Physics behind Exergy

1. Apr 11, 2015

ishendra

Hello all. I know that exergy of of a system is the maximum available work which can be extracted before coming in equilibrium with the surroundings. However, i wanted to understand it at a micro level. Why certain forms of energy have less exergy. I will give you an example on the kind of analysis i want.. Heat is an 'inferior' energy. Now, how i look at it(it may be incorrect) is that because heat is simply effect of random molecular motion, hence, to produce useful work, only a part of that random motion is being used. Hence, only a part is helping in bulk motion of piston etc, thus producing less work. I need similar analysis for other types of energy.Thanks in advance.

2. Apr 11, 2015

rootone

Heat isn't inferior energy, it's just one form of energy.
The temperature of an object is one aspect of an object's total energy, it also has energy in the form of it's mass and the energy binding atomic nucleii together..
Some nuclear reactions can make part of that mass energy available but usually all we do with it is convert it to heat, then use some kind of conventional heat driven machine to further convert the energy into something useful, electricity in particular.

3. Apr 11, 2015

Randy Beikmann

ishendra, I read a very good summary of the background of this in a thermo book by Cengel and Boles. They grade different types of energy in terms of their "quality," based on their versatility. Mechanical energy has the highest quality, being 100% convertible to work or any form of heat. Next highest is high temperature heat, largely convertible to work, 100% convertible to low temperature heat. Lowest is low temperature heat, minimally convertible to work, more so to high temperature heat.
This is similar to your describing heat as "inferior" to mechanical energy. Of course, entropy varies in the opposite direction as energy "quality."
I agree with your thinking that the randomness in direction of thermal energy (heat) is what makes it less useful, because only part of the thermal energy in a material (say a gas) is in the right direction to produce work (by pushing on a piston or a turbine blade). The more degrees of freedom in the gas in an Otto cycle engine, the lower the efficiency. For example, a noble gas has 3 DOF's (translations only), giving it a specific heat ratio k of 1.67. Nitrogen has 5 DOF's and therefore a k of 1.40. If you plug the values for k into the Otto cycle efficiency 1 - 1/(CR^k-1), where CR is the compression ratio.
So in "short," I agree with your thinking.