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International Journal of Thermal Sciences 50 (2011) 2536e2542

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International Journal of Thermal Sciences

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Improvement of thermal performance of spiral heat exchanger on hydrogenstorage by adding copper fins

H. Dhaou*, N. Ben Khedher, S. Mellouli, A. Souahlia, F. Askri, A. Jemni, S. Ben NasrallahLaboratoire d’Etudes des Systèmes Thermiques et Energétique, Rue Ibn Eljazzar, Ecole Nationale d’Ingénieurs de Monastir, Université de Monastir, Monastir 5019, Tunisia

a r t i c l e i n f o

Article history:Received 27 October 2010Received in revised form26 May 2011Accepted 26 May 2011Available online 10 August 2011

Keywords:Metal hydrideHydrogenCopper finsLaNi5

* Corresponding author.E-mail address: dhaou_2000tn@yahoo.fr (H. Dhao

1290-0729/$ e see front matter � 2011 Elsevier Masdoi:10.1016/j.ijthermalsci.2011.05.016

a b s t r a c t

The reaction time of hydrogen in metal-hydride vessels (MHVs for short) is strongly influenced by theheat transfer from/to the hydride bed. In the present work an experimental study of the geometric andthe operating parameters of a finned spiral heat exchanger has been carried out to identify their influ-ence on the performance of the charging process of the MHV. The experimental results show that thecharge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. Inaddition, the effect of different parameters (flow mass and temperature of the cooling fluid, appliedpressure of hydrogen in the case of absorption and desorption) has been discussed and obtained resultsshow that a good choice of these parameters is important.

� 2011 Elsevier Masson SAS. All rights reserved.

1. Introduction

Hydrogen storage alloys, which are a kind of inter-metalliccompound, have received much attention due to their importancein applications such as hydrogen and thermal reservoirs, heatpumps, compressors, batteries, hydrogen purification and recoverydevices, catalysts in hydrogenation and dehydrogenation of organiccompounds, etc. [1]. Although the hydriding kinetics of hydrogenstorage alloys is very important in the design of a hydrogen storagereactor for industrial applications, it has been studied for very fewalloys. This is because it has features like: generating a largeamount of heat with hydrogen absorption, absorbing and desorb-ing hydrogen reversibly and storing hydrogen compactly [2].

The dynamic response of hydrogen storage system is importantin the design of metal-hydride energy conversion systems. Therehave been numerous experimental and theoretical investigations onseveral aspects of hydride storage. Gopal and Murthy [3,4] andMayer et al. [5] investigated transient heat andmass transfer withinthe reactor using a one-dimensionalmodel. Jemni and BenNasrallah[6,7] examined both the dimensional heat and mass transfer withina metal-hydrogen reactor for both absorption and desorptionprocesses using two mathematical models. The sorption process formixtures containing impurities that are inactive or weakly active to

u).

son SAS. All rights reserved.

the hydride forming material was investigated by Shmalkov et al.[8]. Mat and Kalpan [9] and Aldas et al. [10] studied numericallythree-dimensional heat and mass transfer in a metal-hydride bedduring absorption. Jemni et al. [11] conducted an experimental andnumerical study in order to determine the effective thermalconductivity, the equilibrium pressure, and the kinetic-reactions.Demircan [12] examined experimentally the hydrogen absorptionin two LaNi5-H2 reactors. The optimization of hydrogen storage inmetal-hydride beds was investigated by Eustathios et al. [13], twonovel cooling design options are investigated by introducing addi-tional heat exchangers at a concentric inner tube and an annularring inside the tank. Mellouli et al. [14] designed and manufactureda novel design of MHV equipped with a spiral heat exchanger andmentioned that the charge and the discharge time are significantlyreduced. Recently, Eustathios et al. [15], developed effective heatmanagement strategies and novel cooling design options forhydrogen storage systems in LaNi5 metal-hydride reactors by per-forming systematic simulation and optimization studies. Similarly,Kim et al. [24] have studied the effect of different parameters (heatconductance between reactors and reservoirs, pressure ratio of thecompressor, mass ratio of slurry to hydrogen, compressor efficiency,heat of desorption, and fraction of heat conductance associatedwiththe cold reservoir) on the functioning of a compressor-driven,metal-hydride slurry air conditioner. In a previous publication[16], we have measured and modeled the kinetics of hydrogensorption by LaNi5 and two related pseudobinary compounds. Mel-nichuk et al. [17] optimized the aluminium fraction of heat transfer

Fig. 1. Synoptic scheme of the installation.

Table 1List of parameter values.

HydrogenAtomic weight, MH 1 kg/kmolSpecific heat capacity, cpg 14.89 kJ/kg KMetal hydride (LaNi5)Mass, m 1 kgDensity, rM 8300 kg/m3

Molecular weight, MM 432.4 kg/kmolPermeability, k 1 mm2

Porosity, 3 0.5Specific heat capacity, cps 0.419 kJ/kg KReactorInternal diameter, din 80 mmInternal height, hint 110 mmThickness, er 15 mmFinsThickness, e 1 mmLength, l 5 mmSpiral Heat exchangerInternal diameter, d 6 mmLength, L 300 mmFilterDiameter 10 mmPorosity 25 mm

H. Dhaou et al. / International Journal of Thermal Sciences 50 (2011) 2536e2542 2537

fins for a LaNi5 hydrogen storage container, at different absorptiontimes and container diameters.

However, it is known that the performance of hydrogenabsorption to and desorption from metal hydride depends on theheat transfer, mass transfer and kinetic-reactions of the metal-hydride bed. So it is necessary for designing an optimized MHV toenhance the transient heat and mass transfer rate in the metal-hydride bed [25].

Askri et al. [18] established a model to study the dynamicbehavior inside various designs of MHVs and optimization resultsindicate that almost 80% improvement of the storage time can beachieved when the design includes a concentric heat exchangertube equipped with fins and filled with flowing cooling fluid.

All these studies have shown that the charge as well asdischarge times of MHV strongly depend on the successful heattransfer from/to the hydride bed. Thus, different attempts toimprove the effective thermal conductivity of metal-hydride bedshave been undertaken: insertion of aluminium foam [19], integra-tion of copper wire net structure [20], packing in a multilayerwaved sheet structure, microencapsulated metal hydride compact[21], and compacting metal-hydride powder with an expandedgraphite [22]. The integration of the fins represents an importantsolution to improve the absorption and the desorption processes.

Veerraju and Ram Gopal [23] have studied the heat and masstransfer studies on plate fin-and-elliptical tube type metal-hydridereactors. These studies shows that the use of elliptical hydride tubesin place of circular tubes can offer benefits in terms of compactnessand fan power consumption with negligible drop in average heattransfer rate. Marc Linder et al. [26] studied the metal hydrideLmNi4.91Sn0.15 and the applied reaction bed is based on a capillarytube bundle heat exchanger with a large heat transfer surface.Based on experimental investigations of the hydrogen distributionin the capillary tube bundle reaction bed a limitation of the reactionbed dynamics due to insufficient hydrogen transport can beexcluded.

Wanget al. [27] studiednumerically three-dimensionalmodelingof hydrogen sorption in metal-hydride hydrogen storage beds,results indicate that at the very initial absorption stage the bed

temperature evolves almost uniformly, while it varies greatlyspatially at the latter stage. By analyzing the Peclet number, he foundthat the heat convection by the hydrogen flow may play an impor-tant role in local heat transfer. Mellouli et al. [28] developed a two-dimensional mathematical model to optimized heat and masstransfer in metal-hydride storage tanks for fuel cell vehicles, equip-ped with finned spiral tube heat exchangers. Moreover a novelcooling design option is investigated by introducing a heatexchanger consists of two layers of spirallyfinned tubes.Mohan et al.[29] studied the performance simulation of metal-hydride hydrogenstorage devicewith embeddedfilters andheat exchanger tubes. Heatexchanger tube diameters are observed to have marginal influenceon hydriding time at higher values. Design of hydrogen storagedevice with embedded filters and cooling tubes demands a suitable

Fig. 2. MHV based on spiral heat exchanger with helically crimped.

H. Dhaou et al. / International Journal of Thermal Sciences 50 (2011) 2536e25422538

method for the calculation of above geometric parameters satisfyingthe prescribed performance and systemweight.

Constantinos et al. [30] explored the optimal design of multi-tubular MHV and how such a configuration improves the overallthermal efficiency of the metal-hydride storage vehicle MHSV. Thedeveloped model provides significant insight into the problem ofminimizing the storage time by manipulating the parameters thataffect the thermal efficiency of the reactor. Optimization resultsindicate that significant improvements on the total storage timecan be achieved under a safe and economically attractive operation.

Bhouri et al. [31] developed scoping and numerical modelsdescribing phenomena occurring during the loading process in analanate storage system having the configuration of a cylindricalshell, tube and fin heat exchanger. The numerical tool is used toevaluate the influence of varying the fin thickness and the numberof heat exchanger tubes on both the loading and dischargingprocesses. However, for the discharge case, a quicker release rate isobtained since the increase of the fin thickness results in a moreuniform distribution of hydride bed temperature and, thus, a moreuniform distribution of consumed and formed species.

In this work, heat transfer characteristics of a MHV based onspiral heat exchangers with and without fins were investigated.

Experimental results, for the two cases, are compared. In addition,the effect of flow rate and temperature of cooling fluid on thesorption process was studied.

2. Experimental set-up

The schematic of the experimental set-up designed to study theheat andmass transfer characteristics of theMHV is shown in Fig. 1. Itconsists of a reactor, a thermostat bath, a hydrogen tank, a vacuumpump, a water pump and measurement systems. One end of thereactor is connected to the hydrogen tank and the other to thehydrogen receiver through valves and stainless tubes. Moreover weused a filter with 25 mm porosity to avoid the flowing of powderedparticles. During absorption, hydrogen is supplied from the hydrogentank; hydrogen gas released during desorption is collected in thehydrogen receiver. Cold and hot water are used as working fluids inshell side and tube side, respectively. The experiments are done at thecold and hot watermass flow rates ranging between 7 and 13 g/s. Theinlet temperatures of cold and hot water are between 20 and 30 �C,and between 40 and 50 �C, respectively. The cold/hot water enteringthe heat exchanger at the outer channel flows across the helical tubeandflowsout at the inner channel. The temperatureof thehydridebed

Fig. 3. Effect of the copper fins on the absorption process.

H. Dhaou et al. / International Journal of Thermal Sciences 50 (2011) 2536e2542 2539

is measured by three thermocouples which are placed at differentpositions ((r¼ 1 cm, z¼ 2.5 cm), (r¼ 3 cm, z¼ 1 cm) and (r¼ 3 cm,z¼ 2 cm)). Hydrogen gas pressure in the gas tank and receiver aremeasured by a pressure sensor.

The geometrical characteristics of the entire reactor (Reactor,spiral tube and fins) are depicted on Table 1. MHV is manufacturedwith stainless 316-L; it is composed of two parts, namely:

- A cylindrical body: internal diameter¼ 80 mm, externaldiameter¼ 110 mm, interior height¼ 110 mm, thickness of thebottom¼ 15 mm.

- A lid: diameter¼ 180 mm, thickness¼ 10 mm- Set-up mainly consists of a hydride reactor, three referencecylinders interconnected via valves allowing the variation ofthe reference volume (11,400 cm3), a vacuum pump to evac-uate the reactor and the reference cylinders before, absorptionand desorption experiments respectively, a thermostatic waterbath (0e100 �C), a water pump, a data acquisition system anda tank of 99.99% pure hydrogen gas. The hydriding and dehy-driding processes are monitored with temperature and pres-sure measurement via three thermocouples positioned at threelocations within the hydride bed and with a pressure sensorinstalled between the reference cylinders and the hydridereactor. The temperature and the pressure readings are recor-ded on a computer for further processing and interpretation.

The heat exchanger (Fig. 2a) consists of a steel shell anda spirally coiled tube unit. The spiral-coil unit consists of one layer

of spirally coiled tubes with a 50 mm diameter. Each tube is madeby bending a 6 mmdiameter straight stainless tube into a spiral coilof 11 turns. Fig. 2a shows an alternate arrangement for securinga copper fins to a spiral tube wall. The tubewall exterior surface hasa spaced apart spiraled groove formed into a spiral coil. A fin isspiraled onto the spiral tube wall so that base edge is receivedwithin the space between the spirally coiled tubes. Water is used asa working fluid. The chilled water entering the outermost turnflows along the spirally coiled tube and flows out at the innermostturn. The hydride enters the heat exchanger at the center of theshell and surrounds radially across spiral tubes with fins to theperiphery (Fig. 2b).

3. Experimental procedure

The reactor is filled with 1 kg of LaNi5 alloy in granular form. Thebed is activated 26 times by a repeated charging (at 10 bar and20 �C) with high purity hydrogen gas (purity¼ 99.999%) and dis-charging (at 0.1 bar, 40 �C). This reduces the alloy granules into finepowder particles. After activation, absorption and desorptionstudies are carried out. Before the starting of absorption/desorptionprocess, the valves between the hydride reactor on the one handand the hydrogen tank and receiver on the other are kept closed.The bed is brought to the required temperature by pumping theheating/cooling fluid (water) from the thermostat bath through thereactor. When the bed reaches the steady state, the absorption/desorption process is started by opening the valve between thereactor and the hydrogen reservoir/receiver. The amount of the

Pre

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e (b

ar)

Con

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rati

on (

H/M

)

Wat

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utle

t te

mpe

ratu

re (

°C)

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)s(emiT)s(emiT

T = 10 °C

T = 20 °C

T = 30 °C

T = 10 °C

T = 20 °C

T = 30 °C

T = 10 °C

T = 20 °C

T = 30 °C

0 1000 2000 3000 4000 5000

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T= 30°C T= 20°C

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Ave

rage

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tem

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ture

(°C

)

Time (s)

0 1000 2000 3000 4000

10

20

30

40

50a b

c d

Fig. 4. Effect of the water temperature on the absorption process.

Fig. 5. Effect of the water flow on the absorption process.

H. Dhaou et al. / International Journal of Thermal Sciences 50 (2011) 2536e25422540

H. Dhaou et al. / International Journal of Thermal Sciences 50 (2011) 2536e2542 2541

hydrogen absorbed is obtained by measuring the temperature ofthe hydrogen gas and the pressure drop in the hydrogen tank.During desorption, the released hydrogen is collected in thehydrogen receiver, and the amount of hydrogen desorbed isobtained bymeasuring the temperature and the pressure rise in thehydrogen receiver. Water from the thermostat bath is pumpedthrough the reactor so as to remove the heat generated duringabsorption and to supply the heat required during desorption.

4. Experimental results

To demonstrate the effect of the copper fins on the absorptionprocess, we have presented the time evolution of the pressure inthe hydrogen reservoir, average bed temperature and the absorp-tion rate which obtained with and without copper fins (Fig. 3). Itcan be observed from Fig. 3a the effect of fins in the evolutiontemperature in the reactor, at the beginning of the absorptionprocess, there is a sudden decrease in the hydrogen reservoir.Initially the absorption rate is higher than the heat transfer rate,and as a result most of the energy released due to hydrogenabsorption is absorbed by the hydride bed itself, which leads toa sudden rise in the bed temperature as shown in Fig. 3b. FromFig. 3c we note that the time at which 75% of the amount ofhydrogen has been stored is 1000 s with fins and 2000 s withoutfins. In this extremely simplified example the filling time hasalready been reduced by a factor 2 approximately.

Fig. 4 shows (a) the pressure in the hydrogen reservoir, (b)average bed temperature, (c) water outlet temperature and (d) at

0 1000 2000 3000

0

1

2

3

4

Pres

sure

(ba

r)

Time (s)

= 9 g/s

= 7 g/s

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m.

= 13 g/s

0 1000 200

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5

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Time

Con

cent

ratio

n (H

/M)

a

c

Fig. 6. Effect of the water flow

different water temperatures. It can be observed from Fig. 4a that atthe beginning of the absorption process, there is a sudden decreaseof the pressure. Initially the absorption rate is higher than the heattransfer rate, and as a result most of the energy released due tohydrogen absorption is absorbed by the hydride bed itself, whichleads to a sudden rise in the bed temperature as shown in Fig. 4b.However, the bed gradually gets cooling as the energy from thecold-transfer fluid reaches the bed. As a result of this, the equilib-rium pressure rises initially and then drops. Similarly, the wateroutlet temperature change rises initially and then falls gradually asshown in Fig. 4c. It can also be seen that as the water temperaturedecreases, the average temperature of the bed decreases, hence theabsorption reaction rate increases as shown in Fig. 4d.

The water flow plays a significant role during both absorptionand desorption of hydrogen. Fig. 5 shows (a) the pressure in thehydrogen reservoir, (b) average bed temperature, (c) water outlettemperature and (d) absorption reaction rates for different waterflows.

From Fig. 5a we note that a higher value of water flow causesa sudden decrease of the pressure. Fig. 5b shows the variation of thetemperature within the reactor. It is observed that for a weak waterflow, the temperature reaches a peak at the beginning anddecreases gradually toward the initial cooling temperature at theend of the absorption process.

As depicted in Fig. 5c, the maximal value of the water outlettemperature is significantly sensitive to the water flow rate. Fig. 5dproves that the water flow does not have any effect on the storagecapacity of hydrogen. For a water flow of 13 g/s, the absorption rate

m.

m.

m.

m.

m.

m.

0 3000 4000

Ave

rage

bed

tem

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(s)

= 7 g/s

= 9 g/s

= 13 g/s

0 1000 2000 3000 4000

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35

40

= 7 g/s

= 9 g/s

= 13 g/s

Time (s)

b

on the desorption process.

H. Dhaou et al. / International Journal of Thermal Sciences 50 (2011) 2536e25422542

reaches the maximum concentration after 500 s, while for theother two cases it takes nearly about 1000 and 1500 s, respectively,revealing that an increase in the water flow (overall heat transfercoefficient) reduces the absorbing time significantly.

Fig. 6 shows the performance of the hydride bed duringdesorption at different water flow rates. Similarly to absorption, itcan be observed from Fig. 6a that as soon as the valve is opened,there is a gradually rise in the bed pressure due to the desorbedmass of hydrogen. However, due to the very low effective thermalconductivity of the hydride bed, the heat transfer rate betweenwater and the bed is not fast enough to match this reaction rate. Asa result of this, most of the energy required for the reaction issupplied by the sensible heat of the bed, which leads to a suddendrop in the bed temperature as shown in Fig. 6b it can also beobserved that as the flow water is increased, the average bedtemperature increases slightly. Fig. 6c proves that the water flowdoes not have a considerable effect on the rate desorption ofhydrogen.

5. Conclusion

In this study, a spiral-type heat exchanger with fins was insertedin a MHV and an experimental set-up was designed to study theeffect of fins on charge/discharge times. The experimental resultsshow that the absorption/desorption times of the MHV areconsiderably reduced, due to the integration of fins. In addition, theeffect of different parameters (flow mass and temperature of thecooling/heating fluid) has been discussed and obtained resultsshow that a good choice of these parameters is important for thecontrol of the charge/discharge times.

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