Control of Solution Temperature by Air Flow through a Hydroponic Bed Made of Microporous Film

Control of water temperature by air flow through a duct connected to the bottom of a hydroponic bed made of microporous film (MP film) was evaluated. Air and vapor can pass through MP film, whereas water cannot do. The relationship between water temperature in the bed and air flow velocity was investigated using small devices (1m×0.05m) . Water temperature decreased immediately after air flow treatment, and a low velocity (1.2-1.5ms-1) was more effective than other rates (0 m s-1, 10-11 m s-1, 20-21 m s-1) . The temperature under low humidity (RH=31-33.6%) after the air flow treatment was 6.6°C lower than that before treatment. High humidity decreased the effect of the air flow treatment. Spinach grew in a polystyrene bed, a MP film bed and a MP film bed with air flow treatment ; the MP film bed consisted of polystyrene side wall and a MP film bottoms. The average temperature during air flow treatment was 2.8°C lower than that in polystyrene bed, and the maximum difference between the two treatments was 5.0°C during cultivation from July 4 to 10. The decrease in temperature can be controlled by stopping the treatment. These results indicate that a hydroponic bed made of MP film with a ventilation device is useful for controlling the solution temperature.