WHY PURELY ISOTHERMAL HEATING LACKS IN RANKINE CYCLE?

Answer 1

In a Carnot cycle, the inside temperature remains constant during heat absorption. The Cylinder head act as heat source as well as sink . All of the heat addition occurs at the maximum temperature. But in a Rankine cycle, heat is added when the working fluid exits the pump. Hence, heating liquid, boiling liquid, and even the super heat occurs at temperatures lower than the maximum cycle temperature. The maximum cycle temperature occurs when the super heat gas enters the expansion turbine.

A Rankine cycle deals with real processes with system irreversibilties, heat loss and entropy. Carnot cycle is an ideal cycle.

Hope this clears why heat addition in Rankine cycle is not a pure isothermal process.