Abstract
Capacitive De-Ionization (CDI) is becoming a suitable alternative for desalination. The low cost of the materials required and its reduced energy consumption can be critical factors for developing this technique. CDI technology does not require a high-pressure system and the energy storage capability of CDI cells allows it to be reused in other CDI cells, thus minimizing consumption. The goal of the power stage responsible of the energy recovery is transferring the stored energy from one cell to another with the maximum possible efficiency, thus allowing the desalination process to continue. Assuming hysteresis current control is implemented at the DC/DC (direct current) converter, this paper aims to determine the optimum peak current through the inductor in each switching period with a view to maximizing overall efficiency. The geometrical parameters of the desalination cell and the NaCl concentration modify the cell electrical properties. The peak current control of the power stage should be adapted to the cell characteristics so that the efficiency behavior of the whole CDI system can be improved. The mathematical model defined in this paper allows the CDI plant automation using the peak inductor current as control variable, adapting its value to the salt concentration during the desalination process.
Highlights
Water availability is not guaranteed for large regions of the world due to current issues such as population increase, pollution or global warming
The electric model of the Capacitive De-Ionization (CDI) modules must be defined as a function of their geometry and of the salt molar concentration: RS (d,n,M), RP (d,n,M) and C (d,n,M). These values are changing during the desalination process, if we aim to obtain the maximum possible efficiency, the up/down converter must be adapted during the process
The hysteresis control of the inductor current involves a variable operating frequency, because a steady state is never really reached: the input capacitor is discharged while the output voltage increases during the process: ton and toff are constantly changing
Summary
Water availability is not guaranteed for large regions of the world due to current issues such as population increase, pollution or global warming. Desalination of sea water could be a possible solution to this problem, but it is not widely used yet because of the high power consumption required to make water drinkable. There are already several processes being used in this field such as reverse osmosis, distillation or electrodialysis. Reverse osmosis is a widely-used process for large water production; it involves energy consumption around 4 kWh/m3 [1,2]. Other technologies involve high diesel consumption with higher energy requirements and CO2 emissions. It is estimated that the energy required when using CDI + energy recovery control systems can be reduced up to one fourth that required by reverse osmosis [3,4]. The aim of this work is to take advantage of the energy stored in the desalination modules and to re-use it with high efficiency by means of a buck-boost converter
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