CACHE Modules on Energy in the Curriculum

Fuel Cells

Module Title: Application of Heat of Reaction: Hydrogen vs. Gasoline

Module Author: Jason Keith

Author Affiliation: Michigan Technological University

Course: Material and Energy Balances (First chemical engineering course)

Text Reference: Felder and Rousseau, section 4.6

Concepts: Gas law, heat of combustion

Problem Motivation:Fuel cells are a promising alternative energy technology. One type of fuel cell, a proton exchange membrane fuel cell reacts hydrogen and oxygen together to produce electricity. Fundamental to the design of fuel cells is an understanding of heat of reaction of different fuels. In this problem we will use the heat of reaction to determine the energy contained in a hydrogen cylinder, and determine the equivalent number of gallons of gasoline.

Consider the schematic of a compressed hydrogen tank feeding a proton exchange membrane fuel cell, as seen in the figure below. The electricity generated by the fuel cell is used here to power a laptop computer. We are interested in determining the maximum amount of time the laptop can be operated from the compressed hydrogen tank.

Problem Information

Home Problem Statement:

A gas cylinder is full of hydrogen gas at room temperature and 2000 psig pressure. This cylinder has been proposed as a fuel tank for a fuel cell powered vehicle. Note that the volume of the cylinder is 49.9 L.

  1. Determine the energy content in kJ within the cylinder when the hydrogen is combusted to make liquid water. Use the ideal gas law, PV = nRT.
  1. How much gasoline in gallons is this energy equivalent to? Assume liquid gasoline has a heat of reaction of Hr,gasoline-5370 kJ/mol, an average molecular weight of 112 g/mol, and a density of 0.69 g/cm3.
  1. If the hydrogen in the tank is combusted with oxygen to make water, how much liquid water is produced?
  1. Suppose you have a 100 W fuel cell (which can power a laptop computer). What is the maximum amount of time you can run the fuel cell (hint: use the total energy from part a).

Additional information:

Hf H2O = -285.84 kJ/mol (liquid water)

Hf H2O = -241.83 kJ/mol (vapor water)

1st draftJ.M. KeithJune 5, 2007

RevisionJ. M. KeithJuly 25, 2007

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