Dehumidification Load Prediction and Evaluation of Dehumidifying Methods for Greenhouses in Cold Regions

Abstract

Year-round greenhouses in cold regions face the problem of high indoor relative humidity (RH). Greenhouse growers are concerned with being able to estimate the exact amount of dehumidification load (moisture amount) and selecting the most suitable dehumidification technology for their greenhouses. In this study, a dehumidification requirement model (DehumReq) is developed to predict the dehumidification load of greenhouses. The model output is validated with the real dehumidification requirement of a tomato greenhouse, where good statistical agreement was obtained with the maximum coefficient of determination (R2) value of 0.89. The root mean square error (RMSE) values between predicted and actual data are 9.5%, 7.5%, and 8% of maximum load for May, July, and November, respectively. A dehumidification strategy is also developed where the RH control of any greenhouse is recommended for 90% of the time without affecting the crop rather than 100% of the time. The control of indoor RH in a greenhouse 100% of the time is found to be economically impractical. The effects of changing several important greenhouse parameters and the sensitivity of each parameter on the dehumidification load are also analyzed in this study. The air exchange rate is found to be the most sensitive and widely adjustable parameter in controlling and minimizing the dehumidification load. The indoor set-points (temperature and RH) are also found to be highly sensitive, but their ranges of adjustment are small because of the short RH tolerance range of plants. As a part of the study of the most efficient dehumidification technology for commercial greenhouse production, a unique state point liquid desiccant (SPLD) dehumidifier unit is experimentally evaluated at the laboratory at Nortek Air Solutions, Saskatoon, SK, Canada. The new SPLD unit works in two different modes: an internal dehumidifier under the wet mode and an air-to-air heat exchanger under its dry mode of operation. The lab testing indicates that in order to remove the expected amount of moisture (20 kg/hr in the wet mode and a wide range in the dry mode), the unit consumed 3.08 kW under the dry mode and 5.68 kW with an additional 20 kW of heat under the wet mode. The performance of the SPLD unit was also evaluated and compared at commercial greenhouses with three other commercial dehumidifier units, namely a mechanical refrigeration dehumidifier (MRD), direct contact liquid desiccant dehumidifier (DCLD), and heat recovery ventilator (HRV), which represent the refrigeration, liquid desiccant, and heat-exchanger type dehumidification technologies, respectively. The four different dehumidifier technologies were tested in four different greenhouse zones: three of which were used in a potted rose greenhouse (referred as to Greenhouse X) and the other in a tomato greenhouse (referred as to Greenhouse Y). The results found the HRV unit to have the lowest average electricity consumption (0.02 kWh/kg), the cheapest average cost (0.022 $/kg) of moisture removal, and the maximum average moisture removal capacity (189.1 kg/hr) among all four units. However, the HRV unit causes the most average energy loss (0.935 kWh/kg of moisture). The MRD unit was found to be the most energy efficient, as it can add 0.60 kWh of energy in removing 1 kg of moisture rather than consuming energy. The SPLD unit under dry mode showed a similar performance to the HRV unit in terms of electricity consumption (0.046 kWh/kg) and moisture removal cost (0.026 $/kg). The DCLD unit was found to have moderate performance in terms of electricity consumption (0.15 kWh/kg) and the overall energy (0.775 kWh/kg) consumption with good indoor environment control. The SPLD unit under wet mode showed good RH control with the highest average cost (0.068 $/kg) of moisture removal. Based on the results, it can be concluded that no single technology performs well in all performance indices, but good in a few criteria, so greenhouse growers can select the dehumidifier according to their preference.