A new Open Access paper was published in the prestigious journal ES&T, from the group of prof. Meny Elimelech (Yale, USA) which discusses a detailed comparison between electrodialysis (ED) and membrane capacitive deionization (MCDI).
Patel et al. "Energy Efficiency of Electro-Driven Brackish Water Desalination: Electrodialysis Significantly Outperforms Membrane Capacitive Deionization", ES&T (2020). https://pubs.acs.org/doi/abs/10.1021/acs.est.9b07482
According to the authors, "we provide the first systematic and rigorous comparison of the energetic performance of electrodialysis (ED) and membrane capacitive deionization (MCDI) over a broad range of brackish water desalination conditions."
The authors find that:
- The energy consumption of ED is substantially lower than MCDI for all investigated conditions, with the energy efficiency being nearly an order of magnitude higher for many separations.
- Even with idealized operation (complete energy recovery and reduction in energetic losses), the energy efficiency of MCDI remains lower than ED.
Finally, the authors emphasize that "for low feedwater salinities (< ~2 g/L), energy efficiency should be a secondary consideration in the choice of desalination technology, with capital cost, ease and reliability of operation, and additionally required treatment steps taking higher priority."
... and the field of Capacitive Deionization keeps on growing at an increasing speed ! With over 160 scientific papers written and published last year, the total number of CDI publications has grown from ∼25 in 2000, to ∼65 in 2010, to over 1,000 at the end of 2019! CDI papers in 2019 came from many prestigious places including Tsinghua University, Seoul National University, Technion, MIT, Stanford, and Yale. This output from such eminent research groups shows that CDI is taking a leading role in the scientific study of water desalination technologies. Analyzing these publication data for the past decade, an exponential growth can be observed, with a doubling of the publication output every 2.5 year !
Citations to the CDI literature have grown from a number less than 100 per year before 2010, to about 2000 per year at the end of 2015, and close to 10,000 per year in 2019. These statistics also reflect an exponential growth, with a doubling time of 2.0 year. This difference in doubling time (faster for citations than publications) may indicate that CDI papers are more and more cited in papers from outside the CDI-field.
Following the CDI conference recently held in the Republic of Korea, July 2017, twenty scientists active in the field of CDI worked on a joint position paper, putting forward the proposition that CDI defines a class of desalination technologies that share common operational principles and relevant metrics, thereby joining under one common term different CDI cell layouts and chemistries. Thus, according to the position paper, the class of CDI includes electrodes based on carbon materials, but as well electrodes with ion storage based on different chemistries such as using redox materials. The position paper can be downloaded via the link given below.
P.M. Biesheuvel, M.Z. Bazant, R.D. Cusick, T.A. Hatton, K.B. Hatzell, M.C. Hatzell, P. Liang, S. Lin, S. Porada, J.G. Santiago, K.C. Smith, M. Stadermann, X. Su, X. Sun, T.D. Waite, A. van der Wal, J. Yoon, R. Zhao, L. Zou, and M.E. Suss, "Capacitive Deionization -- defining a class of desalination technologies," ArXiv:1709.05925 (2017).
Last week's CDI-E's conference was a big success. 180 participants gathered for three full conference days in the heart of Seoul, Republic of South Korea. The conference, hosted by prof. Jeyong Yoon and his team of Seoul National University, proved invaluable in informing participants of all the latest developments in CDI technology, both from an academic and industrial perspective. The versatile program consisted of a tutorial session, plenary and keynote lectures, regular lectures and two poster sessions and gave food for thought to all attendants. The lively and amiable atmosphere gave the whole conference the right touch of feeling welcome in perhaps one of the most vibrant places in the world, the famous Gangnam district, a place that never sleeps.
Two recent papers with authors from the US, The Netherlands, and Israel, convincingly show the relevance of chemical charge residing in the carbon electrodes ("immobile", or "complementary" charge) to enhance salt adsorption capacity (SAC) of CDI electrodes.
In the more theoretical paper of the two, published OPEN ACCESS in Colloids and Interfaces Science Communications, the theoretical framework is laid out which comprehensively describes the range of recently developed new CDI desalination modes such as inverted-CDI and (what the authors call) enhanced-CDI. Also the occurrence of "inversion peaks" which often develop during normal CDI operation are explained as due to developing chemical charge. In addition, a novel operational mode is described where due to the chemical charge, it becomes possible to enlarge the operating window of CDI and thus to enhance SAC further still. This operational mode of "extended voltage CDI" was not described before.
In the sequel paper, published in Water Research, both the enhanced-CDI regime and the extended-voltage CDI-regime are experimentally validated. In this paper the more advanced amphoteric Donnan model is used to describe the EDL-structure. This model quantitatively predicts the experimental observations of salt storage and charge. An interesting inconsistency is how the measured chemical charge (by titration) can be up to one order beyond the chemical charge derived from comparing the amph-D model to the data.
... and the field of Capacitive Deionization keeps on growing at an increasing speed ! While over 60 scientific publications are now written and published annually, citations to the CDI literature have grown from a number less than 100 per year before 2010, to about 2000 per year at the end of 2015, and this number continues to rise. Analysing citation data for the past 10 years, an exponential growth in citation rate is clearly observed, with a doubling of the citation rate every 18 months !
In a collaboration involving scientists from five different countries in two continents, members of the CDI&E working group used the past year to come with a Perspective-paper on capacitive ionization and electrosorption. Published on invitation in the high-impact journal Energy&Environmental Science (IF=25) as a prestigious Perspective-contribution, the paper is expected to generate attention inside and outside the CDI-field. To help quick dissemination of its content, the authors have chosen for the OPEN ACCESS-format.
As corresponding authors Prof.Dr. Mathew Suss and Prof.Dr. Volker Presser explain: "The idea for this perspective was conceived of during the last CDI-conference in Leeuwarden, the Netherlands, and our aim is that it serves the growing CDI-community in outlining current trends in CDI developments, in standardizing metrics, and to help by identifying 'white areas' in CDI, both experimentally and theoretically. It was a most exhilarating task to work together with so many different authors on different continents to see this paper growing over the year. Many discussions helped us to focus on the most important elements and to converge on the key trends and best metrics for CDI performance. We have done our very best to put together a paper that helps to catalyze scientific and industrial developments in the CDI&E-field."
As reported during the 8th International Conference "Interfaces against Pollution," May 2014, the energy consumption of CDI operation can be significantly reduced by tuning the discharge voltage, which is the cell voltage applied during cell discharge, when the adsorbed salt is released and a concentrated brine stream is produced. Commonly in CDI, the charging voltage is tuned to an optimum value, where salt adsorption is high but leakage currents are still low. The discharge voltage is by default set to zero Volt. Following an earlier study from Bar-Ilan University, Israel, a cooperation of Seoul University (South Korea), Wageningen University and Wetsus (the Netherlands) found and reported on the positive influence of tuning the discharge voltage to values higher than zero. In contrast to the earlier work, it was found that salt adsorption per cycle did not markedly decrease, while the charge efficiency went up to values approaching the theoretical limit of one (unity). This meant that the energy consumption significantly decreased (being inversely proportional to charge efficiency), even without considering energy recovery, something that is possible with positive discharge voltages.
In the same study it was also found that with a non-zero discharge voltage, it becomes easier in CDI to achieve a stable effluent concentration by using constant-current operation; something that before this study was thought to be possible only for membrane-assisted CDI. As senior author prof.dr. J. Yoon remarks: "This was a very insightful study that clearly showed the potential of tuning the operational conditions of CDI to enhance the performance of a CDI cell. It was remarkable how accurately the porous electrode transport theory, using the Donnan concept to describe salt adsorption, could describe the data. For design purposes, such a model is indispensable."
link to journal
link to pdf of paper