Configuration and Field Testing of a Liquid Desiccant Dehumidification System for Greenhouse Applications

Abstract

Agriculture and Agri-Food Canada (AAFC), the Ontario Greenhouse Vegetable Growers Association (OGVG), and Queen’s University’s Solar Calorimetry Laboratory (SCL) are undertaking a joint project to evaluate the energy and crop-yield benefits of operating commercial greenhouses in isolation from the outdoor environment, i.e., eliminating natural or forced ventilation to the exterior. Implementing such a scheme requires “closing” the greenhouse envelope and the installation of an active air-conditioning system to control temperature and moisture levels that could be harmful to crop growth. To this end, a prototype air-conditioning system, centered around a liquid desiccant dehumidifier, was designed, constructed and instrumented such that its thermal and functional performance could be evaluated over extended periods. The prototype unit was installed in a “research” greenhouse located at the Agriculture Canada, Greenhouse and Processing Crops Research Center (GPCRC) located in Harrow, Ontario. Both the novel air-conditioning and monitoring systems were implemented during the course of the thesis and operated for two preliminary crop trials to characterize system performance and identify aspects needing further refinement. Data obtained over these two initial periods, indicated that, the latent and sensible cooling capacity of the novel desiccant system averaged: 2.25 kW and -0.25 kW, respectively, during the severe summer trial; and 1.25 kW and -0.1 kW, respectively, during the milder spring trial. Values obtained from the preliminary monitored data also indicate that the liquid desiccant unit operated at electrical and thermal coefficients of performance (COPs) between 0.74 and 3.1 and between 0.15 and 0.52, respectively. Finally, using the monitored data, a simple regression-based empirical model was formulated to describe the average performance of the liquid desiccant unit. This was attempted to illustrate how performance results could be generalized to assist in the future design of similar commercial-scale systems. The results of this part of the thesis indicated, however, that further test data is required to confidently characterize the unit’s performance. As well, it was concluded that addition instrumentation (specifically, the addition of a meter to measure the flowrate of the regenerator air-stream) would enhance the potential to develop a practical performance correlation.