MD Desalination Characteristics Analysis And Study Of Its Combination System With Absorption Chiller

Global population growth and energy-environmental problems make water short-age increasingly become one of the major issues that constrain human economic and social development.Undoubtedly,wastewater reuse and desalination are effective means to break through this bottleneck.As one branch of many water treatment technologies,membrane distillation(MD)has been applied in fields such as desalination,water treat-ment,purification or concentration,as well as food and medicine.Due to its atmospheric pressure operating condition,excellent separation effect and low energy quality require-ments,MD is attracting more and more attention around the world.However,it also has disadvantages of low energy efficiency and poor economics,especially when driven by conventional energy.Therefore,it is necessary to carry out research on membrane dis-tillation characteristics and related system,in order to provide theoretical supports and practical references for MD system design and application.Firstly,based on the background of MD desalination,a mathematical model of its heat and mass transfer process has been established,and the permeate flux as well as thermal efficiency are used as indexes to evaluate the MD characteristics.Single-factor-based and double-factor-assisted analysis methods were used to simulate and analyze the effects of feed inlet parameters as well as permeate inlet parameters on MD characteristics and its influence mechanism.Results show that the feed inlet parameters are dominate factors that have obvious influence on MD process,within the simulated parameters.When the feed inlet temperature raises from 55? to 75?,the MD permeate flux and thermal efficiency show an increase of 121.030%and 20.07%,respectively.When the feed inlet flow rate raises from 9.0L/min to 11.OL/min,the MD permeate flux and thermal efficiency show an increase of 11.03%and 3.85%,respectively.Secondly,considering the advantages of waste heat,geothermal and solar energy in driving membrane distillation,we combine MD and absorption chiller into a system,in order to achie've the goal of using lo'w-grade heat efficiently,providing pure water and supplying cooling capacity simultaneously.A mathematical model of the whole combination system has been established,and influence of driving temperature,chiller evaporation temperature as well as bypass ratio on the system performance have been studied,too.Results indicate that,within the simulated parameters,both driving temper-ature and bypass ratio have a significant impact on the combination system,while chiller evaporation temperature just has little impact which can be neglected.When the driving temperature raises from 95? to 115?,water production rate and cooling capacity of the whole system show an increase of 119.56%and 81.97%,respectively;at the same time,total efficiency of the whole combination system increases by 7.36%.While the bypass ratio increases from 0 to 1,the water production rate also raises from 0 to the maximum value,but cooling capacity and total efficiency of the whole system show an opposite trend.Within the variation range of bypass ratio(0?1),total efficiency of the whole system decreases from 65%to 18.50%,which has a drop as high as 71.54%.Influence characteristics of bypass ratio imply that the optimal load distribution strategy of the whole combination system is mainly based on refrigeration and accompanied by producing water.To achieve a not too low system efficiency(?30%),the bypass ratio should not be higher than 0.625.Finally,based on MD characteristics analysis and combination system simulation,this study selects the optimal parameters combination that makes the system index best under typical bypass ratio,with some simplified conditions.Effects of different bypass ratios on the optimal parameters combinations of the whole system under condition of consistent MD water production rate have been compared and analysed,too.Results show that the maximum or minimum values of the four indexes(water production rate,cooling capacity,MD thermal efficiency and total system efficiency)of the combination system correspond to the maximum or minimum values of MD module feed inlet pa-rameters as well as absorption chiller's driven temperature,at any studied bypass ratios(except 0 and 1)· That's to say,the change of bypass ratio does not affect the system's optimal parameters combination.The reason is that changing bypass ratio only affects the load distribution of MD end and chiller end,while it has no effect on parameters combination when MD end or chiller end achieves whose best thermal efficiency or COP.Study of this thesis can provide theoretical supports and practical references for membrane distillation characteristics analysis and MD system designing,operating as well as optimization which combined with absorption chiller.