Experimental Investigation And Optimization On The Thermal Performance Of A Novel Beam Down Concentrating System Employing Fresnel Heliostats With Finned Absorber

As a significant form of solar thermal utilization,the beam-down concentrating solar tower system has the advantages of high efficiency solar-thermal conversion,simple structure and favourable wind resistance,thus playing a great potential in providing moderate or high heat energy and replacing traditional fossil fuel.In this paper,the optical characteristics and thermal performance of beam-down concentrating solar tower system based on a finned absorber employing linear Fresnel heliostats are analyzed both theoretically and experimentally.The main contents that has been finished are summarized as follows:Firstly,according to the model of solar position and sun-tracking of heliostats’sub-mirrors,the ray tracing model based on MCRT algorithm is established to study the factors that affect the optical efficiency of the beam-down concentrating solar tower system.After that,the efficiency factor and the heat removal factor of the finned absorber are deduced as the evaluation index of its working performance at different conditions.Secondly,the concentrating behavior and thermal performance of the beam-down concentrating solar tower system are investigated experimentally.Owning to the spot shape of the beam-down concentrating solar tower system,the test circuit involving the finned absorber is designed.The useful energy gained and the corresponding heat loss of the finned absorber at the different inlet temperatures and volume rates are experimentally achieved.When the inlet flow rate is580L/H:the average temperature of the working fluid within the absorber is 90.6℃,the thermal efficiency of system is 44.93%;the average temperature of the oil is 182.8℃,the thermal efficiency of system is reducing to 28.31%.At the same time,the heat loss of the finned absorber increases from 109.77W to 1465.13W.When the inlet flow rate is 400L/H:the average temperature of oil is 91.4℃,the thermal efficiency of system is 42.59%;the average temperature rises to 183.0℃,the thermal efficiency of system is as low as 26.65%.Simultaneously,the heat loss grows from 123.79W up to 1621.87W.By fitting of least squares method,the polynomial relationship between the thermal efficiency and the normalized temperature is obtained;The linear relationship between absorber heat loss coefficient and the difference of the inlet temperature and the ambient temperature is also getted.Through analyzing exergy efficiency combined with ideal Carnot cycle,the optimal value of the working temperature of oil under different inlet mass flow rate is attained:the optimal operating temperature is 142℃at400L/H,while it is 145℃at 500L/H.Based on the above experimental results,the varying characteristics of the optical efficiency of the beam-down concentrating solar tower system using linear Fresnel heliostats as a function of time is showed:the optical efficiency of the beam-down concentrating solar tower system reaches up to 58.42%in the vicinity of noon.The efficiency factor F’and the thermal removal factor F_R is also calculated:When the inlet flow of the working fluid is 580L/H,the heat transfer factor F’of finned absorber is changed from 0.877 to0.759 with inlet temperature increasing,and its thermal removal factor F_R changes from 0.857 to 0.748 simultaneously.When the inlet flow rate of the working fluid is 400L/H,F’changes from 0.864 to 0.736,and F_R changes from 0.844 to 0.724 with the inlet temperature getting higher.Then,the MCRT ray tracing algorithm is used to simulate the optical efficiency of the concentrating solar tower system at different moments.It is found that the concentrating solar tower system is mainly affected by the shadow efficiency the heliostat itself and cosine efficiency related to solar position.Based on the annual optical efficiency of the concentrating solar tower system,the structual parameters of heliostats and cylinderial secondary concentrator is optimized,optimal length and height is obtained:H=15m,L=5.35m.Once again,MCRT ray tracing model is used to simulate the optical efficiency after the optimization of the cooresponding parameters in one day of the experiment period.The optimal efficiency after optimitation is obtained,showing that the best improvement rises up11.2%compared to the values before.Finally,according to the coupled three-dimensional analysis model of MCRT and CFD,the thermal performance of the finned absorber during the photothermal conversion of the concentrating solar tower system is analyzed.By comparing the parameters such as themal efficiency and heat loss of the finned absorber under the experimental conditions,coupled three-dimensional analysis model of MCRT and CFD is validated.Furthermore,the efficiency factor F’and the heat transfer factor F_R of the finned absorber in the simulated working condition are calculated with the heat transfer and structural parameters of the finned absorber.By studying the effect of inlet volume rates on thermal efficiency of finned absorber at different inlet temperature,the optimal inlet flow rate of 670L/H was optimized by the three-dimensional analysis model of MCRT and CFD coupling.Based on the optimized length of heliostat and beam-down concentrator and the best operating condition of the finned absorber,thermal efficiency after optimization is simulated:compared with the results under the same experimental conditions,the thermal efficiency of finned absorbers increases by 21.32%at most;while compared with the simulated results before and after the optimization,the finned absorber’s thermal efficiency increases by 14.51%at most.