Hydrogen storage employing Mg-10 wt% Ni alloy

Int. J. Hydrogen Energy, Vol. 19, No. 7, pp. 605-609, 1994 Copyright © 1994 International Association for Hydrogen Energy Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-3199/94 $7.00 + 0.00

Pergamon

H Y D R O G E N S T O R A G E E M P L O Y I N G Mg-10 wt % Ni ALLOY* J. C. BOLCICH, A. A. YAWNY, H. L. CORSO, H. A. PERETTI and C. O. AYALA Comisi6n Nacional de Energia At6mica, Centro At6mico Bariloche e Instituto Balseiro,? 8400 Bariloche, Argentina

(Received for publication 12 July 1993) Abstract Two prototypes were developed for different capacities and kinetics of hydrogen storage as hydrides: single tube (ST) and multitube (MT) models. The ST can absorb up to 0.3 Nm3H2. It is based on a unique tube design employing an external helicoidal fin. Air is used for head exchange. The MT model can absorb 1 Nm3H2 and is composed of an array of 37 tubes interconnected in parallel. In this case, oil is used for heat exchange. For both models, the hydrogen absorption-desorption performances are presented using the high-temperature hydride former Mg 10wt% Ni (260-300°C). The utilization of a room-temperature hydride former, like rare-earth (LaNis)- and Ti-Zr-based alloys (Laves phases), is also possible. We conclude that the ST model resulted in better kinetics for hydrogen absorption-desorption, lower cost, and more reliable operation than the MT model.

1. I N T R O D U C T I O N Electric energy occupies almost the whole scenario of secondary energy carriers. However, it has an intrinsic limitation to satisfy the storage necessity for many applications in connection with continuous and noncontinuous primary energy sources, either traditional or renewable, like solar or wind energies. Summarizing a full list of qualifying criteria, hydrogen not only offers the possibility of easy production and transportation, but also the capacity to be stored in large quantities. The storage techniques comprise both the gaseous and liquid states. Large volumes and high pressures are needed in the former case, and very low temperatures ( - 253°C) are required in the latter. Other methods concern the formation of a liquid hydride (ammonia, methanol, etc.) [1], or a metal hydride. In the last case, a high density of hydrogen can be achieved, exceeding that of liquid hydrogen [2] (see Table 1).

(a) Amount of hydrogen absorbed per metal unit (storing capacity). (b) Low charge/discharge hysteresis. (c) Kinetics of the metal-hydrogen reaction. (d) Low heat of hydride formation AQ. (e) Heat transfer. (f) Conditions of alloy fabrication and its transformation to powder. (g) Metallurgical and physicochemical stability (composition, phases, particle size) upon manipulation and

1.1. Hydride-forming alloys

Several companies and countries have already developed hydrogen storage units of different capacities, ranging from small fractions to several thousands of Nm3H2 [3-7]. Similar types of base materials as those described in Table 2 were used. Initial applications refer to hydrogen purification I-4, 6], transportation [8, 9], some domestic uses [8, 10] and heat engines based on hydrides [7, 8, 11]. One of the main motivations for this work is the development of wind energy in isolated regions of Argentina, especially in Patagonia, where average wind speed and persistence are very high [12, 13]. Furthermore, distances between small populated areas are very large.

Several alloys can form hydrides in the range of low pressures (1-10 atm) and temperatures in the range 20-300°C. Additional parameters must be taken into account in selecting the proper alloy according to given requirements. Some of them are:

* Work presented at the 9th World Hydrogen Energy Conference, Paris, France, 22-25 June 1992. f Comisirn Nacional de Energia At6mica and Universidad de Cuyo.

use.

(h) Degradation upon absorption-desorption cycles.

(i) Costs. We have produced Mg-10Wt~o Ni alloys, Ti-Zr-based alloys of the AB 2 type of intermetallic compounds, and LaNi 5 alloys, at a laboratory scale (see Table 2).

1.2. Hydrogen storage units

605

606

J.C. BOLCICH et al. Table 1. Hydrogen content and energy density of different storage media Media

Hydrogen content (wt%)

Volume density* (10ZSHatoms l - l )

100.00 100.00 11.76 17.76 7.65 2.10 3.60 1.95 1.5 1.75 1.38

0.5 4.2 5.9 8.7 6.7 11.4 5.9 5.5 7.6 6.0 4.8

H 2 gas (150 atm) Liquid H 2 (20K)

Methanol (20°C) Ammonia ( - 79°C) MgH 2 VH 2

Mg2NiH 4 TiFeH1.95 LaNisH6. 7 ZrMn2H3. 6 ZrMnZ2Feo.sH3. 4

Energy densityt (MJkg 1) (MJ 1-1 ) 141.90 141.90 16.70 25.20 9.92

1.02 9.92 13.21 20.58 14.32

4.48 2.47

11.49 13.56

*Includes neither weight of container nor dead volumes. tReferred to hydrogen equivalent values in metallic hydrides.

Table 2. Hydride forming alloys under study at CAB Alloy and melting technique

LaNis*

Powder fabrication

Absorption desorption conditions

Hydrogen activation

P (atm)

T(°C)

2.1 4.7 8.5

30 50 67

1.3 2.8 4.9

292 318 343

Difficult High P, T

4.5

8

Easy/in situ

--

Hammer milling

Easy/in situ

TiNi*

Attrition

Mg-10~o Nit

Chips + hammer milling Attrition

High temperature Vacuum 300°C/30 atm cycling

TiFe* TiZrCrNi* ZrTiFeMn* ZrTiFeCr*

"

Hf (kcal mol- 1)

-

7.7 + 0.1

I

---

-

17.7 _+ 0 . 1

---

*Electric arc furnace/copper crucible/high-purity argon atmosphere/nonconsumable tungsten electrode. tInduction melting with agitation.

2. E X P E R I M E N T A L Two different prototypes were constructed: single tube (ST) and multitube (MT) models. M T is designed for 1 m a and ST for 1/3 m 3 H 2 absorption. Both prototypes were loaded with powdered Mg-10wt~o Ni alloy. Details of the alloy preparation are given elsewhere [3] (Table 2). The initial ingot, melted in an induction furnace with mechanical stirring, was reduced to fine turnings and subsequently fragmented in a hammer mill. The resulting particles were within 100-200 /tm, as revealed by SEM. Absorption-desorption measurements were made using the standard volumetric method [3].

2.1. S T m o d e l

The ST model was made of a single SS304 tube of 60 mm internal diameter, 500 mm length and 2 mm wall width, with helicoidal fins externally welded (see Fig. la). F o r heating up to the hydride formation-decomposition temperature ( ~ - 3 0 0 ° C in this case), an external removable electric furnace is used. It can be pulled in or out as needed during desorbing or charging with hydrogen, respectively. Only natural convection of air is used here, but, if necessary, there exists the possibility of using forced air convection, with minor changes in the heating system. The inlet-outlet of hydrogen in each prototype goes

HYDROGEN STORAGE EMPLOYING Mg-10 wt% Ni ALLOY

Hydrogen Vatve "~ £

Manometer

-~..

~Fitter

~ e ~ T e m perature q I "~ Indicator thermocouple -

-

Tube

/,l~

exchange

•

Furnace

Fig. l a. Single tube (ST) storage unit prototype.

I Hydrogen VatveFitter

-

~

The ball valve, of 0.5 in. useful diameter, is made of SS material with teflon settlings. In the ST model, the head is interconnected to the storage container by a 20 mm internal diameter SS tube that keeps it about 200 mm away from the hot container body. Temperature is measured by a chromel-alumel thermocouple immersed into the hydrides through a SS can. The whole system is designed to support internal pressures up to 50 atm, necessary for the activation process (i.e. initial hydrogen charging of the alloy). Even when normal use requires lower pressures, this feature allows for carrying out the activation in situ, avoiding manipulation of the activated powder.

Heat

~

Hydrde

607

, Manometer

Temperature Indicator

r =zlr Tubes

pie

Heater

Thermal Oil Fig. lb. Multitube (MT) storage unit prototype. through a metal head, which is provided with a metallic filter, a Bourdon-type manometer, a ball valve and a flange for connection with the gaseous hydrogen line. The filter, with the purpose of avoiding small hydride particles escaping from the storage container, is made of sintered copper particles with an average size of 100 #m.

2.2 M T model

This prototype is composed by an array of 37 interconnected SS316 tubes in parallel, 500 mm in length, 25 mm external diameter and 1 mm wall width each. They are positioned by two grids located at each end (see Fig. lb). Inside these tubes, the hydride forming alloy is lodged. Each tube is connected to a collector chamber through smaller distribution tubes. This chamber is connected to the hydrogen line through a similar metal head as described for the ST. The array of tubes is immersed in a thermalizing oil bath containing an electric heater and shielded chromel-alumel thermocouple. This MT design was conceived with the aim of improving the overall kinetics by an increase of the heat transfer surface. In the MT prototype, the activation of the powder was done in a separate reactor because of structural limitations in the working pressure ( < 10 atm).

3. RESULTS AND DISCUSSION The following comments concern the fabrication of the alloys and their metallurgical characterization: Mg-10wt% Ni has the advantage of easy fabrication process [3] and high hydrogen storage capacity. However, it presents some drawbacks: difficulty of activation (P -~ 30 atm) and high temperatures required for its use (300°C). AB 2 intermetallic compounds (A = Ti, Zr and B = Mn, Fe, Cr, Ni) form Laves phases of the type C14 and C15. They are most advantageous regarding working conditions near room temperature and atmospheric pressure, as well as low AQ. However, our experience indicates a tendency to segregation during solidification, difficult to avoid in a multicomponent alloy. Also, the progressive refinement of particle size after each cycle of absorption-desorption produces a submicron size powder. This may lead to several problems related to material expansion and possible arrival of powder particles in the gas line through the filter barrier. Some alloys show pyrophoric behaviour. In this case, the intrinsic safety conditions for hydrogen storage are not fulfilled.

608 •

J. C. BOLCICH

LaNi s alloy seems to present a better combination of properties, except for its higher cost. The same would be applicable to other rare-earth-based alloys [4, 14].

After the activation of the Mg-10Wt~o Ni alloy powder for the MT model, carried out in a separate installation as stated, absorption characteristics were measured in the same reactor at different temperatures, from an initial gauge pressure of 20 atm (Fig. 2). From this result, a maximum absorption rate is observed in the range of 270-274°C. At lower temperatures, 260-270°C, the kinetics are slightly decreased. Diminishing the temperature to 24(~249°C, the kinetics slow down and when the range 222 236°C is achieved, practically no hydrogen-metal reaction is observed. On the other hand, when the temperature is increased above 274°C, a lower reaction rate was measured (see Fig. 2). From these results it is concluded that the optimum working temperature range for absorption is located around 270 274°C. Equilibrium pressure is reached at that temperature in less than 10 min. The hydrogen pressure evolution during the first stages of activation of the ST model and during absorption steps of the MT model are presented in Fig. 3.

20 t

i .

.

.

_

_

i

i

i

I

i

I

.

-

x

=

i

i

i

I

===== 270-274 c===-': 2 6 0 - 2 7 0 ::::: 300-313 ::-:-':--':--':- 2 4 0 - 2 4 9 =-'-'-'-" 222-236

=

&

"C =C °C *C °C

tlJ

~1o O3 IZ Q.

o

0

5

10 TIME (rain)

15

et al.

The absorption behaviour of the MT appears much slower than expected according to the material behaviour measured in the activation reactor (Fig. 2). This difference could be explained partly by material deterioration due to handling and transference of the activated powder from the activation reactor to the MT assembly. Another possible reason is the very high impedance of the small distribution tubes (2 mm internal diameter, with radial holes less than 0.2-0.3 mm in diameter), which are immersed in each container tube in its whole length. The ST activation curve shows that an incubation time of about 50 h at 30 atm and 300°C is necessary for the initiation of hydrogen absorption, after which an increasing reaction rate is achieved for the initial hydrogen charge. Subsequent activation steps from 30 atm show a decreasing rate which could be related to progressing hydride phase transformation. Figure 4 shows the pressure and temperature evolution during absorption in the ST model starting at 270°C. The curves correspond to consecutive absorption desorption cycles just after activation. In one case, the absorption stage requires about 1 h to reach saturation, while in the other, only 20 min are needed. In the last case, the removable furnace was not only switched off, but also taken off, allowing for better heat dissipation. In both experiments, the temperature jumped from 270°C to 400°C in a few seconds after the beginning of the hydriding reaction. In Fig. 5 we present desorption rates (against atmospheric pressure) as a function of desorbed gas volume, corresponding to the ST and MT prototypes at 300°C. It is clearly noted that the ST model has a much better kinetic response than the MT model. The crossing at 2801 is due to the lower hydrogen capacity of the ST model. Owing to structural and heat transfer design characteristics, both prototypes can also work with roomtemperature hydride formers, like rare-earth (LaNis)and Ti-Zr-based alloys (Laves phases). These alloys are

Fig. 2 Hydrogen absorption of Mg-10wtg/o Ni at different temperatures. 400 '

'

'

'

I

,

,

,

.

I

,

.

,

,

I

,

,

,

,

n" 3 0 0 ,_.30 E ~6

la.l

~- 2 0 0

0

30

60

90

~ 20 E

,

~

i

,

,

i

~I0 ~-" 0

I

0

I

I

I

l

1 O0

I

I

I

i

I

t

,

200 TIME (Hours)

t

,

I

300

i

s

,

.~

.7.

g.

d.

,

400

Fig. 3 Evolution of pressure with time. (©) First stages of actiwttion of ST at 300°C; (A) first stages of absorption of MT at 270°C.

14 0

I 60

30

,

"~,

go

TIME (min)

Fig. 4 Evolution of pressure and temperature in ST during absorption. (&) First cycle; (O) second cycle.

HYDROGEN STORAGE EMPLOYING Mg 10 wt% Ni ALLOY

609

present work, in particular to Pedro Bavdaz, Raul Stuke, Fritz Tutzauer and Carlos Eggenschwiler who collaborated in the machining and assembling of different parts of the prototypes. We especially thank Dr J. P. Abriata for reading the manuscript.

7, 3

Z O I.-

2

REFERENCES

o_1 t~ O tag

~o

0

1 O0

200

300

400

DESORBED VOLUME (liters)

Fig. 5 Desorption behavior of ST (solid line) and MT (dashed line) prototypes at 300°C. currently under study and pilot plant fabrication processes in our laboratory. The storage facilities can either be used for general safe hydrogen accumulation and for hydrogen purification. CONCLUSIONS M g - 1 0 w t % Ni alloy powder was elaborated at pilot plant scale to be used as a hydrogen storage material. Two hydrogen storage prototypes, a single tube model (ST) and a multitube model (MT), of 0.3 and 1 Nm3H2 storage capacity respectively, were designed, constructed and characterized. The ST resulted more advantageous in fabrication and performance. The ST model constitutes a simple, efficient and lowcost hydrogen storage unit. It is adequate for taking advantage of wind energy in the Patagonia region of Argentina.

Acknowledgements-The authors fully acknowledge all those who contributed in one way or another to the concretion of the

1. C. E. Bamberger and J. Braunstein, Hidr6geno, un elemento vers~itil. Energia Nuclear, Espa~a 19, 94 (1975). 2. T. N. Veziro~lu, Hydrogen technology for energy needs of human settlements. Int. J. Hydrogen Energy 12, 99 (1987). 3. G. M. Friedlmeier and J. C. Bolcich, Production and characterization of Mg-10wt~o Ni alloys for hydrogen storage. Int. J. Hydrogen Energy 13, 8 (1988). 4. S. Suda, Metal hydrides. Int. Z Hydrogen Energy 12, 323 (1987). 5. LIEL, Colorado Springs, U.S. Pat. No. 4.600.525. (1986). 6. O. Bernauer, Development of hydrogen hydride technology in the F.R.G. Int. J. Hydrogen Energy 14, 727 (1989). 7. O. Bernauer, Metal hydride technology. Int. J. Hydrogen Energy 13, 181 (1988). 8. H. Butchner, in A. F. Andresen and A. J. Maeland (Eds) The Hydrogen Hydride Energy Concept; Hydrides for Energy Storage, Proc. Int. Symp., Geilo (1977). 9. H. Buchner, Hydrogen use--transportation fuel, Int. J. Hydrogen Energy 9, 501 (1984). 10. R. E. Billings, Hydrogen homestead, in T. N. Veziro~lu and W. Seifritz (Eds) Hydrogen Energy System Vol. 4, pp. 1707-1730. Pergamon Press, Oxford (1979). 11. M. Wierse, R. Werner and M. Groll, Magnesium-hydride for thermal energy storage in a small-scale solar thermal power station. Int. Syrup. on Metal-Hydrogen Systems, Alberta, Canada (1990). 12. C. M. Marschoff, Niches for hydrogen energy applications in Arentina. Proc. 7th Worm Hydrogen Energy Conf., Moscow, Vol. 1, p. 1 (1988). 13. V. R. Barros, Atlas del Potencial E61ioc del Sur Argentino. CREE (Centro Regional Energla E61ica), Chubut (1986). 14. A. Yawny, G. Fridelmeier and J. C. Bolcich, Hydrides for hydrogen deuterium separation. Int. J. Hydrogen Energy 14, 587 (1989).