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Titill
en

Greenhouse climate model : an aid to estimate the influence of supplemental lighting on greenhouse climate

Útgáfa
Júní 2012
Útdráttur
en

GeoGreenhouse project involves the construction of a greenhouse for growing tomatoes in Iceland. The first stage consists of a gross area of five hectares. Due to the peculiarities of such project and because of the unique weather, a greenhouse climate model is advisable to analyze various design solutions.
Iceland's weather has a seasonal change in the length of day and night, creating unique weather phenomena. In midwinter, there is a period where darkness prevails. In midsummer, daylight takes over and night darkness is almost absent during June and July. Artificial lighting and movable blackout screen are therefore needed to have a year round production.
Replacement lighting requires a high density of lighting devices which implicates a big heat gain due their losses. Understanding the influence of the artificial lighting on the greenhouse climate is necessary in order to set up an adequate control strategy.
The developed model includes, in addition of the greenhouse structure, all the equipments for climate controlling, such as: blackout screens, artificial lighting, heating system, fogging system, roof vents, and mechanical ventilation.
Simulation results show that the requirements to maintain the desired indoor climate change depending on the mutual relation between the blackout screen position and the outdoor solar radiation. For instance, if the crop night-phase occurs when the external radiation is low, turning off the lamps is sufficient to lower the air temperature in the greenhouse; while, if the crop night-phase occurs at noon to lower the indoor temperature it is necessary to open the roof vents.
A control strategy which ensures the desired indoor climate has been investigated.
At the beginning, the control methodology considered a 24 hours cycle for the crop photoperiod. A deeper analysis showed a big disparity between the sunlight intercepted by the plant canopy of one zone of the greenhouse and the others. Since the irradiation on canopy leaves is the factor which has the main influence on crop growth, a solution to level off the fruit production along the three zones has been proposed. A reduction of the crop day-length creates a variable phase delay between the crop night-period and the night. Simulation results show that a reduction of 45 minutes in the 24 hours cycle permits to level off the Daily Light Integral to a unique common value.
If on one hand a lower crop day-length permits to uniform the yield production along the three zones, on the other hand it requires a control system independent of the mutual effect between the blackout screen position and the outdoor solar radiation because it is always changing.
Several simulations have been executed in order to evaluate how different control strategies affect the greenhouse indoor climate. A feasible control methodology which permits to create the adequate crop growth environment has been designed and results are given in chapter 4.
Blackout screens are installed to avoid light pollution and to handle the tomatoes photoperiod. Once the artificial lighting control has been defined, the blackout screens are controlled by a clock in series with the signal controlling the lamps. The clock’s signal defines when the foreseen sunlight is enough high to stimulate plant growth. This coupling of the clock signal and the artificial lighting signal permits control of screen closure avoiding any lighting pollution. A solar radiation meter can replace the clock or be included in the system, in such a way that during cloudy days the blackout screens can be kept closed if the outdoor radiation is below a predefined value.
Remaining controlled equipment includes the roof vents and the heating pipes located below the benches. In both cases a proportional controller has been modelled. Due to the difficulty of measuring leaf temperature, the controlled parameter is the air temperature. The desired temperature changes between crop day-period and night-period. Thus, the settings of both controllers vary during the day. The switching between the two configurations must be done in accordance with the crop photoperiod which coincides with the scheduling adopted for the artificial lighting system.
Simulation results show the need to limit the roof vents aperture and to turn the heating system on in advance. Adopting such modification, parameters considered fundamental for crop growth are within an acceptable range. All the settings utilized and simulation results are given.
Although results are promising, the model developed and used has not been validated with any empirical data, thus some modifications on the suggested control system will probably be needed. However, the proposed control methodology can be used at the beginning of the operations when data on the variations due to external factors on the greenhouse climate are unavailable.
KEY WORDS
greenhouse model, artificial lighting, control system.

Birting
30.8.2012


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