Thermodynamic analysis of electrodialysis

Abstract

The work presented in this thesis is motivated by the water and energy problems our world faces today. Desalination can help alleviate the problem of water shortage by increasing the supply of fresh water. However, for desalination to play a major role in the future, it needs to be done in a sustainable manner. Significant progress towards making desalination technologies sustainable can be made by decreasing their energy consumption. This can be done with the help of a better understanding of the thermodynamics of desalination technologies. In this thesis, we present a thermodynamic analysis of electrodialysis (ED). ED is a desalination technology with many applications, and has shown promise in desalinating brackish water and in concentrating high-salinity brines. In order to better understand how the energetic performance of this technology can be improved, we first study the sources of entropy generation at different salinities, and locate areas where possible improvements need to be made under different operating conditions. In the second part, we define a fair set of constraints to allow a fair comparison between different system sizes, designs, and operating conditions. We study the tradeoffs governing the optimal channel height and velocity for brackish-water desalination and for high-salinity brine concentration. In addition, we study the minimum costs associated with the different system sizes, and we compare the differing trends in brackish-water and high-salinity applications. Further, we report optimal values of system size, current density, length, velocity, and cost for the two applications at different unit fixed costs and energy costs. In the third part, we study possible improvements to the energy efficiency of electrodialysis through the use of two electric stages with different voltages, and through the operation using a counterflow configuration. We first look at how a two-stage ED system should be operated for optimal energy efficiency. We then quantify the effect of operating under two voltages in brackish-water desalination and in high-salinity brine concentration. This is done at systems sizes that are shown to be cost effective at different unit fixed costs and energy costs. Finally, we quantify the effect of operating ED in counterflow for the same applications. In the final part, we study the optimal operation of a batch ED system for the desalination of brackish water and seawater, and for the concentration of high-salinity brine. We compare three processes: operation under constant voltage, constant current, and constant entropy generation. We then study the effect of improved operation on the energy consumption and on the system cost of batch ED at different fixed-to-energy cost ratios. It is shown that significant improvements to energy consumption and cost can be made through better system operation, especially in the seawater desalination application.