Concept of the Un-cooled Air in a Greenhouse Cooled by Fogging in Summer: An Aid to Estimate the Cooling Efficiency of a Fogging System

摘要

The current definition of the evaporative cooling efficiency (η_c) can be applied only for a greenhouse with a wet pad-fan system, in which no sensible or latent heat is added to the air during the cooling process. However, η_c of the wet pad-fan system cannot be applied for a greenhouse with a fogging system, in which sensible and latent heat are added to the air during the cooling process. The purpose of this study is to define η_c correctly for a greenhouse with a fogging system and to describe the fogging process. For this purpose, the concept of un-cooled air was developed, which is the greenhouse air gaining sensible and latent heat from the cover, the floor and the plant surfaces. The dry bulb temperature of un-cooled air (T_u) was defined based on the sensible heat added to the air. Sensible heat strongly depends on the convective heat transfer coefficients in the greenhouse (i.e., between the air and the cover, the floor, the plant surfaces). Therefore, different relations expressing each coefficient were examined. η_c was defined using T_u and the measured values of the dry and wet bulb temperatures in the greenhouse with fog. Values of T_u and η_c were estimated in a greenhouse located in the Tokyo area, Japan, and cooled by intermittent fogging on a hot summer day. The results show that value of 71, was found to be not very sensitive to the selection of convective coefficients, with maximum relative error of 4% observed using different expressions for the convective coefficients from the literature. The assumption that the fogging process takes place in the greenhouse air at a constant wet bulb temperature is practically acceptable.