Please describe what exergy is and articulate how it is diff

Please describe what exergy is and articulate how it is different from energy. Write your discussion with a certain system in mind. For example, you might choose to describe a fluid or gas that has been both heated and compressed.

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I am just looking for 4-6 sentences explaining the differences here. Thanks!

Solution

The energy of a body can be defined as the sum of all energies available in the system. It represents the absolute maximum emergy of the system.

On other hand, exergy is the amount of useful work that can be extracted and stored. Not all the energy of the system can be used by a device all the time.

For example, we all know that petrol (gasoline) has a certain calorific value of 45.8 MJ/kg. So, that is all the energy present in the petrol. If you burn petrol for warmth, you get all the 45.8 MJ/kg to be used. So, in this case, all the energy is exergy and available at your disposal.

But take the same petrol and put it into your engine to run your vehicle. Now, we have an efficiency of about 20%. Even if we try to optimize every single aspect to get the best efficiency, we know that we can only reach the limit of carnot efficiency. So, no matter how hard we try, our exergy is significantly smaller than our energy. i.e if efficiency is 20%, our exergy is 20% of our energy.