Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Frequency analysis and resonant operation for efficient capacitive
  deionization   Capacitive deionization (CDI) performance metrics can vary widely with
operating methods. We here develop a theory around and experimentally
demonstrate a new operation for CDI that uses sinusoidal forcing voltage (or
sinusoidal current). We use a dynamic system modeling approach, and quantify
the frequency response (amplitude and phase) of CDI effluent concentration, and
demonstrate that CDI can be modeled as a linear time invariant system. We
validate this model with experiments, and show that a sinusoid voltage
operation can simultaneously achieve high salt removal and strong energy
performance, thus very likely making it superior to other conventional
operating methods. Based on the underlying coupled phenomena of electrical
charge (and ionic) transfer with bulk advection in CDI, we derive and validate
experimentally the concept of using sinusoidal voltage forcing functions to
achieve resonance-type operation for CDI. Despite the complexities of the
system, we find a simple relation for the resonant time scale: the resonant
time period is proportional to the geometric mean of the flow residence time
and the electrical (RC) charging time. Operation at resonance implies the
optimal balance between absolute amount of salt removed (in moles) and dilution
(depending on the feed volume processed), thus resulting in the maximum average
concentration reduction for the desalinated water. Using a generalized tool
based on Fourier analysis, we show that most of the salt removal (~95%) for
square and triangular voltage forcing waveforms is achieved by the fundamental
Fourier (sinusoidal) mode. The frequency of higher Fourier modes precludes high
flow efficiency for these modes, so these modes consume additional energy for
minimal additional salt removed. This deficiency of higher frequency modes
further highlights the advantage of DC-offset sinusoidal forcing for CDI
operation.

24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
18
The categories of this paper are: econ
32
The categories of this paper are: atom-ph
6
The categories of this paper are: astro-ph
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
4
The categories of this paper are: cond-mat
13
The categories of this paper are: quant-ph
29
The categories of this paper are: funct-an
22
The categories of this paper are: chao-dyn
10
The categories of this paper are: q-bio
7
The categories of this paper are: hep-th