Abstract
In hot and sunny regions, extensive solar radiation transmitted into greenhouses makes it essential to select a greenhouse cover with specific characteristics. Reflection and diffusion are the most important properties of covers for reducing solar heating load and scattering solar beams for better growth of plants. Three types of plastic films that are commonly used for covering greenhouses in arid climate were selected for this study. These were reflective-diffusive (RDF), diffusive (DF), and locally produced (CF) films. The films were used to cover three identical twin-span greenhouse models; the radiation components, air temperature (Ti), and relative humidity (RHi) were measured in each model. Transmission of solar radiation through each cover was characterized by determining: (i) The beam that is diffused during transmission, and (ii) the unscattered beam that is transmitted directly through the film. The results show that the diffuse radiation transmitted through the DF, RDF, and CF covers was enhanced, respectively by 77%, 85%, and 109% as a result of diffusing 34%, 33%, and 43% of the transmitted beam radiation by the DF, RDF, and CF covers, respectively during transmission. The diffusive nature of the tested covers increased the ratio of diffuse to direct beam radiation (D/B) from 0.3 outside the greenhouse to 0.77, 0.69, and 0.95 inside a greenhouse covered with DF, RDF, and CF, respectively. At around noon, the CF cover decreased Ti by about 5–10 °C and increased RHi by about 3%–5% compared to those under the DF and RDF covers. However, DF and RDF covers showed almost similar effects in Ti and RHi. The low-price CF cover showed higher diffusive-radiative properties than DF and RDF covers and can serve effectively in arid climate as an alternative covering material.
Highlights
During the last decade, greenhouses have been extensively used for crop production in hot and sunny regions to provide a suitable environment for plant growth [1]
One of the main advantages of using diffusive plastic films for covering greenhouses their ability ability to diffuse a portion of beam radiation during transmission
109% for the DF, RDF, and CF, respectively. These results indicate that the reflective-diffusive film diffuse beam radiation as the diffusive film (DF); the locally produced film (CF) diffused (RDF) has a similar capability to diffuse beam radiation as the diffusive film (DF); the beam radiation much more than the DF and RDF films
Summary
Greenhouses have been extensively used for crop production in hot and sunny (arid) regions to provide a suitable environment for plant growth [1]. This is because in arid regions, as in the Arabian Peninsula, high solar irradiance (up to 1200 W m2 around noon), high air temperature (above 45 ◦ C), and low relative humidity (below 15%) are the common weather during most of the months [2,3]. The radiation heat load can be reduced before it enters the greenhouse by excluding a portion of the incident solar radiation on the greenhouse cover [5,6].
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