Final Report: Solar Desalination with Capacitive DeionizationEPA Grant Number: SU836777
Title: Solar Desalination with Capacitive Deionization
Investigators: Lin, Shihong
Institution: Vanderbilt University
EPA Project Officer: Page, Angela
Project Period: September 1, 2016 through August 31, 2017
Project Amount: $9,554
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2016) RFA Text | Recipients Lists
Research Category: Sustainable and Healthy Communities , P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources
Capacitive deionization (CDI) is an emerging technology for brackish water desalination. It is operated by applying low voltage to drive charged species (e.g., ions) migrating from water to porous electrodes, resulting in desalted product water. When the porous electrodes are saturated, they are regenerated by short-circuiting or applying reverse voltage/current, meanwhile the temporarily stored salt release forming brine. The energy consumed during the charging step could be partially recovered, rendering it an energy efficient approach for water desalination.
There exist many communities that have limited access to large-scale infrastructure of power and water supply. The total dissolved solid (TDS) concentration of the local brackish groundwater is relatively high and requires a treatment process for removing the salt for drinking water. Existing desalination processes are typically classified into two major categories: pressure driven processes such as reverse osmosis (RO) and nanofiltration (NF), and thermal driven processes such as multiple effect evaporation and multistage flash distillation. These technologies confront significant hurdles in being applied for small-scale and off-grid brackish groundwater desalination. For example, RO and NF processes are prone to fouling during extended suspension of system operation, which is not suitable when using intermittently sustainable energy. On the other hand, thermal drive processes require a high level of maintenance and are inherently energy inefficient. Compared with pressure driven or thermal driven processes, CDI technology is advantageous in several aspects, including low voltage requirements and compatibility with local solar energy supply in remote areas.
In this project, a cohesive team, formed by six undergraduate students at Vanderbilt University, developed a system that coupled solar energy with capacitive deionization. One major challenge the team overcame was to design and build a control system that is capable of separating the deionized product water and the concentrated brine. This project not only achieves a sustainable desalination system with practical purposes, but also teaches next-generation engineers the necessity of using multi-disciplinary teams to solve real-world problems.
Design of CDI Cell: The desalination performance of CDI is highly dependent on the cell configuration. In general, based on the operational nature of continuity, CDI cell designs fall into two categories: ‘batch mode’ and ‘single-pass mode’. In batch mode, the salinity in the volume-specified batch drops steadily and levels off during the charging stage, and the stored salt is released into another batch when it discharges. In single-pass experiments, effluent salinity drops shortly after applying the cell voltage, and it rises up to the feed level due to the limit of saturation capacity. The regeneration of the electrodes occurs in the same feed simply by shortcircuiting or reversing the voltage. In this project, we choose to follow the single-pass mode, because it is more similar to a real desalination process allowing water to pass the system only once, instead of being recycled multiple times.
Control System: A conductivity probe was purchased from Atlas Scientific, which is controlled through an Arduino microcontroller. The probe sends measured data to the Arduino for analysis. In order to visualize the data, the microcontroller exports the data to an Excel spreadsheet for data storage and real-time graphing. Also, the Arduino system controls the valve that splits the dilute water and the brine. Once the salinity of dilute water exceeds a prescribed level (800 μS cm-1), the valve is switched, and the effluent of the CDI cell goes to the brine container.
Testing of CDI Cell: Before integrating with the solar power system, the CDI cell is tested with salt water (~10 mM NaCl). A cell voltage of 1.2V is applied across the electrode stack (four parallel pairs). The flowrate of the feed water flowing through the stack is kept around 10 mL min-1. At the commencement of applying voltage, the current diminishes exponentially, while the measured conductivity experiences an immediate drop but rises up again. The charging step ceases before the electrodes reach their limit of saturation, because the conductivity in the deionized water container exceeds 800 μS cm-1. At that specific time point, the valve controlling the effluent automatically switches to the brine container, simultaneously, the cell discharges by short-circuiting. During the discharging stage, the effluent conductivity increases beyond the feed level and a negative current response is detected. It is safe to conclude that the designed single-pass CDI system with multiple cells in parallel is able to desalinate the feed to less than 800 μS cm-1. And the controlling Arduino simultaneously enables the valve switching and discharging when the conductivity in the deionized water container meets the prescribed limit. In that sense, the separation of brine from deionized water is achieved by controlling the flow to a brine container during the discharging stage.
Solar Power System: The ultimate goal of the design project is to provide a viable alternative for producing fresh water for communities in areas that lack water and energy infrastructures. Decentralized CDI treatment process coupling with solar energy is a desirable energy efficient and economical alternative. Unfortunately, we are not able to secure a constant voltage of 1.2V output from the system even though the system itself works fine. The energy supply for the CDI system is one AA battery cell. Nevertheless, it proves that an energy source as simple as a AA battery will suffice in driving the CDI cell.
This project was carried out and completed by the undergraduate students from diverse engineering majors at Vanderbilt University. The future generation of engineers, as a team, designed and fabricated a functional CDI cell that performs reasonably well in desalinating brackish water. In addition, the team made a conductivity measurement and controlling system with the aid of Arduino. Although a solar-powered system was developed, it was not sufficient to drive the CDI system. Instead, a simple AA battery was used to drive the CDI process.
Last, but not least, the project has facilitated collaboration among students with different backgrounds and taught them how to apply the knowledge learned in the classroom to solve the real-world problems.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
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|| Wang L, Lin S. Intrinsic tradeoff between kinetic and energetic efficiencies in membrane capacitive deionization. Water Research 2018;129:394-401.