Theoretical analysis of integration of solar-coal hybrid power generation system based on energy level coupling

Abstract

As a clean, free and non-depleting energy source, solar energy is attracting more and more attention. Owing to the defects such as low intensity and periodicity, the levelized cost of electricity (LCOE) of current solar-only power plant keeps high and the annual efficiency keeps low. Solar-coal hybrid power generation system is the power generation system which integrates the solar energy into the conventional coal-fired power cycle. This hybrid system is thought to be an efficient, economic and low risk approach for promising solar thermal energy utilizing as it possesses the advantages such as higher thermodynamic efficiency and lower LOCE. However, existing studies are mostly case studies, which merely focus on specific types of hybrid power generation systems. Unified theory has not yet been proposed. Based on the existing classification, the solar-coal hybrid power generation is normally classified into power-boosting and coal-saving types. The boosted power output and the saved coal amount are used for evaluating the performance of power-boosting and coal-saving hybrid power generation systems, respectively. It is found that the coal-saving hybrid power generation system is more suitable for practical occasions due to the dispatch of the national grid and the boosted power output could be equivalently transformed into the saved coal amount. Therefore, the coal-saving coefficient, indicating the relative saved coal amount, is chosen as the unified criterion for the performance evaluation of solar-coal hybrid power generation systems. Expressions for the coal-saving coefficient of various types of hybrid power generation systems are primarily conducted based on reasonable hypotheses. Then, the unified theoretical expression for the coal-saving coefficient is derived out. The physical meaning of each item in the unified expression is revealed in details. The relationship and differences between various types of hybrid power generation systems are also expounded based on the unified theoretical expression. The “superposition effect” of the coal-saving coefficient is revealed based on detailed analysis of unified theoretical expression. The input exergy from the solar field, and the efficiency variations of boiler and turbine both influence the thermal performance of the hybrid power generation system. It is found that the coal-saving coefficient is the superposition of input exergy from solar field and the efficiency change of boiler and turbine. Based on detailed analysis of the theoretical expression, it is found that the share of solar energy and the matching of energy levels between the solar thermal energy and the working fluid are the two main factors influencing the coal-saving coefficient. In the purpose of verification, the above theoretical analysis is applied into the case study, where the exhaust stream of last-stage high-pressure heater of a 200 MW coal-only power plant is substituted by solar energy. The analytical results are compared with the published data. The good agreement between them verifies the proposed theoretical analysis. The present study shall serve as the theoretical guidance for the design of newly-built and modified solar-coal power plants. What should be noted is that the proposed unified criterion and theoretical analysis are focused on the thermal performance evaluation of hybrid power generation systems. Other factors, such as environmental and economic aspects, are to be considered in the future study.