Establishment and application of daylighting efficiency model for the front roof covered with dust film in solar greenhouses

Highly efficient use of solar energy depends mainly on the reasonable daylighting design of the front roof in a solar greenhouse. This study aims to establish a daylighting efficiency model for the front roof of a solar greenhouse in a natural environment. Taking Yangling district, Shanxi Province of China, as the study area, the dust accumulation and distribution were also acquired on the surface of greenhouse film under the conditions of no rain for a long time in winter. The light transmittance of greenhouse film was evaluated under different solar incidence angles and ash accumulation. An ash accumulation distribution model and a light transmittance attenuation model were constructed for the greenhouse film, further combining with the calculation model of solar incidence angles. Matlab platform was selected to develop the script for the calculation program of light transmittance for the front roof in a solar greenhouse. The transmittance of the daylighting surface was first simulated using the geographic coordinates of a greenhouse, date and time, the curve shape of the daylighting surface, orientation, and the type of film materials. A field test was also carried out to verify the model in the study area. The results showed that: 1) There was a similar simulation accuracy of the transmittance model and the measured value when considering both the influence of dust accumulation and solar incidence angles. The average absolute error of calculated and measured values at three measuring points were 0.90%, 2.13%, and 2.02%, respectively. The transmittance efficiency of greenhouse plastic film decreased by about 16.2% in the case of ash accumulation, compared with that without ash accumulation. This indicated that there was great significance of ash accumulation to precisely predict the light transmittance. 2) The high transmittance of the winter solstice was taken as the design goal in the study area, while the point of 0.8m away from the bottom of the front roof in the greenhouse was determined as the control point. The optimal heights of control points on the front roof were 0.6 and 0.8m for the greenhouses with the parabolic and the single-oblique curves. Correspondingly, the solar incidence angles of the daylighting surface were within the reasonable range of daylighting angles within 2 h before and after noon on the winter solstice, when the construction orientation was 5° from the south by west. Therefore, the daylighting efficiency and energy capture were higher than those in the other orientations. 3) Two kinds of plastic films were tested to make sure the higher light intensity in the greenhouse. It was found that the white PO film performed better in winter, where the outside environment was in low light level and cold. As a result, it was suggested to select the white PO film as the covering material of the front roof. This finding can provide a promising theoretical foundation to calculate the daylighting efficiency of the film surface, thereby optimizing the daylighting surface structure in a greenhouse.