| The Photovoltaic/Thermal(PV/T)system is an integrated system which can make full use of the solar energy by generating both electric and heat.Utilization of solar energy based on the PV/T system is proved effective and has incurred major international research interest.The fluid flow and heat transfer performance of cooling liquid in the cooling network at the back of the PV/T solar collector is the key influence factor of the entire PV/T system.However,the lack of design consideration on the cooling network in traditional PV/T system leads to poor temperature uniformity of the PV modules and excessive power consumption resulting in limited performance and difficulty in the widespread the integrated utilization of solar energy based on PV/T system.It is worth noting that,in recent years,considerable progress has been made in enhancing heat transfer performance of microchannels through optimized channel structures.Inspired by this,numerical simulation based on the Discrete Ordinates heat radiation theory on PV/T collectors is carried out in an attempt to enhance the performance of cooling network for PV/T system.A three-dimensional steady-state model have been developed to study the fluid flow and heat transfer in the cooling network at the back of PV/T solar collector.Various cooling pipe network structures have been examined,and the major factors that determine the performance of PV/T system have been discussed thoroughly.Based on the numerical results,the optimization methods for the fluid flow and heat transfer of the heat collector are proposed and the influence of the heat collector performance on the PV/T system is studied systematically.To study of the performance of PV/T solar collectors with different channel structures,a three-dimensional steady-state numerical model based on the Discrete Ordinates heat radiation theory is developed.The fluid flow and heat transfer characteristics of the cooling network of PV/T solar collectors under various solar radiation conditions have been examined,and the influence mechanism of the cooling channels on the heat transfer performance of the PV/T solar collectors have been studied quantitatively.The quality factor of the PV/T solar collector under different cooling channel configurations is evaluated.The results show that,compared to the disc shape and the serpentine shape channels the parallel channel presents lowest total pressure drop with highest quality factor and best economical efficiency.In the parallel channel the pressure drop of the entire channel gradually decreases with the increasing number of the branches.And when the number of branches reaches 10,the total pressure drop of the channel reaches a minimum value.The total pressure drop of the Z-channel is higher than that of other channel types.The fluid distribution in each branch of the C-channel is unbalanced.The total pressure drop of the I-channel is lower and the fluid distribution of the branches is more uniform than that of other channel types making the I-channel a reasonable cooling network candidate for the PV/T solar collector.The fluid distribution in each branch tends to be uniform with increasing ratio of the pipe diameter which is beneficial for lowering the average surface temperature of the battery and achieving uniform temperature distribution.The operating condition of the photovoltaic module can be improved accordingly.With the increasing of inlet velocity of the cooling network,the total pressure drop increases despite of the channels type while the surface temperature of the battery layer decreases with improving temperature distribution uniformity of the cell layer.To enhance the heat transfer performance of the PV/T collector with parallel channel cooling network,local structures with varying diameter are added to the cooling channels.Systematic study on the characteristics of the flow field and temperature distribution of the PV/T solar collectors affected by the types,height and distance of local structures with varying diameter have been conducted.The enhancing mechanism of local structures with varying diameter on the cooling processes of the PV/T solar collector is revealed.And the quality factor of the PV/T solar collector is evaluated under different configuration of local structures.The results show that the heat transfer between the fluid and the channel wall is enhanced significantly by adding the local structure with varying diameter.Consequently,the photovoltaic components can work under lower surface temperature with better performance.However,the total pressure drop is increased after adding the local structure with varying diameter due to higher local flow resistance,and the increment of the total pressure drop in the channel with a rectangular local structure is the highest among all types of the local structures.Increasing the height of the local structure and reducing the distance between the local structures can enlarge the heat exchange area between the fluid and the channel wall and increase the vortex intensity in the local structure as well thus enhance the heat transfer and the performance of photovoltaic module.Through the average temperature can be reduced the total pressure drop of the channel is increased indicating more pump power is consumed.A dynamic simulation model of PV/T system is developed using TRNSYS to simulates and optimizes the overall performance of the system.The results indicate that:the increment in the branches of the cooling network,the flow rate of the circling cooling water and the volume of the hot water tank all contributes to higher photoelectric and photo-thermal efficiency of the system,while the increasing rate slow down gradually.A systematic study of enhancing the performance of PV/T system by optimizing design of the cooling network has been conducted.This work not only has academic significance for further understanding of the radiative-convection-conduction coupled heat transfer mechanism of PV/T solar collectors,but will also provide optimization design for PV/T system and key technical support of integrated utilization of electric and heat. |
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