Irreversible processes and performance improvement of desiccant wheel dehumidification and cooling systems using exergy

Desiccant wheels are effective dehumidification devices. The performance of desiccant dehumidification and cooling systems is examined in this paper. Based on a theoretical investigation, six kinds of systems (systems A-F) were analyzed as the system changed from being reversible to being irreversible, which sharply reduced performance. The performance of system E, which is composed of an actual desiccant wheel, an actual heat recovery exchanger, and an actual single-stage heat pump, represents the relatively high standards that actual systems can achieve. Under the designed working conditions, COP and exergy efficiency of system E were 5.0 and 18.3%, respectively. Based on the analysis of a real ventilation cycle, it was found that to improve the system's performance, over-dehumidification should be avoided, and heat sources with low exergy destruction should be utilized. To avoid over-dehumidification, the direct evaporative cooler at the processed air side should be replaced by a sensible heat exchanger. When the electrical heater is replaced by a heat pump system, the performance of such a system can be improved, especially when pre-cooling is adopted. The proposed heat pump-driven system had similar schematic and performance characteristics as system E, with COP and exergy efficiency being 5.01 and 18.0%, respectively, under the same working conditions. (C) 2015 Elsevier Ltd. All rights reserved.