ECE 7800: Renewable Energy Systems Topic 13: Fuel Cells Spring 2010 © Pritpal Singh, 2010

ECE 7800: Renewable Energy SystemsTopic 13: Fuel Cells

Spring 2010

© Pritpal Singh, 2010

Basic Description of a Fuel Cell A fuel cell is an electrochemical device

in which the reactants are continuously supplied and the reaction products continuously removed. (Dr. Tom Reddy)

Historical Development of Fuel Cells1839 - First fuel cell developed by Sir William Grove in England (“gaseous voltaic battery”). 1890 - Mond and Langer built on Grove’s idea and made a 1.5W fuel cell which operated at 50% efficiency. They coined the term “fuel cell”. 1952 – Francis Bacon developed the first practical fuel cell - 5kW alkaline fuel cell which powered a 2-ton fork lift truck. Chalmers demonstrated a 20hp fuel cell- powered tractor.

Historical Development of Fuel Cells (cont’d)

Space exploration accelerated the pace of development of fuel cells. The Gemini missions were fueled using permeable membrane fuel cells and Apollo and Space Shuttle missions were used alkaline fuel cells.

Recently, fuel cells have been developed for back up power applications and vehicular applications.

Basic Operation of a Fuel Cell The basic operation of a fuel cell is

illustrated in the figure below:

Components of a Fuel Cell System• Fuel cell stack

• Humidifier• Cooling system• Controller• DC-to-DC Converter

Chemical Reactions in a Fuel CellAnode: H2 -> 2H+ + 2e-

Cathode: ½ O2 + 2H+ + 2e- -> H2O

Overall reaction: H2 + ½ O2 -> H2O

The net reaction is exothermic and so a cooling system is required to maintain a constant stack temperature.

Thermodynamics of Fuel Cells How much of the chemical energy in the

hydrogen and oxygen can be converted to electrical energy?

In order to answer this question we need to consider some thermodynamics. Three terms need to be considered:

1) Enthalpy, H = U + PV 2) Entropy, S = ΔQ/T3) Gibbs Free Energy, G

Thermodynamics of Fuel Cells (cont’d) Enthalpy

The enthalpy of a substance is defined by: E = U +PV

where U=internal energyP= pressureV=volume

The internal energy includes kinetic energies of atoms/molecules, inter-molecular/atomic forces, and intra-molecular/atomic forces.

Thermodynamics of Fuel Cells (cont’d)Enthalpy (cont’d)

Molecules in a system possess energy in various forms,

- Latent and Sensible energy depending on state (solid, liquid or gas) and temperature;

- Chemical energy, associated with molecular structure;

- Nuclear energy, associated with atomic structure.

Chemical energy is the one important for fuel cells and is best described by enthalpy (Units: kJ/mole).

Thermodynamics of Fuel Cells (cont’d)Enthalpy (cont’d) The reference temp. and pressure at

which the enthalpy is considered zero is 25ºC and 1 atmosphere (STP conditions).

Another way to think of enthalpy is how much energy it takes to form substance from its constituent elements – enthalpy of formation.

Thermodynamics of Fuel Cells (cont’d)Enthalpy (cont’d) In a chemical reaction, the difference

between the enthalpies of the products and the reactants gives the enthalpy released or absorbed in the chemical reaction.

Example:

½ O2 + H2 -> H2O

ΔH = -285.8kJ/mol

Thermodynamics of Fuel Cells (cont’d)

Thermodynamics of Fuel Cells (cont’d)Entropy Recall that the loss of entropy ΔS

when an amount of heat Q is lost in an isothermal process at temp. T is given by:

ΔS = Q T

Thermodynamics of Fuel Cells (cont’d)Entropy (cont’d) Consider a fuel cell in which chemical

energy is converted to electrical energy and waste heat.


In the fuel cell system we can state:

Enthalpy = rejected + enthalpy in (H) heat (Q) out (We)

Since heat is generated in the process of the fuel cell, the entropy of the system must increase. This can be used to determine the maximum efficiency of the fuel cell.

Thermodynamics of Fuel Cells (cont’d) We can rewrite the overall fuel cell

reaction as:½ O2 + H2 -> H2O + Q

where the heat released in the reaction is explicitly included. Assuming an isothermal fuel cell process, the entropy change is then:

ΔS = Q T

Note: The entropy associated with We = 0.

Thermodynamics of Fuel Cells (cont’d)Entropy (cont’d) 2nd Law of Thermodynamics states

that the entropy of a system must increase. Thus,Entropy gain ≥ entropy loss

i.e. Q + ΣSproducts ≥ ΣSreactants

T which gives:

Q ≥ T (ΣSreactants -ΣSproducts) (minimum heat rejected by a fuel cell)

Thermodynamics of Fuel Cells (cont’d) Maximum Fuel Cell Efficiency

The enthalpy supplied by the chemical reaction H is equal to the electricity produced We plus the heat rejected Q,

i.e. H= We+Q

The fuel cell efficiency,η, then is given by: η = We = H-Q = 1- Q

H H HExample 4.9

Thermodynamics of Fuel Cells (cont’d) Gibbs Free Energy The chemical energy released in a

reaction can be thought of in two parts – an entropy-free part, the Gibbs Free Energy ΔG that can be directly converted to electricity or mechanical work, and waste heat, Q.

ΔG = (ΣGproducts - ΣGreactants )

and ηmax = ΔG ΔH


Example 4.10

Thermodynamics of Fuel Cells (cont’d) Electrical Output of an Ideal Cell

For an ideal fuel cell, the electrical output is simply equal to the Gibbs free energy, ΔG. At STP, this is equal to 237.2 kJ/mol H2.

For each mole of H2, 2 e- pass through the load. Using unit conversions, we get:

I(A) = n (mol/s). 6x1023 (molecules H2/mol). 2e-/molecule H2.1.6x1019C/e-

= 192,945n

Thermodynamics of Fuel Cells (cont’d)Electrical output of an Ideal Cell (cont’d)

Power delivered to the load (W) = 237.2kJ/mol x n(mol/s)x 1,000J/kJ x

1W/1J/s = 237,200n

Voltage (V) = P(W) = 237,200n = 1.229Voutput I (A) 192,945n

Note: The voltage decreases to about 1.18V at operating cell temp. of 80ºC in a PEM fuel cell.

Thermodynamics of Fuel Cells (cont’d)Electrical Output of Ideal Cell (cont’d) We can now find the hydrogen rate

needed to generate 1kWh of electricity as follows:

H2 rate = n (mol/s) x 2(g/mol) x 3600 s/h 237,200n (W) x 10-3 kW/W = 30.35 gH2/kWh

Electrical Characteristics of Real Fuel Cells In real fuel cells there are various losses:

- Activation losses associated with the energy required by catalysts to initiate the chemical reactions.

- Ohmic losses due to series resistance in the electrolyte membrane, electrodes and cell interconnects.

- Fuel crossover losses in which fuel passes through the cell without releasing electrons to the load.

- Mass transport losses where the H2 and O2 gases are impeded to reach the electrodes.

Electrical Characteristics of Real Fuel Cells (cont’d)

Electrical Characteristics of Real Fuel Cells (cont’d)

Example 4.11

Types of Fuel CellsThere are six different types of fuel cells:- Proton Exchange Membrane Fuel Cells

(PEMFCs)- Direct Methanol Fuel Cells (DMFCs)- Phosphoric Acid Fuel Cells (PAFCs)- Molten Carbonate Fuel Cells (MCFCs)- Alkaline Fuel Cells (AFCs)- Solid Oxide Fuel Cells (SOFCs)

PEM Fuel Cells Proton exchange membrane (PEM) fuel

cells operate at relatively low temperature (50-80ºC) and high efficiency (45%). They are commercially available in sizes ranging from 30W to 250kW. They are the units being deployed in fuel cell vehicles and for backup power applications.

PEM cells generate 0.5W/cm2 of membrane area at 0.65V/cell and a current density of 1A/cm2.

PEM Fuel Cells (cont’d)Important issues with PEM fuel cells are:1) High purity hydrogen is required

because CO can poison the membrane catalyst.

2) Pt catalyst used on membrane is very expensive.

3) Thermal and water management.

Present cost of PEM fuel cell system ~ $1,000/kW

(c.f. IC engine ~ $50/kW)

Example of a Commercial PEM FC StackMark 1030 Stack from Ballard Corp.

www.ballard.com

Direct Methanol Fuel Cells DMFCs use the same polymer electrolyte

as PEM FCs but directly use a liquid fuel, methanol (CH3OH), instead of pure H2 (easier to handle and transport and higher energy density). The chemical reactions at the anode and cathode are as follows:

CH3OH + H2O -> CO2 + 6H+ + 6e- (anode)

½ O2 + 2H+ + 2e- -> H2O (cathode)

for an overall reaction:

CH3OH + 3/2 O2 -> CO2 + H2O

Direct Methanol Fuel Cells (cont’d) Main components of a DMFC

Courtesy: Dr. Tom Reddy

Direct Methanol Fuel Cells (cont’d) Presently small scale DMFCs (10-100W) have been demonstrated for powering laptop computers, smart phones, and bar code scanners.

Significant technical challenges include: - excessive fuel crossover through

the membrane- reducing catalyst poisoning by CO

and other methanol reaction byproducts DMFCs are relatively new and so research

is 3-4 years behind other types of fuel cells.

Examples of Commercial DMFC Prototypes MTI Micro

Ref: www.mtimicrofuelcells.com

Toshiba DMFC Launched in Oct. 2009Toshiba

Source: http://www.toshiba.co.jp/about/press/2009_10/pr2201.htm

22 Oct, 2009Toshiba Launches Direct Methanol Fuel Cell in Japan as External Power Source for Mobile Electronic Devices

TOKYO—Toshiba Corporation (TOKYO: 6502), a world leader in the development of fuel-cell technology for handheld electronic equipment, today announced the launch of its first direct methanol fuel-cell product: Dynario™, an external power source that delivers power to mobile digital consumer products. Dynario™, together with a dedicated fuel cartridge for refueling on the go, will be launched in Japan, in a limited edition of 3,000 units only, and will be exclusively available at Shop1048 (http://shop1048.jp/), Toshiba's direct-order web site for digital consumer products in the Japanese market. Orders will be accepted from October 22, and shipping will start on October 29.

http://shop1048.jp/

Toshiba DMFC Launch (cont’d)Outline of ProductProduct Model No. Price Start of Delivery Dynario™ (DMFC) PF60A000001 29,800 yen October 29, 2009 Fuel cartridge MC050A00001 3,150 yen (set of 5) October 29, 2009

Note: Above prices include sales tax and delivery in Japan.

Outline of SpecificationsDynario™ (DMFC): Model No. PF60A000001 Fuel cell type: Direct Methanol Fuel Cell Fuel Highly-concentrated methanol (through dedicated cartridge)Output[1] : DC5V-400mA External dimensions: Approx. W150 x D21 x H74.5 mm (when rotary stand is housed)Weight Approx.: 280g (without fuel) Fuel tank capacity: 14ml Operating temperature and humidity range: From 10 to 35 degrees C; from 30 to 90% RHInput: DC5V-500mA Accessories: Output cable, input cable, cover and instruction manual

[1] Maximum performance in hybrid operation using an integrated lithium-ion battery.

Fuel cartridge: Model No. MC050A00001 Content: Highly-concentrated methanol Outside Dimension: W62 x D29.1x H122 mm Weight: Approx. 92g Capacity 50ml

Dynario™ and its logo are trademarks of Toshiba Corporation in Japan.

Phosphoric Acid Fuel Cells Electrochemical reactions are the

same as in a PEM cell but phosphoric acid is the electrolyte. Operating temperatures are much higher (~200ºC). These cells tolerate CO better than PEM FCs but they are sensitive to H2S.

Phosphoric Acid Fuel Cells (cont’d)Installation costs > $80,000 (>$40/kW)• Reliability - recorded up to 9,000 hrs. operation

between outages (better than gas and diesel engines)

• Availability- Already > 90%

Easy to get permitting because of non- polluting operation

Slide courtesy Dr. Tom Reddy

Phosphoric Acid Fuel Cells (cont’d) These fuel cells were introduced into the marketplace in 1990’s and ONSI Corp., International Fuel Cells division, has sold hundreds of 200kW plants (production capacity is 200 PC25 plants @ 200kW/yr.)

Note: The new model is termed Pure CellTM 200

Phosphoric Acid Fuel Cells (cont’d)PureCellTM Model 200 Plant Specifications• Rated Capacity: 200kW / 235kVA • Voltage and Frequency: 480 Volts, 3-Phase,

3-Wire, 60-Hz 400 Volts, 3-Phase, 3-Wire, 50-Hz • Electrical Operation: Grid-connected or Grid-

independent • Thermal Energy Available: 700,000 Btu/hr • Thermal Energy Temperature: 140 ºF (60 ºC)

hot water • Electrical Efficiency: 40%• Total Efficiency (Electric + Heat): 80%• Natural Gas Consumption: 1,900 ft3/hr.• Pollutant Emissions: less than 6 ppmv (total)

Phosphoric Acid Fuel Cells (cont’d) Pure CellTM 200 PAFC

Alkaline Fuel Cells Used in Apollo and Space Shuttle

missions, this type of fuel cell employs a potassium hydroxide (KOH) electrolyte. Main problem with alkaline fuel cells is intolerance to CO2 even at low atmospheric levels.

Molten Carbonate Fuel Cells Molten Carbonate fuel cells operate at

very high temperatures (600-700ºC). Because of this high temperature, reformation of hydrocarbon fuels to generate hydrogen can be done directly. CO from reformers does not poison nickel catalyst in these types of fuel cells. Efficiencies of 50-55% are projected for internally reformed MCFCs and, with combined cycle operation, electrical eff. ~ 65% are projected and cogen. efficiencies ~80% are possible.

Molten Carbonate Fuel Cells (cont’d) The chemical reactions in a MCFC are as follows:

H2 + CO32- -> H2O + CO2 + 2e- (anode)

½ O2 + CO2 + 2e- -> CO32- (cathode)

The carbonate ion (CO32-) is the

conducting ion rather than H+ and the electrolyte is molten lithium, potassium or sodium carbonate.

Molten Carbonate Fuel Cells (cont’d) Fuel Cell Energy has commercialized a 2.8 MW plant. Cogeneration 300kW unit in license with MTU.

Main technical challenge is developing materials to work in the very high temperature corrosive environment of the electrolyte.

Solid Oxide Fuel Cells The electrolyte in a solid oxide fuel

cell is a ceramic material made of yttria and zirconia. This type of fuel cell operates at high temperatures (700-1,000ºC) and is physically smaller than an MCFC for the same power rating.

Solid Oxide Fuel Cells (cont’d) The oxide O2- ion is transported

through the electrolyte. The reactions at the anode and cathode are as follows:

H2 + O2- -> H2O + 2e- (anode)

½ O2 + 2e- -> O2- (cathode)

Efficiencies >60% for electric power and >80% cogen. are projected.

Solid Oxide Fuel Cells (cont’d)Other features of SOFCs include:• Cell can be tubular, planar or

monolithic• Corrosion problems minimized• Kinetics are fast and so natural gas

and air can be used directly


Solid Oxide Fuel Cells (cont’d)Solid oxide fuel cell designs


Solid Oxide Fuel Cells (cont’d)US manufacturers of SOFCs include:

– Siemens/Westinghouse Corp. + Ontario Hydro

– Honeywell– Cerametec– ZTEC– Technology Management, Inc.– SOFCo.


Solid Oxide Fuel Cells (cont’d)

A 100 kW SOFC cogeneration system installed at RWE in Essen, Germany

The major components of the 100 kW system

http://www.powergeneration.siemens.com/products-solutions-services/products-packages/fuel-cells/demonstrations/

Fuel Cell Bus

http://www.eere.energy.gov/afdc/pdfs/41041.pdf

Propulsion system employs UTC Power Pure Motion 120 PEM 120kW fuel cell system. The power system is a hybrid system which Also includes three Zebra (sodium/nickel chloride) batteries.

Hydrogen Generation and Dispenser


Hybrid Drive System


Fuel Cell Bus Experience


Fuel Cell Bus Experience (cont’d)






Fuel Cell Cars (US DOE/NREL) US DOE Freedom CarTM Program (superceded by plug-in hybrid program)

Ref: http://www.eere.energy.gov/afdc/pdfs/42284.pdf

Fuel Cell Cars (CA FC Partnership Program)

Ref: http://www.fuelcellpartnership.org/

Hydrogen Production Four approaches can be used to

produce hydrogen:- Steam reformation of methane- Partial oxidation (see text)- Gasification of biomass, coal

or waste (see text)- Electrolysis of water

Hydrogen Production (cont’d) Steam reformation of methane

This is the most common and lowest cost means for producing hydrogen. After some gas cleanup to remove sulfur, a mixture of natural gas and steam is passed through a catalyst at 700-850 ºC producing syngas comprising CO and H2 as follows:

CH4 + H2O -> CO + 3H2

Hydrogen Production (cont’d) Steam reformation of methane (cont’d) The hydrogen concentration in syngas

is then increased using a water-gas shift reaction:

CO + H2O -> CO2 + H2

The resulting syngas is 70-80% H2. The overall efficiency of this hydrogen production method is 75-80%.

Hydrogen Production (cont’d) Electrolysis of Water Water may be electrolyzed by passing

current through an electrolyte. The reaction is simply:

2H2O -> 2H2 + O2

An electrolysis cell that uses a proton exchange membrane is shown below:

Hydrogen Production (cont’d) Using renewable energy to electrolyze

water holds the promise of can lead to a completely clean, emission-free system that can produce power when needed using the hydrogen for energy storage.

Hydrogen Storage Hydrogen can be stored in two ways:

• Compressed hydrogen in a tank• Metal hydride storage

1 gm. of H2 occupies 11 liters (2.9 gals.) of volume at atmospheric pressure. Thus, in order to store a reasonable amount of hydrogen, it must be compressed.

Hydrogen Storage (cont’d) 5-13 kg of H2 must be stored to achieve a 300 mile range in a fuel cell vehicle. Conventional stainless steel tanks store H2 at 2,500 psi. Carbon reinforced tanks can store hydrogen at 5,000 psi and 10,000 psi. The amount of hydrogen stored at these pressures is 23.5g/liter at 5,000 psi and 38.7g/liter at 10,000 psi. Thus, a vehicle needs 155 liters (41 gals.) of 10,000 psi compressed hydrogen storage for a 300 mile range.

Hydrogen Storage (cont’d)Metal Hydride Storage

Ref: http://escholarship.org/uc/item/6gt782vt

Hydrogen Storage (cont’d) The two simplest metal hydrides are LaNi

and FeTi which operate at low temperatures (~100ºC) and moderate pressures (<100 atm.). These metal hydrides offer good volumetric density (0.10- 0.12 kgH2/liter) but store only a few percent (2-3%) per unit weight of material. This results in a relatively low gravimetric density. An intermetallic metal hydride was demonstrated in a Prius by Texaco Ovonic which stored 3 kg of H2 and weighed 190kg and occupied 70 liters. This vehicle demonstrated a 150 mile range.

Hydrogen Storage (cont’d) Higher gravimetric and volumetric

storage densities can be achieved with higher temperature metal hydrides such as MgH2 and MgNiH2. These operate at temps. 300-350ºC and 5-10 atm. pressure. The metrics for these materials is 3-8 wt.% and 0.13-0.15 kgH2/liter. However, the high temperatures preclude these materials from use in vehicles.

Hydrogen Storage (cont’d) Chemical hydrides, such as sodium

borohydride (NaBH4), undergo chemical reactions to release hydrogen. In the case of NaBH4, the hydride is mixed with water and pumped through a catalyst to release the hydrogen. 29% of the hydrogen energy is released as heat requiring significant cooling. Also, the chemical hydride must be replaced and an infrastructure would be needed to process this material and the corresponding waste.

Documents

ECE 7800: Renewable Energy Systems Topic 13: Fuel Cells Spring 2010 © Pritpal Singh, 2010