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Friday, October 15, 2010

The Hydrogen Economy

This post is part of a series, Nonsense and the Second Law of Thermodynamics. The previous post is entitled Perpetual Motion.

The media often perpetuate the idea that the so-called hydrogen economy is the solution to all of our energy needs.  Hydrogen is abundant everywhere; in fact there are oceans full of hydrogen in the form of water, just waiting to be extracted, oxidized and used as an endless source of energy, right?

Hydrogen, may or may not be a good way to store energy, but hydrogen from water is not a source of energy.  When hydrogen is burned in oxygen, it forms water and releases energy, but it takes energy to reduce hydrogen from water.  One method of doing getting the hydrogen is by electrolysis.

Now burning hydrogen is not the only way to get the energy out, and electrolysis is not the only way to produce hydrogen.  In fact, there are much more efficient methods of conducting these processes.

Regardless of the methodology, however, the fundamental energetics remain the same, and the process is still governed by the first and second laws of thermodynamics.

If water could be electrolyzed with 100% efficiency, and then combusted with 100% efficiency, the energy required would exactly match the energy output.  One can imagine building a perpetual motion machine that would use the energy from the combustion of hydrogen to electrolyze water.

Even such an impossible machine could not do any useful work, because all of the energy would be required to keep it running.

In the real world, we cannot even build such a machine. To operate in the real world, the machine must create some heat that cannot be used to do useful work.  To compensate for this heat, it would be necessary to find an external source of energy.

Hydrogen for Energy Storage and Global Warming

Now hydrogen may be a perfectly good way of storing energy for later use.  Perhaps, a power plan generates hydrogen from water that can be used to run a car.

There are multiple schemes for doing such a thing, some involving catalysts, some involving materials that provide energy to split water.

One of the advantages of using hydrogen in such a manner is that the product of combustion is water.  Burning hydrogen does not produce carbon dioxide or other gases that drive climate change.

Water is a greenhouse gas, but its presence in the atmosphere is governed by temperature, wind, and the balance with water in the oceans and other terrestrial sources.

Water vapor acts as feedback to climate change, and a very important feedback at that, but it is not the driver for such change.

Nuclear Energy

When you hear someone advocating a hydrogen economy to replace a carbon economy , understand that they are most likely advocates of nuclear power.

There is nothing wrong with that: I am an advocate of nuclear power, but do not be fooled by some of the rhetoric of a hydrogen economy; the energy has to come from somewhere!

There are, of course, other potential sources of besides nuclear energy, but instituting a hydrogen economy will not in itself solve our energy problems.

The next post entitled, Heat Can Be Transferred From a Cold Body to a Hot Body: The Air Conditioner, looks at an everyday counterexample to a popular misunderstanding of the second law.

Sources
  • Atkins, P. W. Physical Chemistry, W. H. Freeman and Company, New York, 3rd edition, 1986
  • McQuarrie, Donal d A., Statistical Thermodynamics,  University Science Books, Mill Valley, CA, 1973 
  • Bromberg, J. Philip, Physical Chemistry, Allan and Bacon, Inc., Boston, 2nd Edition, 1984
  • Anderson, H.C., Stanford University, Lectures on Statistical Thermodynamics, ca. 1990.
  • Wikipedia: Hydrogen
  • Rifkin, Jeremy, The Hydrogen Economy: After Oil, Clean Energy From a Fuel-Cell Driven Global Hydrogen Grid, E-magazine, January/February 2003, volume XIV, no. 1

Contents
  1. Introduction
  2. What the Second Law Does Not Say
  3. What the Second Law Does Say
  4. Entropy is Not a Measure of Disorder
  5. Reversible Processes
  6. The Carnot Cycle
  7. The Definition of Entropy
  8. Perpetual Motion
  9. The Hydrogen Economy
  10. Heat Can Be Transferred From a Cold Body to a Hot Body: The Air Conditioner
  11. The Second Law and Swamp Coolers
  12. Entropy and Statistical Thermodynamics
  13. Fluctuations
  14. Partition Functions
  15. Entropy and Information Theory
  16. The Second Law and Creationism
  17. Entropy as Religious, Spiritual, or Self-Help Metaphor
  18. Free Energy
  19. Spontaneous Change and Equilibrium
  20. The Second Law, Radiative Transfer, and Global Warming
  21. The Second Law, Microscopic Reversibility, and Small Systems
  22. The Arrow of Time
  23. The Heat Death of the Universe
  24. Gravity and Entropy
  25. The Second Law and Nietzsche's Eternal Recurrence
  26. Conclusion


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