Modeling of Ionic Shortcut Currents in RED and ED with segmented electrodes - An open-source approach to stack modeling with Python and ngspice
Typ
Examensarbete för masterexamen
Program
Publicerad
2020
Författare
Tenblad, Pauline
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
In the battle against global warming, reducing greenhouse gases is of utmost importance.
Salinity gradient energy is a clean renewable energy source where energy
is generated by mixing waters of different salinity. To harvest this energy reverse
electrodialysis (RED) can be used. High energy efficiency is fundamental for upscaling
the process and make it commercializable. A factor limiting the efficiency
is the occurrence of ionic shortcut currents (ISCC). They arise from voltage differences
within the stack and lead to loop-back of the current, resulting in a lower
current coming out of the stack and thus a lower power production. Previous work
have investigated the impact electrode segmentation has on the power production
and found that electrode segmentation can improve the power production in RED
stacks. These studies have assumed ideal stack properties and failed to take the
occurrence of ionic shortcut currents into account. The object of this project has
therefore been to investigate the role of ionic shortcut currents on the performance
of electrode segmentation and to investigate if the ISCC limit the benefit of electrode
segmentation. A model was created that simulates the process as an electric
circuit. It uses the RED model created by Simoes et al. (Desalination, vol. 492,
p. 114604, 2020) to calculate the internal resistance, the electromotive force and
resistances of the shortcut currents for each cellpair forming the stack. The model
created in this project can be used to simulate both a reverse electrodialysis stack
and an electrodialysis stack (where energy is applied to the system to drive the
salinity gradient, i.e. to desalinate one stream and form a brine stream). It was
found that a 2x2 segmented stack outperformed the unsegmented stack for stacks
with the number of cellpairs composing the stack ranging from 10 to approximately
100 cellpairs. For larger stacks, the power output of the segmented stack dropped
drastically while the power output of the unsegmented plateaued. The results suggest
that the ionic shortcut currents inhibit the advantages of the segmentation
when the stack becomes too big. However, there is still a need to further validate
these results. Further investigations with different cell sizes, flow rates and segmentation
configurations are advised. The results show that for an increased amount of
cellpairs forming the stack, the impact of the ionic shortcut currents is increased.
In future studies, investigating alternatives to reduce the ISCC, for example with
special manifolds, is encouraged.
Beskrivning
Ämne/nyckelord
Salinity gradient energy , Reverse electrodialysis , Renewable energy , Energy efficiency , Ionic shortcut currents , Parasitic currents , Stack design , Current leakage , Modeling , Optimization