Comparison of Two Cooling and Dehumidifying Methods for a Semi-Closed Organic Tomato Greenhouse

Two non-traditional methods of cooling and dehumidifying (C/D) were compared in a semi-closed greenhouse using cold water withdrawals from a saturated water table in order to 1) maintain the greenhouse semi-closed under high solar radiation, 2) evaluate the impact of both systems on temperature on relative air humidity (RH) and on vapor pressure deficit (VPD), 3) increase carbon dioxide levels (CO2) by limiting air external exchanges, and 4) reduce the CO2 consumption. The first studied method was a cold water fan coil system where greenhouse air was cooled through a coil on which the water vapour was condensed. The second method was a cold water curtain installed above the canopy, forcing warm humid air to condense on its surface, thus generating a free convection air movement within the greenhouse. These two heat exchanger methods were compared with the traditional natural ventilation C/D technique. The cooling capacities of both systems varied between 125 and 600 W m(-2). Under sunny summer days, higher than 838 W m(-2) for each system, the water curtain, the fan coil system and the traditional method were able to maintain set 24 h temperature (21.2, 21.2 and 21.9 degrees C), and RH (82.0, 82.4, and 77.5%) corresponding to acceptable VPD levels (0.5, 0.5 and 0.6) in the greenhouse. By maintaining closed-roofs, the water curtain and the fan coil technology maintained higher maximum levels of CO2 (1,046 and 992 mu mol mol(-1)) in the greenhouses compared to the traditional method (550 mu mol mol(-1)). During sunny days, the water curtain and the fan coil demonstrated higher maximum daily temperatures (26.6 and 29.6 degrees C), while cooler minimum temperatures during the night (18.1 and 15.8 degrees C) were observed resulting in a similar 24 h mean temperature. Compared to the water curtain, the fan coil system had the advantage of requiring low maintenance with a reduced fungal disease risk. When comparing the CO2 injection strategies, it was observed that the greenhouse using the fan coil technology injected up to 20% less than the traditional method.