Diffusive transport of water in magnesium chloride dihydrate under various external conditions for long term heat storage: A ReaxFF-MD study

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Abstract

Pure Magnesium chloride hydrates are exceptional materials for long term thermochemical heat storage. In a heat storage cycle, the material is charged by dehydration, taking up heat, and discharged by hydration, generating heat. On charging, the H2O molecules released by the dehydration reaction have to diffuse through the solid material. Gas-solid interactions play an important role in mass and heat transport throughout the volume of the storage materials. Under certain experimental conditions, the mass transport may become a rate-determining step. In order to simulate the complex reaction coupled diffusion process, a reliable MgCl2–H2O reactive forcefield (ReaxFF) is parameterized using a single parameter search algorithm. The parameters of ReaxFF are trained against a set of data obtained from density functional theory (DFT). ReaxFF-MD simulations are carried out on a 2D periodic slab of MgCl2⋅2H2O to simulate various possible operating conditions of temperature, external water vapor pressure, incomplete dehydration layers and various vacancy defects during a charging-discharging cycle. The diffusion coefficient of H2O through the 2D periodic slab of dihydrate is observed as 1.24±0.37×10−10 m2/sec at 300 K. The diffusivity increases with temperature and follows the Arrhenius equation. External water molecules impede the dehydration and promote the diffusion of water. The vacancies of MgCl2⋅2H2O molecules are characterized using vibrational density of states obtained from ReaxFF-MD simulations. The vacancy concentrations for MgCl2⋅2H2O molecules have been varied from 1.38% to 4.16%, which result in a diffusivity enhancement from 32.9% to 107.7% when compared with perfect slab. The incomplete hydration at the surface and the mid-layer increases diffusivity by 76.7% and 75.0%.
LanguageEnglish
Pages93–101
Number of pages9
JournalEuropean Journal of Mechanics. B, Fluids
Volume64
DOIs
StatePublished - 2017

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magnesium chlorides
heat storage
Magnesium
dehydration
Vacancy
Diffusivity
Heat
Molecules
MD Simulation
diffusivity
Water
slabs
Hydration
Mass Transport
Term
heat
water
charging
hydration
molecules

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@article{fc488e4e78a5459bb29b283618a05f67,
title = "Diffusive transport of water in magnesium chloride dihydrate under various external conditions for long term heat storage: A ReaxFF-MD study",
abstract = "Pure Magnesium chloride hydrates are exceptional materials for long term thermochemical heat storage. In a heat storage cycle, the material is charged by dehydration, taking up heat, and discharged by hydration, generating heat. On charging, the H2O molecules released by the dehydration reaction have to diffuse through the solid material. Gas-solid interactions play an important role in mass and heat transport throughout the volume of the storage materials. Under certain experimental conditions, the mass transport may become a rate-determining step. In order to simulate the complex reaction coupled diffusion process, a reliable MgCl2–H2O reactive forcefield (ReaxFF) is parameterized using a single parameter search algorithm. The parameters of ReaxFF are trained against a set of data obtained from density functional theory (DFT). ReaxFF-MD simulations are carried out on a 2D periodic slab of MgCl2⋅2H2O to simulate various possible operating conditions of temperature, external water vapor pressure, incomplete dehydration layers and various vacancy defects during a charging-discharging cycle. The diffusion coefficient of H2O through the 2D periodic slab of dihydrate is observed as 1.24±0.37×10−10 m2/sec at 300 K. The diffusivity increases with temperature and follows the Arrhenius equation. External water molecules impede the dehydration and promote the diffusion of water. The vacancies of MgCl2⋅2H2O molecules are characterized using vibrational density of states obtained from ReaxFF-MD simulations. The vacancy concentrations for MgCl2⋅2H2O molecules have been varied from 1.38{\%} to 4.16{\%}, which result in a diffusivity enhancement from 32.9{\%} to 107.7{\%} when compared with perfect slab. The incomplete hydration at the surface and the mid-layer increases diffusivity by 76.7{\%} and 75.0{\%}.",
author = "A.D. Pathak and {Gaastra - Nedea}, S.V. and H.A. Zondag and C.C.M. Rindt and D.M.J. Smeulders",
year = "2017",
doi = "10.1016/j.euromechflu.2016.12.011",
language = "English",
volume = "64",
pages = "93–101",
journal = "European Journal of Mechanics. B, Fluids",
issn = "0997-7546",
publisher = "Elsevier",

}

TY - JOUR

T1 - Diffusive transport of water in magnesium chloride dihydrate under various external conditions for long term heat storage

T2 - European Journal of Mechanics. B, Fluids

AU - Pathak,A.D.

AU - Gaastra - Nedea,S.V.

AU - Zondag,H.A.

AU - Rindt,C.C.M.

AU - Smeulders,D.M.J.

PY - 2017

Y1 - 2017

N2 - Pure Magnesium chloride hydrates are exceptional materials for long term thermochemical heat storage. In a heat storage cycle, the material is charged by dehydration, taking up heat, and discharged by hydration, generating heat. On charging, the H2O molecules released by the dehydration reaction have to diffuse through the solid material. Gas-solid interactions play an important role in mass and heat transport throughout the volume of the storage materials. Under certain experimental conditions, the mass transport may become a rate-determining step. In order to simulate the complex reaction coupled diffusion process, a reliable MgCl2–H2O reactive forcefield (ReaxFF) is parameterized using a single parameter search algorithm. The parameters of ReaxFF are trained against a set of data obtained from density functional theory (DFT). ReaxFF-MD simulations are carried out on a 2D periodic slab of MgCl2⋅2H2O to simulate various possible operating conditions of temperature, external water vapor pressure, incomplete dehydration layers and various vacancy defects during a charging-discharging cycle. The diffusion coefficient of H2O through the 2D periodic slab of dihydrate is observed as 1.24±0.37×10−10 m2/sec at 300 K. The diffusivity increases with temperature and follows the Arrhenius equation. External water molecules impede the dehydration and promote the diffusion of water. The vacancies of MgCl2⋅2H2O molecules are characterized using vibrational density of states obtained from ReaxFF-MD simulations. The vacancy concentrations for MgCl2⋅2H2O molecules have been varied from 1.38% to 4.16%, which result in a diffusivity enhancement from 32.9% to 107.7% when compared with perfect slab. The incomplete hydration at the surface and the mid-layer increases diffusivity by 76.7% and 75.0%.

AB - Pure Magnesium chloride hydrates are exceptional materials for long term thermochemical heat storage. In a heat storage cycle, the material is charged by dehydration, taking up heat, and discharged by hydration, generating heat. On charging, the H2O molecules released by the dehydration reaction have to diffuse through the solid material. Gas-solid interactions play an important role in mass and heat transport throughout the volume of the storage materials. Under certain experimental conditions, the mass transport may become a rate-determining step. In order to simulate the complex reaction coupled diffusion process, a reliable MgCl2–H2O reactive forcefield (ReaxFF) is parameterized using a single parameter search algorithm. The parameters of ReaxFF are trained against a set of data obtained from density functional theory (DFT). ReaxFF-MD simulations are carried out on a 2D periodic slab of MgCl2⋅2H2O to simulate various possible operating conditions of temperature, external water vapor pressure, incomplete dehydration layers and various vacancy defects during a charging-discharging cycle. The diffusion coefficient of H2O through the 2D periodic slab of dihydrate is observed as 1.24±0.37×10−10 m2/sec at 300 K. The diffusivity increases with temperature and follows the Arrhenius equation. External water molecules impede the dehydration and promote the diffusion of water. The vacancies of MgCl2⋅2H2O molecules are characterized using vibrational density of states obtained from ReaxFF-MD simulations. The vacancy concentrations for MgCl2⋅2H2O molecules have been varied from 1.38% to 4.16%, which result in a diffusivity enhancement from 32.9% to 107.7% when compared with perfect slab. The incomplete hydration at the surface and the mid-layer increases diffusivity by 76.7% and 75.0%.

U2 - 10.1016/j.euromechflu.2016.12.011

DO - 10.1016/j.euromechflu.2016.12.011

M3 - Article

VL - 64

SP - 93

EP - 101

JO - European Journal of Mechanics. B, Fluids

JF - European Journal of Mechanics. B, Fluids

SN - 0997-7546

ER -