Determination and analysis on solar radiation of trapezoidal soil wall and soil surface in solar greenhouse

Abstract: In a solar greenhouse, solar radiation is the only resource of energy, thus functioning as the essential prerequisite for balancing its energy. The wall and the ground are the major recipients of solar radiation as well as the major elements for preserving the energy. In Northern China, 95% of the greenhouses have soil walls; therefore, it is meaningful to study solar radiation in the greenhouses with soil walls. 　　Radiation probes were used to study the law of solar radiation distribution on a trapezoidal soil back wall and ground. The actual solar radiation on the greenhouse wall and ground was measured by balancing the probes with the walls and grounds respectively. That is to say, the probes at the wall were set inclined, while horizontal at the ground. Then, solar radiation was determined at the inner back wall's upper, middle and lower parts, and also at the ground's northern and southern parts. The radiation on the wall surface was determined by setting inclined and horizontal probes, and then the two groups of figures were analyzed. The results showed that the solar radiation determined by the two methods was strikingly different: the radiation quantity determined by setting inclined probes was much higher than that of horizontal probes. The quantity determined by inclined method was the actual solar radiation the wall ground received. Under the condition of closed tuyere, the results showed that: 1) Wall: the daily average solar radiation intensity at the back wall increased successively at the upper, middle and lower parts. On clear days, the total radiation at the three parts was 5.74, 9.70, and 11.08 MJ/m2 respectively, accounting for 21.6%, 36.6%, and 41.8% respectively of the total daily solar radiation. On cloudy days, the total radiation at the three parts were 0.99, 1.50, and 1.92 MJ/m2 respectively, accounting for 22.5%, 34.0%, and 43.5% of the total daily solar radiation. 2) Ground: the total solar radiation in the southern part was always higher than that in northern part. On sunny days, the total radiation at the two parts was 11.00 and 6.74 MJ/m2, accounting for 62.0% and 38.0% respectively of the total ground radiation. On cloudy days, the radiation at the two parts was 3.51 and 2.04 MJ/m2, accounting for 63.2% and 36.8% respectively of the total ground radiation. 3) Wall and ground: the total solar radiation measured along the ground was higher than that measured along the wall. On sunny days, the mean wall radiation was 8.117 MJ/m2 and the ground radiation averaged 8.280 MJ/m2. Therefore, the ground radiation was slightly higher than the wall radiation, but the difference was not so obvious on sunny days. on cloudy days, the total solar radiation fluxes to the wall and ground were 0.984 and 2.068 MJ/m2 respectively. The ground radiation was always higher than wall radiation on cloudy days. In summary, the study is both significant for optimizing the structure of a trapezoidal soil wall and meaningful for further discussing the thermal environment of the greenhouse.