The purpose of this study is to analyze the thermal environment inside greenhouse by means of system identification and to indicate the applicability of system identification. The experiment was carried out in a vinyl house with the floor area of 58 m
2 and the surface area of 190 m
2 in July and August of 1991.
At first, the authors investigated the mechanism which determines the thermal environment inside greenhouse under no ventilation. The assumed mechanism is shown in Fig. 1. The measurement points in this experiment are shown in Fig. 2. Figure 3 is the block diagram of the heat flow which expresses the assumed mechanism of Fig, 1.
The results of identification of a system with an input of radiation outside greenhouse and an output of air temperature at the point 2M are shown in Fig. 4. The calculated air temperature is in good accordance with the measured air temperature. The step response indicates that increase of 1 kW⋅m
-2 in radiation causes increase of about 30°C of inside air temperature. Figure 5 shows the relationship between order and error in identification of the system.
The results of identification of a system with an input of air temperature at the point 2L and an output of air temperature at the point 2M are shown in Fig. 6.
Figure 7 shows the measured and calculated air temperature at the point 2M. Calculation was executed according to the block diagram of Fig. 3 from radiation outside greenhouse.
Furthermore, the authors investigated the mechanism where the thermal environment inside greenhouse is determined by both radiation and ventilation. The block diagram of system which determines air temperature at the point 2M based on radiation outside greenhouse and on-off of ventilating fan is shown in Fig. 8. Figure 9 shows the results of identification, indicating that system identification is effective in analysis of condition with on-off of ventilating fan.
Figure 10 shows the results of identification of the same system of Fig. 4. In Fig. 10, identification was made for every 3 hr and the calculated air temperature was linked. The calculated air temperature is in much better accordance with the measured air temperature than that of Fig. 4, indicating that system of greenhouse is changing slowly and that the latest data is needed for more precise calculation of the system.
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