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Efficiency of Thermally Assisted Capacitive Mixing and Deionization Systems

Moreno, Daniel, Hatzell, Marta C.
ACS sustainable chemistry & engineering 2019 v.7 no.13 pp. 11334-11340
deionization, energy conversion, environmental factors, environmental technology, mixing, temperature profiles
The temperature of an input solution to an environmental technology can vary greatly depending on numerous industrial and environmental factors. For capacitive mixing (CapMix) and deionization (CDI) based technologies which rely on charge storage within an electric double layer (EDL), this temperature dependence affects energy output and energy consumption. Yet, how temperature-dependent EDL properties impact the thermodynamic efficiency for energy conversion and ion separations is less known. Here, we evaluate how isothermal, nonisothermal and variable temperature profiles impact the thermodynamic efficiency of CDI and CapMix cycles operated under both reversible and irreversible (current dependent) conditions. For CapMix (CHC = 600 mM, CLC = 20 mM, ϕ = 50%), reversible system operation resulted in an optimal efficiency of 43% when THC ≠ TLC. For CDI (Cfₑₑd = 20 mM, Cdᵢₗᵤₐₜₑ = 5 mM, α = 50%), optimal thermodynamic efficiencies are attained through the use of nonisothermal (Tfₑₑd ≠ Tbᵣᵢₙₑ) or variable temperature operation, and approached 10%. The introduction of current (25 mA) based irreversibilities, however, reduces the maximum attainable CapMix and CDI thermodynamic efficiency to 25% and 3.2% (25 mA).