A transient model of the interaction between crop, environment and greenhouse structure for predicting crop yield and energy consumption

Abstract

This paper describes a computer-based method of modelling the transient performance of greenhouses. The method was developed to assist in the design of low energy protected cropping structures to be used in the hot, arid inland climates of Australia. Because of the generalized form it has applicability to a wide range of climatic conditions. To facilitate the modelling procedure, a greenhouse is considered to be composed of a number of separate but interactive components. These are the cover, floor, growing medium, air space and crop. A particular feature is the use of a tomato crop model which responds to photo-synthetically active radiation, leaf temperature and CO 2 level. Design criteria were that the greenhouses should use only a small amount of conventional energy for heating when necessary and that they must operate at all times in an essentially sealed condition for continuous CO 2 enrichment. To satisfy the first criterion, solar air heaters, a rockpile thermal store and a moveable thermal screen were incorporated in the simulation model, while the second condition was met by simulating the performance of a total enthalpy wheel and evaporative cooler which dehumidifies and cools without ambient venting of the greenhouse. The paper presents details of the mathematical models of each component and lists the assumptions used with each. The simulated performances of a number of different greenhouse types in winter and summer are presented and an analysis of the simulated crop yields, energy flows and temperatures indicates that the model is simulating the expected trends in greenhouses.