Ideal regenerative rankine cycle

The correct option is "efficiency is increased".

An ideal regenerative Rankine cycle is a modification of the simple Rankine cycle designed to improve thermodynamic performance. Regeneration is the process of extracting a portion of the steam (partially expanded steam) from the turbine and using it to preheat the liquid feedwater before it enters the boiler.

Let us break down why this process leads to an increase in the cycle's thermal efficiency step-by-step:

1. Feedwater Preheating:
In a standard Rankine cycle, cold water from the condenser pump is fed directly into the boiler, requiring a large amount of heat input to raise its temperature to the boiling point. In a regenerative cycle, by mixing or exchanging heat between the extracted turbine steam and the feedwater, the feedwater enters the boiler at a much higher temperature.

2. Reduction in Heat Input:
Since the feedwater is already preheated before entering the boiler, the average temperature at which heat is added to the working fluid in the boiler increases. More importantly, the amount of external heat input ($Q_{\text{in}}$) required to turn the feedwater into superheated steam is significantly reduced.

3. Effect on Thermal Efficiency:
The thermal efficiency ($\eta$) of any power cycle is given by the relation:
$\eta =1-\frac{Q_{\text{out}}}{Q_{\text{in}}}$
By utilizing regeneration, the average temperature of heat addition is raised, which thermodynamically increases the cycle efficiency. Even though the turbine work output per unit mass flow rate is slightly reduced (since some steam is bled off early and does not expand fully to the condenser pressure), the percentage reduction in the required heat input ($Q_{\text{in}}$) is greater, resulting in a higher overall thermal efficiency.