Water use for pad and fan evaporative cooling of a greenhouse in a semi-arid climate

Water availability is a common concern in semi-arid regions, such as Southern Arizona, USA, where more greenhouses are operating due to high solar radiation. Hydroponic greenhouse crop production greatly reduces irrigation water use; however, there is currently no information demonstrating water use of an evaporative cooling system. This project investigated water use by a pad and fan (P&F) cooling system under semi-arid climate conditions. Data were collected for two physically identical, side-by-side, double-layer polyethylene film-covered arched-roof greenhouses (28 m x 9.8 m x 6.3 m) during summer conditions (38.5°C, 15% RH, 845 W m^-2). One greenhouse had mature tomato plants (2.3 plants m^-2) and the other had no plants. Water use was primarily affected by air exchange rate. The average water use by the P&F system was 0.145, 0.182, 0.265, 0.325, and 0.387 g m^-2 s ^-1 for greenhouse air exchange rates of 0.017, 0.037, 0.051, 0.067, and 0.079 m³ m^-2 s^-1, respectively. In the empty greenhouse, the lowest ventilation rate produced the highest average greenhouse air temperature (T_in=33.8°C) and lowest RH (40%). T_in and RH were nearly equal for the three highest air exchange rates. In the greenhouse with plants, the lowest ventilation rate produced the highest T_in (31.1°C) but also the highest RH (69.4%). In general, RH decreased with increasing air exchange rate and the minimum T_in of 27.6°C (RH: 52%) was achieved at the middle air exchange rate (0.051 m³ m^-2 s^-1). It is believed that the lower cooling efficiency of the P&F system at higher ventilation rates caused a reduction in cooling. However, the lower ventilation rates limited air exchange and effectively reduced cooling. If a higher pad cooling efficiency could be maintained at high ventilation rates, a cooler air temperature and higher relative humidity may be achieved, though water use would increase. Finally, when the air exchange rate was controlled to maintain the GH at 24°C/18°C during day and night, water use by the P&F cooling system (14.8 L m^-2 day^-1) was greater than the tomato irrigation system (8.9 L m^-2 day^-1) assuming 100% drainage recovery, which is typical for many high-technology greenhouse facilities.