UC Merced

Freshwater consumption in greenhouse (GH) cooling is a major concern for agriculture in Central California. A spray drying system to cool greenhouses is studied. Such a system can intake drainage water from agricultural fields thus lowering freshwater usage. As an additional benefit of the system, minerals dissolved in the wastewater can be collected in solid form, which would reduce the need for expensive wastewater treatments to meet environmental standards. The system consists of an evaporation chamber (ECH) and a precipitate separation system (PSS). Water droplets ranging from 15 to 30 μm are sprayed into the chamber. Hot outside air moved by fans into the ECH evaporates the water, thus, decreasing the temperature while increasing humidity. The cold and moist air then transports the solid precipitate particles to the PSS. There, a louver-type separator takes advantage of the inertia of the solids. Unable to change direction as fast as the airflow, the particles accumulate in a dust chamber. At the system output, clean cold air is released into the GH to lower the inside temperature and provide an agreeable habitat for plants. The feasibility of the ECH is assessed via thermodynamic and kinetic models and experimental characterization of a prototype chamber in the laboratory. Measured and predicted values for outlet temperature and humidity differ by 2 to 11 %, which is within the combined uncertainty of the measurements and model. A prototype louver-type separator recovers 35 % of the total supplied solids from 1% NaCl feed solution. Given representative conditions, the chamber footprint required to cool a 100 m2 GH ranges from 4 to 10 m2.