Design and Analysis of CDI Techniques for High Energy Efficiency and Energy Recovery

Abstract

Water and energy scarcity by industrialization and population growth has emerged as global crisis to humanity, which cause the demand for efficient desalination technique with low-energy cost. Compared to conventional desalination processes such as reverse osmosis (RO) and distillation, capacitive deionization (CDI), an electrochemical desalination technology using electrical double layer on the electrode, has come into spotlight in terms of environment-friendly and low-energetic process. Since the desalination performance and energy efficiency of CDI process are determined by operation techniques such as constant current operation and energy recovery, design and analysis for these techniques are needed for enhancing energy efficiency. Therefore, in this dissertation, design and analysis of CDI techniques for high energy efficiency and energy recovery was implemented by focusing on the evaluation of energy consumption according to operational modes and energy recovery system. Firstly, salt adsorption capacity (deionization capacity) and energy consumption of two CDI operational modes (CV and CC) were comparatively investigated. As major results, CV mode resulted in faster salt adsorption while CC mode showed much lower energy consumption than CV mode by 26 ~ 30% due to the overall lower cell voltage used in CC mode than in CV mode. Secondly, the successful construction of an energy recovery system in an actual MCDI cell with a buck-boost converter was implemented

the buck-boost converter facilitated the delivery of the energy stored in the MCDI cell into a supercapacitor. The salt adsorption capacity was found to play an important role in the energy recovery and constant current charging was found to be more favorable for energy recovery than constant voltage charging. Lastly, energy recovery ratio in MCDI depending on electrode properties was investigated using constant current operation. Almost the whole carbon electrodes showed energy recovery ratios of 0.5 ~ 0.75 and we found out that not only salt adsorption capacity but also salt adsorption rate play an important role in energy recovery performance. In conclusion, this dissertation focused on design and analysis of operating techniques, CC operation and energy recovery process with investigating energy efficiency and energy recovery according to operating condition. We expect that this dissertation will provide a comprehensive guide for the construction and operation of high energy-efficiency CDI process.