Effect of arid environment on radiative properties of greenhouse polyethylene cover Ibrahim M. Al-Helal * , Abdullah M. Alhamdan Department of Agricultural Engineering, College of Food and Agricultural Sciences, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia Received 30 June 2007; received in revised form 22 October 2008; accepted 20 November 2008 Available online 16 December 2008 Communicated by: Associate Editor Darren Bagnall Abstract Degradation of the radiative properties of a 200 lm-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 radi- ation (PAR), air temperature and relative humidity were made inside and outside two single-polyethylene-covered model structures. Each model structure was 1 m wide, 1 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 °C, as compared to 46.9 °C inside the control structure, while the average of outside temperature was 38.2 °C. The degradation of the polyeth- ylene 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 11 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. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Arid environment; Greenhouse polyethylene films; Transmittance; GSR; PAR 1. Introduction Greenhouses are used in order to create a more favorable environment that is essential for plant growth and productiv- ity, especially in an arid region such as Saudi Arabia. A greenhouse allows plants to grow during any season of the year by controlling light level, air temperature, moisture levels, and carbon dioxide. The greenhouses are usually cov- ered with a material (i.e., glass or plastic) that have the ability to transmit light that provides essential energy for plant growth and production. Intensity, duration and spectral dis- tribution of light affect plant response. Photosynthesis pro- ceeds only with visible light or photosynthetically active radiation (PAR) in the 0.4–0.7 lm waveband. Lozano et al. (1996) emphasized the importance of good solar radiation transmission in increasing greenhouse crop production. In addition, the climatic factors such as air temperature and rel- ative humidity are indirectly affected by the cover material. 0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.11.008 * Corresponding author. E-mail address: imhelal@ksu.edu.sa (I.M. Al-Helal). URL: http://faculty.ksu.edu.sa/alhelal/default.aspx (I.M. Al-Helal). www.elsevier.com/locate/solener Available online at www.sciencedirect.com Solar Energy 83 (2009) 790–798