Journal of Alloys and Compounds 645 (2015) S117–S122

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Niche applications of metal hydrides and related thermal managementissues

http://dx.doi.org/10.1016/j.jallcom.2014.12.2710925-8388/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: mlototskyy@uwc.ac.za (M. Lototskyy).

M. Lototskyy a,⇑, B. Satya Sekhar a, P. Muthukumar b, V. Linkov a, B.G. Pollet a

a HySA Systems Competence Centre, South African Institute for Advanced Materials Chemistry, Faculty of Natural Sciences, University of the Western Cape, Private Bag X17,Bellville 7535, South Africab Mechanical Department, Indian Institute of Technology Guwahati, Guwahati 781039, India

a r t i c l e i n f o

Article history:Available online 30 January 2015

Keywords:Metal hydridesHydrogen storageHydrogen compressionHeat managementThermal management issues

a b s t r a c t

This short review highlights and discusses the recent developments and thermal management issuesrelated to metal hydride (MH) systems for hydrogen storage, hydrogen compression and heatmanagement (refrigeration, pump and upgrade, etc.). Special attention is paid to aligning the systemfeatures with the requirements of the specific application. The considered system features include theMH material, the MH bed on the basis of its corresponding MH container, as well as the layout of theintegrated system.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Metal hydride (MH) materials are able to reversibly absorb anddesorb large amounts of hydrogen in a wide range of temperaturesand pressures. They are characterised by some unique properties,including extremely high volume density of H atoms incorporatedin the metal matrix [1–5], wide ‘‘tuneability’’ of thermodynamicperformances [3–7] and fast kinetics of hydrogenation/dehydroge-nation [3–9]. MH technologies utilising such features are typicalapplication-driven niche areas able to provide very efficient solu-tions for hydrogen handling, particularly for end-use applications,by the tuning of component and phase composition of the parentMH material, system layout and operation parameters. MHs offerseveral advantages over traditional hydrogen storage (e.g. com-pressed hydrogen, liquefied hydrogen, etc.) and processing sys-tems, such as compactness, safety (due to low pressures <10 bar),possibilities of hydrogen supply at pre-defined pressures, absenceof moving parts, long operation lifetime, efficient and technologi-cally flexible energy conversion between low-grade heat and theenergy of pressurised H2 [7,10]. These systems have found a num-ber of promising gas phase applications including hydrogen stor-age and supply, hydrogen compression, heat management(refrigeration, pump, and upgrade), etc. [1–10].

The present work herein briefly highlights features of the mostimportant gas-phase niche applications of MH. Methods for

improving heat transfer performances of the corresponding MHreactors are also described and discussed.

2. Thermodynamic properties of MH and main applications

The chemical bonding of gaseous hydrogen in MH can be writ-ten as:

MðsÞ þ x=2H2ðgÞ¢ MHxðsÞ þ Q ; ð1Þ

where M is the metal, alloy or intermetallic compound forminghydride, MHx, (s) and (g) denote solid and gas phase, respectively.

The direct process of reaction (1), hydride formation/hydrogenabsorption is an exothermic reaction, and the generated heat, Q,is approximately equal to the absolute value of the enthalpy ofreaction (1). The heat has to be effectively removed in order toachieve the desired H2 charge rate. Accordingly, the endothermichydride decomposition/H2 desorption (reverse process of reaction(1)) needs efficient supply of the approximately same amount ofthe heat to provide the necessary rates of H2 discharge.

It has to be noted that the driving force of both direct andreverse processes of reaction (1) strongly depends upon the devia-tion of the actual hydrogen pressure, P, from the value, Peq, of equi-librium dissociation pressure of the MH which is an importantproperty of hydride forming materials. Temperature dependenceof Peq in the plateau region of a Pressure–Composition Isotherm(PCI) is expressed by the van’t Hoff equation:

-160 -140 -120 -100 -80 -60 -40 -20 0-0.16-0.15

-0.14-0.13

-0.12-0.11

-0.10-0.09-0.08

-0.07-0.06

-0.05-0.04

CeNi4.5Mn0.5

Mg2Co

ZrH2

TiH2

Mg2Cu

ZrNi

UH3Mg2Ni

MgH2

ZrV2V75Ti10Zr7.5Cr7.5

VH2(Vi0.89Ti0.11)0.95Fe0.05

LaNi4.3Mn0.7

Pd0.99V0.01

TiFe

CeNi5LaNi5

Ti1.1Cr2ZrVFe

Pd0.972Ir0.028

Ce1.1Ni2.5Cu2.5

CeNi3Cu2TiFe0.9Ni0.1

AlH3

ZrMn3.8

ZrMn2.5Zr0.8Ti0.2MnFe

ΔS

[kJ

mol

H2-1

K-1

]

AB5AB2±xABA2B

Pd alloysbcc-V alloysBinary

ΔH [kJ mol H2-1]

A

B D C

Fig. 1. Hydrogenation enthalpies and entropies of various hydride materials andtheir suitable applications: (A) heat pumps, (B) heat storage, (C) hydrogen storage,(D) hydrogen compression.

S118 M. Lototskyy et al. / Journal of Alloys and Compounds 645 (2015) S117–S122

lnPeq

P0¼ �DS

Ruþ DH

RuT; ð2Þ

where T [K] is the temperature, Ru [J mol�1 K�1] is the universal gasconstant, DH [J mol H2

�1] and DS [J mol H2�1 K�1] represent the asso-

ciated changes in enthalpy and entropy, and P0 is the referencepressure (1 atm).1

Fig. 1 shows DS and DH values for 278 metal hydride materials[7,10]. For the most of the MH materials, DS does not vary signifi-cantly around �111 J mol�1 H2 K�1 which is close to the change ofentropy of gaseous H2 for the reaction (1), of �130 J mol�1 H2 K�1.The plateau pressure, Peq, is mainly determined by the reactionenthalpy, DH, which varies from �166 to �6.6 kJ mol�1 H2 forZrH2 [11,12] and AlH3 [13], respectively. At the same time, in someMH materials the contribution of the entropy of the solid into thetotal entropy change for reaction (1) can be significant thus result-ing in significant deviations of DS from the average value specifiedabove. Binary metal hydrides and solid solution alloys exhibitlower DS, down to �150 J mol�1 H2 K�1, while multicomponentAB2±x metal hydrides can have hydrogenation entropy as high as�45 J mol�1 H2 K�1.

Fig. 1 can guide in selecting metal hydrides for different gasphase applications including:

A – Heat pumps: for efficient operation, the MH material shouldhave high heat effects (|DH| > 30 kJ mol�1 H2) while providingmoderate H2 pressures during the operation. The absolutevalue of the reaction entropy should be as low as possible(|DS| < 110 J mol�1 H2 K�1).B – Heat storage: similarly, the MH material has to have highheat effects (|DH| > 30 kJ mol�1 H2), but highly exothermic MHshould have not too low H2 pressures at operating temperatures(|DS| > 110 J mol�1 H2 K�1).C – H2 storage: for this type of application, low energy consump-tion for hydrogen release is important (|DH| < 30 kJ mol�1 H2).D – H2 compression: this application requires compromisebetween energy consumption for hydrogen desorption and highcompression ratio; accordingly the MH materials shouldhave medium heat effects (|DH| = 20–30 kJ mol�1 H2) and highabsolute values of the hydrogenation entropy (|DS| > 100J mol�1 H2 K�1).

1 Since left-hand side of Eq. (2) contains the ratio Peq/P0, their values can be taken inany but the same units.

3. Thermal management issues

3.1. General features

When the rates of heat dissipation (exothermic H2 absorption)or heat supply (endothermic H2 desorption) from/to the MH mate-rial are lower than the rates of its heating/cooling due to heateffect, DH, of reaction (1), the material reaches an equilibrium tem-perature, Teq, when P = Peq, and no further absorption or desorptionoccurs. The value of Teq can be estimated by a solution of Eq. (2)with respect to the temperature for the each actual pressure(‘‘dynamic van’t Hoff equation’’) as it was suggested by Goodell[14]. Thus, further rates of hydrogen absorption/desorption inany part of the MH material in a reactor will be limited by the rateof heat exchange between the material (elementary volume in MHbed) at T = Teq and the cooling/heating means at T = T0. Accordingly,the integral H2 absorption/desorption rates in a MH reactor can bedetermined only in the course of the solving of, in 3D statement,the Fourier heat equation with a source term [3]:

@T@t¼ r ke

qcPrT

� �� DH

qcP; ð3Þ

where T [K] is the MH temperature, t [s] is time, ke [W m�1 K�1] isthe effective thermal conductivity, q [kg m�3] is the apparentspecific weight and cP [J kg�1 K�1] is the specific heat capacity ofthe packed material, ‘‘+’’ and ‘‘�’’ correspond to H2 desorption andabsorption, respectively.

This approach has been taken in numerous investigations forthe modelling of heat-and-mass transfer in MH beds [15–17]. Atthe same time, the details of the task statement strongly dependupon a number of factors, first of all, effective heat conductivityof the MH bed, its geometry, ways of heat supply and its removal.These parameters related to heat management issues may signifi-cantly differ in numerous applications. However, the commonstrategy in the improvement of heat transfer performances ofMH beds should include the ‘‘maximizing’’ of the term ke

qcP, first of

all, by increasing the effective thermal conductivity, ke. The short-ening of the characteristic heat transfer distance is also veryimportant.

3.2. Augmentation of hydride bed heat transfer

Enhancing the effective thermal conductivity forms the centralobjective of all techniques applied to porous MH beds. Impregna-tions of heat conduction matrices have been found to be very use-ful. The approach includes 2D and 3D structures made of heatconductive metals, e.g. copper or aluminium. Common impedi-ments to the above solution include the parasitic weight of theheat conductive material, disintegration and deterioration uponcycling, and high cost. Representative values of effective thermalconductivity and void fraction achieved by different augmentationtechniques are listed in Table 1.

Suda et al. [24] carried out experimental investigations aimed atimproving the effective thermal conductivity of activated MH. A 3Dstructure of porous aluminium foam increases ke in 9–10 times(4 W m�1 K�1 at 0.5 MPa H2) as compared to the unmodifiedhydride bed. Laurencelle and Goyette [19] numerically studiedthe impact of aluminium foam to enhance hydrogenation reactions.The effective thermal conductivity of the hydride bed was improvedto 10 W m�1 K�1. The reaction rate in the reactor with foamdepends upon the resistance inside the foam cells, and also alongthe path to the reactor wall. With foam, the MH bed thickness couldbe increased without hampering the charging time. The foams withpore density less than 20 PPI (Pores Per Inch) were not recom-mended due to poor heat transfer. Fleming et al. [25] showed that

Table 1Comparison of different heat transfer augmentation techniques.

Heat transfer augmentation technique/hydride bed Eff. thermal conductivity (W/(m K)) Void fraction Hydride-forming alloy

Copper wire matrix (Nagel et al. [18]) 0.5–2.5 0.91 MmNi4.46Al0.54

(Cu)

Aluminium foam (Laurencelle and Goyette [19]) 10.0 0.91 LaNi5

(Al)0.55(LaNi5)

Metal hydride powder composites with copper encapsulation (Kim et al. [20]) 5.0 0.35 LaNi5

PMH compacts with Al. (Ron et al. [21]) 12.3 0.27 LaNi5

(total)6.9 0.25 MmNi4.15Fe0.85

(total)

MH compact with recompressed expanded graphite (Kim et al. [22]) 3.0 – LaNi5

Expanded graphite MH compacts (Sanchez et al. [23]) 19.5 0.39 LaNi4.85Sn0.15

(total)

M. Lototskyy et al. / Journal of Alloys and Compounds 645 (2015) S117–S122 S119

adding metallic foam significantly improved the rate of metal-hydride based hydrogen separation process.

Expanded Natural Graphite (ENG)–MH compacts provide a costeffective solution to the problem [22,23]. Recompressed ENG offers(i) high plasticity under compression loads, (ii) high thermal con-ductivity and (iii) gas permeability in the direction perpendicularto the direction of the compacting. Hydrogen absorption rates werefound to decrease slightly compared to that of the MH powder–Alfoam reaction bed. Further improvement of heat transfer in MH/ENG compacts was suggested by De Rango et al. [26], as an alter-native arrangement with heat conductive fins within an MH hydro-gen storage tank. This layout was successfully applied for large-scale H storage tanks comprising stable MH (MgH2) taken for thecompacting with ENG in the hydrogenated state [27]. For the MHmaterials which form hydrides with dissociation pressure higherthan 1 bar at the room temperature, some variations of the com-pacting technology have been recently suggested by the SouthAfrican co-authors of this review (ML, BGP, VL) [28].

Implementation examples of the heat transfer augmentationtechniques are presented in Fig. 2.

In addition to the internal heat transfer, the solutions intensify-ing heat exchange between the MH bed and the heating–coolingfluid can be very efficient as well, especially, when they are com-bined with the shortening of the characteristic heat transfer dis-tance. The best achievement for this approach was realised byErgenics Inc. [32,33] whereby MH materials were loaded into atubular containment of a small outer diameter, down to 1/1600

(1.588 mm). The intensification of heat exchange between theouter surface of the hydride tube and heating/cooling fluid wasachieved by fins formed by steel wire wound and soldered ontothe hydride tube. The hydride beds in such hydride ‘‘ring mani-folds’’ (the images can be found in [7,30,32,33]) enable very shortcycle times (<1 min), as well as high cooling capacities in the MHheat management systems (from 1545 to 2097 W).

Following the design described in [34], Linder and Laurien [35]developed a metal hydride based cooling system employing twoCapillary Tube Bundle Reactors (CTBR). The researchers observedfast reaction kinetics of about 95 s for desorption and 80 s forabsorption and the reported specific cooling power was850 W kg�1 for 95 s cooling time (in other words 425 W kg�1 forthe complete cycle). Recently, Anbarasu et al. [36] studied theeffect of different arrangements of cooling tubes embedded inthe hydride bed on the hydriding and dehydriding characteristics.Using 2.75 kg of LmNi4.91Sn0.1, the absorption time was signifi-cantly reduced to 5 min when the number embedded cooling tubeswas increased from 36 to 60.

3.3. Specific niche applications

3.3.1. Hydrogen storageThe use of MH materials for hydrogen storage is characterised

by high compactness, safety and ease of operation and consump-tion of low ‘‘quality’’ energy (waste heat) for H2 desorption. Eventaking into account low hydrogen storage weight capacity for‘‘low-temperature’’ MH materials, they can be successfully usedfor a number of wide-scale stationary and special mobile applica-tions when the system weight is not a major and critical issue[37]. However, the main issue related to the thermal managementis in poor H2 charge/discharge dynamics due to heat transfer limi-tations. This effect is especially pronounced for the larger-scalehydrogen storage systems and can be mitigated by the improve-ment of heat transfer inside MH bed (see Section 3.2).

3.3.2. Heat pumps. Heat transformers. Heat storageThe use of Chloro-Fluro-Carbon refrigerants (CFCs) is well-

known to be significant source of pollution to the global environ-ment and great effort has been undertaken to tackle and minimizethis problem through the promotion of the alternative refrigerationand air-conditioning systems. One of the most promising alterna-tives is the use of hydrogen as refrigerant in the metal hydridebased heat pumps (MHHP) [30,38]. MHHPs are characterised bysafe and easy operation (no moving parts), and enable to use thewaste heat (T � 150 �C). The main problem to be solved in thedevelopment of all heat management applications of MH, includingMHHP, is in the careful selection of MH materials with thermody-namic properties for the high- and low-temperature system sidesneeding to be aligned. The selection must take into account therequired operation temperatures: low (Tl), middle (Tm) and high(Th); hydrogen pressures; as well as reversible hydrogen storagecapacities of the materials at the operation conditions. Thus forexample, the combination Zr0.8Ti0.2MnFe/LaNi5 allows operationsat Tl = 25.7 �C, Tm = 50 �C, Th = 145.2 �C and P(H2) = 1.77–5.5 �atm.This example was taken from the publication by Dantzer and Orgaz[39] where detailed guidance for the selection of the MH materialsfor heat management applications was presented.

Another problem is in the low efficiency of MHHPs as the Coef-ficients of Performance (COP) of single-stage MHHPs are typicallybelow 0.5. The efficiency can be increased by minimizing the contri-bution of containment and auxiliary elements (heat exchangers)into the total system weight, without compromising on fast dynam-ics of H2 exchange between MH beds and high heat exchange ratesbetween the MH beds and the heat transfer fluid. The use of CTBRsallowed the system to achieve fast reaction kinetics [35]. The

Fig. 2. Augmentation of hydride bed heat transfer: Aluminium foam (A), corrugated copper band (B), corrugated perforated aluminium band (C), copper fins (D) [29,30];combination of Al foam and Cu fins in MH reactor (E) [30,31]; MH/ENG powder mixture, compacts and Al fins before their loading in MH reactor [28] (F).

S120 M. Lototskyy et al. / Journal of Alloys and Compounds 645 (2015) S117–S122

increase of COP and specific cooling power per MH unit mass can beachieved using a Compressor-Driven (CD) MH systems for coolingand heating applications. Magnetto et al. [32] described the devel-opment of a CD-MHHP for automotive applications (with COPs inthe range of 2.57–2.76) which was superior to conventional R-134a refrigerant system.

Apart from the increase in the effective thermal conductivity ofthe MH bed, special attention in the course of the development ofhigh-efficiency MHHP has to be paid to the balance between therate of hydrogen transfer and the capacity rate of heat transfer flu-ids. A lumped parameter model suggested by Bjurström and Sudain 1989 [40] can be useful for such analysis.

MHHP can be used in various energy saving technologies,including Combined Heat and Power (CHP), utilisation of low-potential industrial heat, residential energy systems, etc.

Metal hydride based heat transformer (MHHT) is a type of MHHPwhich can upgrade the temperature of low grade heat such as indus-trial waste heat, solar energy, and geothermal energy up to the tem-perature in the range of 150–220 �C. It also provides higher heatstorage capacity and offers wider range of working temperaturesas compared to other conventional heat pumps/heat transformers.MHHTs use hydrogen as a working fluid which is environmentfriendly, noise free and vibration free. The performance of MHHTcan be improved by (i) sensible heat recovery, (ii) employing

compact reactors with embedded heat transfer fluid tubes and(iii) optimizing heat transfer enhancement and thermal capacityof the reactor [41,42].

Heat storage is another important heat management applica-tion of MH. Unlike the latent and the sensible heat storages utilis-ing phase change materials, in chemical energy storage systems onthe basis of MH, heat can be stored for a longer period without anythermal insulation [43]. MH based energy storage systems are highenergy dense, deliver heat at near constant temperature, and offerwide range of operating temperatures. They can provide energydensities as high as 2814 kJ kg�1 of hydride [44]. The system per-formance depends upon careful selection of hydride forming alloywithin its operating range. Hydrides with high heat of formationand low dissociation pressures at high temperatures are desirable.The other desirable properties of metal hydride alloys are: highhydrogen storage capacity, flat plateau, low hysteresis, fast kinet-ics, easy activation, and high melting point. Hydrides on the basisof Mg alloys (hydrogen storage capacity of MgH2 is 7.6 wt.% H;DH = �74 kJ mol�1 H2, Peq = 1 bar at T � 300 �C) are suitable forenergy storage applications.

An interesting and promising approach in the development ofthe hydride-based heat management systems is in the combina-tion of MH (‘‘low-temperature’’ side) and catalysed organichydrides (‘‘high-temperature’’ side) characterised by wide range

M. Lototskyy et al. / Journal of Alloys and Compounds 645 (2015) S117–S122 S121

of the operation temperatures (>100–500 �C) and high hydrogena-tion heat effects (about 70 kJ mol�1 H2 that is close to the heat offormation of MgH2). This solution [45] could potentially extendthe operating temperature range of the MHHP and increase theefficiency, providing that the problem with slow H transfer kineticsin organic hydride systems is solved.

3.3.3. MH hydrogen compressorsThermally-driven MH sorption compression is an efficient and

reliable method allowing conversion of energy from heat into acompressed hydrogen gas. This technology offers a good alterna-tive to both conventional (mechanical) and newly developed (elec-trochemical, ionic liquid pistons, etc.) methods of hydrogencompression. MH hydrogen compression is characterised by safeand easy operation (no moving parts); practically unlimited, upto kilobar-range, discharge pressures; good scalability, from fewlitres to several m3 H2 (at STP) per hour; modular design; easy rep-aration/maintenance; possibility to utilise waste heat instead ofelectricity. A comprehensive review on MH hydrogen compressiontechnology was recently published by Lototskyy et al. [7]. Similarlyto the MH heat management systems, the main disadvantages ofMH compressors are associated to low efficiency and low produc-tivity originated from slow dynamics of hydrogen uptake andrelease in MH reactors (compression elements). Similarly toMHHPs, the efficiency can be increased by the minimization ofthermal masses of MH containers and their auxiliary elements(heat exchangers, filters, etc.); the use of heat recovery solutionsis promising as well. The productivity can be increased by theimprovement of heat transfer performances of the MH reactors,as described above.

Promising niche applications of the MH compressors cover verywide areas, including laboratory facilities, cryogenics, space, H2 fill-ing stations, utilisation of waste heat, temperature sensors & actu-ators, as well as a number of special H2-consuming technologicalprocesses (e.g., powder metallurgy).

4. Concluding remarks

This paper briefly describes thermal management issues in var-ious niche applications of MH including (i) H2 charge/dischargedynamic performance and (ii) energy efficiency.

The main factors influencing the H2 charge/discharge dynamicsare as follows:

� MH material characteristics (H storage capacity, thermody-namic and thermal properties).� Pressure/temperature operating conditions.� Reactor configuration and geometric parameters.� Thermal design of the overall MH reactor.

The optimisation mainly relates to the improvement of heattransfer performances, by the introduction of a heat transfermatrix in the MH bed, including wire meshes (copper or alumin-ium), metal foams, expanded graphite. An alternative approachcan be in the development of MH containers with short character-istic heat transfer distance in the MH bed and improved heat trans-fer coefficient between the container wall and heating–coolingfluid.

Note that for portable and mobile hydrogen storage applica-tions, heat transfer intensification may be counterproductive dueto the added weight of the thermally conductive materials whichdo not participate in H2 sorption. This problem can be mitigatedby: (i) the use of ENG, and/or (ii) the development of ‘‘hybrid’’systems using unstable MH in combination with gaseous H2 atmoderately high pressures [46].

The following main factors affect the efficiency of MH systems:

� Thermal masses of MH container and auxiliary elements (heatexchangers, filters, etc.).� Heat recovery.� Heat transfer.

Heat transfer is the major factor directly controlling H2 sorption/desorption rates in MH beds and affecting the efficiency of energyconversion. The improvement of the heat transfer characteristicsis crucial.

Special attention and care in the optimisation of MH reactorsrelated to the heat management issues has to be paid to aligningsystem features (MH material, MH bed on its basis in MH container,as well as layout of the integrated system) with the requirements ofthe specific application.

For hydrogen storage, as well as for the increase of energy effi-ciency in other applications, the mass of the MH container andauxiliary elements (heat exchangers, filters, etc.) should bedecreased.

Acknowledgements

The contribution of South African co-authors into this work wassupported by the Department of Science and Technology (DST)within the HySA Programme (project KP3-S02 – On-Board Use ofMetal Hydrides for Utility Vehicles), Eskom Holdings Limited, andImpala Platinum Limited; South Africa. Investments from theindustrial funders have been leveraged through the Technologyand Human Resources for Industry Programme (THRIP), jointlymanaged by the South African National Research Foundation(NRF) and the Department of Trade and Industry (DTI); projectsTP1207254249, TP2010071200039, and TP2011070800020. MLalso acknowledges NRF support via incentive funding grant 76735.

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