Limitations and Feedback Effects in Liquid Desiccant Air Conditioning Systems

摘要

Liquid desiccants in air conditioning systems (LDAC) sometimes are seen as advantageous compared to solid desiccants as integrated cooling of the solution is possible, heat and mass transfer between liquid and gas phase is better than in solid-gas systems, and the solution can more easily be stored and used for load shifting. As the sorption units (dehumidifier and regenerator) incorporate additional components (e.g. internal heat exchangers) and control strategies (e.g. volume flow control) the overall system is subject to internal feedback and limitations. Studies to foster system understanding are only available on a very basic level. To fully understand the working principle of a LDAC-system and detect optimization potentials, the feedback effects between the system components and the sorption process within the sorption unit have to be identified. This study aims at investigating the specific feedback effects of solution heat exchanger quality, solution to air mass flow ratio, and recirculation ratio on the steady-state system performance. An efficiency-NTU approach is used to describe the characteristics of the sorption column. Property data of LiCl is applied exemplary for the calculations. Parameters of the sorption column are calibrated with experimental data. Results show that the additional heat exchanger significantly determines the sorption process efficiency of a single sorption unit. With respect to enthalpy efficiency an optimum mass flow ratio can be defined in dependence of the heat exchanger quality. The solution recirculation rate determines the internal solution conditions and therefore has a significant influence on the process. The findings allow for a better understanding of LDAC systems and provide a more systematic approach to sorption based air-conditioning. A psychrometric presentation of the process is used to ease the comprehension of the underlying feedback effects.