Humidity is the most complicated climatic parameter to control in the horticulture industry. Maintaining set points for the moisture is a challenge for control and monitoring devices. Relative : relative humidity varies with air temperature, and plants continuously release moisture to the air. AdditionallyHowever, humidity control is vital crucial for the crop health of the crops and the prevention of diseases. Fig. 1 demonstrates that humidity in the horticulture industry is a big challenge. Too dry of an environment slows down the plant growth, while can reduced stem lengths and leaf sizes, whereas excessive moisture causes plant diseases [8]. Moreover, humidity control is rather energy-intensive, especially in colder climates. Therefore, humidity control plays a vital role in climate control of greenhouses. (Comprehensive review on dehumidification strategies for agricultural greenhouse applications - ScienceDirect)
the general approaches to humidity control in greenhouses, namely conventional natural ventilation, forced ventilation, maintaining a high temperature, condensation on a cold surface by using refrigeration-based systems or heat exchangers, and adsorption by a hygroscopic material such as various types of desiccants.
diseases such as botrytis and fungal diseases, limit nutrient uptake due to reduced transpiration, and cause condensation on the cover, reducing the solar radiation entering the greenhouse. Generally, it is suggested that greenhouse air RH should be kept in the range of 60–80% for healthy growth. Low humidity leads to reduced stem lengths and leaf sizes, which inhibit plant growth [14]. Also, few fungi develop in low RH. On the other hand, higher humidity levels increment the possibility of condensation on leaves, especially at night, which results in developing Botrytis and other fungal diseases. At high humidity levels, plants cannot evaporate water from their leaves, so the uptake of nutrients such as boron and calcium may be limited. Moreover, condensation on the greenhouse cover can reduce the solar radiation entering the greenhouse by as much as 23% [15]. Bakker [16], for instance, has reported that high humidity in greenhouses has a significant influence on light interception which leads to the decrement of photosynthesis and leaf area index (calcium deficiency), an increase in the number of the leaves, and growth of fungi [17]. The principal sources of moisture in greenhouses are plant transpiration and the evaporation of water from the soil. The combination of transpiration from plant and evaporation from the soil is referred to as evapotranspiration. It must be noted that preventing excessive humidity levels is particularly challenging in closed greenhouses which are gaining popularity in colder climates [18].Crop transpiration and evaporation of water from soil (combined - ‘evapotranspiration’) are the main sources of moisture in greenhouses.
The simplest approach to control humidity in greenhouses is through natural ventilation: on opening the vents, warm, humid air from the greenhouse flows out and is replaced by cold, dry air from outside. Removal of humidity by natural ventilation, however, unavoidably results in loss of heat, both sensible and latent heat. Some alternate approached to limit the heat loss while reducing humidity are dehumidification using forced ventilation and heat exchangers, heat pump dehumidifiers, and hygroscopic chemicals.
Higher relative humidity set points and reducing crop transpiration can further lower the energy use of dehumidification.