A Molecular Dynamics Study on
Porous MgSO4 Formation and
Diffusion Behaviors
Mechanical Engineering
Energy Technology
ir. H. Zhang
dr. S. V. Nedea
dr.ir. C. C. M. Rindt
Huaichen ZHANG
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Water vapor sorption in salt hydrates is a
promising method for compact, low loss,
and long-term heat storage (Fig. 1). The
energy released from salt hydrates is
characterized by slow (de)hydration rates
and changes in the crystal nucleation
stucture (pore formation). These rates are
determined by the transport of vapor
through the porous structure. For better
understandings, the processes are investigated on molecular level.
As water molecules evade from the crystal, the slab shrinks and
pores are formed. These pores can also be identified as initial
cracks due to the volume shrink in the micro grain particles. The
composition ratio rN is defined as rN=n(H2O)/n(MgSO4).
Hot water
MgSO4(s)+7H2O(g)
(a)rN=1.2
MgSO4•7H2O(s)+heat
(b)rN=0.6
(c)rN=0.1
Figure 3: Top view of simulation box, suggesting the formation of pores
Reactor
Indoor
Unit
Summer
Solar collector
Air 300K
Dry air 450K
Winter
Water
Cold water
Pump
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Local diffusion coefficients are calculated at various positions
and at various dehydration stages (Fig. 4).
Humid air
Dry air
300K, RH=0.6
0.20
Figure 1: Seasonal Heat Storage System
Dxyz (10-9m2/s)
0.020
rN=7.0
rN=6.0
rN=5.0
rN=4.0
rN=3.0
rN=2.0
0.18
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·
A slab of crystalline Epsomite (MgSO4 7H2O)
is selected for dehydration simulation at
450K and 20mbar vapor pressure, combined
with periodic boundary conditions (Fig. 2).
0.16
0.14
0.018
0.016
0.014
0.12
rN=7.0
rN=6.0
rN=5.0
rN=4.0
rN=3.0
rN=2.0
0.012
0.10
0.010
0.08
0.008
0.06
0.006
0.04
0.004
0.02
0.002
0
49.5
Dz (10-9m2/s)
50
50.5
51
51.5
52
z (nm)
(a) Diffusion constant Dxyz
52.5
0
49.5
50
50.5
51
51.5
52
52.5
z (nm)
(b) Diffusion constant Dz
102.036nm
2.036nm
Figure 4: Diffusion Coeficients at Various locations in various dehydration stages
Z 5.9
54
nm
X
Y
5.949nm
Figure 2: Periodic simulation box
/ eindhoven multiscale institute
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(1) In the center region, the diffusion is approximately isotropic
and slow, as oppose to that on the surface regions.
(2) On the surfaces, the diffusion parallel to the crystal surfaces is
much faster. This indicates water molecules tend to move along
the pores’ internal surfaces rather than to penetrate into the crystal.
(3) Due to the dramatic volume change, pores are formed under
surface tension.
(4) The adsorption of water onto the slab surface inhibits
complete dehydration.