Microclimate conditions in ventilated wet-walled greenhouses in a subtropical climate: spatial variability

Abstract

Spatial variations of microclimatic conditions in enclosed north–south (N–S) oriented single-arched greenhouse polycarbonate structures, with a wet-wall providing evaporative cooling at the S end, were investigated and displayed online in near real-time. Temperature-controlled fans at the N end extracted air. Air temperature, wind speed and water vapour pressure were measured at many positions to investigate spatially changing conditions. Large N–S spatial variations in air temperature and relative humidity occurred away from the wet-wall and east–west (E–W) positions. Typically, on hot days, at positions furthest from the wet-wall, morning E–W air temperature differences exceeded 14 °C and −6 °C for afternoon. N–S spatial air temperature differences were usually smaller in magnitude. Furthest from the wet-wall, E–W relative humidity differences exceeded 20% in the morning and less than −10% in the afternoon. Heated-needle anemometer wind speed showed reasonable correspondence with three-dimensional sonic anemometer measurements. Needle anemometer wind speeds were greatest nearest the wet-wall, decreasing significantly towards the mid-position. With fans on intermittently, wind speed was lowest near the wet-wall, increasing at the mid-position. There were significant spatial air temperature differences, displayed in near real-time using a web-based system, and in other microclimatic parameters investigated resulting in a very complex and changing microclimate.

Keywords: greenhouse micrometeorology, greenhouse polycarbonate structure, needle anemometers, spatial air temperature measurements, web-based display of spatial greenhouse microclimate

South African Journal of Plant and Soil 2014, 31(3): 137–143