Question

The diagram shows a heat engine. In which area of the diagram is unusable thermal energy detected?

Answer 1

Nope, I disagree with the former answer. The answer is definitely Z. W area (boxed with red outline) is represented as the hot reservoir while Z area is the cold reservoir (boxed with blue outline). X area is the heat engine itself and Y area is the work produced from thermal energy from hot reservoir. Typically, all heat engines lose some heat to the environment (based from the second law of thermodynamics) that is symbolically illustrated by the lost energy in the cold reservoir. This lost thermal energy is basically the unusable thermal energy. The higher thermal energy lost, the less efficient your heat engine is. 

Answer 2

The answer is Z is unusable