Solar Energy Vol. 65, No. 5, pp. 271–284, 1999 1999 Elsevier Science Ltd  Pergamon PII: S0038–092X(99)00002–X All rights reserved. Printed in Great Britain 0038-092X / 99 / $ - see front matter HEAT TRANSFER MODELLING OF SCREENHOUSES ² G. DESMARAIS, C. RATTI and G. S.V. RAGHAVAN Department of Agricultural and Biosystems Engineering, Faculty of Agriculture and Environmental Sciences, Macdonald Campus, McGill University, Ste-Anne de Bellevue, (QC) H9X-3V9, Canada Received 13 February 1998; revised version accepted 16 December 1998 Communicated by J. OWEN LEWIS Abstract—Several mathematical models to represent the behavior of the inside temperature of screenhouses were developed. These models take into account different screenhouse configurations as well as the fact that the roof can be built of a single or double layer of cover. The influence of four types of commercial screens (Lumite 32 and 50, and Tildenet 32 and 50) on inside screenhouse temperatures was analyzed. The concept of cooling through water spray was mathematically represented. Experimental data on real screenhouses was obtained at the Benin station of the International Institute of Tropical Agriculture (IITA-Benin), Republic of Benin, Africa. Four small scale experimental screenhouses having different structural configurations were tested. The experiments were done for different types of screens. Three important parameters required to complete the mathematical representation (transmissivity of the cover, t ; natural ventilation, N ; and global v heat transfer coefficient, U ) were determined independently from experimental screenhouse data. Comparison between simulation and experimental results showed that the mathematical models were appropriate to predict inside screenhouse temperatures. The differences between experimental and predicted inside temperatures were not greater than 28C for all the experiments tested. It can be concluded that a simple lumped approach such as the one presented in this article, can be safely used for prediction of single and double roof screenhouse inside temperatures, with and without water spray, once the values of t, N and U parameters have been determined. v  1999 Elsevier Science Ltd All rights reserved. 1. INTRODUCTION not only used to grow plants but also insects. In such climates, the overheating of the inside The greenhouse concept was developed many greenhouse temperature is a frequent problem, decades ago based on the selective transmissivity which is very prejudicial to living plants and of glazing to long or short wave radiation. This insects. An innovation to the well-known green- selectivity allows most of the sunlight radiation house concept, the ‘screenhouse’, is a promising (short wave radiation) to be transmitted through technology with respect to its inherent capacity of the cover while reducing thermal losses e.g. long providing near-ambient conditions by passive wave radiation, from the screenhouse. The green- cooling (natural ventilation, radiation absorption house technology has been useful in order to grow and evaporative cooling). plants and flowers in climates where natural The screenhouse concept, put forward by Ros- weather conditions do not permit the cultivation sel and Ferguson (1979), offers a number of of such species outside. This technology is nowa- advantages over the greenhouse or glasshouse in days quite well developed, allowing small and humid tropical climatic conditions. The major medium size greenhouses be easily purchased. advantage is its intrinsic zero energy input needs Structures such as greenhouses and growth during operation. The system is inherently natu- chambers must also provide, at reasonable ex- rally ventilated by means of the air going through pense, protection of experimental material against the screens that cover the structure. However, insects and airborne pathogens, as well as close to overheating is still a problem under many con- ambient temperature and sunlight. In many tropi- ditions. cal or subtropical countries, the greenhouses are The screens used as a covering for the screenhouse restrict the outside air from getting inside the screenhouse, resulting in low ventila- ² Author to whom correspondence should be addressed, Dept. tion rates especially in low wind conditions. A ´ ´ des Sols et Genie Agroalimentaire, Universite Laval, judicious choice of screen coverings has to be FSAA Ste-Foy, (QC) G1K 7P4, Canada. Tel.: 11-418- made. Screening is selected in relation to its 656-2131, ext. 4593; fax: 11-418-656-3723; e-mail: cristina.ratti@sga.ulaval.ca ability to exclude or include small insect pests. 271