Finite-time Thermodynamic Optimization Of Solar-powered Hollow Fiber Membrane Liquid Dehumidification System

The promotion of high efficiency refrigeration technology points out a new direction for reaching the goal of double carbon.As a novel desiccant cooling technology with high efficiency and energy saving,membrane solution dehumidification technology has the potential to become a research limelight in the field of solution dehumidification.The solar-driven hollow fiber membrane solution dehumidification system takes the clean and renewable solar energy as the heat source.The solar energy provides regenerative heat for the recycling of the solution,which can reduce energy consumption and improve energy efficiency effectively.Hollow fiber membrane module assembled with high selective composite membrane is used as dehumidifier.Air and solution contact indirectly in the module for heat and mass exchange,which can ensure air quality and avoid gas-liquid entrain and mold pollution.Many scholars have studied the heat and mass transfer mechanism,thermodynamic performance and structure optimization of hollow fiber membrane module,and the thermodynamic performance of membrane dehumidification system driven by different heat sources are also studied.For this system,the research mainly focuses on the thermodynamic performance analysis,experimental research and energy matching,and there is no further study on the thermodynamic optimization of the system.Therefore,this paper adopts the optimization theory of finite-time thermodynamics to strengthen heat and mass transfer processes of the system respectly,which can reduce the irreversible losses and improve the energy utilisation efficiency.The main research contents are as follows:（1）Establish the mathematical model of the system.Mathematical models of each heat and mass transfer component were established,and experimental verification was carried out with a test rig.The relative errors were calculated to be within±10%,indicating the feasibility of the model.（2）The heat transfer process of the system is optimized by using the finite time thermodynamics theory.To minimize entropy generation and minimize entransy dissipation,heat transfer optimization equations were established and compared with the heat transfer strategies of constant fluid temperature and constant heat flow rate.The results show that the entropy generation of the system increases from 0.615 W/K to 0.648W/K,and the entransy dissipation increases from 63775.7 W·K to 66279.3 W·K.The components with the largest and smallest entropy generation and dissipation are the solution-solution heat exchanger and the dehumidifier,respectively,which are mainly related to the heat transfer temperature difference and latent heat changes associated with mass transfer.Under four different heat transfer strategies,the entropy generation of the solar collector with heat leakage is the smallest,the temperature difference on the hot side is the largest and the nonlinear trend is the most obvious.While for the heat exchanger without heat leakage and the hollow fiber membrane module,under the strategy of minimize entransy dissipation and constant heat flow rate,the temperature change law of the hot side is the same.Under the strategy of minimize entropy generation,the temperature difference of the hot side is the largest.（3）The mass transfer process of the system is optimized.The mass entransy dissipation number is defined and its influencing factors are analyzed.Study the influence of different operating conditions on equivalent wet resistance.Based on the optimization objective of minimizing the mass entransy dissipation,the mass transfer optimization equation was established to obtain the optimal configuration of fluid humidity.The results show that the mass entransy dissipation number decreases with the increase of the NTU_m（number of mass transfer units）and the efficiency.Under the same mass flow ratio,air temperature has the strongest impact on the equivalent wet resistance.Under the setting condition,the wet resistance varies between 15.91 s/kg and 23.89 s/kg.Because the humidity difference of the hollow fiber membrane module is small,the humidity of the fluid in the module changes approximately linearly with time.The minimum mass entransy dissipation of the system is 1.367×10^-6 kg/s.