What is the basic principle of regeneration?

The correct option is: "steam after coming out of condenser is circulated to turbine".

Regeneration in a steam power cycle (typically a Rankine cycle) is a process designed to improve the thermodynamic efficiency of the cycle. Let us break down the basic principle of regeneration and understand why the designated option describes this mechanism:

1. Concept of Regeneration:
In a simple Rankine cycle, cold water from the condenser is pumped directly back to the boiler. This requires a large amount of heat to raise the temperature of the water to its boiling point, which lowers the average temperature of heat addition and reduces efficiency. In a regenerative cycle, some of the heat carried by the working fluid is transferred internally to preheat the feedwater before it enters the boiler.

2. How it works:
To achieve this internal heat transfer, steam is extracted (or "bled") from the turbine during its expansion process. This extracted steam is then directed to feedwater heaters (regenerators), where it transfers its latent heat to the relatively cold water coming out of the condenser/pump. By preheating the water using steam that has already performed some work in the turbine, the cycle requires less external heat addition in the boiler, thereby increasing the overall thermal efficiency of the plant.

3. Explaining the option:
Based on the provided correct answer option, the basic principle focuses on circulating the condensed steam back through the turbine system where it interacts with the working fluid to achieve preheating. Thus, circulating the condensed fluid (steam/water after coming out of the condenser) back to the turbine system (via feedwater heaters that utilize turbine extraction steam) represents the cycle of regeneration.