| Compared with water-cooled or heat pipe cooling PV/T system,the air-cooled PV/T system is more compact,lighter,easier to install,and lower in cost.Therefore,it has been widely used.Cooling channel,as a key component of air-cooled PV/T system,the heat transfer characteristics in the cooling channel not only have an important influence on the temperature distribution and variation of the PV panel,but also determine the change of the thermoelectric conversion performance of the system.In order to design the air-cooled PV/T system operating conditions more reasonably,improve the electrical efficiency of system and the ability to collect solar energy,it is necessary to understand the heat transfer characteristics in the cooling channel carefully.With the boundary condition of variable heat flux,the numerical study on heat transfer of cooling channel was proceeded in air-cooled PV/T system whose cooling channel is above the PV panel(case1).Besides,the heat transfer characteristics and performance of air-cooled PV/T system with cooling channel below the PV panel(case2)were compared and analyzed under the same condition.It is found that:(1)In the case1 system,the temperatures of glass cover and PV panel increase significantly in the entrance region and then gradually increase along the flow direction.Meanwhile,owing to radiation influence of cooling channel,the temperature of glass cover slightly decreases at the outlet while the temperature of PV panel rises.Moreover,the effect of radiation inside the cooling channel on the thermoelectric conversion performance of the case1 system is greater than that of the case2 system,but the case2 system has a better cooling effect on the PV panel than the case1 system.(2)When the temperature difference between ambient temperature and inlet temperature is small or the solar radiation intensity is large,the convective Nusselt number of glass cover of case1 system is prone to mutation.However,when the temperature difference gets larger or the solar radiation intensity is small,the convective Nusselt number of glass cover is unlikely to change.In addition,the convective Nusselt number of glass cover increases with inlet flow rate and ambient wind speed.(3)Under the circumstances that the inlet flow rate,solar radiation intensity,or ambient wind speed changes,the size and change trend of the convective Nusselt number of PV panel of the case1 system is almost the same with the case2 system.However,the convective Nusselt number of PV panel of the case1 system changes more obviously than case2 system when the inlet temperature changes.(4)Solar radiation intensity plays a leading role on the average electrical efficiency and inlet temperature has the greatest impact on the thermal efficiency among other parameters regardless of the location of the cooling channel in the air-cooled PV/T system.At the same time,the trends of total energy efficiency and thermal efficiency are uniform,and the tendencies of average electrical efficiency and total exergy efficiency are very similar with each other.Meanwhile,for case1 system,total exergy efficiency attains a maximum value at inlet temperature of 298.15 K while case2 obtains the optimum value at inlet temperature of 295.65 K.(5)From the perspective of the first law of thermodynamics,in the process of each parameter studied in this paper,the total energy efficiency of the case1 system is always higher than that of the case2 system.If the PV/T system pays more attention to the amount of energy,or is more biased towards the supply of heat,the case1 system is preferred.At the same time,from the perspective of the second law of thermodynamics,in some situations,the case1 system has greater total exergy efficiency than the case2 system.In other situations,the case1 system has lower total exergy efficiency than the case2 system.If the PV/T system is more focused on the quality of energy,or more in favor of the supply of electrical energy,it should conduct specific analysis to determine the merits of case1 system and case2 system performance. |
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