Study On Performance Of ’Heat To Power’ Cycle Of Solution Concentration Difference Driven By Low-grade Heat

As a novel way to utilize low-grade heat,Chemical Heat Engine(CHE)technology now has received more and more attention.Different with the ’heat to mechanical work’ cycle,CHE can accomplish ’heat to power’ cycle based on the Salinity Gradient Energy(SGE)of solutions.As one of new and promising technologies of CHE,Reverse Electro-Dialysis Heat Engine(REDHE)mainly consists of thermal separation unit and Reverse Electro-Dialysis(RED)unit.Firstly,the brackish solution is separated by LGH in the thermal separation unit to form high and low concentration solutions,which is called ’heat to SGE’ process.And then these two solutions flow through the RED unit where SGE is converted directly into electricity,which is called the ’ SGE to power’ process.Nowadays,the energy conversion efficiency of REDHE is low since the relevant studies concerning it is just in the initial phase,which limits the popularization and application of it.Therefore,the aim of this paper is to study how to improve the efficiency of REDHE.And it is expected that this paper can provide some theoretical and experimental guidance for the future development of REDHE technology.RED stack is the core component of REDHE,and the ability of RED stack to convert SGE:mainly determines the energy conversion efficiency of REDHE.Therefore,the study of this paper starts with RED stack.In Chapter 2,the model of RED under the background of REDHE was built through screening and reconstructing the existing models of RED,and then the new model was verified by experiments.Results showed that the calculated results of the model are in good agreement with the experimental results.In Chapter 3,in order to improve the ability of converting SGE,Multi-stage Reverse Electro-dialysis(MSRED)is used to replace the single stage RED,and relevant researches had been carried out.In the process of theoretical research on MSRED,the model in Chapter 2 is firstly extended to the MSRED level to deeply compare and analyze two different control strategies of MSRED.The results showed that compared with the independent control strategy,the series control strategy is more convenient to control MSRED and more convenient and effective to manage the output power,so it is more suitable for practical use.Next,it was found that it is beneficial for improving the performance of MSRED by reducing the velocity of feed solutions,increasing the thickness of concentrated solution compartments,reducing the concentration of dilute solution and adopting the counter-flow arrangement of solutions,through studying the influence of different parameters and arrangements of solutions on the performance of MSRED.Finally,some above measures to improve MSRED performance were verified through experiments focusing on the MSRED in series control.In Chapter 4,combining with Low-temperature Multi-effect Distillation(LT-MED)and the series control MSRED,a miniaturized MED-MSRED heat engine was presented.By analyzing the cycle characteristics of the engine,influences of operating parameters and effect number of the MED on the performance of the heat engine was studied.The results showed that the energy conversion efficiency of the heat engine could be improved by appropriately increasing the inlet temperature of hot water in MED,increasing the initial molality of brackish solution and increasing the effect number of the MED.When the initial temperatures of hot water and cooling water are 80℃ and 20℃ respectively,the maximum energy conversion efficiency of the engine is 1.27%.In Chapter 5,research on the irreversibility of the MED-MSRED heat engine was carried out by exergy analysis to explore the factors limiting the improvement of energy conversion efficiency of the heat engine.The results showed that the preheater,condenser and MSRED were the three main exergy destruction sources.Exergy efficiency of the heat engine can be improved by lowering the inlet temperature of hot water in MED,appropriately increasing the initial molarity of brackish solution and increasing the effect number of the MED.When the initial temperatures of hot water and cooling water are 80℃ and 20℃ respectively,the maximum exergy efficiency of the engine is 6.3%.