What is the difference between available and unavailable energy?

The heat absorbed portion that is transformed into work is therefore helpful and accessible for our use, hence it is termed as available energy. But the energy or heat which is of no use is termed as unavailable energy.

Explanation:

A engine requires heat from the reservoir and transforms portion of it into function. The extra heat is expelled (generally atmosphere) to  cold reservoir. The refused heat can't be equal to zero, as per the 2nd law of thermodynamics. The heat absorbed portion that is transformed into work is therefore helpful and accessible for our use, hence it is termed as available energy. But the energy or heat which is of no use is termed as unavailable energy.