Influences of solar frequency divided north wall structure on the light and thermal environment in Chinese solar greenhouses

The Chinese solar greenhouse is a distinctive cultivation facility in China, mainly distributed in northern China for crop overwintering production. The north wall plays an important role in regulating the temperature and light environment of the solar greenhouse, however, asymmetrical structure design leads to an uneven distribution of light and heat from north to south within the greenhouse, struggles to meet the production demands of winter crops. The light intensity on the canopy of the north area is lower than that the south during the day, and the nighttime temperature gradient in greenhouse gradually decreases from north to south. Hence, there is a necessity to construct a multifunctional wall structure with thermal insulation and heat storage capabilities, as well as light-reflective supplementary capabilities. In this research, a direct-absorption type infrared frequency division supplementation light plate (IFDSLP) composed of PVC hollow board, reflective film (polyester aluminum plated film, VMPET), and rubber insulation layer was developed. A water-based nanofluid was synthesized using antimony in oxide (ATO) and tungsten trioxide (WO₃), which was incorporated into the cavity of the PVC hollow board as a heat transfer and light-splitting medium. The ATO-WO₃ nanofluid stock solution used in the experiment consists of a volume fraction of 2.4% ATO and a volume fraction of 97.6% WO₃ stock solutions. In order to avoid agglomeration of nanoparticles and improve the stability of nanofluids, a volume fraction of 4.0% mixed solution of diethanolamine (C₄H₁₁NO₂) and triethanolamine (C₆H₁₅NO₃) was used as the surfactant. With an optical path of 10 mm and a volume fraction of 0.005%, the average transmittance of the fluid in the photosynthetically active band of 300-800 nm was 79.6%, and the average absorptivity in the near-infrared band was 85.4%. This study designed and evaluated the light distribution and heating efficiency of an active heat storage and release system (AHS), combined with nanomaterial optics in a solar greenhouse in Beijing. The heat collection area of the IFDSLP was 7.2 m², where direct solar radiation can be obtained, photothermal regulation test was conducted on clear winter days to evaluate the comprehensive performance of TFDSLP. During the day, sunlight irradiates the IFDSLP system, the nanofluids absorb the infrared band and convert it into heat, and the visible band is reflected into the crop canopy, the heated nanofluid is circulated with the heat storage tank to complete the heat storage cycle. At night, when the air temperature in the greenhouse is lower than the threshold, the high-temperature nanofluid in the heat storage tank flows back to IFDSLP and releases heat to the greenhouse to complete the exothermic cycle. The results demonstrated that IFDSLP system in conjunction with the liquid storage device enhanced canopy lighting and improving heat storage and release capabilities of the greenhouse, under the conditions of a 10 mm thick nanofluid layer and a 100 L nanofluid capacity, the total solar energy utilization of the IFDSLP system reached 71.9%. This represents a 25% - 28% increase in light radiation compared to traditional rear wall setups. The photothermal conversion efficiency was 36.4%, resulting in an average nighttime air temperature increased by 1.5 - 2.0 ℃. IFDSLP provides reflective supplementary light for crops in the greenhouse. The average increase in plant growth factors was about 20 percentage points, and the uneven distribution of light in the canopy of crop in the north and south was improved and tended to remain stable. Leveraging the existing solar greenhouse structure, this system can further enhance the photothermal environment within the greenhouse, providing an effective means to optimize the utilization of the full spectrum of solar energy.