Effect of arid environment on radiative properties of greenhouse polyethylene cover

Degradation of the radiative properties of a 200 pm-polyethylene film caused by exposure to the harsh environment of Riyadh, Saudi Arabia has been investigated over a period of 13 months. Measurements of global solar radiation (GSR), photosynthetically active radiation (PAR), air temperature and relative humidity were made inside and outside two single-polyethylene-covered model structures. Each model structure was I m wide, I m long, and 0.75 m high. The east side of each model structure was open for natural ventilation. One of the model structures was exposed to the environment during the 13-month period, while the other was kept indoor in a room to protect the polyethylene film from environment and was used as a control when measurements were carried out. Results showed that exposure to the environment reduced the polyethylene film transmittance to GSR and PAR. The reduction in solar radiation transmitted minimized air temperatures inside the structure. The average summer daytime temperature inside the exposed structure was 45.7 degrees C, as compared to 46.9 degrees C inside the control structure, while the average of outside temperature was 38.2 degrees C. The degradation of the polyethylene transmittance progresses rapidly during the first three months of exposure. The relative losses of GSR and PAR transmittance were 9% and 15.3%, respectively, after three months. However, the losses decreased to 5% and 8%, respectively, after I I months due to rainfall in the area. Radiation transmittance was found to be dependent on season of the year. The averages (standard deviations) of GSR and PAR transmittance for the control film during winter months were 0.8 (0.01) and 0.75 (0.00), respectively. Whereas, during summer months, the averages (standard deviations) were 0.75 (0.02) and 0.71 (0.02), respectively. The averages (standard deviations) of GSR and PAR transmittance for the exposed film during winter months were 0.77 (0.03) and 0.70 (0.03), respectively, as compared to 0.68 (0.01) and 0.61 (0.02), respectively, during summer months. (C) 2008 Elsevier Ltd. All rights reserved.