Assessing greenhouse climate using CFD: a focus on air humidity issues

Abstract : Humidity control is a key factor in greenhouse management as it impacts the growth of most greenhouse crops. Excessive humidity and the presence of droplets may enhance the development of fungal diseases or affect optical properties of the cover material. For that reason, growers have to adapt their climatic strategy in order to warranty an optimal environment to the plants. The mechanisms involving water vapour transfer inside greenhouses include air exchanges processes through vents or leakage, plant transpiration, condensation and evaporation. To analyse and predict these mechanisms, modelling is a promising approach. Among the numerical tools, Computational Fluid Dynamics (CFD) appears as a powerful mean to simulate the climate inside greenhouses. Important developments in CFD have been made in the last decade to assess air humidity patterns. They involve specific requirements: in particular the resolution of the species transport equations both for water vapour and air. Submodels also have to be activated for taking account of sink or source terms of water vapour. Recent developments include the transpiration process at canopy level as well as condensation phenomena or droplets transport associated to fog systems. Water vapour transfers are also generally coupled with heat transfers. CFD studies concerning air humidity inside greenhouses have been conducted at different scales: the leaf, the canopy and the greenhouse itself. Small scale works were carried out essentially to assess singular phenomena and provide validation before extending applications to larger scales. At the scale of the greenhouse itself, the main concern is how to find solutions for dehumidifying air. The ventilation-heating combination is still commonly used. Its main drawback however is its high-energy cost. Research is still on-going on low-energy consumption systems for dehumidification and CFD proved to be an efficient tool to find alternative solutions to the simultaneous ventila¬tion-heating process. Other topics of interest are cooling systems based on evaporative pads or fogging.