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

Abstract: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 2.5-5.3 ℃. 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.