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[en] Highlights: • Five configurations of a DEC system are analyzed in five climate zones. • DEC system model configurations are developed in Dymola/Modelica. • Performance analysis predicted a suitable DEC system configuration for each climate zone. • Results show that climate of Vienna, Sao Paulo, and Adelaide favors the ventilated-dunkle cycle. • While ventilation cycle configuration suits the climate of Karachi and Shanghai. - Abstract: Performance of desiccant evaporative cooling (DEC) system configurations is strongly influenced by the climate conditions and varies widely in different climate zones. Finding the optimal configuration of DEC systems for a specific climatic zone is tedious and time consuming. This investigation conducts performance analysis of five DEC system configurations under climatic conditions of five cities from different zones: Vienna, Karachi, Sao Paulo, Shanghai, and Adelaide. On the basis of operating cycle, three standard and two modified system configurations (ventilation, recirculation, dunkle cycles; ventilated-recirculation and ventilated-dunkle cycles) are analyzed in these five climate zones. Using an advance equation-based object-oriented (EOO) modeling and simulation approach, optimal configurations of a DEC system are determined for each climate zone. Based on the hourly climate data of each zone for its respective design cooling day, performance of each system configuration is estimated using three performance parameters: cooling capacity, COP, and cooling energy delivered. The results revealed that the continental/micro-thermal climate of Vienna, temperate/mesothermal climate of Sao Paulo, and dry-summer subtropical climate of Adelaide favor the use of ventilated-dunkle cycle configuration with average COP of 0.405, 0.89 and 1.01 respectively. While ventilation cycle based DEC configuration suits arid and semiarid climate of Karachi and another category of temperate/mesothermal climate of Shanghai with average COP of 2.43 and 3.03 respectively
[en] Liquid desiccant dehumidification systems have drawn a great deal of attention in the HVAC industry due to its great energy saving potentials. Numerous studies have been conducted to investigate the relationships between the operating conditions and the system performance. However, it seems that the existing relationships were built improperly since almost all of them were established through the incomplete single-factor tests, rather than the full-factorial tests. This makes the existing work unable to clarify the overall significance of the various operating conditions on affecting the system performance. To address this unexplored issue, an L18 × L8 cross-product orthogonal array together with the statistical analysis method (ANOVA) was adopted in this work to investigate the significance of operating conditions in promoting the system performance (i.e. sensitivity analysis) and stability (i.e. stability analysis). 144 experimental and simulation runs were conducted within the ultrasonic atomization liquid desiccant dehumidification system (UADS) as the example to demonstrate the analysis. It was found that though direct influence on the system can be exerted by all the operating conditions, their significance differed markedly. Based on the analysis, the operating conditions can be classified into four types while the optimal conditions for the UADS were also figured out and validated. - Highlights: • We clarified significance of operating conditions on performance of UADS by ANOVA. • Sensitivity of operating conditions on UADS performance was analyzed and ranked. • Significance of operating conditions on UADS stability was revealed and ranked. • The optimal running conditions for UADS were figured out and validated.
[en] The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator
[en] This study presents a model-based optimization strategy for an actual chiller driven dehumidifier of liquid desiccant dehumidification system operating with lithium chloride solution. By analyzing the characteristics of the components, energy predictive models for the components in the dehumidifier are developed. To minimize the energy usage while maintaining the outlet air conditions at the pre-specified set-points, an optimization problem is formulated with an objective function, the constraints of mechanical limitations and components interactions. Model-based optimization strategy using genetic algorithm is proposed to obtain the optimal set-points for desiccant solution temperature and flow rate, to minimize the energy usage in the dehumidifier. Experimental studies on an actual system are carried out to compare energy consumption between the proposed optimization and the conventional strategies. The results demonstrate that energy consumption using the proposed optimization strategy can be reduced by 12.2% in the dehumidifier operation. - Highlights: • Present a model-based optimization strategy for energy saving in LDDS. • Energy predictive models for components in dehumidifier are developed. • The Optimization strategy are applied and tested in an actual LDDS. • Optimization strategy can achieve energy savings by 12% during operation
[en] The feasibility of using solar energy instead of electricity for air conditioning and refrigeration has been investigated as part of a trend to explore sustainable and low-carbon technologies that minimize the use of fossil fuels. Solar desiccant cooling systems provide direct air conditioning for indoor spaces using solar energy, with the added benefit of full outdoor air provision. The system can handle the space sensible and latent loads while achieving good indoor air quality. This paper proposed an effective optimization method for a solar desiccant cooling system developed by a simulation model. The parameters for suitable operation and control were determined. The objective of optimization is to maximize the solar fraction against the involvement of auxiliary electric heating. This is a constrained optimization problem since the room temperature should be maintained within a comfortable range. As such, traditional numerical optimization methods cannot be used. However, the evolutionary algorithm (EA) can effectively handle the constrained, nonlinear and multidimensional engineering problems such as those encountered in this solar desiccant cooling system. This study included a comparative evaluation of the 3 major paradigms of the EA in an effort to evaluate a suitable approach for the current engineering application with demanding computational function calls. A model of the solar desiccant cooling system was built and its year-round performance was maximized. 17 refs., 2 tabs., 10 figs
[en] Owing to the stringent indoor air quality (IAQ) requirements and high cost of desiccants, one of the major concerns in liquid desiccant technology has been the carryover, which can be eliminated through indirect contact between desiccant and air. Membrane contactors using microporous semipermeable hydrophobic membranes have a great potential in this regard. This communication investigates the performance of semipermeable membrane based indirect contactors as dehumidifiers in liquid desiccant cooling applications. Experiments on different types of membrane contactors are carried out using lithium chloride (LiCl) solution as desiccant. The membrane contactors consist of alternate channels of air and liquid desiccant flowing in cross-flow direction. Hydrophobic membranes form a liquid tight, vapor permeable porous barrier between hygroscopic solution and moist air, thus eliminating carryover of desiccant droplets. In order to provide maximum contact area for air–desiccant interaction, a wicking material is sandwiched between two membranes in the liquid channel. It is observed that vapor flux upto 1300 g/m2 h can be achieved in a membrane contactor with polypropylene (PP) membranes, although the dehumidification effectiveness remains low. The effect of key parameters on the transmembrane vapor transport is presented in the paper. - Highlights: • Indirect membrane contactors developed to avoid carryover in liquid desiccant system. • Dehumidification effectiveness and vapor flux reported under varying conditions. • Vapor flux upto 1295 g/m2 h in polypropylene contactor with high area density. • Dehumidification effectiveness with LiCl solution varies within 23% to 45%
[en] In drying processes it is necessary to appropriately control air humidity and temperature in order to enhance water evaporation from product surface. The aim of this work is to investigate several HVAC configurations for product drying based on desiccant wheels, in order to find systems which reach high primary energy savings through the appropriate integration of refrigerating machines, adsorption wheels and cogenerative engines. Simulations are carried out for different values of sensible to latent ambient load ratio and the effect of ambient and outside air conditions is evaluated for each configuration. It is shown that primary energy savings can reach 70–80% compared to the reference technology based on a cooling coil. With respect to works available in literature, the results of this study keep a general approach and they can be used as a simple tool for preliminary assessment in a wide range of applications. -- Highlights: ► Several HVAC systems for product drying based on desiccant wheels are investigated. ► The sensible to latent ambient load ratio influences the choice of the best system. ► Energy savings can reach 80% compared to the technology based on a cooling coil. ► Simulation results can be used for preliminary assessment in many applications.
[en] Highlights: • A novel compression-absorption refrigeration THIC air-conditioning system is proposed. • All of the condensation heat of the ARS can be recovered for desiccant solution regeneration. • Effects of key parameters on the performance of the system are analyzed. • The PEE of the novel system is 34.97% higher than that of traditional system. • The generation temperature of system can decrease from 100 °C to 60 °C. - Abstract: The vapor compression system driven liquid desiccant dehumidification to realize temperature and humidity independence control (THIC) is a very good idea for the utilization of condensation heat. Absorption refrigeration system (ABS) is widely applied all over the world, due to its energy saving and environment protection; meanwhile, large amount of condensation heat is discharged to the environment. In this study, a hybrid compression-absorption refrigeration air-conditioning (AC) system combined with liquid desiccant dehumidification is proposed. All of the condensation heat from the condenser of the ABS can be recovered to regenerate the diluted liquid desiccant solution. Meanwhile the evaporation temperature in the ABS rises since the latent load is handled by liquid desiccant, which improves the performance of the ABS. The PEE of the proposed system is 34.97% higher than that of traditional absorption refrigeration AC system at the same operation condition. And compared with the traditional absorption refrigeration AC system, the generation temperature of the proposed system can decrease from 100 °C to 60 °C due to the existence of the compressor.
[en] Solar driven rotary desiccant cooling systems have been widely recognized as alternatives to conventional vapor compression systems for their merits of energy-saving and being eco-friendly. In the previous paper, the basic performance features of desiccant wheel have been discussed. In this paper, a solar driven two-stage rotary desiccant cooling system and a vapor compression system are simulated to provide cooling for one floor in a commercial office building in two cities with different climates: Berlin and Shanghai. The model developed in the previous paper is adopted to predict the performance of the desiccant wheel. The objectives of this paper are to evaluate and compare the thermodynamic and economic performance of the two systems and to obtain useful data for practical application. Results show that the desiccant cooling system is able to meet the cooling demand and provide comfortable supply air in both of the two regions. The required regeneration temperatures are 55 deg. C in Berlin and 85 deg. C in Shanghai. As compared to the vapor compression system, the desiccant cooling system has better supply air quality and consumes less electricity. The results of the economic analysis demonstrate that the dynamic investment payback periods are 4.7 years in Berlin and 7.2 years in Shanghai.
[en] This report summarizes an investigation of the performance of two active desiccant cooling systems that were installed as pilot systems in two locations-a college dormitory and a research laboratory-during the fall of 1999. The laboratory system was assembled in the field from commercially available Trane air-handling modules combined with a standard total energy recovery module and a customized active desiccant wheel, both produced by SEMCO. The dormitory system was a factory-built, integrated system produced by SEMCO that included both active desiccant and sensible-only recovery wheels, a direct-fired gas regeneration section, and a pre-piped Trane heat pump condensing section. Both systems were equipped with direct digital control systems, complete with full instrumentation and remote monitoring capabilities. This report includes detailed descriptions of these two systems, installation details, samples of actual performance, and estimations of the energy savings realized. These pi lot sites represent a continuation of previous active desiccant product development research (Fischer, Hallstrom, and Sand 2000; Fischer 2000). Both systems performed as anticipated, were reliable, and required minimal maintenance. The dehumidification/total-energy-recovery hybrid approach was particularly effective in all respects. System performance showed remarkable improvement in latent load handling capability and operating efficiency compared with the original conventional cooling system and with the conventional system that remained in another, identical wing of the facility. The dehumidification capacity of the pilot systems was very high, the cost of operation was very low, and the system was cost-effective, offering a simple payback for these retrofit installations of approximately 5 to 6 years. Most important, the dormitory system resolved numerous indoor air quality problems in the dormitory by providing effective humidity control and increased, continuous ventilation air