Photothermal Characteristics Of A New Composite Parabolic Concentrator Coupled With A Vacuum Tube Air Solar Collector Syste

Solar air collectors are a typical solar thermal utilization device widely used in fields such as agricultural product drying,space heating and ventilation,and dehumidification.By using a compound parabolic concentrator(CPC),the output temperature can be raised to the medium temperature range.This thesis designed a CPC-coupled vacuum tube collector with a flat plate absorber and conducted intensive research on its optical characteristics,photothermal conversion,and heat transfer characteristics.This provides a theoretical basis and practical guidance for the application of medium-temperature air collector and thermal storage systems.The main content and innovative conclusions of the thesis are as follows:(1)A design method was proposed for a concentrator surface shape that does not use the principle of edge rays,combining standard CPC surface shapes and design ideas.By using deformation methods such as translation,rotation,and truncation on the surface shape of the standard CPC,the optical efficiency curve of the collector is analyzed using the Monte Carlo ray tracing method.A multi-dimensional evaluation index based on optical efficiency and concentration time is used to obtain an optimized design method for a“separated absorber-concentrating surface”type concentrator collector with certain adaptability.To verify the effectiveness of this design method,an optical experimental verification platform was built for experimental verification.The results show that the optical efficiency of the new collector is similar to that of the“upper flat-plate absorber”standard CPC collector under the same conditions.The maximum absolute error between the optical simulation results and the experimental values is less than 1.5 mm,indicating that the optical simulation results are reliable.(2)A mathematical model about photothermal conversion and heat transfer was established through a combination of simulation and experimental verification.The heat collection characteristics of the collector were simulated using programming and ANSYS/Fluent.A corresponding photothermal conversion experimental verification platform was built for experimental verification,and the temperature distribution,velocity distribution and field synergy distribution of the collector were analyzed.The velocity distribution,temperature distribution and field synergy angle distribution of the collector unit were analyzed under the working conditions of axial north-south placement and southward inclination to the local latitude.Then,a comparison was made with the traditional one-pass air collector to discuss the thermal performance and advantageous parameter range of heat output under different inlet flow rates,inlet temperatures and irradiance.The results show that the maximum relative error between the simulated output temperature and the experimental value is less than 10%(°C),which is within an acceptable range for engineering.Compared with the one-pass vacuum tube collector coupled with CPC,the novel CPC-coupled collector has advantages in terms of output temperature,thermal efficiency,adaptability to flow rate changes and potential for temperature rise in series and parallel connections under most conditions.When the average solar irradiance after concentration is 800～1600W/m~2,the highest outlet temperature of the novel collector can reach 181.17°C,while that of the comparison collector only reaches 153.25°C.At a normal inlet temperature of 25°C and a flow rate of 1.02 m~3/h,the highest output temperature of a single tube for the novel collector can reach 146.05°C;while at a flow rate of 3.58 m~3/h,the thermal efficiency reaches its maximum value(59.21%).Field synergy simulation results show that the novel collector has greater potential for enhanced heat transfer.The most significant effect is achieved by strengthening in the middle part,followed by the inlet.(3)Optical optimization schemes and their effects for CPCs were investigated.The optimization method of adding baffles and its underlying principles were analyzed.The adjustment method of the rotating focusing surface and the changing patterns of the affected parameters during this process were explored.A new dimensionless evaluation index was established to analyze the changes in the index and optical advantage intervals in different seasons and typical cities,as well as the impact of geographical location on thermal performance.The results showed that CPCs have good focusing effects in spring and autumn but require adjustment in summer and winter.Adding the baffle with a height of 4.1 mm can increase the average optical efficiency from 81.25%to 83.59%,while expanding the range of high optical efficiency outward by about 2°.The diffuse radiation received increased by 5%～10%(average 6.94%),and direct radiation received increased by 1%～7%(average2.31%).A new dimensionless evaluation index G was established considering The amount of radiation received and occupied space.The results showed that the optimal tilt angles in summer and winter are 10°and 40°respectively,with the new index increasing by 2.5 times and 1.8 times compared to the original values,and the amount of radiation received also significantly improved.Performance for different cities showed consistent trends,but each city had slightly different optimal adjustment tilt angles and performance improved.When adjusting the optimal tilt angle in different cities,winter performance can be improved to 2.4～2.9 times the initial value,while summer performance can be improved to 1.8～2.5 times the initial value.