(222av) The Potential of Improving the Thermal Efficiency of Steam Rankine Cycles By Reheating

The steam Rankine cycle has been in development for more than 100 years. Its basic principle has been described in many thermodynamic textbooks. The common measures for improving the thermal efficiency of the cycle are also presented in these books, e.g. increasing the temperature and pressure of the main steam and using superheating, reheating and regenerative preheating of the boiler feedwater. The primary reason for using reheating is to reduce the moisture content of the exhaust steam from the last stage of the steam turbines. Reheating may increase or decrease the thermal efficiency depending on the comparison between the mean temperatures of the heat input with and without reheating [1]. A key parameter to be optimized is the reheating pressure. One-stage reheating is commonly applied in modern power plants, while two-stage reheating is less commonly used due to the increased complexity (thus increased investment cost) and diminished increment of the thermal efficiency. More than two-stage reheating has not been applied in practice and less discussed in literature. Infinite-stage reheating represents an ideal isothermal expansion process [2].

 With increasing concerns on CO₂ emissions and fossil fuel depletion, the improvement in the thermal efficiency of steam Rankine cycles is important. An interesting question is how the thermal efficiency can be improved by various measures in the long-term future. This paper investigates the improvement potential of using reheating from the thermodynamic point of view. The thermal efficiencies for one-, two-, three- and infinite-stage reheating are presented. The near optimal reheating pressures are obtained by sensitivity analysis for various efficiencies of the steam turbines. The mean temperature of heat input is compared. The results show that a higher thermal efficiency can be obtained if the exhaust steam from the last stage of the steam turbines is superheated when two or more reheating stages are used.

  References

[1] H. Spliethoff. Power generation from solid fuels. Berlin, Heidelberg, Germany: Springer-Verlag Berlin Heidelberg; 2010.

[2] C. Fu, R. Anantharaman, K. Jordal, T. Gundersen. Thermal efficiency for coal to power: from theoretical to practical assessments. Accepted to be presented at the 26^th International Conference on Efficiency, Cost, Optimization, Simulation and Enviromental Impact of Energy Systems, 16-19July, 2013, Guilin, China.