Why is heat unable to be used as an energy source?

Answer 1

First of all, that's not entirely true. It is only when heat energy is uniformly spread out (different objects have the same temperature) that it is unusable. If heat is concentrated in one place, part of the energy can still be used.

It is a fundamental law in our Universe, that there are irreversible processes. The reason for this is hard to understand, but scientists have a measure for this, a rather abstract quantity called "entropy" - and the way it is defined, entropy will always increase in any process (or, in the borderline case, not change, or hardly change) - but it can never decrease. It has been found by experience, and by many thought experiments, that no such process exists.

Answer 2

Heat can be used as an energy source through methods like geothermal power or solar thermal systems. However, in many cases, heat on its own may not be a viable energy source because it requires a temperature difference to transfer and convert into a usable form like electricity. This limitation can make it less efficient compared to other energy sources that can be easily converted into useful forms of energy.