Coupling Mechanism And Integration Optimization Of Parabolic Trough Solar Aided Coal-fired Power Generation System

Electric industry is the most important energy industry for Chinsese economic development.Recent years witnessed the increasingly prominent problems of fossil fuel shortage and pullution emmision in China.To optimize the energy structure,conserve energy and reduce emission,it is urgent to further expand the share of power generation by renewable energy as well as improve the efficiency of energy utilization.Solar-aided power generation(SAPG)system collected solar thermal to preheat feedwater to save coal consumption or boost power generation.This technology features in diversity of integration modes,the stability of power output,the consistency of power demand from grid side and the flexibility of operation modes.Meanwhile,SAPG technology meets Chinese satisfaction of rapid solar energy development and its investment decrease.This paper aims to develop system modelling and study the coupling mechanism,thermodynamic characteristic,annual economic performance and design optimization of SAPG system.The major research contents are summarized below:(1)Modelling development and algorithm optimization of SAPG systemIn this paper,the model of key components in SAPG system are established or updated.The model of parabolic trough solar field with single-axis tracking could study the energy conversion process with various design parameters and radiation conditions.The matrix thermal balance equation for preheating system could do the off-design calculation of turine sub-system,which accelerated the calculation speed as well as reduce iteration times.The "black box" model of boiler and oil/water heat exchanger were updated in order to take the heat transfer calculation between each working fluid into account.The updated models could study coupling effect to boiler,optimize the heat exchanger area and temperature difference after integrating solar.Based on the established or updated models above,this paper designed two algorithms for thermodynamic calculation of SAPG system.The "black box algorithm" used the "black box" models of boiler and oil/water heat exchanger,which assumed parameters of main steam,reheated steam and inlet/outlet heat transfer fluid(HTF)unchanged before/after solar integration.The "modified algorithm" used the updated models of boiler and oil/water heat exchanger for the thermodynamic calculation of SAPG system.This algorithm could not only calculate parameters of main steam and reheated steam without replying on the above-mentioned assumption,but also do the off-design research of oil/water heat exchanger in order to get the amount of heat transfer and the inlet/outlet parameters of each fluid.(2)Steady thermodynamic characteristic and coupling mechanism of SAPG systemBased on the updated model and modified algorithm of SAPG system,this paper studied the relationship among the model accuracy,thermodynamic characteristic of SAPG system and integration modes.It showed the "black box algorithm" cannot be used in the situation when the pressure of replaced extraction steam was lower than that of inlet reheated steam.The integration of solar led to the increase of reheat ratio and the decrease of outlet reheated steam temperature,which was against the fundamental assumption for the "black box algorithm".With the outlet temperature of reheated steam decreasing,the safety of boiler's operation was threatened and its capability of doing work sacrificed,which worsened the cycle efficiency of SAPG system and efficiency of solar thermal to electricity conversion.Both of the "black box algorithm" and the "modified algorithm" could be used in the situation when the pressure of replaced extraction steam was lower than that of inlet reheated steam.The integration of solar did no effect on the reheat ratio and heat distribution of boiler,therefore,outlet temperature of reheated steam,the cycle efficiency of SAPG system and efficiency of solar thermal to electricity conversion could remain unchanged.With the pressure increase of extraction steam to be replaced,the cycle efficiency of SAPG system and the conversion efficiency of solar thermal to electricity become higher as well.Through coupling optimization of SAPG system,it showed using solar thermal to replace the third stage of extraction steam performs best in terms of its highest conversion efficiency of solar thermal to electricity,solar power generation and the safety of boiler.(3)Key parameters optimization methods of SAPG systemBased on the research of the energy conversion efficiency and annual economic performance of SAPG system,the universal methods for parameter optimization had been proposed.Not relying on the former empirical value,these optimization methods could further improve the efficiency of solar energy conversion and lower the investment cost.It showed that the mean temperature difference and HTF temperature drop would decide the heat exchanger area,heat transfer efficiency and the needed solar aperture area.This paper defined the ratio of annual collected thermal between single-axis and dual-axis tracking modes,which evaluated the ability to collect solar energy under various tracking modes.It showed that the N-S titled axis tracking performed best,with 11.4%and 33.8%higher than N-S horizontal axis tracking and E-W horizontal axis tracking,respectively.Through the analysis of solar-to-electricity conversion and annual economic performance,the distance between adjacent collector rows,solar aperture area and capacity of thermal energy storage(TES)could be optimized.It showed that the optimized distance of adjacent collector rows was determined by land cost and tracking mode;the optimization of solar aperture area and TES capacity aimed to maximize annual solar-to-electricity efficiency and minimize levelized cost of energy.(4)The establishment of collector 3-D model,thermal-structure coupling analysis and its safety performanceBased on the thermal-structure coupling analysis and 3-D modelling of collector,the distribution of temperature,temperature gradient and stress could be studied with the conditions of variable radiation and heat transfer coefficient.It showed that the collector could be divided into three zones,e.g.,corrugated part,straight part and their transitional region.The highest temperature,maximum Von Mises stress and thermal strain lay on transitional region between corrugated part and straight part.The increase of DNI or the decrease of heat transfer coefficient between HTF and inner pipe surface would lead the increase of maximum Von Mises stress,even higher than the yield limit of the pipe material.Finally,based on the steady evaluation of collector safety,its dynamic safety characteristic was analysed under the condition of abrupt DNI increase.Meanwhile,the variation of the dangerous collector zone,the highest temperature,temperature gradient and Von Mises stress with time was studied as well.