DEHUMIDIFICATION OF GREENHOUSES IN COLD REGIONS
Greenhouses in cold regions often exhibit conditions of high relative humidity (RH). Frequent high RH levels of over 80% result in poor yields and undesirable crop quality, as well as poor working conditions for employees. The objective of this study was to seek an effective and economical method(s) to control RH in greenhouses of cold regions. Three methods were tested and evaluated: finned tubing condensation, air-to-air heat exchangers, and domestic mechanical refrigeration dehumidifiers. Temperature based ventilation and RH based ventilation were used for comparisons. Finned tubing condensation using chilled water was tested in an environmental chamber. The condensation rates were obtained and statistical models were developed for the selected finned copper tubing with aluminum fins under various room and water conditions. This method was proved highly energy intensive and costly, thus discarded in the field experiment. The field experiment was conducted in a Saskatchewan greenhouse. The air-to-air heat exchangers and domestic mechanical refrigeration dehumidifiers were tested against the control treatment which was the temperature based ventilation control, i.e. relying on infiltration in winter and ventilation using exhaust fans in the other seasons. The field experimental results proved that dehumidification was needed most of the year with high RH occurring from April to November. The heat exchangers and dehumidifiers controlled RH very well in winter, early morning and night in other seasons, and saved heating cost; however, in mild and warm weather from about 9 am to noon the RH was high before the exhaust fans operated at full capacity. When the ambient air was humid during the warm season, the heat exchangers were ineffective for RH control. The dehumidifiers controlled RH not as well as the heat exchangers mainly due to the low capacity. Both methods added extra heat to the greenhouse in the warm season, which was desirable in early morning and at night when ambient temperature was low and heating was still needed but undesirable during daytime when cooling was required in the greenhouses. Comparing energy efficiency, the moisture removal index (MRI) of the dehumidifiers was around -0.629 kW-h/L (produced 0.629 kW-h energy per liter of water removed), the heat exchangers’ MRI was 0.916 to 1.020 kW-h/L (consumed 0.916-1.020 kW-h energy to remove 1 L of water), while the RH-based ventilation required 1.099 to 1.373 kW-h/L, and the finned tubing condensation required more than 7.2 kW-h/L. If natural gas was the heat source, the dehumidifier method was the most economical with annual average energy cost of $0.018/L, approximately 60% and 50% of those of the heat exchangers and exhaust fans, respectively. If thermal coal was used as the heat source, the heat exchanger was the most economical with an annual average energy cost of $0.016/L, as compared to $0.019/L and $0.035/L for the dehumidifiers and exhaust fans, respectively. The mechanical dehumidifier method is energy efficient and effective year-round, and its operating cost is low, thus is recommended for greenhouse dehumidification in cold regions. However, a complete economic analysis that includes the capital cost is needed to evaluate the economic feasibility of the various methods. The heat exchanger is also recommended which can supply CO2 in winter but it is ineffective in humid weather. Proper sizing of the dehumidification requirement is the key to success.
DegreeMaster of Science (M.Sc.)
DepartmentAgricultural and Bioresource Engineering
ProgramAgricultural and Bioresource Engineering
CommitteePredicala, Bernardo; Noble, Scott
Copyright DateDecember 2011
Greenhouses, dehumidification, heat exchanger, dehumidifier, energy efficiency