| dc.description.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. | en |
