Analysis of performance and regeneration method of internally-heated liquid desiccant regenerator

Abstract: Liquid desiccant cooling system, as a good alternative to traditional electric refrigeration air conditioner, is environmentally friendly and can be driven by low-grade energy while it can improve indoor air quality and has high energy storage capacity. The regeneration technique, a key technique in the liquid desiccant cooling system, must be developed before it is widely applied in variety of buildings. The present methods of solution regeneration have electrodialysis, membrane energy exchanger, ultrasonic atomization regeneration, packing tower, and so on. The first three have complicated structures and high costs for their application on a large scale. The packing tower regenerator because of its simple structure and being driven possibly by low-grade energy has attracted many attentions across the world. In packing tower regenerators, the internally-heated liquid desiccant regenerator is a kind of high-efficient solution regeneration device. To improve the reliability and economy of internally-heated regeneration technique, the mathematical models of pre-heated and internally-heated regeneration are established based on the energy and mass conservation between solution and air as well as the energy conservation between heated water and solution in this paper, which include parallel flow, counter flow and cross flow with 2 kinds of different flow directions of heated water respectively, and their theoretical performances are numerically simulated and compared with each other. As for the regeneration performances affected by the ways of heating solution and flow directions of heated water, the simulation results show that regeneration performances of internally-heated type are about 2-4 times that of pre-heated type in most conditions, which means the internally-heated regenerator has a better performance. And the regeneration performances are greatly influenced by the flow-rate ratios of solution to air and heated water to air. With the decrease in flow-rate ratio of solution to air and the increase in flow-rate ratio of heated water to air, the regeneration performances of the internally-heated regenerator are increasingly better than that of the pre-heated regenerator. At the maximum point of the internally-heated (flow-rate ratio of solution to air is 0.1 and flow-rate ratio of heated water to air is 0.95, flow-rate ratio of air is 1 kg/s), the rate of evaporation is calculated to be 20 times that of the pre-heated at its maximum point (flow-rate ratio of solution to air is 0.4, flow-rate ratio of heated water to air is 0.65). The flow direction of heated water in internally-heated regenerator is divided into 2 conditions: Heated-water is parallel to solution (DirectionⅠ) or counter to solution (DirectionⅡ). It is also found the regeneration performances, when the heated water flows counter to solution, are superior to heated water paralleling to solution and are increased by 5% at most. As for the effects of the numbers of heat transfer units (NTU1 and NTU2), the regeneration performances in general increase with the increase in NTU1 and NTU2, and a fitted curve combining NTU1 with NTU2 occurs that presents the rapidest increase in regeneration performance with the increasing of NTU1 and NTU2. Besides, it is also exposed that parallel type shows the largest concentration difference of solution and the cross type is about 97% of that, while the counter type only reaches about 87% as much as parallel type in the worst condition. The results in this paper can offer theoretical supports for the optimal design of internally-heated regenerator.