Yearly numerical evaluation of greenhouse cover materials

Abstract

The greenhouse internal microclimate, and consequently the plants growth, is formed from the combination of available solar radiation and ventilation. Transpiration and photosynthesis are affected by the PAR levels, as well as by the temperature, the wind velocity, and the air humidity etc. factors that vary along a day and along a year. The appropriateness of cover materials in a greenhouse depends on the crop as well as on the season of the year. Cover materials with high transmissivity in Photosynthetically active radiation (PAR) spectrum are desired during the winter but require significant ventilation during summer. Conversely covers with low transmissivity reduce the cooling requirements during summer but may prevent appropriate PAR to reach the plants during the winter. Consequently the evaluation of cover materials performance should be done along the whole year. In the present work the performance of four cover materials namely: Three-layer co-extruded film (3L), Ethylene vinyl acetate film (EVA), Thermal polyethylene film (TPE) and Rose Polyvinylchloride-based fluorescent (VPVC) were examined in terms of available PAR, temperature and air velocity in the plants’ level, for a tunnel type tomato greenhouse with side openings sited in the Central Greece.In order to evaluate the importance of the above mentioned parameters, the Ansys Fluent Computational Fluid Dynamic (CFD) code was used for a 2D simulation of transport phenomena inside the greenhouse like, the aerodynamics and the heat and radiation transfer in four wave length bands (Ultra Violet (UV), PAR, Near Infrared Radiation (NIR) and Infrared Radiation (IR)), using the method of finite volumes. The solar radiation was modeled with the Discrete Ordinates (DO) model and the daily variation of external solar radiation was introduced via User Defined Functions (UDFs) written in source code form. Plants were approached as porous materials. Unsteady simulations concerning eight characteristic days of the year (equinoxes, solstices and four intermediate days) were performed for 4 examined materials using climatic parameters regarding the area of Larissa, located in Central Greece. During the winter days the side openings are considered to be only 20% open with respect to the summer opening of them.Cover materials were numerically evaluated in terms of: (a) the available PAR distribution at crop level, (b) air velocity fields, (c) flow patterns’ discrepancies, and (d) the developed temperature vertical gradient distribution, during day time operation. From the simulations was concluded that the best material for the specific crop was the EVA.