Theoretical Research On Radiation Characteristics Of Solar Collectors With Multiple Configurations Based On Monte Carlo

With the rapid development of social economy and technology,problems such as energy crises and environmental pollution are becoming increasingly significant.Solar energy is the most abundant and sustainable clean energy source on earth,and photothermal conversion is the simplest form of solar energy utilization.Direct absorption solar collectors allow the working medium to absorb heat from solar radiation directly,reducing unnecessary energy consumption compared to conventional collectors and increasing the photothermal conversion efficiency of the collectors.Researchers have found that nanofluids have strong light absorption properties,making them effective at improving the light absorption properties of collectors.Currently,common direct absorption solar collector configurations include flat plate collectors and circular tube collectors.However,due to the larger optical thickness and complex optical path of a tri-prism collector,it may have a better opto-thermal conversion efficiency.Therefore,this study focuses on the direct absorption solar collector configuration,and investigates the influence of three different collector configurations,namely flat plate,circular tube,and tri-prism,on the radiation characteristics of the collector.A program was developed by combining Mie scattering theory and Monte Carlo method to calculate the spectral absorptivity（α）,spectral reflectivity（R）,spectral transmissivity（T）,and bidirectional reflectance distribution function（BRDF）of direct absorption solar collectors with different configurations.To investigate the effect of external conditions on the radiation characteristics of the collectors,five different nanofluids（Si O₂@Ag,Si O₂@Au,Si O₂@Ni,Ti O₂@Ni and Ti O₂）were selected as the working media and studied by varying the parameters of nanofluid type,nanofluid volume fraction,nanoparticle size,and light incidence angle.To improve the photothermal conversion efficiency of the collector,two absorbing elements,Si and Ti O₂,were coupled in the collector to study the effect of coupled absorbing elements on the absorption of the collector.The results show that the tri-prism collector has the largest spectral absorption among the three configurations of direct absorption solar collectors when the light incidence angle is 0^o.For flat plate collectors,the spectral absorptivity of nanofluid decreases with increasing aspect ratio when the spectral absorptivity of nanofluid is approximately less than 0.72,while it increases with increasing aspect ratio of flat plate collectors when the spectral absorptivity of nanofluid is approximately greater than 0.72.The spectral absorption curves of the circular tube collector and the flat plate collector with an aspect ratio of 1:1 are similar,but when the spectral absorption of the nanofluid is approximately greater than 0.72,the spectral absorption of the circular tube collector is similar to that of the flat plate collector with an aspect ratio of 1:3.The spectral absorption of the Si O₂@Ni nanofluid is the largest,while the spectral absorption of the Ti O₂nanofluid is the smallest.Additionally,the larger the volume fraction of nanofluid,the larger the spectral absorption.In the near-infrared band,the nanoparticle size has a significant effect on the spectral absorbance.In the 0.30-1.00μm band,the spectral absorbance decreases with the increase of nanoparticle size,while in the 1.00-1.50μm band,the spectral absorbance increases with the increase of nanoparticle size.When the incidence angle is less than 60^o,the absorption rate of flat plate collectors increases with an increase in the incidence angle.However,when the incidence angle is greater than 60^o,the absorption rate decreases with an increase in the incidence angle.On the other hand,both round tube collectors and trigonal collectors exhibit a decrease in absorption rate with increasing incidence angle.Under the same conditions,the Si coupled absorber element is more effective than the Ti O₂ coupled absorber element in improving the absorption rate of nanofluid in the collector.