Dehumidification Requirement Modelling And Control Strategy For Greenhouses In Cold Regions

Plant transpiration and evaporation from soil surface or growth media in greenhouses produce a large amount of moisture. High relative humidity (RH) favors fungus growth and causes plant diseases thus dehumidification is essential in greenhouses. In this study, a novel model (DehumReq) is developed to predict the dehumidification requirement of greenhouses. The model is based on the crop characteristics, greenhouse environment, and outdoor weather conditions. The year-round predictions give the variation of hourly dehumidification requirement and identify the dehumidifier capacity required for RH control. The model output is validated by the actual dehumidification requirement of a tomato greenhouse. A good agreement is obtained with the coefficient of determination (R-2) values up to 0.89 and root mean square error (RMSE) values of less than 10% of maximum dehumidification load between the predictions and measured values. The dehumidification load strategy is also developed using the model, which recommends use 90% of assurance rate for RH control (for setpoint 70, 75, and 80% RH) to keep the load at a reasonable level without causing long high RH period to affect the crops; whereas 100% assurance rate would triple the loads. Using this model and strategy, the greenhouse growers can predict the dehumidification needs and select suitable dehumidifiers. Due to the high loads, it is recommended that in cold region, ventilation using exhaust fan or heat/energy exchangers may provide most of the humidity control while refrigeration or hygroscopic dehumidifiers will supplement when ambient air is humid (summer or mild seasons) or heating cost is a concern (winter season). It is also recommended that the greenhouse ventilation and dehumidification systems should have a temperature-relative humidity (T-RH) control strategy to provide effective and economic T-RH control.