Capillary-driven Low Grade Heat Desalination

Problem with fresh water has been expected to become worse in the coming decades,with water scarcity occurring globally,even in regions currently considered water-rich.Typical water-saving projects or South-North Water Diversion Project just realize space transfer of water resources,no addition of fresh water resources,which is difficult to fundamentally solve the problem of water shortage.The situation can be tackled only if mankind finds some ways to produce fresh water efficiently.Currently,water scarcity has been one of the most serious global challenges,and the unique methods to increase fresh water supply beyond what is available from the hydrological cycle are desalination.However,huge investments and high energy consumption limit its desalination application in large scale.Global shortage of energy resources can hardly match the ever-increasing energy consumption of desalination processes.This problem can be solved unless a novel desalination method with a high coefficient of heat transfer under low temperature difference can be proposed.Restricted by diffusivity transfer mechanism,MSF or MED desalination technology has closed to theoretical limits,which is difficult to obtain larger breakthrough.It is an inevitable tendency to access to new sources of fresh water through new methodologies.The development of heat and mass transfer in micro/nano porous provides the possible breakthrough and innovation.The typical application is the loop heat pipe,which is a closed evaporation-condensation system capable of transporting large quantities of heat at low temperature differences based on capillary force within a porous wick without any additional power.This article presented a capillary-driven seawater desalination method,focused on the fundamental research of the new technology,in order to break through the key scientific problems related to the hydrodynamic characteristics of water desalination system driven by capillary forces and phase transition and solute transport at microscale.Relevant results will serve as the theoretical basis for the development of the seawater desalination demo.Specific content involves the following four aspects:(1)A simplified laboratorial-scale process scheme of the desalination system has been designed and established.Different from conventional LHP,the condensed liquid is the desalinated water,flowing out along the desalinated water line under gravity.The condensed liquid flows upward along the seawater line under capillary force when supplement to the evaporator is the saline water.The capillary force is provided by the meniscus in the wick for the continuous operation of desalination,which draws the seawater through the wick to vapor-liquid interface and no external pumping power is needed.Experimental results proved that nearly 18.27 kg/(m2·h)of freshwater could be produced at 42? of heat source.The total dissolvable solids(TDS)of the produced freshwater is about 30 mg/L,far beyond the standard for drinking water.More importantly,the proposed system could be operated adaptively in the range of 33-56 ? compared to the conventional phase-change processes commonly operated within 70-100 ?.(2)Heat transfer and thermodynamic process have been discussed to explain the operation principle based on the laws of energy conservation and momentum conservation.The effects of heat flux,operation height,length of vapor line,heat leak on the performance of the OLHP were studied in detail.(3)A mathematical model was developed to clarify the concentration migration process and the experimental study was investigated.A steady-state convection-dispersion equation was proposed to analysis the solute movement in vertical direction,and the governing equations of steady-state concentration distribution in the wick was established.The solute concentation in the vapor-liquid interface can be controlled in a certain scope to avoid crystallization by smart structure design in porous wick.(4)An industrial desalination unit has been established at Guangdong Yudeam power plant(Guangdong,China).The low temperature steam around 60 ? extracted from#2 steam turbine is considered as the heat source for the desalination unit anticipating a fresh water production rate of 45.7 kg/h,whose TDS is just 40.3mg/L.The meniscus,in the wick,can self-adjust the balance by the angularity change of meniscus curvature such that it can meet the varied pressure gradient to work smoothly at 55-70?.For wide operation temperature adaptability,particularly at lower temperatures,the presented desalination technique may have huge potentials for the applications of low-cost and low-grade waste heat.