Session: 4.1.1 - Renewable Energy Systems

Paper Number: 108772

108772 - Employing Variable Current/Voltage Control Schemes to Develop Carnot-Analogous Mixing Engines for Salinity Gradient Energy Extraction 

Capacitive mixing (CapMix) is an energy harnessing technique using the salinity gradient between high and low concentration water streams, such as at esturaries where river water empties into the ocean. Unlike other salinity gradient energy extraction methods, such as reverse electrodialysis and pressure-retarded osmosis, CapMix can circumvent the use of costly and fouling-prone membranes by storing the salt ions within porous carbon electrodes. The ions are stored within electrical double layer in the electrode pores, whose structure changes based on the surrounding cell concentration, and then subsequently released into a lower concentration water stream to generate net electrical energy, sometimes called “blue energy” [1]. The process is similar to heat engines which extract work or power from a temperature difference, and different operating modes of CapMix cycles have been examined to maximize power output, efficiency, and other related parameters [2]. 

Traditional thermodynamics principles would suggest that a Carnot analog would maximize efficiency and thus be fully reversible assuming no irreversible heat transfer occurs. Electrochemically, the analogs for temperature and entropy are chemical potential and number of ions, respectively [3]. In transient cycles, continuous mode operation will generally retain the initial concentration in the cell, enabling constant chemical potential if kept at a constant temperature. The total number of ions is the cell is more complicated and depends on both charge and concentration, which would require precise dependence on the applied current or voltage in the cell, as well as flow rate. If studying a theoretical, steady-state CapMix cycle, temperature is the only remaining variable that can be altered if a constant number of ions stage is to be realistically employed.

This work will aim to develop control schemes for the voltage, current, and flow rate profiles over time to determine the feasibility of replicating a Carnot analog cycle. Previously, temperature was employed as a variable parameter to assist in maintain constant chemical potential/number of ions stages [4]. This operation, while interesting to study and analogous to a Carnot cycle electrochemically, still cannot be considered a true Carnot analog due to the large amount of heat transfer required to employ the temperature changes. Additionally, it was deduced that, at least in a transient cycle, there remain enough free parameters that number of ions can be maintained using a variable current/voltage profile. However, this will constrain the allowable range of concentrations and sizing of the cell. Transient modeling results will be exploring examining a range of mixing concentrations and cell sizes to determine how a Carnot cycle can be most feasibly operated. The results will also consider different types of electrochemical models to best reflect the nature of the electrochemical double layer as a function of the concentration within the cell. Finally, considerations to make for practical, real-world implementation based on the initial simulated results obtained, will be discussed.

1.    Moreno DA. Using Temperature Variations to Demonstrate Analogous Carnot Heat Engines for Salinity Gradient Energy via Capacitive Mixing. InASME Power Conference 2022 Jul 18 (Vol. 85826, p. V001T07A006). American Society of Mechanical Engineers.
2.    Janssen M, Härtel A, Van Roij R. Boosting capacitive blue-energy and desalination devices with waste heat. Physical review letters. 2014 Dec 24;113(26):268501.
3.    Moreno D, Hatzell MC. Using Thermodynamics Principles to Optimize Performance of Capacitive Mixing Cycles for Salinity Gradient Energy Generation. InASME Power Conference 2019 Jul 15 (Vol. 59100, p. V001T12A007). American Society of Mechanical Engineers.
4.    Boon N, Van Roij R. ‘Blue energy’from ion adsorption and electrode charging in sea and river water. Molecular Physics. 2011 Mar 30;109(7-10):1229-41.

Presenting Author: Daniel Moreno Missouri State University

Presenting Author Biography: Dr. Daniel Moreno is an Assistant Professor of Mechanical Engineering at Missouri State University's Cooperative Engineering Program, with a joint appointment in the PAMS department and a courtesy appointment at Missouri University of Science & Technology. He received his Bachelor’s degree in Mechanical Engineering at the Cooper Union for the Advancement of Science and Art in New York City. He received his Master’s and Ph.D. at Georgia Institute of Technology, also in Mechanical Engineering. Dr. Moreno’s teaching expertise is in the thermal sciences. His research integrates thermodynamics concepts in ME with the multi-disciplinary field of electrochemistry to promote renewable energy technologies. Projects that he has worked on consisted of applications in batteries, fuel cells, carbon capture, and capacitive technologies for water desalination. Presently, Dr. Moreno is interested in exploring innovative, reliable energy resources to foster a cleaner environment and eliminate harmful atmospheric emissions.