Abstract
In arid climates, the optical properties of plastic-covered greenhouses deteriorate very quickly. To examine the effect of greenhouse shape and orientation on the degradation rate of cover optical properties, four greenhouse prototypes were constructed in two shapes (duo-pitched roof and tunnel), covered with a 200 µm thick low-density polyethylene film. Two types were oriented in the North–South direction, the other two in the East–West direction, and all were exposed for one year to an arid climate. Samples were taken from the different surfaces of each cover for testing. The total transmittance (Ts) and reflectance (Rs) of the samples were measured and averaged to obtain the whole cover properties (T and R). Measurements were carried out periodically every 30 days for the four prototypes. The degradation behavior of the optical properties of each cover surface (Ts, Rs) and the whole cover (T, R) was investigated for the four film covers during the exposure time. Results show that the degradation rate of Ts depends on the surface location and the cover orientation. Among the different surfaces of the four prototype covers, the maximum difference in the Ts value between the E and N surfaces for the tunnel cover oriented in the N–S direction was 15.5%. Although the variation of the Ts value among the different cover surfaces was found, the time dependences of the whole cover transmittance (T) for the four covers tested were almost similar. Accordingly, the shape and orientation of the small size greenhouses did not significantly affect the degradation rate of the cover optical properties. In the four covers tested, the reduction in the global solar radiation transmittance (T) was 27–31% after one-year exposure compared to the new film. Nonlinear correlation was developed to predict the degradation rate of the cover transmittance as a function of the accumulated solar irradiance.
Highlights
Each prototype was covered with a 200 μm thick low-density polyethylene (LDPE) film, which is popular in the local market and commonly used for cladding greenhouses in Saudi Arabia and the Arabian Peninsula
The low relative humidity (RH) induces dry weather throughout the year that negatively affects the lifetime of the film and its optical properties (Figure 2a)
The degradation of the optical properties of plastic film covers is due to the combined effects of climatic factors, i.e., Tam,max, SE, V w, and RH)
Summary
The arid climatic conditions of the Arabian Peninsula are usually characterized by hot and long summer seasons and dusty and dry weather. The ambient temperature often exceeds 45 ◦ C, the intensive solar radiation flux reaches 1200 Wm−2 , and the relative humidity of the air drops below 10% [1]. Cultivation in greenhouses is necessary to protect crops from such harsh environments and enhance and manage crop production throughout the year [2]. Most of the greenhouse covers in the Arabian Peninsula are made of low-density polyethylene (LDPE) film owing to its relatively good mechanical and optical properties, combined with its competitive market price [3,4]. LDPE films have high solar and thermal radiation transmittances; this allows easy thermal energy release from the greenhouse and reduces the greenhouse cooling requirement in summer
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