gang Wang School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beinong Rd 2, Beijing 102206, China; Institut fur Energietechnik, Technische Universitat Berlin, MarchstraBe 18, Berlin 10587, Germany Yongping Yang 1 School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beinong Rd 2, Beijing 102206, China e-mail: yyp@ncepu.edu.cn Changqing Dong School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beinong Rd 2, Beijing 102206, China Tatiana Morosuk Institut fur Energietechnik, Technische Universitat Berlin, MarchstraBe 18, Berlin 10587, Germany George Tsatsaronis 1 Institut fur Energietechnik, Technische Universitat Berlin, MarchstraBe 18, Berlin 10587, Germany e-mail: georgios.tsatsaronis@tu-berlin.de Systematic Optimization of the Design of Steam Cycles Using MINLP and Differential Evolution The process synthesis and design optimization of energy conversion systems can be mod eled as a mixed integer nonlinear programming (MINLP) problem. The nonconvexity potential and the combinatorial nature of the objective functions and constraints largely suggest the application of heuristic search methods for global optimization. In this paper, a modified differential evolutionary algorithm is applied to a MINLP problem for optimizing the design of steam cycles based on a complex superstructure, containing a variable number and varying positions of reheatings, varying layouts of the feedwater preheating train, and a boiler feedpump turbine with steam extractions. The energy-savings potential from the existing system design was studied. The optimization of a 262 bar/600 °CI 605 °C unit with a single reheat shows that an efficiency improvement between 0.55 percentage points (PP) and 1.28 PP can be achieved. The optimal design of steam cycles over 650 °C was found to be different from those of the designs under cur rent steam conditions: a transition throttle pressure, above which the benefits of steam temperature elevation can be completely realized, is critical and, accordingly, three design zones associated with the match of throttle pressure and the steam tempera ture level are clearly identified with recommended ranges of reheat pressures. [DOI: 10.1115/1.4026268] 1 Introduction Electricity generated from unsustainable fossil-fuel-fired thermal power plants is dominating and will still dominate the electricity supply markets in the near future in most countries [1,2], especially in the developing ones such as China [3], Poland and South Africa. Power generation from fossil fuel combustion, especially coal combustion is regarded to be not very efficient and harmful to the environment due to the large emissions of green- house gases and pollutants, including SOx, NOx, particle matters, heavy metals, etc. As a consequence, the efficiency improvement of thermal power plants is much more emphasized nowadays than before, in order to cope not only with the ever-increasing fuel depletion rates but also with the ever-demanding environmental requirements [4]. The overall performance of thermal power plants during a cer- tain period depends on many internal factors [5] such as the initial technology selection, system design and optimization, additional system integration for waste heat utilization, auxiliary power 'Corresponding author. Contributed by the Advanced Energy Systems Division of ASME for publication in the Journal of Energy Resources T echnology . Manuscript received June 18, 2013; final manuscript received December 12, 2013; published online March 4, 2014. Assoc. Editor: S. O. Bade Shrestha. consumption, device states in operation and optimal control strat- egies with trade-offs of economic and security benefits, and some external factors including fuel quality, the allocated plant load, the state of the environment as well as local restrictions on pollu- tant emissions. Among these factors, the initial technology selec- tion, the design and optimization of the thermal power system especially of the steam cycle are the basis to realize a better ther- modynamic, economic, and environmental performance as well as a safe operation. The technology selection is mainly represented by the levels of the throttle and reheat parameters, according to which the subcritical, supercritical, ultra-supercritical (USC) and advanced ultra-supercritical (700 °C and higher-pressure live steam, AUSC) cycles are distinguished. According to the current status of technology, efficiencies of 45% on a lower heating value basis are achieved when the throttle parameters of main steam are limited at about 300bar and 600 °C [6], Perspectives of technol- ogy development aiming at a temperature of 700 °C and at pres- sures higher than 300 bar with plant efficiencies above 50% have been announced in Refs. [7-9]. Pressure and temperature of main steam are not completely independent and should be properly combined so that the exhaust steam from the turbine is neither superheated (a situation that might be caused by reheating) nor too wet (a situation that might be caused if no reheating is used) [10]. One of the thermodynamic benefits of improving steam Journal of Energy Resources Technology Copyright ©2014 by ASME SEPTEMBER 2014, Vol. 136 / 031601-1