日光温室顶部菲涅尔透镜的光热分离特性研究

为了进一步提高日光温室内主动蓄放热的热能利用效率,该研究在日光温室内的顶部空间,构建了基于曲面菲涅尔透镜的直散分离系统,该系统对顶部区域的空间利用率为25.8%。利用光学仿真软件对不同入射角的太阳光进行追踪,并对该曲面菲涅尔透镜在典型日条件下的接收效率和焦斑分布进行分析,得到一日内的变化规律。在直射光集热测试方面,正午时段内,该系统的集热效率可以达到45%。对比散射光环境对温室的影响,发现试验区全天光照度减小约为10%~40%。该文以主动集热土垄加温系统提升栽培土垄温度作为试验组,并与不加温对照组进行了比较。试验结果表明,系统可提高土垄温度4.5~5.0℃。连续晴天情况下,土垄加温系统的COP(coefficient of performance)为1.5~1.9。研究表明此新型温室集热方式可提高空间利用率,改善温室内光热环境,同时利用午间强直射光集热,实现太阳能综合利用。

Sunlight-heat separation characteristics of Fresnel lens on the top of Chinese solar greenhouse

The common south roof materials of Chinese solar greenhouses is transparent plastic film.Generally speaking,they have no significant advantage for adjusting incidence lights of greenhouses.Given that the solar irradiance above the light saturation point is meaningless for plant growth,as in summer,the strong sunlight at noon is surplus,that is,surplus light is unused.In order to improve the efficiency of the solar energy utilization in Chinese solar greenhouses,a separation of direct and diffuse light system based on curved Fresnel lens is established in the non-planting zone on top of the Chinese solar greenhouse.The system uses 25%of the space in the non-planting zone.The sun light is tracked from different angles of incidence,by using optical simulation software,and the receiving efficiency and focal spot distribution of the curved Fresnel lens under typical daily conditions are analyzed to obtain the rule of change within a day.Regarding direct light heat integration test,the heat integration efficiency of the system can reach 45%at noon.The results showed that compared with control group,heating system which was designed could increase the soil ridge temperature by 4.5-5.3℃.In the case of continuous sunny days,the COP of heating system was 1.5-1.9.The heating system was composed of Fresnel lens,vacuum tube,water tank,heating pipes under soil ridge and water submersible pump.The experiment was conducted from 16th Dec.2018 to 16th Feb.2019.Tomato was used as a model plant in the greenhouse.Compared with the impact of scattered light on plants inside the greenhouse,it is discovered that the illuminance of a whole day in the test area is reduced by 10%-40%.The study shows that the new type of greenhouse heat integration method can enhance space utilization and improve the light and heat inside the greenhouse.Also,heat is integrated by using strong direct light at noon to realize comprehensive utilization of the solar power.Test method:The direct light at the noon perpendicular to the specular surface is concentrated by the curved Fresnel lens and projected on the solar vacuum tube receiver to achieve the photothermal conversion.Part of the scattered light not intercepted by the receiver passes through the lens and falls on the plant blade at the bottom.Both the lens concentrator and the Chinese solar greenhouse are arranged in the east-west direction to receive sunlight,and the Fresnel lens concentrator is suspended in the non-planting zone at the top of the greenhouse using plastic ropes and fixed pulleys.The test system consists of 2 rows of 20-meter-long Fresnel lenses with a total heat collection area of 26 m2.The system also includes support mechanisms and receivers.The program can be controlled according to the local latitude,so that the angle between the center line of the curved Fresnel lens and the horizontal line is fixed to the maximum value of the sun's altitude angle of the day.In the experiment,the location was Beijing,which would be 73°on summer solstice and 26.5°on winter solstice.