Study On Thermal Performance Enhancement Of Porous Media-assisted Flat-plate Solar Collector

As a new type of energy that can replace fossil fuels,solar energy is one of the most promising clean energy sources.The utilization of solar energy is significant for improving the energy crisis and protecting the ecological environment.Currently,a variety of solar photothermal conversion methods have been developed.However,as the continuous updating of the methods of photothermal conversion,traditional flat-plate solar collector with low thermal performance has not met the actual needs.Improving the thermal performance of solar collector has become a major research focus for all scholars.In this paper,a new type of flat-plate solar collector structure is proposed,in which a porous substrate with a cavity structure is installed near the upper absorber to promote the flow and heat mixing.The effects of cavity number（N）,cavity depth（H_D）,chamfered cavity structure design,porous media material,inlet mass flow rate（（?））,Darcy number（Da）,and insulation side arrangement of porous substrate on the thermal performance of collector are numerically analyzed.Both the Darcy Brinkman-Forchheimer extended model and the energy equation based on local thermal equilibrium（LTE）model are used to describe the thermo-flow fields in porous media zones.The results show that more cavities significantly improve the fluid flow mixing,and the global Nusselt number increases by 9.7%to 33.0%compared with one cavity structure.The cavity depth is smaller and the thermal performance of collector is better.The effect of Darcy number on the thermal performance of collector is obvious.When the Darcy number increases from 10^-5 to 10^-2,the PEC increases by 2.77 to 6.28 times.Increasing the flow rate can improve the global Nusselt number of collector,and the peak value of global Nusselt number is 60.81.When the porous media material is copper,the global Nusselt number can achieve the maximum of 115.94.Compared with the non-chamfered designs,the PEC value of collector increases by the maximum of12.5%due to the chamfered structure design of porous cavity.Furthermore,the porous substrate is arranged on the insulation wall,the advantages of porous media are not fully demonstrated,and the thermal performance of collector is not significantly enhanced.Based on the above research,in order to further improve the thermal performance of flat-plate solar collector,the nanofluids with high heat transfer are used as the heat transfer media.The effects of nanofluid types and concentrations on the thermal performance of porous collector under different Darcy numbers and flow rates are studied.The results showed that the Darcy number increases from 10^-5 to 10^-2,and the PEC value increases by 6.19 to 6.30 times.Compared with water as the heat transfer media,the PEC value of collector increases by the maximum of 7.73 times.Increasing the flow rate can accelerate the heat transfer rate of channel.When the flow rate increases from 0.024 kg/s to 0.072 kg/s,the global Nusselt number of collector increases by 13.6%to 14.2%.Increasing the concentration of nanoparticles slightly reduces the global Nusselt number,but it also increases the convective heat transfer coefficient and reduces the friction coefficient,thereby enhancing the comprehensive thermal performance of collector.When the concentration increases from 1.0%to 3.0%,the PEC increases by 4.3%to 13.4%.Compared with water,the PEC value increases by the maximum of 21.4%.In addition,when Cu-Water nanofluid is used as heat transfer media,the convective heat transfer coefficient,global Nusselt number,and PEC value of collector are all the highest.In general,the use of nanofluid enhances the thermal performance of porous collector.Fig.37 table 4 Ref.80...