# What it meant by quality of energy?

1. Feb 14, 2015

### KOUSIK

I know that energy has quantity but what it means that energy has quality also??
what it meant by quality of energy?

2. Feb 14, 2015

### SteamKing

Staff Emeritus
I'm afraid you need to supply more context here. If you can provide any references which talk about 'quality of energy', that would be helpful.

3. Feb 14, 2015

### KOUSIK

Reference: Thermodynamics:An Engineering Approach by Yunus A.Cengel and Boles
I read about it in second law of thermodynamics chapter
"The use of second law is not limited to identifying the direction of processes. The second law also asserts that energy has quality as well has quantity. The first law is concerned with quantity of energy and transformations of energy from one form to another form with no regard of its quality.Preserving the quality of energy is a major concern to enineers, and the second law provides the necessary means to determine the quality as well as the degree of degradation of energy during a process. More of high temperature energy can be converted to work, and thus it has a higher quality than the same amount of energy at a lower temperature."

4. Feb 14, 2015

### SteamKing

Staff Emeritus
This passage seems to be discussing what used to be called 'availability', rather than quality. This is also called 'exergy' in some texts:

http://en.wikipedia.org/wiki/Exergy

For instance, water has a relatively large specific heat capacity, so a large body of water contains a staggering amount of heat. The major problem in extracting some of this heat from a body of water in order to do work is that usually there is little temperature difference between the upper portions of the body of water and the atmosphere. Thus, you could say, the quality of this heat is rather low. OTOH, if you take some of this water and put it into a boiler, converting it to steam, you have made a high quality source of energy.

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