To increase the thermal efficiency of a reversible power cycle, one can either increase Th with Tc constant, or decrease Tc while keeping Th constant, aiming to minimize the ratio of Tc/Th. However, due to entropy and the second law of thermodynamics, there are natural limits to the efficiency that can be achieved.
To increase the thermal efficiency of a reversible power cycle operating between two reservoirs at temperatures Th and Tc, one should increase Th while keeping Tc constant, or decrease Tc while keeping Th constant. However, the greatest efficiency improvements are obtained when the ratio Tc/Th is made as small as possible. In practical terms, this means aiming for the highest possible temperature of the hot reservoir and the lowest possible temperature of the cold reservoir. Nevertheless, natural limits imposed by entropy dictate that there will always be some heat transfer to the cold reservoir (Qc) and a perfect conversion of heat to work is impossible due to the second law of thermodynamics.
The entropy concept implies that there's always an inherent loss of available energy to do work when transferring heat from a hot to a cold reservoir. Thus, while attempts to increase a power cycle's efficiency by manipulating Tc and Th are valid, the second law of thermodynamics bounds this efficiency, reflecting the natural limits on how much useful work can be obtained.