Feasibility study for water recovery system for pad-and-fan cooled greenhouse in semiarid climate

In arid and semiarid regions, greenhouses use large amounts of water for both evaporative cooling and irrigation, reducing the system water use efficiency (crop yield per greenhouse water input). Pad-and-fan evaporative cooling systems are widely used to provide a cooled and moistened air stream to the plants. As air travels through the greenhouse, moisture is added to the air by canopy evapotranspiration, increasing its dew point. It is hypothesized that a portion of water lost by canopy evapotranspiration can be recovered and reused in irrigation and/or cooling by placing a condensation surface with a temperature lower than the dew point of exhaust air. Sump water temperature often gets as low as the outside wet bulb temperature (15-18°C in dry summer, Arizona), and therefore, it may be usable for condensers. In the present study, we monitored the dew point temperature of the greenhouse exhaust air and the temperature in the pad sump water as potential coolant, in a large-scale commercial greenhouse, and the collected data were used for estimating condensation rates during the typical summer months (May to September) in southeastern Arizona, USA. The simulation was based on a condenser energy balance model, validated experimentally inside a greenhouse. The result showed that, although further optimization is necessary, the proposed water recovery system, using the pad sump water as condenser coolant, could significantly reduce the system water use of semiarid greenhouses during summer.