There is a large push to develop appropriate technologies to treat not only seawater but brackish waters in order to increase the availability of water. Brackish water treatment requires significantly less energy than seawater desalination. To put the energy demand into perspective, the thermodynamic minimum energy require for separation, estimated by the Gibbs free energy of mixing, is between 1-3 kWh per m³ for seawater (35 g/l) and only 0.2-0.9 kWh per m³ for brackish streams (5 g/l). This energy savings quickly compounds at industrial and power generation sites which require over 1 billion m³ of water daily¹. Furthermore, brackish waters are not geographically limited, available at both coastal and non-coastal regions.

The treatment of brackish rather than seawater can yield energy savings; however, achieving high energy efficiency during low saline separations processes remains a significant challenge. This has been demonstrated using state of the art desalination technology (reverse osmosis and distillation) that in general thermodynamic efficiencies are low (~5%)². Furthermore, most common desalination technologies require additional components (heat or pressure exchangers) to recover energy. One emerging technology which aims to improve thermodynamic efficiencies associated with brackish water and simultaneously has the capability to recovery energy is termed capacitive deionization (CDI)³. CDI removes minority ions from a mixture through electroadsorbing the charged constituents when a potential is applied across an electrochemical cell. The ions stored in the electric double layer (EDL) can then be discharged into a brine solution to regenerate active surface area. During this discharge process energy can be recovered. To date, the majority of CDI energy efficiency studies have focused on evaluating systems levels losses associated with the materials, or have evaluated individual processes (charge or discharge) in order to improve performance. While optimizing individual processes provides significant insight into mechanistic losses, evaluating a complete cycle is critical in order to evaluate 2^nd law limitations. Here we will evaluate the challenges associated with discharging into a brine solution, and the potential energetic and exegetic savings which can occur through non-adiabatic system operation. 

1. Maupin, Molly A., et al. Estimated use of water in the United States in 2010. No. 1405. US Geological Survey, 2014.

2. Demirel, Yaşar. "Thermodynamic analysis of separation systems." Separation science and technology 39, no. 16 (2004): 3897-3942.

3. Hatzell, Kelsey B., Marta C. Hatzell, Kevin M. Cook, Muhammad Boota, Gabrielle M. Housel, Alexander McBride, E. Caglan Kumbur, and Yury Gogotsi. "Effect of oxidation of carbon material on suspension electrodes for flow electrode capacitive deionization." Environmental science & technology 49, no. 5 (2015): 3040-3047.