Thermochemical Energy Storage Using Salt Hydrates

Thermochemical Energy Storage Using Salt Hydrates

Thermochemical Energy Storage Using SaltHydrates

Ganesh Balasubramanian, Mehdi Ghommem,Muhammad R. Hajj, Ishwar K. Puri

Department of Engineering Science and MechanicsVirginia Tech, Blacksburg, VA 24061

William P. Wong, Jennifer A. Tomlin

Science Applications International Corporation:::::::::::::::::::::::::::::::::::

2010 ASME IMECE, Vancouver, Canada November 15, 2010


Thermochemical phase change materials (TCM) for energystorage

TCMs impregnated into mesoporous materials like silica and zeolites

Inorganic salt hydrates eg.MgSO4.7H2O

High volumetric heat capacity,low cost, easy availability

Absorption of heat to releasecoordinated water

Release of stored energy duringhydration

Epsom Salt

Balasubramanian, G., Ghommem, M., Hajj, M. R., Wong, W. P., Tomlin, J. A., and Puri,I. K. Modeling ofthermochemical energy storage by salt hydrates, International Journal of Heat and Mass Transfer,53(25-26):5700-5706, 2010



Seasonal thermal energy storage

MgSO4·7H2O︸︷︷︸Salt hydrate

Addition of heat(solar)−−−−−−−−−−−−−−−−−→ MgSO4︸︷︷︸

Anhydrous salt

+ 7H2O︸︷︷︸Water vapor



Seasonal thermal energy storage

Water released and heat stored when salt hydrates are heated abovetheir dehydration temperature during warmer ambient periodsHeat obtained from dehydrated salts by passing low temperaturewater vapor during cooler periods



Objectives of the investigation

Design and optimize system geometry

Characterize mechanisms

Search for optimal material

Compare performance of different potential TCMs using numericaland analytical experiments

Predict optimum properties

Quantitative analysis

Characterize total energy stored in dehydrated salts under differentthermal conditions

Energy loss by water vapor release from system



Configuration of the 2D simulation domain

2-dimensional squaresimulation box filledwith porousMgSO4 · 7 H2O

Heat flux applied fromthe top boundary

Insulation of the otherboundaries varied byintroducing heat lossesthrough them



Conservation of energy, mass and chemical species

Conservation of energy

∂

∂t

[(MhNhCh + MsNsCs + MgNgCg )T

]= ∇(K∇T ) + rMhNh∆H

r = A exp(− E

RT), K = βhKh + βsKs + βgKg

The contribution of convective thermal and mass transport due to themovement of water vapor released from the salt hydrate is neglected

Conservation of mass

Mh∂Nh

∂t+ Ms

∂Ns

∂t+ Mg

∂Ng

∂t= 0



Conservation of energy, mass and chemical species

∂

∂t

[(MhNhCh + MsNsCs + MgNgCg )T

]= ∇(K∇T ) + rMhNh∆H

(1)

Mh∂Nh

∂t+ Ms

∂Ns

∂t+ Mg

∂Ng

∂t= 0 (2)

Chemical kineticsFirst order chemical reaction

∂Nh

∂t= −rNh (3)

Stoichiometry∂Ns

∂t= −∂Nh

∂t(4)



Significant nondimensional parameters in the model

∂

∂t

[(ηh + ηs + ηg )T

]= K∇2T + Dmηhχ exp(−E

T)

∂ηh∂t

= −Dmηh exp(−E

T)

∂ηh∂t

+Ch

Cs

∂ηs∂t

+Ch

Cg

∂ηg∂t

= 0

∂ηs∂t

= −MsCs

MhCh

∂ηh∂t



Significant nondimensional parameters in the model

∂

∂t

[(ηh + ηs + ηg )T

]= K∇2T + Dmηhχ exp(−E

T)

∂ηh∂t

= −Dmηh exp(−E

T)

Symbol Description

Dm Modified Damkohler Number

= Rate of thermochemical energy transferRate of heat diffusion

χ Dimensionless Thermochemical Heat Capacity

=Enthalpy of dehydration of MgSO4 · 7 H2O

Heat capacity of MgSO4 · 7 H2O per unit mass

q̂ Dimensionless Heat Flux

= Input heat fluxDiffusive heat flux



Temperature rise at different locations

The local temperature of the uppermost boundary increases linearly (in regionI) until the reaction temperature (T = 1) is reached after which there is anabrupt transition to a higher temperature during desorption (region II)



Transient evolution of concetration of different components

The hydrated salt concentrationdecays as the temperatureincreases and its anhydrous formis produced

The increased concentration inthe anhydrous component of thesalt and the free water vapor arereflected in the ηs and ηgprofiles



Effect of insulation on process performance

The time required to initiate thedesorption process for hydratedsalts decreases nonlinearly as qincreases

Performance ratio

π = 1− Energy lost

Energy supplied= 1− QgV∫ te

trq dt

As the input heat flux increases,the time required for the salthydrate to undergo the reactiondecreases

When heat is lost from theinsulated boundaries (α ≤ 0),the effective energy available forthe hydrate to undergo thechemical reaction is smaller



Temperature rise at different locations

A larger dehydration enthalpy ensures that all of the imposed flux isutilized towards the release of water vapor and a negligible fraction ofthe input energy diffuses through the system

Heat diffusion in the system is enhanced by improving the thermalconductivity of the hydrate, resulting in smaller Dm values

Rapid thermal conduction implies that the hydrate layers take longer toretain sufficient energy to complete the thermochemical desorptionreaction



Thermochemical phase change materials could have significant implications forlong-term energy storage applications

1 A mathematical model to investigate the capability of salt hydrates tostore thermochemical energy during their dissociation into anhydroussalts and water vapor when they are supplied with external heat

2 A parametric study provides suggestions to improve process performance,e.g., by properly selecting materials for thermochemical energy storage

3 The process performance is improved by introducing a smaller heat fluxand considering materials that have larger thermal conductivities, higherspecific heat capacities, and lower thermochemical desorption rates

4 A future approach to material design during the next phase of ourresearch will use the parameter optimization method that we havedeveloped

Questions ?


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Thermochemical Energy Storage Using Salt Hydrates

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