Can Rankine cycle achieve Carnot efficiency?

In summary, the Rankine cycle is a practical thermodynamic cycle that utilizes real-world properties and cannot achieve the efficiency of the idealized Carnot cycle. This is due to factors such as entropy generation and limitations on turbine exit quality and pump capabilities. While it can approach the Carnot efficiency, it will always be less efficient in reality.
  • #1
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.
 
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  • #2
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
R. Boukhanouf - University of Nottingham said:
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.
 
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  • #3
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.
 

1. Can the Rankine cycle ever achieve Carnot efficiency?

No, the Rankine cycle cannot achieve Carnot efficiency. This is because Carnot efficiency is only attainable in a theoretical system with no friction or energy losses, which is not possible in a real-world system.

2. What factors affect the efficiency of the Rankine cycle?

The efficiency of the Rankine cycle is affected by several factors, including the temperature and pressure of the working fluid, the size and efficiency of the turbine and pump, and the efficiency of heat exchangers.

3. How does the Rankine cycle compare to other thermodynamic cycles?

The Rankine cycle is one of the most commonly used thermodynamic cycles for power generation. It is more efficient than the Carnot cycle, but less efficient than the Brayton cycle. It is also more practical and cost-effective than other cycles, such as the Stirling cycle.

4. Can the Rankine cycle be modified to improve efficiency?

Yes, the Rankine cycle can be modified to improve efficiency. This can be done by using higher temperature and pressure working fluids, incorporating reheating and regeneration processes, and improving the design and efficiency of components such as turbines and pumps.

5. What are the real-world applications of the Rankine cycle?

The Rankine cycle is commonly used in steam power plants to generate electricity. It is also used in geothermal power plants, where hot water or steam from underground is used as the working fluid. The Rankine cycle can also be used in refrigeration systems, such as in air conditioning units.

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