SIMULATED EFFECTS OF VARIOUS ENVIRONMENTAL MANAGEMENT PRACTICES ON WATER CONSUMPTION IN OPEN AND CONFINED GREENHOUSE SYSTEMS

Abstract

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.m instead of 0.01 m3/s.m 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. INTRODUCTION To provide economically optimal micro-environments for plant growth, producers can use or control the number of glazing layers, insulation curtains or screens to reduce long-wave radiation losses at night, reduced ventilation rates, evaporative coolers, and shading devices to control incoming solar radiation. In addition, a Rankine power cycle heat pump that was developed by Yildiz et al. (1993) holds promise for reducing water consumption, winter heating requirements and warm weather cooling loads. These systems involve complex tradeoffs between initial and operating costs for plant responses to various environmental factors and the strategies used to regulate temperature, humidity and CO2 levels in the crop canopy. Special attention must also be given to the operational strategies associated with the use of heat pumps, especially in maintaining acceptable relative humidity levels within greenhouses. A dynamic simulation model was developed and validated to provide an accurate prediction of greenhouse energy and moisture exchanges as a function of dynamic environmental factors (Yildiz and Stombaugh, 2006). This model was used to predict transpiration rates and water consumption, and to evaluate the operational strategies associated with heating and cooling using the proposed heat pump and a conventional system. The heat pump was evaluated for both open and confined greenhouse systems, and these were compared to a conventionally ventilated and heated greenhouse. The specific objective of this study was to evaluate the effects of canopy size, relative humidity, light management levels, CO2 enrichment and minimum ventilation rates on water consumption in three different greenhouse systems (conventional, open heat pump, and confined heat pump) in winter, spring, and summer.