ANNALS OF THE FACULTY OF ENGINEERING HUNEDOARA 2006, Tome IV, Fascicole 3, (ISSN 1584 – 2673) FACULTY OF ENGINEERING HUNEDOARA, 5, REVOLUTIEI, 331128, HUNEDOARA 161 THE AIR LAYER THICKNESS INFLUENCE ON GREENHOUSES MEMBRANE COVER HEAT TRANSMISSION Olivera ECIM, Ivan ZLATANOVIC, UNIVERSITY OF BELGRADE, FACULTY OF AGRICULTURE, SERBIA Summary: Heating constitutes the major energy requirement for greenhouses. Measures that minimize heating requirement include designs that minimize surface area and cover heat losses, maximize the penetration of solar radiation, use heat conserving covering materials, and features that store day time solar energy for release into the greenhouse at night. As well as protecting plants against adverse climate and pests, a greenhouse provides an elevated temperature during the day. Energy efficiency improvement is currently one of the mayor concerns in greenhouses design development. Plastic membrane covered greenhouses thermal losses are larger comparing to the glass covered, due to cover low thermal properties and thickness. In order to decrease energy losses, heat transfer through the two layer foil cover with air layer inside, has been investigated, particularly heat transfer coefficient dependence on Nu and Ra number and natural convection onset in air layer within two plastic foils. Keywords: greenhouses, natural convection, heat transfer coefficient, air layer, temperature, foil, energy losses, 1. INTRODUCTION Designing a system for saving environmental conditions in agricultural objects, especially in greenhouses, is a complex process, due to large number of influential factors and inter-dependence: environmental hydro-meteorological parameters, plant species growth and yield parameters, as well as agricultural economic income restrictions /1/. In plant-growing structures, it is necessary to coordinate expenses of the system for keeping environmental controlled conditions, and find necessary environmental control degree in production, i.e. permitted level of deviation from ideal conditions, which will not endanger product quality. Since dominant expenses during greenhouse exploitation are the ones for energy used to compensate closed space heat loss, the main objective of the energetic optimization of these structures is to reduce coefficients of covered areas heat transmission. Usual covers nowadays are single or double-layered constructions made of glass or plastic materials. Plastic covers are thin, light foils, and put over cheap and light constructions, using minimal effort and material. Due to inferior thermal characteristics, plastic covers’ heat losses are considerably bigger than with glass covers (with similar interior temperature conditions).