Theoretical Analysis And Experimental Investigation On A Novel Chiller With Open-cycle Rotary Desiccant Cooling

Rotary desiccant air conditioning is advantageous in being free from CFCs, using lowgrade thermal energy and controlling humidity and temperature separately. Thus, it hasbeen recognized as a viable alternative to vapor compression refrigeration for spacecooling. Ongoing research and development works suggest that the optimization of cycleconfiguration and the reduction of adsorption effect have significant potential forimproving the resource utilization rate and enhancing the sensible and latent handlingability of rotary desiccant dehumidification and air conditioning cycle. Up to now, varioustypes of cycles have been proposed and investigated, which have contributed greatly to theimprovement and the promotion of the technology. For the purpose of breaking theobstacle of limited temperature reduction encountered by conventional desiccant coolingcycle, a novel rotary desiccant cooling cycle is proposed in this paper. Especially, thetechnologies of isothermal dehumidification and regenerative evaporative cooling areincorporated by this cycle. In addition to dehumidification, the cycle is capable ofproducing chilled water, thereby realizing independent temperature and humidity controlwith low grade heat source.Firstly, the thermodynamic characteristics of the basic ventilation cycle are analyzed.Appropriate methodology and thermodynamic indexes for evaluating the performance ofthe rotary desiccant cooling cycle are developed. Based on this, a novel rotary desiccantcooling cycle incorporating isothermal dehumidification and regenerative evaporativecooling is proposed. It can provide dry air and chilled water simultaneously and overcomesthe problem of limited sensible handling ability encountered by conventional cycles.Furthermore, the cycle can also be designed as a standalone chilled water plant. As a rotarydesiccant dehumidification-based chilled water producing technology, it would expanddesiccant cooling to a boarder niche application. In view of air conditioning, the novelcycle remove latent heat load by two-stage dehumidification, and sensible heat load by the produced chilled water. Hence, it is similar to the conventional hybrid two-stage rotarydesiccant cooling cycle in system configuration. The main difference lies in the treatmentof sensible heat load. The hybrid two-stage cycle usually adopts conventional coolingequipment like air-source heat pump, and the novel cycle uses the produced chilled water.Both the air conditioning ability and the energy utilization rate of the novel cycle are muchhigher than those of the conventional cycles. With similar energy utilization rate, thesupply air specific exergy of the novel cycle is about10%higher than the conventionaltwo-stage cycle. Compared with hybrid two-stage rotary desiccant cooling cycle, both thethermal coefficient of performance and the exergy efficiency of the novel cycle areincreased by about30%.Secondly, hybrid two-stage rotary desiccant cooling cycle, which is similar to theproposed cycle, is studied. The operating characteristics, the seasonal performance and thesuitability of the hybrid cycle are investigated and discussed. It is found that, standalonesolar driven two-stage rotary desiccant cooling cycle can’t accomplish the treatment ofsensible heat under high humid conditions. Auxiliary cooler must be incorporated. Thehybrid cycle not only has favorable cooling capacity and energy utilization rate, but alsocan be applied to a wide range of operating conditions. Based on the typical meteorologicalyear data of Beijing, Shanghai and Hong Kong, the seasonal average thermal coefficientperformance of the cycle is around0.9, with a solar fraction and electric power saving ratearound30%. Being similar to the hybrid cycle in principle, the prospect of the proposedcycle is suggested to be favorable.Thirdly, the experimental setup of the novel cycle is configured. The isothermaldehumidification and regenerative evaporative cooling-based chilled water producingprocess is tested. The operating characteristics of the novel cycle are analyzed incomparison with those of the conventional cycle. The results demonstrate that theproduced chilled water of the novel cycle can remove the sensible heat effectively. It istherefore proved to be a promising alternative to the hybrid two-stage rotary desiccantcooling cycle. The supply chilled water temperature of the chilled water producing processis around15.1-20.7oC, with a thermal coefficient performance of about0.3-0.6. Thethermal coefficient of performance of the novel isothermal dehumidification andregenerative evaporative cooling-based cycle is about0.8-0.9. With the same moistureremoval, the temperature of the supply air is lowered greatly in comparison with the conventional cycle. Especially, under the conditions of humid and high humid, theeffective cooling capacity of the conventional cycle becomes much low due to limitedtemperature reduction ability. In these cases, relatively higher grade heat source is neededto realize standalone operations. Nevertheless, the novel cycle overcomes this problem andis more promising for low grade heat sources utilization.Finally, on the basis of the thermodynamic analysis and experimental investigation,the model of the novel isothermal dehumidification and regenerative evaporativecooling-based rotary desiccant cooling cycle is established. The characteristics of heat andmass transfer, the attainable handling region and the sensible and latent heat handlingability of the cycle are analyzed. Investigations on attainable handling region suggest thatthe cycle is applicable to a wide range of weather conditions. Favorable supply chilledwater and qualified supply air can be provided by the cycle with heat source of50-90oC inmost cases under the climates of temperate, humid and even high humid. Moreover,compared with the conventional desiccant cooling cycle, the novel cycle can handle air to amuch lower temperature without diminishing in thermal performance. This is of greatbenefit to breaking the obstacle of limited temperate reduction encountered byconventional desiccant cooling cycle.