The Concept of Entropy and its Importance

A Simple Explanation of Entropy

<Based on my book: http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=ISBN+148409834X&gt;

The concept of entropy originates from the second law of thermodynamics. Energy tends to flow from a concentrated form to a dispersed form. For instance, petrol (gasoline) is an example of energy stored in a concentrated form. When an automobile engine burns petrol, the energy stored in petrol is released and gets dispersed in various forms. The energy is partly dispersed as hot gases, as heat transferred to the parts of the engine, and partly as mechanical work that gets done in the engine to move the automobile. Finally, all the energy obtained by burning petrol gets dispersed in the surroundings as heat. The energy so dispersed is not accessible anymore to do any useful work and such energy is termed as the unavailable energy. Entropy is essentially the unavailable energy and the second law of thermodynamics states that the entropy of the universe always increases. This implies that every energy conversion adds to unavailable energy.

To do any useful work, we need concentrated sources of energy and the ability to control when and how much energy will be released and utilized. In this process the concentrated source of energy gets dispersed and a part of the energy becomes unavailable. It is not possible to create a concentrated source of energy without giving a larger energy input that must come from some other concentrated source! For instance, hydrogen gas has a stored energy content of 123 MJ/kg. It has more stored energy per kg than petrol. However, hydrogen gas is not found in nature in the basic molecular form (H₂) required for combustion. To obtain hydrogen gas by electrolysis of water, it is necessary to provide an energy input that is larger than the energy contained in hydrogen. Quite obviously, it is meaningless to carry out the electrolysis using electricity obtained by burning coal. There will be multiple energy conversions in the entire process and primary energy spent per kg of hydrogen so obtained will be much larger than the 123 MJ obtained by combustion of hydrogen. Petroleum is a concentrated source of energy that was created in the crust of the earth as a part of various geological processes. While it is possible to create petroleum products in the lab, the primary energy required to create such products will always be larger than what is obtained by their combustion.

These examples illustrate the fact that as energy sources are consumed, it is not possible to create them by processes that do not use some other concentrated source of energy. In other words, the depletion of energy sources is an irreversible process. The only energy source that won’t deplete for about a billion years is the sun. Hence, the civilization cannot hope to be dependent on the fossil fuels forever. The decline of fossil fuels is inevitable and at some point in the future, it will be necessary to switch entirely to the use of solar energy, either in direct or indirect form.

Another important point that comes up based on the concept of entropy is that before proposing or advocating any new method of energy conversion or a new source of energy, detailed thermodynamic analysis is an absolute necessity. In absence of such an analysis, a wonderful idea in the lab may not provide any net energy.

To sum up, entropy and the second law of thermodynamics occupies a prime position among the laws of nature. Most of the struggle to find alternative sources of energy is essentially a struggle against entropy, which makes it so difficult. It is fairly common to read in newspapers about entrepreneurs, enthusiasts, and economists to have found a solution to the energy crisis. Yet, none of them turn out to be real solutions that are actually workable. It will be worthwhile to use an old quote from Sir Arthur Stanley Eddington that sums up the problem that most enthusiasts do not realize:

“If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations – then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation – well these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.”

This quote may be modified in the current context to be:

“You may find the elusive Higgs Boson at CERN by putting together the most brilliant scientific minds and spending billions of dollars; but finding a solution to the energy crisis that somehow beats the second law thermodynamics is a hopeless quest.”