Environmental simulation of a greenhouse system in Kenya

Abstract

Greenhouse systems are becoming important as more Kenyans venture into horticultural farming. The environment of a greenhouse is an important factor that determines the quality of horticultural produce. This study was done in this area with respect to Kenyan conditions. The broad objective of the study was to develop a model to be used to simulate the environment of a greenhouse system in Kenya. The specific objectives of the study were to: Identify the pertinent physical parameters which affect the environment of a greenhouse system; use parameters identified to develop a mathematical model for simulation of environment of a greenhouse system; use computer simulation to solve the model developed; and verify and then validate the computer simulation model developed, using experimental data collected from a physical model greenhouse. The pertinent physical parameters identified from established works were: Solar heat gain; furnace heat; heat from equipment; plant respiration; photosynthesis; evapotranspiration; thermal radiation exchange between the greenhouse and its surroundings; conduction through the greenhouse floor; conduction through the greenhouse cover; ventilation; infiltration and ex-filtration through cracks; and condensation. A one dimensional mathematical model was developed based on energy balances on six elements of the greenhouse system which were: Cover; air; vegetation; soil surface; first soil layer and second soil layers. Non-linear differential equations were used to represent mathematically the interactions between the six elements. A dynamic computer simulation program (GREENSIM) was developed in DELPHI-5 environment for numerical solution of the simultaneous differential equations using the fourth-order Runge-Kutta method. The inputs into the computer simulation program included: Global solar radiation, the external temperature and relative humidity, and average external wind speed. The outputs were: Cover, air and soil surface temperatures and relative humidity of the air in the greenhouse. The computer simulation model developed was verified and then validated using a five days data collected during July 2003, from a naturally ventilated, polyethylene covered, single even-span greenhouse situated at University of Nairobi Field Station, Kabete Campus. Good aqreernents were obtained between the simulated and measured values. The correlation coefficient (R2 value) between the measured and simulated cover temperature, interior air temperature, soil surface temperature and relative humidity were: 0.92; 0.96; 0.76; and 0.80 respectively. Sensitivity analyses done showed that global solar radiation, vents area, initialization and wind speed had influence on the model output. The computer model can be used to test the effects of changing design parameters on the environment of a greenhouse. It can also be used to predict and analyse the behaviour of microclimate of a particular design of greenhouse under different climatic conditions, without need for expensive experimentation, so long as meteorological information about the particular area is known. It is a tool which can be used for rational decision making about the most appropriate design of a greenhouse system.