| Compared with the solar water heating system,the solar air heating system has the advantages of not easy to crack and overheat,simple system operation and maintenance.Compared with flat-plate air collector,evacuated tube solar air collector has the characteristics of small heat loss and good heat collection performance.Therefore,the evacuated tube solar air heating system is particularly applicable in rural areas with inconvenient maintenance and safety requirements,especially in plateau rural areas such as Qinghai and Tibet with abundant solar energy resources.Mastering the resistance characteristics and air volume distribution principle of solar evacuated tube air collector system is the key to ensure the expected heating effect.Due to the small air heat capacity,the solar air collector system generally requires large area,and the resistance design and balance in the system are complex.In practical application,the physical properties of air temperature change dramatically after absorbing solar radiation heat,and the flow process and thermal process are relatively complex,resulting in the fluctuation trend of internal resistance of the collector array.It is difficult to use the resistance calculation method under steady-state working conditions to describe it.How to analyze and calculate the resistance of solar air collector system under severe fluctuation is one of the engineering problems faced by the evacuated tube solar air collector system in practical application.In this paper,the flow process and thermodynamic process in the evacuated tube solar air heating system were analyzed in detail,and the main influencing factors of resistance characteristics in the system were determined.The mathematical model of resistance and air volume distribution of evacuated tube solar air collector system was established,and the comprehensive evaluation index of collector system performance based on resistance and thermal performance was proposed.The numerical simulation method was used to study the influence factors(solar irradiance,ambient temperature,inlet temperature,collector air volume and other environmental and operational factors,the number of series collectors and parallel rows,tube diameter and other connection forms)of evacuated tube solar air collector,series collector array and evacuated tube solar air collector resistance variation.The performance of the heating system was evaluated by using the comprehensive evaluation index of system performance with the optimization objective of more uniform air flow and temperature distribution of the heating field and higher heating efficiency.Finally,according to the system resistance calculation results,the simplified resistance calculation table was summarized.According to the research on the resistance characteristics of evacuated tube solar air collector system,the following specific conclusions were obtained :(1)According to the variation law of collector array resistance,the influence degree of each factor on the resistance of single row tandem collector system is ranked as follows : the number of tandem collectors > air volume > solar irradiance > collector inlet temperature > ambient temperature;(2)In the study of the resistance characteristics of evacuated tube solar air collector field,the connection form and air volume have a significant impact on the system resistance.Reducing the proportion of N(number of branch series collectors)and Nrow(number of parallel branches of collector field)and reducing the air volume of the system can significantly reduce the resistance of the collector system.(3)The collector resistance was proportional to the solar radiation intensity;With the increase of the diameter of the main pipe of the system,the system resistance showed a decreasing trend of change rate.In addition,the ambient temperature and the inlet temperature of the system have little influence on the system resistance;(4)Lower solar irradiance,lower inlet temperature,larger system air volume and larger collector main pipe diameter are more beneficial to the overall performance of the collector field when more uniform air volume and temperature distribution in each branch of the collector field and higher average collector efficiency are the optimization objectives.(5)When the heating area of the collector field is constant,increasing the proportion of N/Nrow and reducing the number of parallel branches of the collector field when the number of branch series collectors is constant can make the air flow and temperature distribution of each branch of the system more uniform and the comprehensive performance of the system better.In this paper,the resistance calculation model of evacuated tube solar air collector system was established,and the resistance calculation table of unit area collector was summarized.The resistance of evacuated tube solar air collector,collector array and collector field under different design parameters can be designed and calculated,which provides a basis for the selection and design of power equipment of evacuated tube solar air collector system. |
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