1

Fuel Cells(http://w

ww

.stanford.edu/group/fuelcell/images/fuel%

20cell%20com

ponents.jpg)Lecture prepared with the able assistance of Ritchie King, TA

2

A Comparison of Two Engines

Internal-combustion engine Electric motor

Only 33% efficient at best 80-90% efficientAir emissions Zero direct emissionsPeaky torque-rpm curve Broad torque-rpm curve

(needs a transmission) (does not need a transmission)Power loss in idle No idleIrreversible energy conversion Regenerative brakingBig and heavy Small and light

(250 hp in 600 lbs = 0.7 kW/kg) (75 kW in 13 kg = 5.8 kW/kg)Noisy Quiet

So, why don’t we have electric motors in our automobiles today?

Because we do not have good enough batteries to store the electricity on board of the vehicle !

Benefits of an electric motor

Direct drive –

No driveline losses

More efficient – This number is smaller

No idling

3

Hydrogen Fuel Cells

• A fuel cell is a way to generate electricity on board to power an electric motor. Onboard electricity generation eliminates the need to store the electrical energy in a battery.

• The technology is based on a catalytic electrochemical process. The catalyst itself does not change; so, unlike batteries, fuel cells don’t go bad.

• However, hydrogen is only a carrier of energy, not an energy source. This means that fuel cells are only as clean as the ultimate energy source used to generate the hydrogen from the original energy source (coal? sun? wind?).

• Hydrogen (H2) is, in some ways, the ideal fuel:- It has no carbon → the product of combustion is only water (H2O)- Of all fuels, it contains the most energy on a per-weight basis

(142.5 MJ/kg as opposed to 43 MJ/kg for gasoline and 29.8 MJ/kg for ethanol)

• The major hurdle is that hydrogen is a gas with an extremely low temperature of liquification.

How do fuel cells work?

• All fuels cells have– an anode (+)– a cathode (–)– an electrolyte separating the two.

• A fuel flows to the anode and an oxidant to the cathode – the resulting chemical reaction produces electricity.

• Fuel cells are typically classified according to the type of electrolyte used.

4

A typical fuel cell

(http://www.visionengineer.com/env/fc_structure.shtml)

Fuel Cell Types

100-250Aq. KOHAlkaline

70-100Sulfonated PolymersProton Exchange

Membrane

700-1000YSZSolid Oxide

500-700(Na,K,Li)2CO3Molten Carbonate

150-220H3PO4Phosphoric Acid

Operation Temperature (oC)ElectrolyteFuel Cell Type

(http://www.superprotonic.com/our_technology.htm)

5

Comparative Advantages of Proton Exchange Membrane (PEM) Fuel Cells

• Higher Power density → Fewer cells needed in stack

• No electrolyte corrosion or safety concerns

• Lower operating temperature→ In a car, it allows for instant start-up.

How do PEM fuel cells work?

e- e-

Pressure forces H2 into catalytic membrane Electronegativity of

O2 attracts electrons

H2 air

water

N2

circuit

6

Animated cartoon

http://www.bigs.de/en/shop/anim/bz01.swf

Technical characteristics

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The complete fuel-cell system is more than the cell stack…

Examples of PEM fuel-cell vehicles

www.fuelcellsworks.com/Perth_Fuel_Cell_Bus.jpg

www.ase.org/content/article/detail/1926

8

http://www.sunline.org

GM’s concept

9

... and there even exists a fuel-cell motorcycle !

This motorcycle is not just quiet, it is silent!

(http://ww

w.envbike.com

)

Remember…

Hydrogen is not a source of energy, but rather a carrier of energy.

In other words, it takes energy to produce hydrogen fuel.

To understand how much energy is needed, we need to look at a little chemical thermodynamics.

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Thermodynamics of hydrolysis and water formation

222 21energy OHOH +→+

energy21

222 +→+ OHOH

A

B

The splitting of water (Reaction A) is an endergenic (endothermic) reaction, meaning that it requires a net input of energy because the products are inherently more energetic than the original molecule.

Conversely, Reaction B is exergenic (exothermic), meaning that it creates a net release of energy to the environment.

However, the “energy” terms in Reactions A and B are not equal due to the Second Law of Thermodynamics.

The Second Law of Thermodynamics

The Second Law of Thermodynamics has been expressed in many different ways over the years.

Perhaps the most well known form is the following:“The entropy of the universe is always increasing.”

This means that the universe is becoming more disordered with every chemical reaction. The splitting and recombination of H2O is no exception.

222 21 OHOH +→

Water, being a single compound, is more ordered than the two components, H2and O2, taken separately. Thus, it takes some energy to cleave water to create the more disordered H2 and O2.

The energy associated with creating the disorder, or entropy, ultimately dissipates and cannot be recovered to do useful work.

11

Enthalpy and Gibbs Energy

The enthalpy change of reaction (ΔH) captures the notion of energy changes for chemical reactions.

In contrast, the change in Gibbs energy (ΔG) of reaction discounts the change of entropy, retaining only the usable energy.

The two are related by the following equation:

STHG Δ−Δ=Δ

where T is the temperature in Kelvin and ΔS is the change in entropy.

Because absolute temperature is always positive and entropy increases with every reaction, the above equation tells us that the change in Gibbs energy is always less than the change in enthalpy, which is precisely what we would expect.

For the above reaction, we have:

ΔH = 286 kJ/mol H2O

ΔG = 237 kJ/mol H2O @ 298 K = 25oC

Now, if it takes ΔH to cleave water into hydrogen and oxygen, and we can only get ΔG back through using hydrogen as fuel, then the maximum efficiency we can possibly attain for the entire process, from hydrogen production to automobile propulsion, is:

Back to Water

222 21 OHOH +→

%8383.0kJ/mol 286kJ/mol 237

===thermalη

Typically, however, actual efficiencies are much smaller, on the order of 40%.

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Hydrogen Production

Remember: Hydrogen is only as clean as the fuel source used to produce it.

Basically, hydrogen can be produced in one of two ways:

– through a series of high-temperature chemical reactions, or

– by using electricity to split water (electrolysis).

Possible chemical reactions for hydrogen production from a hydrocarbon:

Steam reforming (750 – 800oC)

CH4 + H2O (steam) → 3H2 + CO

Water gas shift (200 or 350oC)

CO + H2O (steam) → H2 + CO2

Dry reforming (600 – 700oC)

CH4 + CO2 → 2H2 + 2CO

Partial oxidation of methanol (250oC)

CH3OH + H2O (steam) → 3H2 + CO2

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An interesting question:

What is the minimum number of windmills it would take to produce enough hydrogen to power the US automobile fleet?

What would that number be for New Hampshire alone?

ww

w.hydrogennow

.org/Facts/Wind/w

ind.htmNow, let us consider wind as the energy source to generate electricity to be used for electrolysis.

What we need to know

• Rate of electrolytic hydrogen production per unit power

• Fuel economy of hydrogen fuel cell cars

• Number of miles driven annually in the US

• Typical capacity of a windmill

Note: Often, this kind of information is best found on the U.S. Energy Information Administration website: http://eia.doe.gov

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Electrolytic Hydrogen Production

A typical value for the amount of energy needed to produce hydrogen by electrolysis is 367 kJ/mol.

(Note that this is appreciably higher than the ΔH value, 286 kJ/mol).

This means that 1 kW of electricity can produce 1/367 = 0.00272 mole/s or 0.00545 g/s of H2.

Equivalently, 1 MW can produce 0.00545 kg/s.

(Berry, Gene D. “Hydrogen Production” Encyclopedia of Energy, 6th ed. Elsevier 2004)

Experience with existing prototypes reveals that a hydrogen fuel-cell car has a fuel economy of 199 km/kg.

This means that a fuel-cell car can go 199 km (= 124 miles) on 1 kg of hydrogen.

Put another way, to have a typical range of 600 km (= 400 miles), a hydrogen car needs a tank that can hold 600/199 = 3.02 kg of hydrogen. That does not seem much.

In 2003, US personal vehicles traveled a total of 2,594 billion miles (4.17 x 1012

km), equivalent to 8,750 miles per person per year in the US (everyone, including children and others who don’t drive!).

New Hampshire residents drove a total of 14,251 million miles (2.29 x 1010 km) in the same year, equivalent to 10,880 miles per person in the state (again including children and others who don’t drive).

(Sources: http://www.eia.doe.gov/oiaf/aeo/pdf/aeotab_7.pdfVital Signs 2006: Economic and Social Indicators for New Hampshire, 2001-2004. Economic & Labor Market Information Bureau, January 2006)

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Capacity of average wind turbine

Typical wind turbine has nameplate capacity of about 1 MW

– Nameplate capacity is max capacity –of course, average production will be less than this and will depend on local wind patterns.

Source: http://www.eia.doe.gov/oiaf/servicerpt/rps2/supplement.html

Meeting the US driving demand

windmills920,121s101563.3

year1

year 1km 1017.4

km 199H kg 1

/sH kg 00545.0MW 1

MW 1 windmill1

7

122

2

=×

×

××××

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Is this possible?

122,000 windmills have a collective capacity of 122 GW. The EPAestimates that the US has over 2,500 GW of potentially available wind resource, meaning that 5% of total available capacity would be used to power the US automobile fleet.1

Remember, however, that the value for miles traveled used here was for 2003. The EIA projects that by 2025, annual vehicle miles traveled will be 3,791 billion, which would require about 178,200 windmills and 7.1% of total available wind resource.2

1http://www.eia.doe.gov/oiaf/servicerpt/rps2/supplement.html2http://www.eia.doe.gov/oiaf/aeo/pdf/aeotab_7.pdf

Is it going to happen?

Not without a huge push.

The EPA estimates that by 2025, the US will have developed a total wind power capacity of 47 GW.

If the EIA projections prove accurate, this capacity will only account for 26% of the energy needed.

(http://www.eia.doe.gov/oiaf/servicerpt/rps2/supplement.html)

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Meeting the New Hampshire demand

windmills670s101563.3

year1

year 1km 1029.2

km 199H kg 1

/sH kg 00545.0MW 1

MW 1 windmill1

7

102

2

=×

×

××××

Does this make sense?

According to the US census, in July 2005, the population of the US was about 296.4 million while the population of New Hampshire was about 1.31 million.

If everybody were driving the same amount everywhere in the States, we would expect New Hampshire to need about 540 windmills.

http://www.census.gov