. In mobile applications, however,

hydrogen must be used for higher system efﬁciency, but on-board reformation is generally

too complex, and pure hydrogen storage is preferred.

Hydrogen generation can be accomplished through chemical, electrochemical, bio-

logical, and other means. Around 90% of hydrogen generation is presently accomplished

through a chemical reformation process of carbon-based fuels, and this should continue

into the near future as it is a well-established technology. In the future, carbon sequestration

could be used with fuel reformation to reduce the environmental impact. Electrochemically,

electrolysis is a well-established technique to separate hydrogen and oxygen from water and

is useful for local small-scale or back-up generation. However, it is less efﬁcient than other

methods and requires a source of inexpensive and excess capacity electricity and freshwater.

Biological-based hydrogen generation processes offer an opportunity to recover hydrogen

from products normally considered waste. Biological methods to produce methanol or other

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8.5 Problems 449

alcohols can also be exploited to produce hydrogen-rich fuel stocks. Other new technolo-

gies for hydrogen generation will almost certainty continue to be developed, considering

the vastly different fuel stocks and other local factors.

Hydrogen delivery consists of transport from generation stations and delivery into the

fuel cell storage container. Delivery in the short term will likely continue to be from small-

scale local reforming stations and by truck as a cryogenic fuel. Introduction of a distributed

hydrogen infrastructure would likely initially spread from ﬂeet vehicle applications, with

long-term distributed generation networks eventually connected by pipeline infrastructures.

APPLICATION STUDY: A GROWING HYDROGEN AND

ELECTRON INFRASTRUCTURE

In 2004, The United States produced over 17 million cubic meters of hydrogen for various

industrial processes [33], a minor fraction of which were fuel cells. Assuming that over

the next 50 years 100 million hydrogen-powered cars averaging 15,000 miles per year will

be on the road, how much additional capacity to produce hydrogen will be needed? If the

hydrogen required is produced by electrolysis, how much additional electrical capacity

must be added to the existing infrastructure? Can an argument for fuel reformation as a

better alternative be made? Note that you will have to use your knowledge to make some

reasonable assumptions about the efﬁciency and size of fuel cell stacks for automotive

applications. Next, analyze the possibility of delivering this hydrogen by compressed gas

or as cryogenic liquid. You can assume a normal hydrogen delivery truck carries 530 kg

in the gas phase or 3370 kg in the liquid phase and will travel an average of 100 km per

day. Do these choices make sense? What would you recommend for the mode of delivery

of this much fuel in the future?

PROBLEMS

8.1 Estimate the maximum rate of hydrogen availabil-

ity that would be required to supply a 100-kWe maxi-

mum output PEFC stack for an automotive application

in grams in per second. Assume an anode stoichiometry

of 1.2.

8.2 Determine the minimum gravimetric storage fraction

that would be required for a cell phone application to pro-

vide 4800 mAh of energy with a 28-g hydrogen fuel storage

container.

8.3 Determine the gravimetric storage fraction required to

achieve 7 kg storage for an automotive application in the

same volume proﬁle as two 25-gal gasoline tanks. Assume

the walls of the gasoline tank are negligibly thin.

8.4 Determine the weight of a magnesium hydride fuel

storage system with 6 kg of hydrogen in a 2 wt. % storage

media. Compare this to the weight of a car. Is this reason-

able? What storage in weight percent would be reasonable?

8.5 Make a plot of the weight of a metal hydride hydrogen

storage vessel used to power a fuel cell to provide a lap-

top computer with 53 Wh of energy versus the hydrogen

storage weight percent.

8.6 Consider hydrogen and oxygen production by electrol-

ysis. Is it possible to have a water-powered car using on-

board electrolysis with the oxygen and hydrogen generated

to power a fuel cell? What would be the main drawbacks of

this approach?

8.7 What fraction of compression work would be saved

by using high-pressure electrolysis at 20 MPa followed by

regular compression to ﬁll a compressed hydrogen tank to

70 MPa, compared to only compression from 0.1 MPa?

8.8 Determine the temperature increase in a hydrogen

storage tank that is adiabatically compressed from 0.1 to

70 MPa. Discuss why this effect limits the rate of hydrogen

ﬁlling the storage tank.