Advanced materials for solid state hydrogen storage: “Thermal engineering issues”

Accepted Manuscript Advanced Materials For Solid State Hydrogen Storage: “Thermal Engineering Issues” S. Srinivasa Murthy, E. Anil Kumar PII:

S1359-4311(14)00284-1

DOI:

10.1016/j.applthermaleng.2014.04.020

Reference:

ATE 5544

To appear in:

Applied Thermal Engineering

Received Date: 20 December 2013 Accepted Date: 10 April 2014

Please cite this article as: S. Srinivasa Murthy, E. Anil Kumar, Advanced Materials For Solid State Hydrogen Storage: “Thermal Engineering Issues”, Applied Thermal Engineering (2014), doi: 10.1016/ j.applthermaleng.2014.04.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Hydrogen has been widely recognized as the “Energy Carrier” of the future. Efficient, reliable, economical and safe storage and delivery of hydrogen form important aspects in achieving success of the “Hydrogen Economy”. Gravimetric and volumetric storage capacities become important when one considers portable and mobile applications of hydrogen. In the case of solid

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state hydrogen storage, the gas is reversibly embedded (by physisorption and / or chemisorption) in a solid matrix. A wide variety of materials such as intermetallics, physisorbents, complex hydrides/alanates, metal organic frameworks, etc. have been investigated as possible storage media. This paper discusses the feasibility of lithium- and sodium- aluminum hydrides with

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emphasis on their thermodynamic and thermo-physical properties. Drawbacks such as poor heat transfer characteristics and poor kinetics demand special attention to the thermal design of solid

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state storage devices.

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ADVANCED MATERIALS FOR SOLID STATE HYDROGEN STORAGE: “THERMAL ENGINEERING ISSUES” S. Srinivasa Murthy1*, E. Anil Kumar 2 Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai- 600036, India 2

Discipline of Mechnical Engineering, Indian Institute of Technology Indore, Indore-453446, India

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1

ABSTRACT

Hydrogen has been widely recognized as the “Energy Carrier” of the future. Efficient, reliable,

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economical and safe storage and delivery of hydrogen form important aspects in achieving success of the “Hydrogen Economy”. Gravimetric and volumetric storage capacities become

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important when one considers portable and mobile applications of hydrogen. In the case of solid state hydrogen storage, the gas is reversibly embedded (by physisorption and / or chemisorption) in a solid matrix. A wide variety of materials such as intermetallics, physisorbents, complex hydrides/alanates, metal organic frameworks, etc. have been investigated as possible storage media. This paper discusses the feasibility of lithium- and sodium- aluminum hydrides with emphasis on their thermodynamic and thermo-physical properties. Drawbacks such as poor heat

state storage devices.

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transfer characteristics and poor kinetics demand special attention to the thermal design of solid

Keywords: Solid state hydrogen storage, physisorption, chemisorption, alanates, complex hydrides,

Corresponding author Fax: +91-44-2257-0545/-4652 E-mail address: [email protected]

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*

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thermo-physical properties

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1. INTRODUCTION Depletion of fossil fuels and the increased awareness of their negative ecological impacts have necessitated the need for environmentally benign alternatives. Characterized by its high gravimetric energy content and clean conversion, hydrogen is considered as a very promising

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alternative. Cost effective, efficient and safe means of storage and delivery of hydrogen is essential to make this a reality.

Hydrogen in compressed, liquefied or solid state is the possible means of storage. Compressing hydrogen to very high pressures (about 700 bar) or liquefaction at cryogenic temperatures (20 K)

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present several safety and design challenges [1, 2]. Hence, solid state hydrogen storage emerged as a safe and viable alternative, especially for mobile and portable applications.

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Hydrogen can be physisorbed by sorbents such as porous carbon and zeolite, whereas metal hydrides and complex hydrides can store hydrogen through chemisorption and chemical reaction respectively. Important requirements for potential applications include high storage capacity, low cost, favorable operating temperatures and pressures, low hysteresis, easy activation, high cyclability, low density and low specific heat. Intermetallics and complex hydrides can be used to store hydrogen reversibly at their respective operating conditions, with suitable techniques for

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heat transfer enhancement. The DoE (USA) targets for on-board hydrogen storage in automobiles is given in Table 1[3].

Intermetallics like LaNi5 are easily reversible at moderate temperatures and pressures and exhibit fast sorption kinetics. However, they are characterized by poor gravimetric storage capacity (1.5-

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2.5 wt.%). Table 2 shows the comparison of the gravimetric storage capacity of some intermetallic hydrides[4]. High capacity hydrides like MgH2, are reversible only at temperatures above 250°C [5] i.e., beyond the optimum temperature range of operation for PEM fuel cells

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(~80°C) [6]. High density and cost of the reversible low temperature hydrides also limit their practical usability. The volume change associated with these hydrides during absorption and desorption cycles are also higher than complex hydrides [7]. With inherent low thermal conductivity of hydrogen storage materials, heat transfer is often the major rate limiting parameter [8-10]. Temperature of the storage bed has a significant impact on the reaction rate. Minimizing of refueling time is an important objective in the development of mobile hydrogen storage devices. Appropriate temperature levels need to be maintained in the storage bed so as to get maximum sorption performance. Excess cooling and overheating of the bed lead to poor 1

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sorption performance [11, 12]. The initial reaction rate is a characteristic of the material used while the later reaction is controlled by heat transfer [13]. It has been observed that absorption reaction commences from cooler regions of the storage bed, normally near to heat exchanger surfaces [9]. A metal hydride reactor embedded with multi tubular heat exchanger previously

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reported by the author, improved sorption kinetics by enhancing heat transfer within the bed [11]. Major developments in metal hydride based hydrogen storage were recently reviewed by this author [14].

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2. COMPLEX HYDRIDES

Complex hydrides, mostly alanates or borates of lithium, sodium and potassium have hydrogen

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retaining capacity up to 18 wt.%. These hydrides based on sodium and lithium store 7.4 wt.% and 10.5 wt.% respectively [15] while borates like sodium borohydride and lithium borohydride have higher theoretical storage capacities of 10.4 wt.% [16] and 18 wt.% [17] respectively. Their high hydrogen content, lower weight and lesser costs make them attractive for portable and mobile applications. However, their low thermal conductivity and high heat of reaction limits their application in practical systems. They also decompose in a multi-step reaction at high

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temperatures and pressures, exhibiting slow kinetics and poor reversibility. LiAlH4 can be easily dehydrogenated in a three step reaction similar to NaAlH4, but its hydrogenation occurs at very high pressures[15]. The third dissociation step of sodium and lithium alanates occurs at temperatures 425°C and 680°C respectively [15, 18, 19], which is beyond the desired

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temperature range for practical hydrogen storage applications. LiBH4 can reversibly store over 8 wt.% hydrogen at temperatures above 325°C with magnesium additives and Titanium based

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catalysts [20]. Desorption of the compound at low temperature is negligible (0.3 wt.%). Another complex hydride, Lithium nitride (Li3N) releases most part of its capacity only above 230°C [19]. Mixtures of such hydrides have been tried by different investigators to improve the overall sorption characteristics [21-24]. Among different mixtures of LiAlH4 and LiBH4, 2:1 ratio specimen exhibited a maximum desorption of 6.6 wt.% H2 in the presence of TiCl3 catalyst precursor at temperatures below 220°C [25]. In this paper thermodynamic properties, modeling of kinetics, measurement of thermo physical properties of Alnates are reviewed. Heat and mass transfer characteristics, Numerical Simulation, Design optimization of hydrogen storage devices (with NaAlH4) are discussed. 2

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2.1. Sodium Aluminum Hydride Ashby et al. [26, 27], were one of the earliest to synthesize sodium aluminum hydride and study its sorption characteristics. Sodium aluminum hydride, NaAlH4 decomposes into sodium hexahydride (Na3AlH6) and aluminum, accompanied with evolution of hydrogen in the first step.

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In the second step Na3AlH6 decomposes into sodium hydride (NaH), aluminum metal and hydrogen at elevated temperatures and pressures[28]. Dehydrogenation returns the crystalline alanate[29]. These are shown in the following reactions: 3NaAlH4 ↔ Na3AlH6+ 2Al+ 3H2

(Reaction 1)

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Na3AlH6+2Al+3H2 ↔ 3NaH+3Al+4.5H2

(Reaction 2)

The first and second reactions release 3.70wt.% and 1.85 wt.% of H2 respectively.Sodium

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alanate, in its pure form takes nearly 22 hours to attain saturation at 200°C and much slower at lower temperatures. The rate of reaction is almost negligible at 160°C. The studies also revealed the effect of Titanium doping in sodium alanates. Ti doping was found to increase the sorption kinetics drastically, simultaneously improving cyclability and storage capacity. Addition of catalysts in small amounts remarkably improved its sorption performance [30]. These results generated greater interest in the development of complex hydrides for portable applications.

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Jensen et al. [31] reported that significant decomposition of neat alanate occurs only above the melting point of the material at 183°C, while melting of the material doesn’t affect the reaction kinetics [7]. Several studies were conducted subsequently to understand modified kinetics and dopant action in the lattice. Considering the favorable operating conditions and available

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thermodynamic data, sodium alanate can be used for practical hydrogen storage applications. Moreover, alanates can be produced in laboratory scale from NaH and aluminum powder with

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aluminum reduced TiCl3 [32]. 2.1.1 Role of catalyst

It is well established that catalysts improve the sorption kinetics drastically. The above

mentioned reactions of the sodium alanate at 200°C were completed within an hour when doped with 2 mol% of Ti(OBu)4precursors[30]. Characteristic fast and slow kinetic regimes were observed in the desorption curve indicates the two step reaction [33]. It has been confirmed by different studies that the addition of Titanium as catalyst enhances the sorption capacity and improves reversibility under moderate conditions while maintaining important thermodynamic properties [6, 15, 30, 31, 33-37]. Zidan et al.[35]confirmed superior decomposition of 3

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Ti/Zrdoped Sodium alanate below 100°C. Addition of catalysts into pure alanate in any manner decreases dehydrogenation temperature and enhances kinetics [38]. While Ti doping was found superior for NaAlH4, Zirconium was reported as better catalyst for Na3AlH6 [35] even though both were found to improve cyclability and promote faster

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dehydriding. The effects of several catalyst precursors like TiCl3, TiCl4, Zr(OPr)4, Ti(OBu)4 have been studied. Though the latter two had considerable effect on bed performance, they were found to lower the cyclic capacity due to damage caused by the oxygen on the hydride surface. This may be accounted to the loss of material due to oxide formation [7, 39]. The liquid organo-

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metallic alkoxide precursors also affect the purity of hydrogen liberated. Sandrock et al. [7] reported that 2 mol % of alkoxide leads to 22% additional weight on the system.

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The loading of Titanium also found to have varied the effect on catalysis. During absorption, 0.9 mol % of Ti doped sample failed to show noticeable response while 2-9 mol % doped samples reached saturation within 2 hours. Though higher loading of dopant improves rate of dehydriding, it has a negative impact on hydrogen storage capacity. The addition of Ti catalyst results in the decrease of the activation enthalpy and thereby reaction temperature. Significant differences were reported in activation enthalpies of NaAlH4 and Na3AlH6 as shown in Table 3

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[34].

The dopant action is irrespective of the halide used with Titanium [37, 40]. Further, titanium powder alone as dopant did not produce any effect on the reaction kinetics [40]. However, the radius of cation is a critical parameter for effectiveness of catalyst.

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Studies have revealed different mechanisms by which the catalysis can occur. First principle method [41], X-ray diffraction pattern [42-45], neutron inelastic scattering technique [41],

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scanning electron microscopy [38, 39]electron dispersive spectroscopy [39] and Mossbauer spectroscopy [15] were commonly used to understand the role of catalysts. While some studies [30, 39, 42], reported that the dopants are segregated and remain in the surface of the material, the others suggested that they substitute for sodium in the bulk of the lattice. 2.2. Lithium Alanates

LiAlH4, was first synthesized by Finholtet al. [46] as early as in 1947 and was used as a reducing agent of organic compounds. Desorption of hydrogen proceeds in three-step reaction with the decomposition of lithium tetrahydroaluminate into trilithiumhexahydroaluminate, aluminum and hydrogen [47-50]. 4

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3 LiAlH4 → Li3AlH6 + 2 Al + 3 H2

(Reaction 3)

Li3AlH6 → 3 LiH + Al + 3/2 H2

(Reaction 4)

3 LiH + 3 Al → 3 LiAl + 3/2 H2

(Reaction 5)

The reaction is initiated by the melting of LiAlH4 which transforms into solid Li3AlH6 and Al

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during release of hydrogen. The maximum hydrogen storage capacity of LiAlH4 is 10.6 wt.% [50] which decomposes according to above reactions with the release of 5.3 wt.% by reaction 3, which occurs at around 423- 448 K [47-49]. Reactions 4 and 5 occur at 453- 493 K and 623- 673 K with a hydrogen release of 2.6 wt. %, and 3.6 wt. %, respectively [48, 49, 51, 52]. The reaction

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temperature depends upon heating rate [53]. Since the decomposition of LiH occurs at very high temperature, only 7.9 wt. % hydrogen is considered available for practical applications [49, 50].

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Reactions 3 and 4 are endothermic processes with ∆H of 22 to 58 kJ/(mol H2) [49-51, 54] respectively and reaction 5 is an exothermic process with ∆H of 116.3 kJ/(mol H2) [54]. Doping LiAlH4 with Ti-based and V-based catalysts result in lowering of decomposition temperatures of Reactions 3 and 4. The addition of VCl3 and TiCl3 x 1/3AlCl3 in LiAlH4 significantly decreases the decomposition temperature [48, 49]. Among the two, VCl3 is more effective [48]. It has been shown that doping with TiCl3 x 1/3 AlCl3 facilitate isothermal

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dehydrogenation at 373-548 K at a time scale of 50 min to a few hours. Ball milling of pure LiAlH4 decreases the decomposition temperature by 60oC of reactions 3 and 4[49, 52]. Pure LiAlH4 does not decompose during ball milling at ambient temperatures [55], whereas LiAlH4 doped with catalytic additives e.g. TiCl3, TiCl4, VCl3 and FeCl3 rapidly decomposes during

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milling [48, 55]. Doping with TiCl3 and TiCl4 results in decrease in desorbed amount of hydrogen because it eliminates the first step of hydrogen decomposition. The doping was done

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by ball milling the mixture of LiAlH4 and TiCl3/TiCl4. XRD studies for LiAlH4 doped with TiCl3 and TiCl4 showed the presence of Li3AlH6, which indicated that the solid state reaction, LiAlH4→ Li3AlH6+Al+H2 (first step of desorption), had occurred during ball milling. Also addition of the titanium lead to loss of storage capacity as titanium was reduced by alanate ion during ball milling process. While, doping with TiH2 and other elemental iron does not affect the desorption capacity [52]. The comparison between maximum hydrogen desorption capacity of LiAlH4 with different catalyst in the temperature range 430-450°C is presented in Fig. 1. Unmilled LiAlH4 shows

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higher desorption capacity than other catalyst doped LiAlH4. Significant decrease in desorption capacity after 1 hour ball milling can be seen. This decrease is because of high energy involved during ball milling process. The decrease in desorption capacity of TiH2 doped LiAlH4 is very less and close to ball milled LiAlH4. It can be

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considered that the decrease in desorption is due to ball milling alone [52]. The desorption capacity of TiCl3 doped and TiCl4 doped LiAlH4 is very less because it eliminates the release of hydrogen in Reaction 3, rather other catalyst doped LiAlH4 shows approximately same desorption capacity. Doping with catalyst results in decrease in decomposition temperature and

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desorption capacity in each step of reaction.

Figure 2 shows the hydrogen desorption capacity of TiCl3 x 1/3AlCl3-doped LiAlH4 at different

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temperatures. The release of hydrogen is more with the increase in reaction temperature whereas the reaction time decreases.

2.2.1DehydridingBehaviour of Carbon Admixed LiAlH4

Authors group has extensively [56, 57] investigated dehydrogenation kinetics of LiAlH4 Carbon composites. The effect of carbon content and catalytic effect of carbon nanofibres on dehydrogenation kinetics were studied. Four different types of carbons, Vulcan XC72R

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(250 m2 g−1), Black Pearls 2000 (1500 m2 g−1), CDX 975 (240 m2 g−1) and Mesoporous carbon (1400 m2 g−1) were used. Carbon nano fibers were synthesized using Ni-Cu catalyst. 2.2.2 Effect of type of carbon

The effect of carbon on the dehydrogenation kinetics is visualized by isothermal

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dehydrogenation curves for all LiAlH4-C (Vulcan XC72R, Black Pearls 2000, CDX 975 and Mesoporous carbon) composite samples as shown in Fig. 3. Same isothermal dehydrogenation temperature was applied to all composites. For comparison the

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dehydrogenation kinetics of pure LiAlH4 milled for 45 min, decomposed at the same temperature (403 K) was included as a reference. It was observed that dehydrogenation kinetics improved considerably by addition of various carbon materials. The enhancement is nearly 4-5 times for the Black Pearls 2000, CDX 975, mesoporous carbon and ~2 times for the Vulcan XC-72R in comparison to pure LiAlH4 (at 45 min). The dehydrogenation profile of the Vulcan XC-72R admixed LiAlH4 resembles the as-received LiAlH4 curve. On the other hand, the Black Pearls 2000, CDX 900, mesoporous carbon containing composites give higher rates significantly altering the curve profile. It suggests that the reaction does not follow the same mechanism in all 6

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the cases. It follows different paths in the case of Black Pearls 2000, CDX 975 and mesoporous carbon admixed LiAlH4. Among the four carbons, Black Pearls and CDX 975 show high desorption kinetics. The rate is higher for the mesoporous carbon in the initial stages and drops down slowly. The rate is not

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directly proportional to surface area of the carbon. The order of dehydriding reaction rate is, Black Pearls ~ CDX >mesoporous carbon > Vulcan XC72R. Hence the change in the rate cannot be solely attributed to the surface area of the carbon.

2.2.3 Dehydrogenation Kinetics of CNFs Admixed LiAlH4Composite

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The isothermal dehydrogenation kinetic traces of composite materials based on lithium aluminum hydride admixed with carbon nanofibers and Vulcan XC72R as shown in Fig. 4.

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These curves were plotted by calculating the total hydrogen released (at 408 K under atmospheric pressure) in terms of weight percentage with respect to the dehydrogenation time under dynamic conditions.

For comparison, the dehydrogenation kinetics of pure LiAlH4 milled for 45 min, decomposed at the same temperature, 408 K is included. The dehydrogenation kinetics improved considerably

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by the addition of Vulcan XC72R carbon and remarkably with carbon nanofibers.

3. MODELLING OF SORPTION KINETICS Several numerical models have been formulated to define the hydrogenation/ dehydrogenation processes. An isotropic and uniform hydride bed with thermo-physical

Hardy[58].

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properties independent of hydrogen loading, temperature and cyclinghas been reported by

As suggested by El Osairy et al. [59, 60] for analysis of LaNi5H6 system, the rate of reaction can

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be defined as a function of temperature, pressure and composition.  dc j    = f (T ) × f ( P ) × f ( C )  dt 

(1)

The function of temperature in the rate equation is represented by the Arrhenius term. The pressure function is the driving force for sorption represented by the differential between supply pressure and equilibrium pressure. The last term represents the concentration of reactant as a function of hydrogen to metal atom ratio. Even though they used a first order reaction to arrive at the composition term, the order of reaction may be different for alanate. The power term is used

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to represent the order of the reaction in United Technologies Research Center (UTRC) model [58].  dc j   dt  P  ∆H R ∆SR ln  e,i  = − R  1bar  RT

 χi  × ( Ck ) 

(2)

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where,

E  −  i  P − Pe ,i   i  RT  × ( −1) ×   = Aie  P  e ,i 

(3)

Similar to intermetallics, hydriding undergoes nucleation and growth in alanates. Rudman [61] has explained the use of nucleation theory in the development of hydriding and dehydriding kinetic models. Accordingly, Lozano et al. [12] used the Johnson- Mehl- Avrami (JMA) equation

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to arrive at the composition term. This term is defined as a function of transformed fraction(∝), which is the mass ratio of hydrogen (mH2 ) absorbed to the maximum absorption capacity

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(mH2,max ). This function can be obtained by numerical fitting with experimental values. Both absorption steps in the hydride formation can be represented with JMA model with n = 1.3. Model suggested by Kircher and Fichtner[62] reported different values of n equal to 1.1 and 1.4 for first and second reaction steps respectively. α=

mH2

mH 2 max

(4)

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The model put forward by Lozano et al. [12] considers continuous formation of reaction products. The effect of reactant materials as well as the inert components present in the bed was included in the model. But, the models suggested by Luo and Gross [63] represented independent formation of the products. They considered the first hydrogenation step as a first order reaction

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and second step as a second order reaction. Due to the presence of excess aluminum during the first reaction, only the concentration of NaH affects its progress. Hence it is represented as a first

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order reaction. Lozano et al. [12] used low bed thickness cells to achieve isothermal condition during experiments. Same reaction rate can be achieved at two sets of pressure and temperature values. Higher pressures yield better kinetics at constant temperature. Due to higher plateau pressure of second absorption reaction, it progresses considerably slower than the former. The rate of dehydrogenation increases with increase in temperature [33] and decrease in pressure with impact of temperature being more significant. Hydrogen back pressure is a major parameter controlling the desorption process. The rate of dehydrogenation approaches zero when the hydrogen back pressure equals the equilibrium pressure for the product formation. The desorption reaction can be considered to occur sequentially. The desorption PCI curves show a 8

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delay or a break when the desorbed weight fraction approaches 2.5 % [63, 12]. Lozano et al. [12] approximated the first desorption reaction to a zero order reaction i.e., the rate of desorption remains constant throughout the desorption step and second step as a first order reaction (similar to[33]) with index ‘n’ in JMA equation evaluated as unity. First order conditions are valid only > 0.7. First and the second desorption reaction steps commences only above 114°C

P-Pe,i Pe,i

and 147°C irrespective of hydrogen pressure.

The kinetic model proposed by Luo and Gross [63] is given below:

dt

Formation of Na3AlH6

d(H wt.%) dt

-Eaa2

dt

-Ead1

 (1.67-H wt.%); (H wt. %< 1.67)

(6)

 (H wt.% -1.67);(1.67< H wt. %< 3.9)

(7)

Peq1

=Koa2e RT ln 

=Kod1e RT ln 

(5)

Pappl

Pappl Peq2

Decomposition of NaAlH4 d(H wt.%)

 (3.9-H wt.%); (1.67< H wt. %< 3.9)

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=Koa1e RT ln  -Eaa1

d(H wt.%)

Ped1

Pappl

Decomposition of Na3AlH6

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Formation of NaAlH4

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when 

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-Ead2 d(H wt.%) Peq2 =Kod2e RT ln   (H wt.%);(H wt. % < 1.67) dt Pappl

(8)

The rate of reaction is calculated using UTRC kinetics model [58, 64] as given below: r1F =Ceqv A1F exp -

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Formation of Na3AlH6

r1B =Ceqv A1B exp -

E1B Peq1 (T)-P(C,T)   RT Peq1 (T)

r2F =Ceqv A2F exp -

E2F P(C,T)-Peq2 (T)   RT Peq2 (T)

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Decomposition of Na3AlH6

Formation of NaAlH4

E1F P(C,T)-Peq1 (T)   RT Peq1 (T)

Decomposition of NaAlH4

r2B =Ceqv A2B exp -

E2B Pe2 (T)-P(C,T)   RT Peq2 (T)

9

(9)

(10)

(11)

(12)

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m3 =

 

8 

=

=

r   ! – C'() (T)

 ()  r     2 "#$



r' 

 () "#$

+2

+

if P ≥ P01 (T)

(13)

if P ≥ P01' (T)

(14)

if P < P 013 (T) and C  (t) ≥ 0

8 () "#$ +'2

– C () (T)

+'

  r  if P < P 013' (T) and C ' (t) ≥ 0  '2 "#$ (m +m ) m2 = - [ 1 3 3 ] ! ()

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m1 =



Ceqv = C10 + C20 + C30

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According to the kinetic model by Lozano et al. [12], Rate of reaction is defined as, -Ea dα =ǩ(T,p)=AeRT *f p,peq  dt For first absorption step,

αSI→II =

mSII mSII +mSI

dαSI→II 1 <= =1.33 kSI→II (1-αSI→II ) :ln ; dt 1-αSI→II

(15) (16)

(17)

(18)

.25

For second absorption step,

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fSI→II p,peq  =

αSII→III =

p-peq 

(20)

peq

mSIII mSIII +mSII

(21)

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dαSII→III 1 <= =1.33 kSII→III (1-αSII→III ) :ln ; dt 1-αSII→III

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(19)

.25

fSII→III p,peq  =

p-peq  peq

(22)

(23)

For first desorption step,

αSIII→II =

mSII mSIII +mSII

dαSIII→II =kSIII→II dt

10

(24) (25)

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For second desorption step,

peq

@AII→I =

< + 1.04 > 2

p-peq  ? peq

CAI CAII + CAI

dαSII→I =kSII→I (1-αSII→I ) dt

peq

p-peq  < - 0.46 > ? peq 2

(26)

(27) (28)

(29)

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fSII→I p,peq  = ;

(p-peq )

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fSIII→II p,peq  = ;

(p-peq )

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The subscripts I, II and III represent NaH, Na3AlH6 and NaAlH4 respectively. 4.DETERMINATION OF THERMO-PHYSICAL PROPERTIES Dedrick et Al. [65] determined thermal wall resistance and effective thermal conductivity of the alanate bed with respect to composition, cycle number and hydrogen pressure using numerical and analytical methods. The effect of bed temperature is found to be negligible. Thermal conductivity in the range of 0.6 W/m-K was obtained at moderate operating pressures.

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Improvement of thermal conductivity will result in better heat transfer and thereby better kinetics [9]. Gross et al. [66] reported 0.2 W/m-K as thermal conductivity of alanate bed which is less than that of intermetallic hydrides. Poor conductivity could be attributed to high inter-particle thermal resistance as in metal hydrides. Hardy and Anton [67] reported 0.325 W/m-K as the

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thermal conductivity for bed in their kinetic scoping model. Lozano et al. [68] estimated alanate bed conductivity at 120°C in the range of 0.55 W/m-K. Van Hassel et al. [69] experimentally

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found the thermal conductivity of alanate based bed using TPS method while homogenization method can be utilized when considering the microscopic bed behavior [70]. The effective thermal conductivity of the bed can also be measured by using transient techniques as demonstrated for intermetallics [71-75]. The effective thermal conductivity values of Sodium alnate beds reported in the literature are summarized in Table 4. The specific heat capacity of a hydride is determined by the amount of hydrogen content given by Neumann-Kopp rule[76]. The value of heat of reaction for both absorption and desorption reaction steps were calculated by Bogdanovic et al. from PCI measurements [15]. The enthalpy of formation and entropy of 11

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formation are given in Table 5. Sandrock et al. [34] have reported the activation enthalpy and rate constant at different doping levels. Kiyobayashi et al. [77] determined the activation enthalpy and reaction rate of dehydrogenation of Ti/Zr doped sodium alanate. Castrilloet al. [78] defined thermal conductivity as a function of contact factor and porosity.

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Bennett et al. [79] studied the effect of varying thermal conductivity using a model similar to Sun and Deng model [80] to take into account the effect of bed temperature and pressure and reported significant increase in accuracy.

An experimental setup for measuring effective thermal conductivity of hydride was constructed

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at authors’ lab according to ASTM E-1225 Standard [81]. PTFE disk was used as reference material. The PTFE disk thermal conductivity was measured using a Square Guarded Hot Plate

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(SGHP) apparatus. The hydride bed and PTFE are in disk shape with 85 mm diameter and 20 mm thickness. They are sandwiched between hot and cold plates. Hot plate is heated with a disk heater and cold plate is cooled by circulating coolant. Required boundary conditions are imposed and one dimensional heat transfer conditions are achieved. The temperature gradients in metal hydride bed and reference material are measured at steady state. The heat transfer in series through the metal hydride bed and reference material at steady state was calculated from

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Fourier’s law and from this; the ke of metal hydride bed is calculated. The complete details of experimental setup, procedure and data reduction are given in author’s recent paper [82]. Effective thermal conductivity is measured at different operating conditions and the effects of hydrogen gas pressure and concentration, bed temperature are studied.

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At a particular average temperature, as hydrogen pressure changes concentration of the hydride bed also changes. Hence, practically it is not possible to study the effect of gas pressure alone

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while using hydrogen as filling medium. So inert gases namely argon, helium and nitrogen are also used as filling gases to study the effect of gas pressure. Figure 5 shows the variation of effective thermal conductivity (ke) of MmNi4.5Al0.5 for different filling gases. As expected for packed beds, ke increases with increasing pressure in the form of the tilted “S” shaped curve. This is obviously due to the increase in gas thermal conductivity with pressure. This variation may be divided into three regions. In the low pressure region up to about one bar, ke of MmNi4.5Al0.5 bed is independent of pressure. In this region the density of the gas is low in the voids of the bed. Therefore the mean free path of the gas molecules is large compared to the void size, which is assumed to be equivalent to the average particle size of the bed. However, with 12

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increase in gas pressure, density of the gas in the pores increases due to which the mean free path of the gas decreases. When the mean free path of the gas molecules approaches the size of the voids, collision of gas molecules is high resulting in higher gas conductivity. In this region of about 1 to 15 bar gas conductivity increases with pressure and thereby ke also increases. With

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further increase of gas pressure, the mean free path of the gas molecules is much lower than the effective size of the voids. In this region, the process of heat transfer by the gas is similar to that in infinite space and do not influence the conductivity of the gas. Thus for gas pressures above 15 bar, keremains nearly constant with increase in pressure. For most of the metal hydrides the

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middle region (where the influence of pressure on ke is felt) lies in plateau region of hydriding or dehydriding process. Hence, in addition to the effect of hydrogen pressure, the effect of

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concentration should also be considered. However, with inert gases as filling medium the effect of gas pressure alone is seen. Effective thermal conductivity of the bed is found to be proportional to the thermal conductivity of the filling gas. Thermal conductivity of hydrogen is the highest of all the gases. The thermal conductivity of helium is nearer to that of hydrogen (kHe= kH2*0.8). At pressures near to 0.1 bar, ke is independent of the filling gas, because heat conduction at these low pressures is due to conduction in particle grid. The effect of gas

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conductivity on ke increases with pressure. The difference between values of ke obtained under hydrogen and helium atmospheres increases with pressure as the concentration increases and stabilizes at the maximum concentration. In the pressure independent region, the difference between ke values obtained from hydrogen and helium atmospheres approximately represents the

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increase in ke due to hydrogen concentration [82]. To estimate ke at wide range of operating conditions, it is required to develop a mathematical or

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empirical relation. Several mathematical models have been proposed in literature for the prediction of ke of packed beds of porous materials. Due to the differences in basic characteristics between metal hydride beds and conventional packed beds, these models are unsuitable for metal hydride packed beds. Author’s group has extended models developed by Zehner and Schlunder[83], Yagi and Kunii [84], Dietz [85] and Masamune and Smith [86] to incorporate the consequences of hydrogen absorption and desorption. The extended models are then used for simulation of experimental data on ETC of MmNi4.5Al0.5 hydride packed bed [87]. The results obtained from the extended models are compared with the experimental results as shown in Fig. 6. The extended models of Yagi-Kunii and Zehner-Schlunder predicted the values 13

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of ETC which are very closely matching with the experimental results. Extended MasamuneSmith model was also very close in the high pressure region while it shows some deviation in the lower pressure region. The reason behind this could be consideration of pendular voids by Masamune-Smith. In case of MH beds, the geometry of voids cannot be assumed to remain same

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for all ranges of pressure. The swelling of hydride particles results into variation in the void geometry also. Dietz model, although shown the trend similar to that obtained in experimental results, but the values were under-predicted throughout the range of pressure. Dietz has suggested the contact between particles to be point contact and also has fixed the geometry of

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bed and arrangement of particles in the bed. Moreover variation in porosity due to hydrogen absorption could not be incorporated into this model as the model did not suggest the ke to be

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directly dependent on porosity of bed.

5.EFFECT OF HEAT AND MASS TRANSFER

Early studies in hydrogen storage revealed that proper heat transfer mechanism can improve reaction rate[8-10]. An overview of heat and mass transport in metal hydride systems were presented by the author [14]. Though the mechanism of hydriding is different in complex

remains the same.

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hydrides and intermetallic hydrides, the effect of heat and mass transfer in the bed largely

Complex metal hydrides, similar to intermetallic hydrides are characterized by low thermal conductivity. Due to large heat of formation, insufficient heat transfer rate lead to sensible

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heating of the bed leading to higher temperature rise in sorption bed during absorption and the reverse for desorption, resulting in poor system performance. The effect of heat transfer controlled reaction rate is predominantly higher for hydride beds than mass transfer influenced

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rates [88]. Further, high temperature operation of bed lead to pulverization. The principal bed parameters contributing to heat and mass transfer are thermal conductivity of powder bed, hydrogen and the bed porosity [89], while the operating parameters have a significant effect on the existing values. The thermal conductivity of bed increases on hydrogen absorption, while it’s augmented also with temperature increase coupled with increase of pressure in moderate range [14, 65, 70, 82, 90]. Cycling of bed increased the conductivity by a factor of 1.3, while decreased permeability by half [91]. Decreased particle size as a result of pulvarisation due to cycling and high temperature operation, increases the packing density at 14

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lower part of the reactor [92, 93]. Expansion and contraction of metal powders during operation result in poor thermal contact with the reactor wall which consequently increases thermal wall resistance [94]. It must be noted that even the wall thickness of the storage device has significant influence on sorption performance [66]. Hence, a heat transfer oriented design of hydrogen

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storage device is essential [95, 96]. Apparently, the break in the absorption curve signifying the transformation of Na3AlH6 was found to reduce as an effect of cycling [91]. Several heat transfer augmentation techniques have been hence suggested as discussed in the following sections. 5.1 Material Modification and Processing

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Any change in the composition of neat alanates amounts to material modification. It was mentioned in the section on catalysis that the presence of impurities in the form of doping,

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additives etc. enhances reaction kinetics. Metal hydride powder combined with additives, such as carbon, expanded natural graphite, graphite pellets and diamond powder can improve the thermal conductivity of bed thus improving heat transfer [5, 38, 69, 76, 89, 97-100]. Porous metallic matrix formed by compacting LaNi5 with powders of aluminum, copper and nickel sustained several cycles while improving sorption [101]. Chemical modification through such additives enhances sorption performance while preserving thermodynamic properties, though the presence

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of additives results in a loss of hydrogen capacity [89, 102]. 10 % loss is reported for graphite additions [100]. This decrease can be compensated to some level by adding more quantity of alanate compacts and a buffer volume [69]. The presence of an expansion volume increases the thermal resistance at the heat exchanger, reducing heat transfer and kinetic performance while

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convective heat transfer gains importance [103]. Metal hydrides encapsulated in copper have higher thermal conductivity at higher packing fractions and are cyclically stable. But, their kinetic characteristics deteriorate with number of cycles [102]. While compaction of hydrides

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increases conductivity and density, porosity and permeability decreases. This leads to significant mass transfer limitations [104]. It has been observed that the mass transfer controlled reaction rate dropped below heat transfer controlled rate in an annular disc type reactor filled with hydride compacts [88].

Sodium alanate, ball milled in the presence of carbon could absorb and desorb hydrogen faster at lower temperature and even faster than neat form in the absence of titanium catalyst [5]. Pelletized alanates with graphite additives can be used for reversible hydrogen storage. Pelletized alanate, over a range of 69-345 MPa resulted in improved density and increased effective bed 15

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thermal conductivity to 9.09 W/m-K. Thermal conductivity of the material kept increasing with pellet die pressure while porosity declined, of which no major detrimental effects are reported [74, 75]. At lower pressures, graphite added alanate pellets have higher conductivity than Ti doped ones, while the trend reverses at high pressures. Decomposition of the material may occur

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during pelletization decreasing the effective thermal conductivity. This could probably be the reason for lower conductivity value of Ti doped alanate pellets compared with neat alanate pellets. Cycling leads to expansion of pellets back into powder form resulting in degradation of conductivity, though no negative impact on kinetics and storage capacity were observed over 50

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cycles [74]. Mechanical confinement of pellets can solve this problem. The graphite additives also slowed down the degradation of thermal conductivity of mechanically confined pellets.

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Enhanced thermal conductivity of bed is found to decrease the volume expansion [105]. Pelletized hydride powder composites encapsulated with copper reported a thermal conductivity of 5 W/m-K and porosity of 0.35 [106]. However, longer activation times are reported for pelletized bed [74].

Ball milling of the catalyzed sample was also found to improve the kinetics considerably[37]in addition to increase in density [107]. Anton [37] studied the effect of milling time on reactor

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performance. To achieve maximum catalytic activity and homogenization longer milling time is necessary. However, it leads to dehydrogenation during the milling process and tend to decrease the reaction rate [37]. This can be prevented by proper cooling of the vial during ball milling [45]. Ball milling alone was found to be good enough to improve the sorption kinetics and low

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temperature operation [5]. The refinement of microstructure during ball milling does not change the thermodynamic properties of complex hydrides while it results in the better dispersion of

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additives in the specimen [108].

5.2 Improved Heat Transfer Mechanisms The heat transfer in the hydride bed needs to be augmented to achieve effective thermal

management in hydride reactors. Even though external cooling of reactorsis preferable from heat transfer and sorption kinetics points of view [109], the importance of heat removal through internal heat transfer enhancement methods cannot be ignored [73]. Various such methods have been proposed to enhance hydride bed heat transfer. High heat removal rates can be achieved by introducing annular metal inserts [110], copper wire matrix [90] and metallic foam [8, 111, 112]. The presence of aluminum foam enhances the effective thermal conductivity of hydride beds [8]. 16

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LaNi5 based storage device inserted with aluminum foam [111] increased the effective thermal conductivity of hydride bed to 10 W/m-K. The shape, size and porosity of the hydride bed are significant in defining its effective conductance. Larger fraction of aluminum in the reactor improves sorption while decreases the storage capacity [113]. Using numerical techniques, Wang

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et al. [99] evaluated the effective thermal conductivity of the bed against fraction of aluminum presence in the bed. Improved surface area for heat transfer was obtained using multiple heat exchangers and hydrogen filters embedded in the reactor. Various configurations with different numbers of such tubes were studied by the author [11]. A multiple tubular reactor with annular

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jacketed external cooling improved heat transfer over the one without such arrangement [104]. It must be noted that the rate of reaction is higher near the heat exchanger surface and attains

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saturation faster [114] while the reaction front propagates from heat exchanger surface [11]. Effect of thermal mass of heat exchanger tubes affects sorption characteristics to a lesser extent [115] and the influence of the coolant side heat transfer coefficient increases initially and saturates at larger values [114].

For larger bed sizes with higher heat transfer requirement, greater fraction of aluminum presence is required [113]. The best way to enhance heat transfer towards outside is to choose a more

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favorable geometry to increase the thermal flow in the direction of largest exchange surface [116] or use radial fins originating from heat exchanger surface [117]. Variation with charging time and effect of operational parameters in a spiral coil heat exchanger with and without fins was studied by Dhaouet al. [118].Souahalia et al. [119, 120] conducted parametric studies on a

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metal hydride based hydrogen storage device with a finned heat exchanger tube. Effect of longitudinal fins on improvement in kinetics is more than the transverse fins, though by a very

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narrow margin [121]. Improved thermal conductivity using graphite additions enabled to increase the fin pitch distance by three folds, well above the DOE prescribed minimum [100]. Bhouri et al. [122] proposed a metal hydride reactor inserted with aluminum honey comb structure, with and without external cooling to provide effective conduction path. In a similar device, introducing heat exchanger tubes between metallic inserts or using a number of honeycomb unit cells for internal cooling further enhanced sorption performance [94]. Further, the same authors proposed shell and tube type bed in which sodium alanate was filled in tubes with internal fins [123] and studied effect of finned reactors in absorption and desorption. Raju and Kumar [105] proposed a multi-tubular reactor with aluminium fins. Hardy and Anton [64, 17

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67] evaluated the heat and mass transfer performance of sodium alanate based two dimensional (without fins) and three dimensional (with fins) models with multiple heat exchanger tubes and hydrogen filters. The presence of fins improved sorption kinetics implying the importance of heat transfer as the rate controlling parameter[64, 67, 118-120, 123, 124]. Fins with perfect

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contact between solid-solid interfaces are desirable, otherwise leading to performance degradation[123]. The thickness of fin need not be considered as an important parameter while absorption as even small thickness can significantly improve sorption. Performances of such systems are limited only by intrinsic kinetics. But, during desorption the kinetics is significantly

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enhanced by increase in fin thickness. However, larger fin thickness lowers gravimetric and volumetric capacities. Number of cooling tubes improve performance in both sorption processes

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while desorption process is more sensitive to cooling tube requirement. During desorption, the presence of more fins and cooling tubes is important to impart heat into the hydride bed. Too small a value (less than .025 cm) of honeycomb cell wall thickness decreases sorption performance while increases slightly in the range of 1mm. The region near fins and heat exchanger surfaces desorb at much faster rate. The effect of heat transfer surface area is more significant in this case than effective thermal conductivity [97, 123, 124]. Gambini et al. [125]

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determined the effect of operational parameters using lumped parameter numerical model. The temperature and pressure of hydride system decreases with increase in power output while heat flux follows the reverse trend. The aspect ratio of the reactor is an important parameter when the conductivity of the bed is low [99]. Low value of radius to height ratio is preferred for maximum

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sorption performance [114]. Not the least, it was also reported that geometric configuration of the reactor does not affect the mechanism of hydriding[11].

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The temperature profile of the reactor bed is significantly influenced by the coolant flow rate.While improving kinetics, increased flow rate has no effect on the amount of hydrogen absorbed was observed [94, 126]. Under active cooling, influence of aluminum foam inserts were more pronounced [99]. The sorption performance is also significantly affected by hydrogen inlet design. Sintered steel filter on a cylindrical orifice supported by a cap with 4 holes reported remarkable improvement in sorption. This resulted in higher temperature peak indicating more formation of hydride [120]. The sorption parameters are strongly influenced by bed pressure and coolant temperature. Higher temperatures favored desorption while lower coolant temperatures were preferred for absorption [76]. 18

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Giving an another outlook, Chung et al.[127] developed a heat pipe based metal hydride model for thermal management which had obvious advantages over conventional cooling systems. Though the reactor performance improved over the one without heat pipe, its applicability on

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practical systems can be ascertained in comparison with finned models with heat exchangers.

6. NUMERICAL STUDIES ON SORPTION PERFORMANCE IN STORAGE DEVICES Numerical simulation of sorption performance of hydrogen storage devices can be achieved by solving a set of coupled equations governing heat and mass transfer. Mayer et al.

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[128] obtained solutions by simultaneously solved for heat transfer by conduction and mass transport at constant pressure while the effects of convective and radiation heat transfer were

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neglected. Convective heat transfer favors the reaction kinetics initially, while it inhibits in the latter part of reaction due to transport of heated hydrogen into the bulk [129]. The effect of exchange of heat with hydrogen gas may not be significant but the pressure drop for hydrogen flow will be significant in larger beds.

This effect can be modeled using Darcy’s law

considering the storage bed as homogenous [9]. The equilibrium pressure at which the reaction takes place can be determined using the equation given by Nishizaki et al. [130], considering the

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effect of plateau slopes and hysteresis. The effect of plateau slope and hysteresis were neglected in many studies[11, 64, 67, 126, 127]. Jemni and Nasrallah [131] expressed equilibrium pressure as a function of hydrogen to metal atomic ratio (H/M). Same authors [132] also proposed that the effect of hydrogen concentration can be neglected for absorption while it is critical for

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desorption. The effect of driving forces can be modeled by using logarithmic functions [61]. Though the effect was demonstrated by Miyamoto et al. [133], linearly dependent functions [134] similar to shrinking core models prevail [135]. A normalized pressure dependent function

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as suggested by Ron [136] can be used for studying overall reaction rates. In a three dimensional model, energy interactions and flow of the coolant fluid may also be considered [64, 66]. Several of the numerical studies conducted in the early days used finite difference method [9, 116, 128, 137, 138] to arrive at the solution. Commercial packages such as FEM based COMSOL Multiphysics®[11, 64, 67, 94, 121-124, 139, 140], CFD based ANSYS FLUENT® [92, 117], PHEONICS® [126] and gPROMS®[110, 141, 142] were used to simulate the sorption performance of storage bed and plot results effectively. Recent developments such as Lattice

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Boltzmann and molecular modeling for simulating hydrogen sorption are not yet reported in literature, but may give better insight to the thermal design of the storage device.

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6.1Numerical Studies on Sorption Performance in Storage Devices with Heat Transfer Augmentation Structures Mohan et al. [143]have studied heat and mass transfer simulation of cylindrical, liquid cooled, hydrogen storage device with and without aluminum honey comb structure. This aluminum structure with multiple cells imparts good heat transfer properties, light weight and

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structural stability. In this study, TiCl3 catalyzed NaAlH4 is used as the storage material. As hydrogen is inducted to the bed, reaction commences. The hydrogen absorption in sodium alanate consists of two consecutive reactions:

1/3 Na3AlH6 + 2/3 Na3AlH6 +H2

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NaH + Al +1.5H2

1/3 Na3AlH6 + 2/3 Na3AlH6+H2

NaAlH4

(Reaction 6) (Reaction 7)

These reactions can proceed one after the other under suitable thermodynamic conditions. As soon as NaH and Al hydrogenated to Na3AlH6 by reaction 6, Na3AlH6 and Al can react further with H2 to form NaAlH4 by reaction 7. These two reactions are associated with corresponding heat release upon absorption of H2.

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This heat can be transported to the coolant by means of honeycomb structure. The numerical simulations of the above computational model are carried out in COMSOL Multiphysics®[144] commercial software. The 2D geometrical model in suitable file format is generated in

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AutoCAD® drawing package. It is subsequently exported to COMSOL Multiphysics® for further pre and post processing.

Figure 7 shows the effect of aluminum honeycomb with fins on average bed temperature of the

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bed. The presence of aluminum structure enhances the heat transfer rate and thereby bed temperature and its respective peak decreases. This causes corresponding rise in hydrogen sorption rate, as shown in Fig. 8.

7. DESIGN AND OPTIMIZATION OF STORAGE DEVICES The reactor performance has been reported to be sensitive to activation energy of the material, effective thermal conductivity, heat of formation and equilibrium pressure [74, 145]. The design of a reactor must consider the above factors. Multiple heat exchanger type and metal insert type reactors, though improve performance, result in the increase of system weight thereby 20

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reducing the gravimetric capacity of the system. Portable systems demand low system weight and high gravimetric and volumetric hydrogen storage capacity. The design procedure for any reactor must take into consideration the influence of heat and mass transfer and the mechanical design elements [146]. The author identified bed size of the reactor as a significant parameter in

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the design [11] and proposed a methodology to obtain the minimum system weight at specific charging levels [139]. The selection of bed material and bed size was the critical parameters as reported by Castrillo et al. [78] for reactor optimization.

Optimization of hydride reactor, by identifying critical parameters improves cycle time. Proper

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positioning of annular metal inserts and suitable coolant flow rate improved absorption time by 60 % [110]. The sorption performance increases with increase in surface to volume ratio of

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hydride tube. Optimum coolant velocity along with number and position of heat exchangers are critical for better performance [141]. Optimization of finned tubes can be done by increasing fin radius and decreasing fin thickness keeping the fin volume constant. Longer fins at smaller fin spacing provide better sorption [147, 148]. Garrison et al. [121] reported that small coolant size tube and closer fins are necessary for high gravimetric hydrogen storage. Lozano et al. [140] optimized a model equipped with an internal heat exchanger tube. Owing to larger bulk density

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of powder compacts, more amounts of material than neat alanate powder can be stored in similar systems. Hence compacts can be easily optimized for weight considerations. Also, it encourages to define the mass of hydrogen absorbed rather than using weight percentage. Hence addition of compacts decreases the parasitic weight of the system. The storage device is optimized for

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maximum hydrogen absorption at minimum time with respect to coolant flow rate by an online controlled optimization strategy using an explicit parametric controller [142] and open-loop

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optimal controller [149]. The coolant flow rate and temperature at the exit of annular heat exchanger were identified as the major parameters [142].

8. IMPORTANTASPECTSOF THE OPERATION OF PRACTICAL SYSTEMS At high packing fraction and long cycle life, the stress on the reactor walls due to volume

expansion of hydrides might be significant. Pulverization of hydride powder results in the shifting of material towards the lower end of the reactor. This may lead to considerable engineering problems [93, 146]. Okumura et al. [93] have reported the dominance of tangential stress developed in the system due to non-uniform expansion in the system diameter. 21

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Impurities present in the hydrogen gas lead to accumulation of foreign materials into the metal hydride voids, resulting in the decrease of hydrogen partial pressure and an early temperature peak. This leads to decrease in sorption performance. The blowing off the impurities repeatedly results in higher loss of hydrogen [150].

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Hydrogen has a specific energy of 142 MJ/kg [2] and normal fuel cell demand is 1.2 MJ/km i.e., a distance of 100km can be travelled using 1 kg of hydrogen in automobiles. Wenger et al. [151] reported the major constraints of a fuel cell as high volumetric storage, efficient heat transfer, low temperature operation and cold start. Raju and Kumar [105] developed a sodium alanate

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based model to numerically evaluate the performance of the system during driving conditions making a way forward in the realization of hydrogen energy for mobile application. Though

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characteristics of sodium alanate are well suited for fuel cell operation, poor gravimetric storage limits its applicability in portable systems for transportation [152] unless a highly efficient design in developed. Maeda et al. [153] have discussed the applicability of hydrogen storage cell in a cogeneration system. The system also focuses on utilizing the heat of reaction for other applications such as air-conditioning to improve overall system efficiency.

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9. CONCLUDING REMARKS

Simple intermetallic hydrides, which have been widely investigated, are in advanced stages of practical applications. However, for mobile applications, efforts to improve their gravimetric storage densities have not been very successful. Due to this, attention is being given

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to complex metal hydrides which show significant promise. There are severe challenges to employ these materials in practice, the major ones being, lack of thermophysical and reversibility data. Optimal thermal design of such devices is one of the keys to the success of hydrogen as the

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fuel of the future.

ACKNOWLEDGEMENTS Thanks are due to the Department of Science and Technology, Ministry of New and

Renewable Energy, Government of India, for their funding support towards hydrogen storage research.

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Prof. S. Srinivasa Murthy is grateful to his colleagues Prof. B. Viswanathan and Prof. M. P. Maiya and to his former doctoral students Dr. L. Hima Kumar and Dr. G. Mohan for their

NOMENCLATURE

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contributions to the research results presented here.

Pre-exponential factor of Arrhenius formula [s-1]

A1B

Pre-exponential factor of Arrhenius formula of Na3AlH6 decomposition [s-1]

A1F

Pre-exponential factor of Arrhenius formula of Na3AlH6 formation [s-1]

A2B

Pre-exponential factor of Arrhenius formula of NaAlH4 decomposition [s-1]

A2F

Pre-exponential factor of Arrhenius formula of NaAlH4 formation [s-1]

C

Concentration of H2 [mole H2/m3 of interparticle void]

C1

Concentration of NaAlH4 [mole/m3]

C2

Concentration of Na3AlH6 [mole/m3]

C3

Concentration of NaH [mole/m3]

C10

Initial concentration of NaAlH4 [mole/m3]

C20

Initial concentration of Na3AlH6 [mole/m3]

C30

Initial concentration of NaH [mole/m3]

Ea

Energy of activation of Arrhenius formula [J/mol]

Eaa1

Activation energy of NaAlH4 formation [J/mol]

Ead1

Activation energy of NaAlH4 decomposition [J/mol]

Eaa2

Activation energy of Na3AlH6 formation [J/mol]

Ead2

Activation energy of Na3AlH6 decomposition [J/mol]

E1B

Energy of activation of Arrhenius formula of Na3AlH6 decomposition [J/mol]

E2B E2F ʄ

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E1F

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A

Energy of activation of Arrhenius formula of Na3AlH6 formation [J/mol]

Energy of activation of Arrhenius formula of NaAlH4 decomposition [J/mol] Energy of activation of Arrhenius formula of NaAlH4 formation [J/mol] Function of p and peq that acts as diving force for the absorption reaction, -

H wt. %

Weight percent of H in sample

d(Hwt .%) dt

Rate of change of H in alanates formed (+) or decomposed (-), as H wt. %

k

Bed thermal conductivity [W/m°C] 23

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Rate constant [s-1]

ke

Effective thermal conductivity of the bed [W/m°C]

K0al

Pre-exponential factor of rate constant of NaAlH4 formation[s-1]

K0a2

Pre-exponential factor of rate constant of Na3AlH6 formation [s-1]

K0dl

Pre-exponential factor of rate constant of NaAlH4 decomposition [s-1]

K0d2

Pre-exponential factor of rate constant of Na3AlH6 decomposition [s-1]

m H2

Mass of hydrogen that could be desorbed [kg]

mH2 max

Maximum mass of hydrogen that could be desorbed [kg]

m1

Rate of concentration of NaAlH4

m2

Average rate of concentration of NaAlH4 and NaH

m3

Rate of concentration of NaH

p

Hydrogen back pressure [bar]

Pappl

Applied pressure [bar]

Pe,i

Plateau pressure at temperature T [bar]

peq

Equilibrium pressure [bar]

Peq1

Plateau pressure of NaAlH4 at a given temperature [bar]

Peq2

Plateau pressure of Na3AlH6 at a given temperature [bar]

R

Gas constant [J/molK]

r1B

Rate of dissociation of NaAlH4 from Na3AlH6 [mol/m3-s]

r1F

Rate of formation of NaAlH4 from Na3AlH6 [mol/m3-s]

r2B

Rate of dissociation of Na3AlH6 from NaH [mol/m3-s]

r2F

Rate of formation of Na3AlH6 from NaH [mol/m3-s]

SI

Mixture of 3 molNaH, 3 mol Al and 9/2 mol H2

SIII T

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SII

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ǩ

Mixture of 1 mol Na3AlH6, 3 mol Al and 3 mol H2 3 mol NaAlH4

Temperature [K]

Greek Symbols ∆HR

Enthalpy of reaction on a molar basis of species [J/(mole)]

∆SR

Entropy of reaction on a molar basis of species [J/(mole K)]

α

Transformed fraction 24

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REFERENCES [1] A. Zuttel, Materials for hydrogen storage, Materials Today 6 (9) (2003) 24-33. [2] L. Schlapbach, A. Zuttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353-358.

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[3] Targets for onboard hydrogen storage systems for light-duty vehicles, Freedom Car and Fuel Partnership,US Department of Energy. http://www1.eere.energy.gov/hydrogenandfuelcells/storage/pdfs/targets_onboard_hydro_stor age.pdf. [4] P. Muthukumar, Studies on metal hydride based thermal devices for compression and hydrogen storage, Ph.D. thesis 621.1:546.11(043) MUT (2004) IIT Madras.

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[5] A. Zaluska, L. Zaluski, J.O. Strom-Oslen, Sodium alanates for reversible hydrogen storage, Journal of Alloys and Compounds 298 (1-2) (2000) 125-134.

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[6] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345-352. [7] G. Sandrock, K.J. Gross, G.J. Thomas, C.M. Jensen, D. Meeker, S. Takara, Engineering considerations in the use of catalyzed sodium alanates for hydrogen storage, Journal of Alloys and Compounds 330-332 (2002) 696-701. [8] W. Supper, M. Groll, U. Mayer, Reaction kinetics in metal hydride reaction beds with improved heat and mass transfer, Journal of the Less-Common Metals 104 (2) (1984) 279286.

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[9] H. Choi, A.F. Mills, Heat and mass transfer in metal hydride beds for heat pump applications, International Journal of Heat and Mass Transfer 33 (6) (1990) 1281-1288. [10] D.W. Sun, S.J. Deng, Study of heat and mass transfer characteristics of metal hydride beds, Journal of Less-Common Metals 141 (1) (1988) 37-43.

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[11] G. Mohan, M.P. Maiya, S.Srinivasa Murthy, Performance simulation of metal hydride hydrogen storage device with embedded filters and heat exchanger tubes, International Journal of Hydrogen Energy 32 (18) (2007) 4978-4987.

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[12] G.A. Lozano, C. Na Ranong, J.M. Bellosta von Colbe, R. Bormann, G. Fieg, J. Hapke, M. Dornheim, Empirical kinetic model of sodium alanate reacting system (II) Hydrogen desorption, International Journal of Hydrogen Energy 35 (14) (2010) 7539-7546. [13] G. Mohan, M.P. Maiya, S.Srinivasa Murthy, The performance simulation of air-cooled hydrogen storage devices with plate fins, International Journal of Low-Carbon Technologies 5 (2010) 1-10. [14] S.Srinivasa Murthy, Heat and mass transfer issues in the design of solid state hydrogen storage devices, Keynote Talk in Proceedings of International Heat Transfer Conference (IHTC 14) (2010) Washington DC USA. [15] B. Bogdanovic, R.A. Brand, A. Marjanovic, M. Schwickardi, J. Tolle, Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials, Journal of Alloys and Compounds 302 (1-2) (2000) 36-58.

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[146] F.S. Yang, G.X. Wang, Z.X. Zhang, X.Y. Meng, V. Rudolph, Design of metal hydride reactors - A review on the key technical issues, International Journal of Hydrogen Energy 35 (8) (2010) 3832-3840.

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[147] S. Mellouli, F. Askri, H. Dhau, A. Jemni, S.B. Nasrallah, Numerical simulation of heat and mass transfer in metal hydride storage tanks for fuel cell vehicles, International Journal of Hydrogen Energy 35 (4) (2010) 1693-1705. [148] S.N. Nyamsi, F. Yang, Z. Zhang, An optimization study on the finned tube heat exchanger used in hydride hydrogen storage system-analytical method and numerical simulation, International Journal of Hydrogen Energy 37 (21) (2012) 16078-16092. [149] C. Panos, K.I. Kouramas, M.C. Georgiadis, E.N. Pistikopoulos, Dynamic optimization and robust explicit model predictive control of hydrogen storage tank, Computers and Chemical Engineering 34 (2010) 1341-1347. [150] D. Dunikov, V. Borzenko, S. Malyshenko, Influence of impurities on hydrogen adsorption in a metal hydride reactor, International Journal of Hydrogen Energy 37 (18) (2012) 1384313848.

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[151] D. Wenger, W. Polifke, E. Schmidt-Ihn, T. Abdel-Baset, S. Maus, Comments on solid state hydrogen storage systems design for fuel cell vehicles, International Journal of Hydrogen Energy 34 (15) (2009) 6265-6270. [152] B. Bogdanovic, U. Eberle, M. Felderhoff, F. Schuth, Complex aluminium hydrides, ScriptaMaterialia 56 (10) (2007) 813-816.

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[153] T. Maeda, K. Nishida, M. Tange, T. Takahashi, A. Nakano, H. Ito, Y. Hasegawa, M. Masuda, Y. Kawakami, Numerical simulation of the hydrogen storage with reaction heat recovery using metal hydride in the totalized hydrogen energy utilization system, International Journal of Hydrogen Energy 36 (17) (2011) 10845-10854.

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List of Figures Fig. 1

Comparison of desorption capacity of unmilled LiAlH4, ball milled LiAlH4, LiAlH4doped with TiH2, AlCl3, Ti, Fe, C-black, V, Ni, FeCl3, TiCl4 and TiCl3 at temperature 430-450°C Variation of Hydrogen desorption capacity and reaction time with temperature for

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Fig. 2

TiCl3 x 1/3AlCl3-doped LiAlH4 Fig. 3

Dehydrogenation profiles for the (
) pure LiAlH4 (•) 5 wt. % Vulcan XC 72R admixed LiAlH4 (♦) 5 wt. % mesoporous carbon admixedLiAlH4 (
) 5 wt. %

Fig. 4

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CDX 975 admixed LiAlH4 () 5 wt. % Black pearls 2000 admixed LiAlH4

Dehydrogenation kinetics of LiAlH4 admixed with CNFs, LiAlH4admixed with

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Vulcan carbon and LiAlH4 Fig. 5

Variation of effective thermal conductivity with pressure for different filling gases

Fig. 6

Comparison of ETC values obtained from extended model with experimental results

Effect of Al-honeycomb and fins on average temperature of bed

Fig. 8

Effect of Al-honeycomb and fins on hydriding

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Fig. 7

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List of Tables DOE targets for on-board hydrogen storage for automobiles [3]

Table 2

Hydrogen storage capacity of some metal hydrides [4]

Table 3

wt. % of H2 absorbed and activation enthalpy at different doping levels [34]

Table 4

Thermal conductivity of Sodium alanate as reported by various authors

Table 5

Value of dissociation enthalpy and entropy [58]

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Table 1

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Table 1. DOE targets for on-board hydrogen storage for automobiles [3] 2017 Goals Ultimate

System gravimetric capacity System volumetric capacity Operating temperature Operational cycle life System fill time (5kg)

% kg H2/L system o C Cycles Min

5.5 0.04 40/60 1500 3.3

7.5 0.07 40/60 1500 2.5

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Units

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Storage parameter

Table 2. Hydrogen storage capacity of some metal hydrides [4]

1.49

MmNi5

1.46

Ti

4.0

TiFe

1.86

TiCr1.8

2.43

Zr

2.2

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LaNi5

1.77

ZrMn2

Material

Capacity in wt. % of H2

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ZrCr2

1.82

Ca

4.8

CaNi5

1.87

CeNi5

1.5

Mg

7.66

MgNi

3.6

Mg + 2 wt. % Ni

7.48

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Table 3. wt. % of H2 absorbed and activation enthalpy at different doping levels [34]

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Doping level (mol % Ti)

Wt. % of hydrogen

Activation enthalpy NaAlH4

Na3AlH6

0

5.12

118.1

120.7

0.9

4.94

72.8

97.1

2

4.25

79.5

97.1

4

3.85

80

97.5

6

2.91

78.5

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Table 4. Thermal conductivity of Sodium alanate as reported by various authors Thermal conductivity (W/m-K)

Dedrick et al. [65]

Condition.

0.5

1 bar

0.68

100 bar

1.84 mol % TiCl3

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Reference

0.62 g/cm3

Gross et al. [66]

0.2

4 mol. % TiCl3

0.62 g/cm3

Hardy [58]

0.325

4 mol. % TiCl3

0.72 g/cm3

Undoped

1.37

Ti doped pellets

~8.5

Doped with 5 mol % expanded graphite

~6

3 mol % ENG

~12

6 mol % ENG

~30

16 mol % ENG

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9.09

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Table 5. Values of dissociation enthalpy and entropy [58] ∆ࡴࡾ /R

∆ࡿࡾ /R

3NaH+Al+1.5H2→ Na3AlH6

- 4475

- 14.83

Na3AlH6+2Al+3H2→3NaAlH4

- 6150

- 16.22

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Sorption reaction

345 MPa

300 MPa

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Highlights

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Advanced materials suitable for solid state hydrogen storage are discussed. Issues related to thermodynamic and thermo-physical properties of hydriding materials are brought out. Hydriding and dehydriding behaviour including sorption kinetics of complex hydrides with emphasis on alanates are explained.

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Key words

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Solid state hydrogen storage, physisorption, chemisorption, alanates, complex hydrides, thermophysical properties