Studies On Two-stage Imaging Concentrating Solar Thermal Application

The concentrating solar thermal(CST)technology has not been yet achieved in the industrial level subject to the discontinuity and instability of solar energy resource.One solution is to implement dispatchable operations by equipping with thermal energy storage(TES)units.For the flexibility of design,the two-stage imaging concentrating optical system can couple with large TES devices more easily to approach higheffective stable thermal storage/fuel generations.In this thesis,we focus on several key issues in two-stage imaging CST applications.A series of new designs and improved methods is proposed and the relevant mathematical modeling and numerical simulation work are then conducted.The main content can be concluded as follows:For the solar concentrating,a novel two-stage dish(2SD)concentrator system is presented.Different from the traditional 2SD,the new design composed of paraboloid primary and hyperboloid secondary mirrors has a special hollow structure which enables overlap concentrating.Monte Carlo ray-tracing(MCRT)method and optical geometrical principle are used in the modeling and analyses.Simulating work are conducted by the Trace Pro(?) tool and the in-house validated Matlab(?) code.The comprehensive performance is compared between the novel and traditional 2SD systems in aspects of optical efficiency,concentration ratio,intercept factor,irradiation homogeneity of focal spot,etc.,under different optical and tracking error conditions.For the solar thermal absorbing,a new design of a cavity receiver coupled to a2 SD concentrator is presented.A 1-D steady cavity heat transfer model is established based on the radiosity method while the data regarding incident irradiation distribution and view factor at the cavity inner walls are obtained by using MCRT method.Through simulating with the in-house code,main parameters of the new cavity design are optimized for approaching the best solar-to-thermal efficiency.The receiver fitted to a conventional and novel 2SD are both discussed.The later system configuration shows superior performance compared to the conventional one,in particular in terms of compact structure,uniformity of the incident flux and temperature distribution,and solar-to-thermal efficiency.In addition,we also propose a forced air circulation system to reduce the convective heat loss across the aperture of a cavity receiver.The function of the proposed system was well validated by analyzing convective losses under different conditions of inclination,airflow direction and rate.The basic analysis tool is a 3-D transient turbulent(k-?)model employing the Fluent(?) CFD software.For the high temperature TES part,a new design of integrated receiver-storage(IRS)unit is proposed which includes a novel heat transfer enhanced method by using a circulation air fan.The in-house programmes based on a 1-D decoupled and a 2-D coupled two-phase transient numerical heat transfer models are respectively developed.Thermal charging and discharging processes of 30 cycles are then simulated.The thermal performance of IRS-system is comprehensively assessed by calculating radial temperature difference inside beds,radiative absorbing efficiency,charging/discharging thermal efficiency and overall energy conversion ratio.For the thermochemical hydrogen generation part,a novel fixed-bed solar reactor system via ceria redox is presented for water splitting.Transient lumped parameter method is used for modelling the whole system including reduction,heat recuperation and oxidation processes.Based on the present technology conditions,the heat recovery effectiveness of solid phase and the overall thermal efficiency are first evaluated with optimized parameters of cavity numbers,nitrogen gas flow rate,and reduction temperature.Then,sensitivity analyses regarding gas phase heat recovery effectiveness and optical concentration ratio are also conducted.In addition,a new concept is proposed to improve the reduction efficiency with a novel nitrogen feeding strategy considering the nonlinear character of solar reducing process.Genetic algorithm(GA)is used to find the global optimal solution regarding nitrogen feeding strategy with variable flow and the corresponding best thermal efficiency.This study can be regarded as the design reference and theoretical basis for the relevant research and development regarding two-stage imaging CST technology.