CFD Modeling of a Liquid Desiccant Dehumidifier

Abstract

The demand for air conditioning is estimated to increase rapidly due to rising world population and the effects of climate change. It is expected that the use of air conditioners will increase by a factor of five by 2050, which stresses the need for more environmentally friendly and energy-efficient systems. In many locations, dehumidification is an important aspect of air conditioning, increasing energy consumption. An alternative to conventional vapour-compression cooling systems are systems where the outdoor-air is dehumidified by a liquid desiccant solution. Typical liquid desiccant dehumidifiers (LDD) pass building-air over a concentrated liquid desiccant solution that absorbs moisture from the air stream, decreasing the solution concentration. The solution is subsequently heated to drive off the moisture allowing it to be regenerated. The design of an efficient and compact dehumidifier section is crucial. To achieve this goal, a thorough understanding of the heat and mass transfer processes is required. Therefore, this thesis presents an analysis of the heat and mass transfer in an idealized section of a LDD. This was accomplished by modelling the flow of water and air in an adiabatic, vertical, parallel-plate channel. The effects of the inlet mass flow rate and temperature were investigated. The results show that the water temperature and inlet flow rate have a significant effect on the condensation processes. The inlet air flow rate was found to have the least impact on the performance. Three cases were studied for a water/lithium-chloride liquid desiccant dehumidifier, i.e. internally cooled parallel-flow and counter-flow, and adiabatic counter-flow. A parametric study indicated the best performance for the internally cooled (isothermal) case, however, for long channels, a uniform heat flux boundary condition should be better.