If energy is conserved why do we worry about world energy sources?

Entropy is the reason of worrying about world energy sources.

Explanation: Entropy is the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. The amount of entropy is also a measure of the molecular disorder, or randomness, of a system. For example, There are some reactions which are irreversible because some of the energy in the reaction is lost to environment instead of converting totally to the products. This lost of energy is entropy. Due to which the energy of universe is losing irreversibly. That's why we worry about world energy sources.