Can Rankine cycle achieve Carnot efficiency?

[Nirmal Jayanth]

Will a rankine cycle have efficiency equal to the carnot cycle, if the mean temperature of heat addition in rankine cycle is equal to the source temperature of Carnot cycle, and the mean temperature of heat rejection is equal to the sink temperature of carnot cycle.

My understanding is that in rankine cycle during heat addition, the working fluid temperature keeps on increasing at a constant pressure, where as heat is added from a source at a constant temperature. Hence the heat transfer occurs at a finite temperature difference, Thus the rankine cycle is internally reversible but not completely reverible. And its efficiency will be lesser than carnot efficiency.

 

[Mech_Engineer]

The carnot cycle is an idealized reversible cycle which doesn't take into account entropy generation or efficiency losses. The Rankine cycle utilizes "real world" properties of steam and compressors/turbines. As a result, the Rankine cycle can only approach the Carnot efficiency but never meet it.

Some more reading here: https://www.researchgate.net/post/Why_Rankine_cycle_is_less_efficient_than_the_Carnot_cycle

The Carnot cycle is a hypothetical cycle with no consideration to entropy generation. it assumes perfect heat transfer in the heat source and heat sink. The working fluid also remains in the same phase which is unlike the Rankine cycle where the fluid undergoes phase change. Under similar heat source and heat sink temperature a practical thermodynamic cycle such as Rankine cycle would achieve at best about 50% of the Carnot cycle efficiency.

 

[FQVBSina]

If we are going to investigate an ideal case, then there are assumptions you can make to have the Rankine cycle approach Carnot efficiency. Such assumptions may include: the turbine exit quality can be neglected, the pump may pump a mixture, and the components in the cycle does not generate entropy.

However, not assuming these assumptions is critical in a real Rankine cycle as pump cannot pump a mixture, and turbine exit quality of less than 0.9 will quickly damage the turbine. So in short, a real Rankine cycle cannot achieve Carnot efficiency.