The objective of this study was to evaluate the effects of relative humidity, light management, minimum ventilation rates, CO2 enrichment and canopy size on water consumption in three different greenhouse systems (conventional, open heat pump, and confined heat pump) in winter, spring, and summer months. Using different relative humidity set points resulted in almost the same relative humidity regimes within the confined greenhouse system, resulting in similar transpiration rates. No difference was observed in transpiration rates in the open system in winter either, because the inside relative humidity levels never reached the 70% and 80% set points. Some differences were observed in spring and summer. Up to a 5.1% reduction was observed in transpiration rates by going from a 70% set point to an 80%. Maintaining an average solar radiation level of 250 W/m2 instead of 350 W/m2 inside the greenhouse reduced the transpiration rate approximately 12.5% at both relative humidity set points. Using a minimum ventilation rate of 0.005 m3/s.m2 instead of 0.01 m3/s.m2 reduced the transpiration rates about 16%, 11%, and 3% in winter, spring, and summer, respectively. The higher decrease in winter was caused by the increase in inside relative humidity when lower ventilation rate was used. Using a CO2 enrichment level of 1000 ppm compared to an enrichment level of 350 ppm resulted in transpiration rates that were predicted to be slightly lower in all the three greenhouse systems used. This decrease was 14% in the confined system, and by about 5% in both the conventional and open heat pump systems. The partial canopy stands (0.4 m) had approximately 7%, 5%, and 6% higher transpiration rates than the full canopy stands (2.0 m) in the conventional, open heat pump, and confined heat pump systems, respectively.


Bioresource and Agricultural Engineering | Horticulture

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