| A water-cooled PV/T system,which is increasingly concerned for its convenience,no need for secondary heat transfer and high efficiency,was proposed in this paper.The unit area mass flow is a operating parameter obviously affects the performance of the system and the existence of the optimal flow rate has been confirmed by many scholars.In view of the two key problems existing in the current research,including valuing the optimal flow rate without consideration of operation energy consumption and the lack of the optimal flow control law.Variable flow control strategy based on the value of solar radiation,two engineering application schemes have been proposed,and operation effect of variable flow control strategy in different solar resource zones has been analyzed.The water-cooled PV/T hot water system experimental test platform was constructed and the mathematical model was developed.Comparing the experimental results with the simulation results,the results showed that the simulated values of parameters were in good agreement with the experimental values.The comprehensive efficiency,which combines the photovoltaic and photo-thermal energy gain with pump consumption,is chosen to be the evaluation criterion of optimal flow rate.Taking the example of summer conditions in Nanjing as an example,the trend of the performance of the system is analyzed with the model,and the corresponding flow range is given.The result shows that the comprehensive efficiency increased slowly and then decreased rapidly.The optimal flow rate exists which makes the maximum comprehensive efficiency 47.5%is 0.022kg·m-2·s-11 in this condition.Most of the studies have speculated that solar radiation intensity has the greatest impact on the optimal flow rate according to the experimental results.While other parameters cannot be controlled in the experiment,and there is a lack of theoretical verification.The impact of flow change on the water-cooled PV/T system performance under different environmental parameters has been conducted using normalized coefficient method in this paper,and the results show that solar radiation intensity plays a decisive role in valuing the optimum flow rate.Meanwhile,the simulation results show that the same radiation intensity under different conditions corresponds to the same optimal flow rate.In short,solar radiation intensity plays a decisive role in valuing the optimal flow rate.At present,flow control strategy mostly starts from the temperature difference between import and export and the demand of water temperature of users.In this paper,the variable flow operation strategy of water-cooled PV/T system based on solar radiation intensity is put forward innovatively.The control law which can be used to calculated the optimal flow rate in any condition is m=1.45×10-5·I+0.0108.Meanwhile,combining the variable flow operation strategy with the user demand,the real-time flow control scheme and average flow operation scheme are proposed.The former,which gains the highest comprehensive income,is suitable for large-scale application,and the solar radiation intensity is read in real time and the best flow value under this working condition is calculated according to the control law to control the running flow.The latter that is economical and practical is suitable for small systems,which can regulate the corresponding operation flow of each season through the three gear switches of pumps.The effects of two variable flow schemes on the performance of water-cooled PV/T system in different solar radiation resource zones?Hohhot,Nanjing,Xi'an,Chengdu and Guiyang?are compared and analyzed.The simulation results show that with the fixed flow operation(0.05kg·m-2·s-1)as the reference object,the system that adopts the real-time flow control scheme improves the comprehensive income achieving 3.93%in the winter conditions of the most abundant solar resources in Hohhot area,22.3%and 57.3%respectively in the winter condition of the short solar energy resources in Guiyang and Chengdu region.It can be seen that the variable-flow operation scheme has obvious advantages in the areas of short solar energy resources,and the system performance is better under low radiation conditions such as winter.Finally,the application strategy of variable flow scheme is put forward based on the simulation results of all regions throughout the year.The variable flow operation scheme is suitable for the general zone of solar energy resources and the city short of solar energy resources,and it is not recommended to apply in the city with abundant solar energy resources.The variable flow operation scheme,which is not recommended for summer use,should be used first in winter and other low radiation intensity condition. |
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