Fundamental Properties Of The Suitable Working Solutions For The Heat Driven Hydrogen Production System With The Reverse Electrodialysis Method

Exploring the renewable energy and improving energy utilization efficiency have become a common concern in modern society.Based on the principle of salinity gradient energy(SGE)power generation with the open-type reverse electrodialysis(RED)method,a closed-type low-grade thermal energy driven electricity generation/hydrogen production cycle was proposed in previous work of our research group,by means of integrating the RED method with the absorption refrigeration and thermal desalination technologies.The working process is described as follows: firstly,the thermal energy is converted to SGE between the concentrated and diluted solutions by using the solution distillation method;then,with the mixing of solutions(separated by membranes)as well as ionic transmembrane migrations,the ion current inside the RED stack arises and potential difference at both ends are formed as per the RED principle;At this situation,hydrogen and oxygen can be produced by means of oxidation and reduction reactions at the phase interface of electrodes at both ends of the RED stack.The waste working solution from the outlet of the RED stack is collected and delivered back to the solution regeneration device to restore their concentration.Working solution,as the "blood" of a cycle system,is a key factor affects the energy efficiency,process structure,economy and stability of the closed-type RED heat-hydrogen conversion system.Both the traditional NaCl solution and new acetate(KAc,Na Ac)solutions were selected as candidates in this work.The basic properties(thermodynamic properties,electrochemical properties,transport properties,and risk)of which were comprehensively assessed.The results showed that the acetate solutions have obvious advantages over traditional NaCl solution in solubility,activity coefficient ratio and other key physical-chemical properties;moreover,it has obvious advantages in the aspect of power density and heat conversion efficiency.The results indicated that the KAc solution has great development potential in this field.To further improve the energy conversion efficiency of the closed-type RED system,the inorganic salt-solvent-modifier ternary working solution idea was suggested in previous work of our research group.Two or more kinds of solvents will be used to show their respective specialties in the thermal separation and electrochemical reaction processes.In this work,the solubility of acetate(KAc,Na Ac)in water,organic solvent and binary mixed solvent was determined by laser dynamic method.The influences of temperature and solvent properties on solubility were analyzed,and then made a comparison between the laser dynamic method and the static solubility test method.Furthermore,five empirical models were used to correlate the solubility data.The van’t Hoff analysis method was used to analyze the thermodynamic properties variation of salt in the process of dissolution from the perspective of energy.The theoretical cycle model of a closed-type low-grade thermal energy driven electricity generation/hydrogen production was established by integrating the air gap diffusion distillation method,open-type RED method and alkaline electrolyzed water hydrogen production method.The optimal KAc solution was used as the working solution,and the single factor control variable method was used to investigate the effects of solution concentration,flow rate,temperature on the output characteristics(U-I curve,power density,conversion efficiency,etc.)of the reverse electrodialysis hydrogen generation system.It lays the foundation of further optimizing the operation conditions to improve the hydrogen production.