Articles
GREENHOUSE ENERGY AND RESOURCE USE ¿ COOLING IN SEMIARID GREENHOUSES
Article number
797_42
Pages
299 – 306
Language
English
Abstract
Several challenging studies focusing on optimizing ventilation and evapotranspiration during evaporative cooling in semiarid climate and less water use have been carried out.
Some results are reviewed and discussed in this paper.
Predictions based on the steady-state energy balance indicated that to maintain an air temperature set point, a decrease in ventilation rate led to a decrease in required evapotranspiration, resulting in an increase in relative humidity.
Experimental studies were carried out to demonstrate the predictions.
Results for a fog-cooled greenhouse showed that the inside relative humidity decreased with an increase in ventilation rate as expected from the predictions, while the water use for fog cooling increased.
However, it was of interest that the reduction of water use under low ventilation was mostly due to the reduction of plant transpiration because the transpiration rate decreased linearly with a decrease in vapor pressure deficit of the air.
A simple control algorithm that incorporated the adjustment of vent openings and the control of fogging was proposed and tested.
It demonstrated a possibility of maintaining relative humidity and air temperature simultaneously within a desirable range while reducing the water use.
A study for pad-and-fan cooling showed similar but slightly different results.
It demonstrated that there were no significant reductions in average air temperature or relative humidity above a level of ventilation rate, while the water use by the pad increased linearly with increasing ventilation rate.
The daily water use by computer-controlled ventilation rate was less than the fixed ventilation rate control.
Another challenge is to recover water from the greenhouse exhaust air, since a large amount of water for evaporative cooling and irrigation is exhausted through ventilation.
The feasibility of using a condenser chilled by the pad sump water to recover water is discussed.
Some results are reviewed and discussed in this paper.
Predictions based on the steady-state energy balance indicated that to maintain an air temperature set point, a decrease in ventilation rate led to a decrease in required evapotranspiration, resulting in an increase in relative humidity.
Experimental studies were carried out to demonstrate the predictions.
Results for a fog-cooled greenhouse showed that the inside relative humidity decreased with an increase in ventilation rate as expected from the predictions, while the water use for fog cooling increased.
However, it was of interest that the reduction of water use under low ventilation was mostly due to the reduction of plant transpiration because the transpiration rate decreased linearly with a decrease in vapor pressure deficit of the air.
A simple control algorithm that incorporated the adjustment of vent openings and the control of fogging was proposed and tested.
It demonstrated a possibility of maintaining relative humidity and air temperature simultaneously within a desirable range while reducing the water use.
A study for pad-and-fan cooling showed similar but slightly different results.
It demonstrated that there were no significant reductions in average air temperature or relative humidity above a level of ventilation rate, while the water use by the pad increased linearly with increasing ventilation rate.
The daily water use by computer-controlled ventilation rate was less than the fixed ventilation rate control.
Another challenge is to recover water from the greenhouse exhaust air, since a large amount of water for evaporative cooling and irrigation is exhausted through ventilation.
The feasibility of using a condenser chilled by the pad sump water to recover water is discussed.
Publication
Authors
S. Sase
Keywords
control algorithm, evaporative cooling, fog, pad-and-fan, water recovery
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