Cooling Efficiency of Fogging Systems for Greenhouses

Abstract

The cooling efficiency η c of wet pad and fan systems, which is currently used for fogging systems, is defined assuming that the dry bulb temperature in the greenhouse T db , i is lower than the outside dry bulb temperature T db , o and is measured directly at the outlet of the wet pad. However, in the case of η c of a fogging system, T db , i is typically measured at different locations in the greenhouse to obtain an average value. The inside air is sensibly heated mainly by solar energy transmitted into the greenhouse under sunny summer conditions. Then T db , i is often higher than T db , o , giving negative values of η c . Even though, T db , i is lower than T db , o , the current definition of η c gives unrealistic values due to the latent heat added to the inside air. In this study, η c of a fogging system was newly defined, taking into consideration the sensible and latent heat added to the greenhouse air. The definition is based on the energy and mass (water vapour) balance of the un-cooled air in the greenhouse during the operation of fogging. The value of η c was determined for an experimental greenhouse with a floor area of 26 m 2 and contained 150 potted tomato plants. The greenhouse was cooled intermittently at a fogging rate of 10 g s −1 for three different fogging times of 30, 60 and 90 s with interval times of 90, 180 and 270 s, respectively. The results showed that a fogging cycle of 60 s on, 180 s off provided a relatively higher value of η c than other cycles. Under this condition, two approximate expressions of η c could be used to estimate the η c value with an error less than ±5%.