Cooling strategies for greenhouses in summer: Control of fogging by pulse width modulation

Abstract

The possibilities for improving the control of greenhouse fogging systems, were studied by comparing several combinations of ventilation cooling techniques, shade screening and low-pressure fogging. The study was divided into three parts: experiments, modelling and simulations.In the first part of the paper, ten combinations of five cooling techniques were tested during the summers of 2002 and 2003 in a 132 m 2 greenhouse with a steel structure and a single-layer methacrylate cover located in Madrid, Spain. An analysis of variance of the climatic parameters was carried out to determine which combinations produced significant differences in inside temperature or relative humidity. Comparing the values for the inside to outside temperature difference, the combination of a shade screen and above-screen fogging achieved a difference in temperature almost the same as that for under-screen fogging, but the relative humidity was significantly lower.In the second part of the study a dynamic model was developed (2002) and validated (2003). The mean absolute error obtained for inside temperature was similar in the fit and the validation and it was less than 1.5 °C in both cases.The model was used to simulate the inside air temperature for a fog system working without shading, and above and under a shade screen. Control algorithms were developed for reducing system water consumption. In the three cases a simple on/off control with a fixed fogging cycle was compared with a pulse width modulation (PWM) strategy, in which the duration of the fogging pulse was increased as a function of inside temperature. The strategies with PWM applied to the fog system were able to reduce water consumption by 8-15% with respect to the strategies with a fixed fogging cycle.

Experimental evaluation of a radiant heated floor coupled to an air-to-water heat pump for the cooling of greenhouses

“…The work of Perdigones et al (2008) was used to provide data regarding other cooling strategies (collected in the same experimental greenhouse in 2003). The methods and materials used in this and the present work were similar.…”

“…A previous work (Perdigones et al, 2008) showed that shading screen reduced ventilation rate in the experimental greenhouse by 18.2%.…”

Effect of Night Interruption with Mist and Shade Cooling Systems on Subsequent Growth and Flowering of Cymbidium ‘Red Fire’ and ‘Yokihi’

“…First Red (Katsoulas et al, 2001) and Lycopersicon esculentum (Romero-Aranda et al, 2002). A further advantage of the mist-cooling is the establishment of uniform temperature and humidity conditions throughout the greenhouse (Perdigones et al, 2008). Withner (1974) reported that mist cooling is effective due to both the cool initial temperature of the water and its evaporative effects on the leaf surfaces in Cymbidium.…”

“…A number of cooling techniques, such as natural ventilation, white-shading, shade screens, and evaporative cooling (e.g. wet pad-fan, misting, and fogging systems) are used, instead of transporting the plants to higher altitudes, to maintain the temperature and humidity of a greenhouse at acceptable levels during hot periods (Perdigones et al, 2008).…”

Analysis and Optimization of the Fan-Pad Evaporative Cooling System for Greenhouse Based on CFD

“…The cooling technique is essential for greenhouse in tropical or subtropical regions, such as natural ventilation, whitening, shade screens, and evaporative cooling [1]. Evaporative cooling is the most efficient way for maintaining lower temperature and increasing humidity in greenhouses [2].…”