Simulation and control of ventilation rates in greenhouses
Introduction
Many attempts have been made to calculate the microclimate, especially the air exchange or ventilation, for commercial greenhouses [1], [2], [3], [4], [5]. Usually ventilation is used for replacing warm, humid, low-CO2 greenhouse air with cooler, drier external fresh air, but the plant organs status—their temperatures, reactions and water stress—form the actual focus of the control system. Insufficient ventilation can lead to too high plant temperatures, too low vapor pressure deficit (VPD) or severe CO2 depletion, all of which may be harmful to the plants. On the other hand, excessive ventilation may waste energy by necessitating additional heating in the winter or artificial cooling in hot weather; it may also impair the protection the greenhouse gives the plants against winds, insects and airborne diseases. Low humidity in the greenhouse, which may result from excessive ventilation, can cause high transpiration, which creates water stress in plants. Furthermore, ventilation must also be adjusted to enable efficient control of CO2 enrichment; if there is excessive ventilation the CO2 input must be increased to replace that which is removed [6]. Since plant transpiration plays a dominant role in the control of foliage and air temperatures in the greenhouse, the water status and transpiration of the plants must also be taken into consideration in programming the use of ventilation to adjust the greenhouse air temperature, humidity and CO2 level, and to control the temperature and reactions of the plants [7].
Ventilation can be generated by using fans (forced ventilation), or by exploiting wind and thermal buoyancy to create air flows through ventilation openings (natural ventilation). Complicated internal circulation and secondary air flows in the greenhouse are created by both forced and natural ventilations [6], [8]. Neither forced, nor natural ventilation create complete mixing or uniformity of the conditions in the greenhouse. It is very difficult, adequately, to predict the air flows and the air exchange rates created by either natural or forced ventilation in the commonly used greenhouses, because understanding of the physical processes that drive forced and, especially, natural ventilation is not sufficiently advanced [6], [9]. There is little information on the coupling between the heat and mass exchanges between the plants and the ventilating air, and their interactions with the physiological reactions of the plants makes the situation even more complicated [6].
Sophisticated techniques have recently been developed for the visualization and quantification of airflows and transpiration in greenhouses [10], [11]. They include the use of: thin thermocouples on static grids [12]; sonic anemometers [13]; and greenhouse models in wind tunnels [6] or saline water tanks [14]. Recent advanced models for characterization and study of these fluxes have included elaborate computer software tools, such as computational fluid dynamics (CFD) programs [15], [16], [17], [18], [19]. Not all of these methods take account of the contribution of the plants or of the external conditions, including wind velocity and direction, but the main disadvantage of all of these models and means is their complexity and their inability to provide results in real-time as required by a control system.
The present paper describes a simple model that has been developed for use in a control system and is capable of describing the ventilation of the greenhouse system with adequate accuracy as required by the control. This model is based on the use of simple energy and mass balances [20], [21], [22]. The full height of the greenhouse is divided into three vertically stacked horizontal segments. The naturally induced air flows, both internally and between the greenhouse and the environment, are presented as horizontal and vertical vectors that affect the temperature and humidity in these horizontal segments. The energy and vapor balances for the three segments connected by the airflow vectors comprise the ventilation model and describe the microclimate within these segments. These equations also account for the heat exchange between the internal atmosphere and the canopy, which absorbs radiation and transpires.
Section snippets
Experimental system and measurements
Input and calibration data for the model were collected during 3 years in a multi-span Azrum type greenhouse (Fig. 1) at the Besor Experiment Station in Israel (31°16′N, 34°24′E). Each span was 7.5 m wide by 22.5 m long, with ridge and gutter heights of 5.5 and 4 m, respectively. The ridges were oriented north-south (Fig. 1). The cladding material was a 150 μm polyethylene film with terrestrial infrared and UV absorbing additives (Ganeigar Co., Israel). The polyethylene walls could be rolled up
The basic model
The ventilation model comprises three vertically stacked horizontal segments (layers) (Fig. 2), each assumed to be a completely mixed region. This partitioning is justified by the observed vertical temperature profile within the greenhouse, during the middle of a summer day (see results), when the greenhouse is completely opened, i.e. the sidewalls and roof gaps are rolled up for maximum ventilation and cooling.
- 1.
The bottom region consists of the layer of air that covers the entire canopy of the
Measured climatic parameters
Typical daytime temperatures within the greenhouse, which has rolling sides and openings in the roof for ventilation, behave as depicted in Fig. 4. This mid-July behavior lasts for most of the summer, June–September.
Generally, all the temperatures change in a sinusoidal fashion, reaching their peak during the middle of the day, but not necessarily at the same exact time. For example, it can be seen (Fig. 4) that during the morning the temperature of the leaves, Tp, is higher than the outside
Discussion
The good agreement that was obtained between the airflows (gi0)mass and (gi0)energy, calculated from mass and energy balances, respectively, can be considered to confirm the validity of this model. Reducing the model to four unknowns, with seven equations still available, provides the means of double-checking the validity of the approach (at least as long as the assumptions appear to hold). Nevertheless, some reasonable assumptions are necessary for reducing the number of unknowns. If the early
Conclusions and future research
A simple model, which enables the calculation of greenhouse natural ventilation, is presented. The model is based on three vertically stacked layers of air within the greenhouse. These segments or layers of air are interconnected among themselves and between them and the environment by cross and vertical airflows carrying vapor and energy corresponding to their origin. The model comprises simple mass and energy balance equations describing the changes of temperature and content of vapor in each
Acknowledgements
This research has been supported in part by the Ministry of Agriculture, Israel, under contracts 306-0417-01 and 645-0029-00. The Technion researchers were also supported by the David & Miriam Mondry Research Fund, contract no. 034-175 and by the Fund for the Promotion of Research at the Technion, contract no. 034-198.
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