Optimization experiment of light transmittance and active lighting mechanism of solar greenhouse

Abstract: Solar greenhouses without heating systems have been utilized extensively for vegetable production in Northeast China since the 1980s. Solar greenhouses play a critical role in self-researched and self-developed greenhouse structure and technologies in China. Solar greenhouses rely on sunlight, of course, as their primary energy source in the absence of a heating system. They are simple in structure, and relatively inexpensive to build and maintain through the winter. Sunshine is a scarce resource from November to next February, so the available light must be very efficiently utilized. Optical light transmittance is the most important characteristic of the solar greenhouse. Unfortunately for greenhouse designers, there exist few theories related or methods for analyzing the light transmittance characteristics of transparent materials. When natural light propagates from its origin into the greenhouse, a complex process of reflection and refraction is generated in the rough air/lighting-surface interface; the relationship between the inclined angle of the lighting surface and the transmitted natural light intensity was investigated at length in this study. We investigated the light transmission properties of typical Chinese solar greenhouse materials according to solar energy application theory. We developed a corresponding mathematical model, which was then used to simulate and analyze variations of indirect solar radiation in a typical greenhouse under varying parameters, (such as structural shape, latitude, azimuth, skeleton, covering material). We found that direct light intensity for a solar greenhouse could be improved by changing the inclined angle of the lighting surface from 25° to 35°; this improvement was predicted according to classical optical theory and verified through experimentation. The transmitted direct light intensity remained nearly constant when the incident angle was less than 10°, and then declined steadily when the incident angle exceeded 10°. Direct light intensity transmittance values were 85.68%, 76.47%, and 64.72% at the incident angles of 20°, 30°, and 40°, respectively. The transmitted direct light intensity significantly decreased when the angle of incidence exceeded 40°; the transmittance values were 53.38% and 39.67% at the angles of 50° and 60°, respectively. There was a highly significant correlation between the theoretical analysis and experimental results, which confirmed that the light intensity transmittance of the greenhouse lighting surface can be dramatically increased by slightly increasing the inclined angle of the lighting surface. The theoretical and experimental values of light intensity transmittance improved by 22.8% and 20.7%, respectively, when raising the incident angle from 25° to 35°. By comparison, the calculated values were, again, in good agreement with experimental values; the deviations in the calculated values can be amended through further research. The conclusions presented here may provide a theoretical reference for future studies on greenhouse lighting surface optimization design, as well as a practical reference for future solar greenhouse developers.