This is a Thermodynamics problem Thank You I need to design

This is a Thermodynamics problem. Thank You!

I need to design a power plant to supply electricity. I am given the highest temperature of 1000 degree F and the lowest temperature of 80 degree F. I am also given the choice of two cycles; Rankine and Carnot. Which of these two cycles will you select in your design and why? Give all the possible reasons.

Solution

Carnot cycle is an ideal cycle and it is theroretically the most efficient cycle. However, it is impractical as it requires heat addition and heat rejection at constant temperature. In reality, this can happen only during phase change process or when the heat addition / rejection process is extremely slow. Therefore, Carnot cycle is difficult to be used in real world even though it is the most efficient cycle.

Rankine cycle is a more real-world cycle compared to Carnot cycle. Its efficiency is lower than Carnot. It requires heat addition / rejection at constant pressure instead of constant temperature. Powerplants usually run on Rankine cycle.

For the given case, Carnot eff = 1 - T_low / T_high

= 1 - (80 + 460) / (1000 + 460)

= 0.6301 or 63.01%

This is really high value. Typical Rankine cycles have efficiency ~40%.