The thermal efficiency of the Carnot cycle is given by: η = 1 - T2/T1.
So, the higher the value of the T1 higher will be the thermal efficiency. Here Carnot cycle is an ideal cycle and it can not be achieved in reality. But it can be used as a reference for real working designs like Rankine cycles.
Therefore, to increase the thermal efficiency of the Rankine cycle the temperature at which the heat is added into the system should be as high as possible.
There are two ways through which mean temperature of heat addition can be done.
1. Superheating the steam
2. Expanding the steam in stages and reheating the steam after every stage.
Reheating is a commonly used practice in which efficiency is improved by 4-5%.