(224a) Evaluating Combined Heat and Power Deionization Systems for Efficient Water Reuse at Thermoelectric Power Plants

Authors: 
Hatzell, M., Georgia Tech
Efforts to increase both the United States and global water supply has primarily taken place through seawater and brackish water desalination. According to the International Desalination Association, the United States capacity for desalination increased in 2013 by 50% from 4 million to 6 million m3/day. Unlike most areas of the world, nearly 77% of all desalination in the United States comes from the treatment of brackish water. Brackish water has lower salt concentrations (~<10 g/l) than seawater (35 g/l) and thus the energy consumed to remove salt ions in theory is much less than seawater desalination. Current technologies utilized for seawater desalination, reverse osmosis (RO) and Membrane distillation (MD), require a significant amount of energy regardless of the type of water (brackish or seawater). Furthermore, these technologies have approached their thermodynamic energy limits (1-3 kWh m–3), indicating that further energy savings cannot be achieved. RO and MD are energy intensive technologies because they operated through extracting the main component water (H2O) from the salt (NaCl). Alternatively, capacitive deionization (CDI), an emerging separation-based technology, operates by adsorbing the minority component (NaCl) from the feeding brackish water. Traditional CDI cells operate in a cyclic process where the capacitive electrodes are brought in contact with a concentrated salt solution (e.g. brackish water) and charged. Once the charged electrodes are fully saturated with salt ions, the electrodes are regenerated through discharging the electrodes in a brine solution. Analogous to a P-V diagram for traditional heat engines, the enclosed area of the charge versus voltage represents the energy that is consumed during this charging-discharging process. Until recently the CDI three step intermittent process has been relatively untouched in terms of literature and system operation. In this study, we explore the effect of temperature on salt removal with CDI. Experimentally, we use traditional electrochemical characterization methods, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), to evaluate the capacitance and resistance changes with temperature. Here it was found that through increasing the operating temperature from 25°C to 45°C, the specific capacitance of carbon electrodes increased from 34.60±2.15 to 42.56±3.57 F/g, and the cell resistance decreased from 20.20±2.97 to 15.57±2.52 ohm. Additionally, the salt adsorption capacity decreased from 13.82±1.13, to 6.58±1.01 milligram NaCl/mg carbon. This indicates that although the electrochemical performance of CDI can be increased with temperature, the ion storage within the electric double layer is hindered at higher temperatures.