| The chemical potential energy between solutions of different concentrations is called salt(concentration)difference energy.The salinity difference in nature mainly exists between the river water and the sea water in the estuary.There are two main ways to convert salt difference energy into electricity based on membrane technology: Pressure Retarded Osmosis(PRO)and Reverse Electrodialysis(RED).At present,the research on RED heat engine is still insufficient,and improving the power generation efficiency of RED heat engine is still the main direction of research.Our research group proposed a novel air-gap diffusion distillation(AGDD)-RED chemical heat engine.The AGDD device uses low-grade heat as the driving heat source,and firstly separates the salt solution into concentrated and diluted solutions with a large concentration difference.Then the two solutions enter a series multi-stage RED stacks to generate electricity,realizing the energy conversion process from "heat-salt difference energyelectricity".This technology can realize the closed circulation of solutions inside the device,avoiding the problem of efficiency drop caused by solution impurities.In this paper,the mathematical model of the AGDD-RED engine is established,mainly including the heat transfer and mass transfer model of AGDD under the ε-NTU method,as well as the electrical output characteristics and the internal mass transfer model of the RED stack.By solving the established mathematical model,the influence of the variations of various operating parameters(feed solution concentration,feed mass flow rate,concentration ratio,hot water inlet temperature,cold water inlet temperature,output current)and structural parameters(air-gap thickness,air-gap height)on the main output parameters and efficiency of the heat engine system is analyzed.The research results show that: under the design conditions and excluding electrodialysis,the smaller the current,the more stacks in series.In the series multi-stack system,the output voltage of the first stack is the highest,and the output voltage of each stack decreases gradually along the solution flow direction,and the decrease of the output voltage gradually decreases from the first stack to the last stack.The output power of the multi-stage stacks increases first and then decreases with the increase of the current.The solution thermal separation unit loses the most during the energy conversion process.Improving the thermal separation efficiency of the solution thermal separation unit is the focus of improving the performance of the heat engine.In addition,variations in operating and structural conditions affect the performance output parameters of heat engine system.For the heat engine system,under the given operating conditions and structural conditions,there are optimal operating parameters: the feed solution concentration is 2mol/L,and the concentration ratio is 1.4.The increase of the inlet temperature of hot water is beneficial to improve the energy conversion efficiency of the heat engine system,while the increase of cold water temperature is not conducive to the improvement of the energy conversion efficiency of the heat engine.The increase of feed flow rate is beneficial to the power output,but it is harmful to the energy conversion efficiency of the heat engine.The increase of air-gap thickness is not conducive to the energy conversion efficiency of the heat engine system,but the increase of air-gap height is beneficial to the energy conversion of the heat engine,and both variations have little effect on the power generation of the stack. |
