Study on heat and mass transfer of internally heated liquid desiccant regeneration for solar-assisted air-conditioning system

Abstract

Traditional air-conditioning (AC) systems have several limitations, such as the poor humidity control, energy wastage, and the creation of wet surfaces that become breeding grounds for mildew and bacteria. The problems become more serious in warm or hot and humid climates. As an energy-saving and environmental-friendly alternative, the Liquid Desiccant AC System (LDAC) becomes a good solution to remove the extra moisture of air by desiccant absorption. Its major energy required is low-grade thermal energy, such as the solar thermal energy provided by solar collectors. To improve the system performance and to utilize the thermal energy efficiently, the internally cooled/heated dehumidifier/regenerator has drawn many attentions recently. Furthermore, due to its low possibility of droplets carried by the air and low pressure drop, the falling film dehumidifier/regenerator has become a promising type for liquid/gas contacting. As most energy consumes for regenerating the solution, this research focused on the regeneration process. However,'the study of literature shows that previous investigations on this system were insufficient. This research aimed to investigate the heat and mass transfer of the internally heated liquid desiccant regeneration experimentally and theoretically by considering the change of wetted area, film thickness and mass transfer coefficient under the insufficient wetting conditions, and to develop a 3-D theoretical model for predicting the fluid characteristics and system performance more accurately. Based on the buildings' load profiles and weather data of Hong Kong, the dynamic operation performance of the whole Solar-assisted LDAC (SLDAC) was evaluated, and several system optimizations were proposed and studied to reduce the energy consumption. The wetted area and mass transfer coefficient are key parameters affecting the system performance, and also indispensable parameters in the system evaluation. By testing a single channel internally heated regenerator, the influencing factors of wetted area, film thickness and mass transfer coefficient were investigated experimentally, as well as their effects on the mass transfer performance. The film sizes under different operation conditions were recorded by a thermal camera, and LiCl was chosen as the solution. The increase of wetted area benefits the mass transfer performance greatly, while the performance reduced with the increasing film thickness. The initial width of the film was affected by the solution distributor thickness most greatly, and changed significantly with the contact angle and solution mass flow rate. When the surface was pre-wetted to be hydrophilic, the initial width dramatically increased, and the contraction of falling film was effectively weakened. The film contraction was also reduced with the higher working surface temperature, while it was aggravated with the increase of solution temperature. Furthermore, under insufficient wetting conditions, the mass transfer coefficient decreased with the increase of solution mass flow rate. It also decreased with the increase of solution distributor thickness and solution temperature, and increased with the increasing air mass flow rate. Additionally, the contact angle of solution was experimentally found to be determined by the surface roughness, solution concentration and temperature. Based on the experimental results, a theoretical model with an analytical solution was developed for accurately calculating the wetted area of the internally heated regeneration, by describing the transverse flow caused by the Marangoni effect. The model could be divided into three parts, including the model for initial wetted width, the model for contact angle and the model for contraction distance along the flow direction. The calculation results were compared with the experimental ones with small average errors of 8.8%, 12.3% and 10.8% respectively for the three parts. The film thickness, plate surface temperature, and solution temperature, concentration and contact angle were numerically found to significantly influence the wetted area. An empiric formula of mass transfer coefficient was also developed with the multi linear regression, showing an acceptable error compared with the experiments results. Then, a 3-D model with a numerical solution of internally heated regeneration was developed for describing the heat and mass transfer among the air, solution and extra hot water in all three directions. The insufficient wetting condition, the change of film thickness due to the mass transfer and film deformation, and the effect of contact angle were considered. The calculation results were compared with those obtained by other existing theoretical models, and showed a closer trend to the experimental data, especially in the prediction of the influences of solution mass flow rate, hot water temperature and working plate surface temperature. The parameter study showed that the thickness of solution distributor has the greatest impact on the moisture removal rate, and the change of inlet solution temperature results in the most obvious change of the regeneration efficiency. The changes of solution concentration and hot water temperature also significantly affect the system performance. Though the increase of mass flow rates of solution, air and hot water benefits the mass transfer, the effect of excessive mass flow rates are slight. Although the model was developed based on the internally heated regeneration, it is also suitable for the adiabatic system by introducing of plate surface temperature. Furthermore, the new model could also be applied for the dehumidification process considering the similar heat and mass transfer mechanism between the dehumidification and regeneration of liquid desiccant. Though the 3-D model could predict the system performance more accurately, it is complex and inconvenient for investigating the dynamic operation performance. A simplified numerical model of internally cooled/heated dehumidifier/regenerator was developed accordingly by defining three kinds of effectiveness, i.e. enthalpy effectiveness, moisture effectiveness, and temperature effectiveness. With the multi linear regression, the statistical correlations of effectiveness were developed by using the heat and mass driving forces and other related parameters as variables. The results were compared with those by the 3-D model with an acceptable error of 14.7% for dehumidifiers and 7.1% for regenerators. With the simplified model, to evaluate the operation performance and energy potential, a dynamic simulation of the SLDAC was conducted by employing four nested iteration calculation loops. As a case study, the AC load profiles of three typical commercial buildings in Hong Kong were investigated. Results showed that only up to 12.5% of electricity consumption could be saved annually if the cooling tower is used as the only cooling source of the dehumidifier due to the high dehumidification demand in summer. But, by introducing an extra cooling coil for the dehumidifier and a heat exchanger for the regenerator, the electricity saving percentage could be significantly improved to 36.7%. Additionally, the energy consumption of solar-assisted systems is affected by the installation area of solar thermal collectors, and the energy saving could not be achieved until the area is larger than the minimum required value. For our case, the minimum installation area is 0.22 m2/kW (peak load) for office buildings. Academically, this research contributes a 3-D theoretical model for predicting the heat and mass transfer of falling film liquid desiccant system more accurately by employing the newly developed models for the wetted area and mass transfer coefficient during the dehumidification/regeneration process. It provides a useful reference for researchers and engineering to improve the wetted area, to enhance the heat and mass transfer, and to optimize the system performance. Furthermore, as the falling liquid film are widely employed in many industrial applications, such as vertical condensers, film evaporators, absorption towers and heat exchangers, our results could also be applied for the performance evaluation and optimization in these areas.