CACHE Modules on Energy in the Chemical Engineering Curriculum: Fuel Cells

Jason Keith1, Don Chmielewski2, H. Scott Fogler3, Valarie Thomas3

1 Department of Chemical EngineeringMichigan Technological University

2 Department of Chemical and Biological EngineeringIllinois Institute of Technology

3 Department of Chemical EngineeringUniversity of Michigan

Outline

• Introduction and Motivation

• What is in each module?

• Where are the modules?

• What are the modules?

• Module walk-through

• Conclusions / Acknowledgments

Introduction and Motivation

• Alternative energy component missing from most departments

• Fuel Cells have been discussed in the political arena as an alternative energy solution– Need to educate ChE’s in this area

• Growth in number of fuel cell textbooks– Most do not have homework problems

• Modules can rapidly infuse new technologies into the Chemical Engineering Curriculum

What is in each module?

• Problem motivation• Reference to related sections and pages in popular

ChE texts• Example problem statement• Example problem solution• Home problem statement• Home problem solution• Link to web resources• Non-ChE textbook resources• Notes to instructor

Where are the modules?

• Current beta test website:http://www.chem.mtu.edu/~jmkeith/fuel_cell_curriculum

• Currently available for use by anyone!

• Ultimately linked through CACHE website

What are the modules?

• Mass and Energy Balances– Application of Heat of Reaction: Hydrogen vs.

Gasoline– Material Balances on a Fuel Cell– Energy Balances on a Fuel Cell– Generation of Electricity Using Recovered Hydrogen

• Thermodynamics– Equation of State for Fuel Cell Gases– Thermodynamics and Fuel Cell Efficiency– Vapor Pressure / Humidity for Fuel Cell Gases

• Fluid Mechanics– Friction Factor in Bipolar Plate Channel

What are the modules?

• Heat and Mass Transport– Conduction and Convection Heat Transfer in Fuel

Cells– Microscopic Balances Applied to Fuel Cells– Diffusion Coefficients for Fuel Cell Gases

• Kinetics and Reaction Engineering– Nernst Equation and Fuel Cell Kinetics– Using Plug Flow Reactor Equations for Fuel Cell

Voltages

What are the modules?

Module walk-through

• How long can you power a laptop computer with a type K hydrogen cylinder (49.9 L)?

H2 tank Fuel Cell

H2 feed line

Computer(Electric Load)

AnodeGas

Chamber

CathodeGas

Chamber

H2 and H2O out

Air in

Air out

Module walk-throughCourse: Material and Energy Balances

(Stoichiometery)Title: Application of Heat of Reaction: Hydrogen

versus gasolineMotivation: Use the heat of reaction to determine

the energy contained in a hydrogen cylinder, and determine the equivalent number of gallons of gasoline.

Reference: Felder and Rousseau, Section 4.6(3rd ed.)Each module has reference to popular text(s) forthe course

Module Example Problem

Example Problem: Determine energy generated for the combustion of a gallon of gasoline

• Step 1) Determine Hr for gasoline components

Hr,C7H16 = 7 HCO2 + 8 HH2O - HC7H16 – 11 HO2

Hr,C7H16 = -4816 kJ/mol

• Step 2) Similarly: Hr,C8H18 = -5461 kJ/mol

Each example has an easy to follow step-by-step

approach

Module Example Solution• Step 3) Weighted average (87% n-heptane and

13% isooctane): Hr,gas = -5370 kJ/mol

• Step 4) Determine mass in grams of components in 1 mol of gasoline: 13.0 g n-heptane and 99.2 g isooctane

• Step 5) Determine volume of these components:

The total volume is 162 cm3 or 0.043 gal

• Step 6) Determine energy per gallon:

-5370 kJ/mol /(1 mol/0.043 gal) = -125,000 kJ/gal

Module Hwk Problem

• How many kJ are there in a K cylinder of H2?

• How many gallons of gas is this equivalent to?

• How many gallons of water do you make from the hydrogen in the gas cylinder?

• What is the maximum time you could power a 100 W laptop with this H2 cylinder?

• Currently open to everyone; final version will be password protected• Recommend printing example and statement and giving to students as a handout

• Rest of step-by-step solution available at end of talk

Module Hwk Solution

• How many kJ are there in a K cylinder of H2?

Assume liquid water product, Hr,H2O = -286 kJ/mol

From ideal gas law, cylinder contains n = 278 mol

Energy content = -n Hr,H2O = 79000 kJ

Conclusions / Acknowledgments

• Fuel Cell Modules are for your use!• Contact one of the authors to participate• Acknowledgments of Partial Support:

– CACHE Corporation– JMK: DOE(DE-FG02-04ER63821),

NSF(DMI-0456537), and the Michigan Space Grant Consortium

– DJC: Argonne National Laboratory– HSF / VT: Vennema Professorship and

Thurnau Professorship

Module Hwk Solution

• How many kJ are there in a K cylinder of H2?

Assume liquid water product, Hr,H2O = -286 kJ/mol

From ideal gas law, cylinder contains n = 278 mol

Energy content = -n Hr,H2O = 79000 kJ

Module Hwk Solution

• How many gallons of gas is this equivalent to? Assume 112 g/mol and density 0.69 g/cm3

Recall gasoline heat of reaction = -5370 kJ/mol

Thus, hydrogen cylinder is equivalent to 14.7 mol gasolineConverting to mass we have 1646 g and then converting to volume gives 0.63 gal

Module Hwk Solution

• How many gallons of water do you make in a fuel cell?

Stoichiometry tells us 278 mol of H2 react to form 278 mol H2O

This is equivalent to 5000 g or 1.32 gal

Module Hwk Solution

• What is the maximum time you could power a 100 W laptop with this H2 cylinder?

To determine the upper bound on time we assume all hydrogen is converted into electricity (100% efficiency).

At a power of 100 W, 79000 kJ of energy would be consumed in 219 hours. In reality, you could expect the fuel cell to operate for 100 hours.