EP3152475A1

EP3152475A1 - Metal hydride bed, metal hydride container, and method for the making thereof

Info

Publication number: EP3152475A1
Application number: EP15806940.1A
Authority: EP
Inventors: Mykhaylo Lototskyy; Moegamat Wafeeq DAVIDS; Bruno G. POLLET; Vladimir Linkov; Yevgeniy KLOCHKO
Original assignee: University of the Western Cape
Current assignee: University of the Western Cape
Priority date: 2014-06-09
Filing date: 2015-06-08
Publication date: 2017-04-12
Also published as: WO2015189758A1; EP3152475A4; CN106687737B; CN106687737A

Abstract

A metal hydride which is disposed in a metal hydride container which includes a mixture of powdered hydride forming material which is able to form hydride when interacting with gaseous hydrogen at pressure higher than the atmospheric pressure at the ambient temperature, and a binder which combines high thermal conductivity, plasticity and high porosity under hydrostatic pressure; and a plurality of heat conductive fins which are disposed in the inner space of a gas-tight containment of the metal hydride container filled with the mixture and have a firm thermal contact with the inner surface of the said space.

Description

METAL HYDRIDE BED, METAL HYDRIDE CONTAINER, AND METHOD

FOR THE MAKING THEREOF

FIELD OF INVENTION

The present invention relates to hydrogen storage, supply and compression systems which use metal hydride (MH) materials.

More particularly, the invention relates to composition, layout and method of the forming of the content of a metal hydride container, or a metal hydride bed, as well as method of making the metal hydride container.

BACKGROUND TO INVENTION

Metal hydrides provide efficient hydrogen storage for various applications, when system weight is not a critical issue. Due to very high volumetric density of atomic hydrogen accommodated in the crystal structure of the MH metal matrix, hydrogen storage in MH is very compact. At ambient temperatures the equilibrium of the reversible interaction of MH material with hydrogen gas can often take place at modest, <1 -10 bar hydrogen pressures. Thus, hydrogen storage using MH is intrinsically safe and benefits from avoiding use of potentially unsafe compressed hydrogen gas and energy inefficient liquid H2. Endothermic dehydrogenation decreases temperature of the MH leading to decreased rates of hydrogen evolution. This, in turn, is an intrinsic safety feature of use of the MH, allowing to avoid accidents even in case of rupture of the hydrogen storage containment. The use of MH also allows to implement simple, efficient and safe technology of thermally-driven hydrogen compression characterised by absence of moving parts and possibility to use waste heat instead of electricity for the hydrogen compression.

At the same time, slow charge / discharge limited by heat transfer is a serious drawback of the MH hydrogen storage, supply and compression systems requiring special engineering solutions to overcome.

A typical improvement of heat exchange between heating / cooling fluid and metal hydrides in medium-to-large scale hydrogen storage tanks (> 1 Nm³ H2 / > 20 kg MH) was presented by 0. Ulleberg, et al¹. Hydrogen supply is facilitated by the usage of ABs-type MH material (Lao.ssCeo.isNis) characterised by H2 equilibrium pressure at room temperature higher than the required H2 supply pressure (~1 bar). Intensification of heat transfer in the MH is achieved by passing heating / cooling fluid (water) through internal heat exchanger additionally equipped with heat conductive fins disposed within MH powder. This solution was shown to be efficient for both hydrogen storage¹ and thermally driven hydrogen compression². However, the introduction of the internal heat exchanger complicates layout of the containment and significantly increases its costs. In addition, it reduces hydrogen storage capacity of metal hydride tank due to additional weight and volume of the heat exchanger.

' 0. Ulleberg, M. Lototskyy, B. Ntsendwana, Ye. Klochko, J. Ren. Metal Hydride Hydrogen Storage Units for LT PEMFC Power Systems. In: 18th World Hydrogen Energy Conference (WHEC 2010); May 16-21, 2010, Essen/ Germany, Parallel Sessions Book 4, p. 101 - 108 ² M. Lototskyy, Ye. Klochko, V.Linkov, P.Lawrie, B.G.Pollet. Thermally Driven Metal Hydride Hydrogen Compressor for Medium-Scale Applications. Energy Procedia 29 (2012) 347-356 Typical solutions to the problem mainly associated with external heating and cooling of the containment were presented in a patent by Lototskyy and Linkov³ and references therein. The hydride container according to these solutions comprises a high-pressure containment filled with the hydride material (as a rule, in a powder form) and comprising heat distributing means. The heat distributing means (preferably, a plurality of transversal heat-conductive fins), together with the hydride material, forms a metal hydride bed. Apart from the storage of hydrogen chemically bound in the MH material, the role of the metal hydride bed, which has to have high enough effective thermal conductivity, is in providing fast transfer of the heat generated / absorbed in the MH during hydrogen absorption / desorption to / from cooling / heating accessories.

The general approach to the layout and forming of MH bed presented above requires a number of additional improvements towards intensification of the heat transfer, lowering labour consumption and associated costs, avoiding significant reduction of hydrogen storage capacity, as well as increase of operational safety. The latter is closely related to the MH filling density.

It is known that the increase of the MH filling density is expected to result in the enhancement of the effective thermal conductivity⁴. On the other hand, too high filling density, exceeding 61 % of the real (crystal) density of the material in the hydrogenated state, is detrimental for the safety when the lattice expansion during hydrogenation can generate high stresses in the MH bed and, in turn,

³ M.Lototskyy, V.Linkov. Hydride container. Patent ZA 2009/02427

⁴ K.C. Smith, T.S. Fisher. Models for metal hydride particle shape, packing, and heat transfer. Int J Hydrogen Energy 37 (2012) 13417- 13428 deform or destroy the containment⁵. Furthermore, pulverization of MH in the course of hydrogen absorption / desorption cycling causes concentration and agglomeration of MH particles in the lower parts of the containment that, in turn, can result in high stresses in its bottom parts even at the lower MH filling densities⁶. That is why when MH material is taken in a powdered form, its filling density is always a compromise between good performances and safety. To provide the safe operation, the filling density is usually kept below 50% of the real density of the hydrogenated MH material.

A simple and efficient method of the forming the MH bed is in the compacting of the MH powder and a heat-conductive material, including expanded natural graphite, ENG⁷. Recompressed ENG combines two important properties, (i) high plasticity under compression loads and (ii) high thermal conductivity and gas permeability in the direction perpendicular to the direction of the compacting. Further improvement of heat transfer in MH / ENG compacts was suggested by De Rango et. al⁸ as their alternative arrangement with heat conductive fins in an MH hydrogen storage tank. As a rule, this layout is applied for quite stable MH's (e.g., MgH2) which are taken for the compacting with ENG in the hydrogenated state. For the MH materials which form hydrides with dissociation pressure higher than 1 bar at a room temperature (these materials

⁵ K. Nasako, Y. /to, N. Hiro, M. Osumi. Stress on a reaction vessel by the swelling of a hydrogen absorbing alloy. J Alloys Compels 264 (1998) 271-276

⁶ M. Okumura, K. Terui, A. Ikado, et al. Investigation of wall stress development and packing ratio distribution in the metal hydride reactor. Int J Hydrogen Energy 37 (2012) 6686-6693

⁷ A. Rodriguez Sanchez, H.P. Klein, M. Groll. Expanded graphite as heat transfer matrix in metal hydride beds. Int J Hydrogen Energy 28 (2003) 515-527

⁸ P. De Rango, A. Chaise, D. Fruchart, P. Marty, S. Miraglia. Hydrogen storage tank. Patent application US 2010/0326992 Al must be used in "hybrid" hydrogen storage systems and hydrogen compressors utilising MH), making the MH / ENG compacts is possible either from the non- hydrogenated hydride forming materials, or from their hydrides stabilised, for example, by an exposure in carbon monoxide or sulphur dioxide⁹. In the former case, disintegration of the formed compacts during hydrogenation is highly possible due to a significant volume increase of the MH filler. The latter approach requires the usage of highly toxic gases for the MH stabilisation.

It is an object of the invention to suggest a metal hydride bed and method of its forming to assist in overcoming the aforementioned problems.

SUMMARY OF INVENTION

According to the invention, a metal hydride bed disposed in a metal hydride container includes:

(a) . a mixture of a powdered hydride forming material which is adapted to form hydride when interacting with gaseous hydrogen at pressure higher than atmospheric pressure at the ambient temperature, and a binder which combines high thermal conductivity, plasticity and high porosity under hydrostatic pressure; and

(b) . a plurality of heat conductive fins which are disposed in the inner space of a gas-tight containment of the metal hydride container

⁹ S. Corre, M. Bououdina, D. Fruchart, G. Adachi. Stabilisation of high dissociation pressure hydrides of formula Lai-_xCe_xNi₅ (x=0-0.3) with carbon monoxide. J Alloys Compds 275-277 (1998) 99- 104 filled with the mixture and have a firm thermal contact with the inner surface of the said space.

The gas-tight containment may be made as a cylinder with two end caps, one of which is equipped with a hydrogen input / output pipeline.

The heat conductive fins may be perforated; they may be disposed inside the containment transversally, and uniformly distributed along the cylinder axis in the mixture of the hydride forming material with the binder.

The hydride forming material may include ABs-type intermetallide.

Alternatively, the hydride forming material may include AB2-type intermetallide and / or BCC solid solution alloy. In this case the hydride forming material may additionally comprise the ABs-type intermetallide in the amount of 10% as respect to the total weight of the hydride forming material.

The binder may be expanded natural graphite.

The ratio of weight of the hydride forming material to its total volume in the metal hydride bed may vary from 0.50 to 0.60 of the real density of the hydride forming material in the hydrogenated state.

The mixture may be taken as a loose powder where the expanded natural graphite is taken in the amount from 1 to 2% as respect to the weight of the hydride forming material. In this case the hydrogen input - output pipeline may be connected with a tubular filter longitudinally disposed in the metal hydride bed. Alternatively, the mixture may be taken as a compact where the expanded natural graphite is taken in the amount from 15 to 20% as respect to the weight of the hydride forming material. In this case the compact may have an axial hole throughout the length of the metal hydride bed, the hydrogen input - output pipeline may be equipped with an in-line gas filter, and the axial hole may additionally comprise a porous member.

The forming of the metal hydride bed when the mixture is taken as the loose powder may include the following steps:

(a) . of pressing the heat conductive fins in the gas-tight containment;

(b) . of weighting and mixing the powders of the hydride forming material and the expanded natural graphite;

(c) . of filling the containment with the powdered mixture.

Alternatively, when the mixture is taken as the compact, the forming of the metal hydride bed may include the following steps:

(a) . of weighting and mixing the powders of the hydride forming material and the expanded natural graphite;

(b) . of separating the mixture to two equal portions;

(c) . of compacting the pellets from the first portion of the mixture;

(d) . of installing the porous member on the axis of the containment; (e) . of alternate pressing the material and the heat conductive fins in the gas-tight containment in the following sequence: i. the loose powder from the second portion of the mixture; ii. the heat conductive fin; iii. the pellet; iv. the heat conductive fin;

(f) . of final compacting the metal hydride bed.

The compacting pressure for making one pellet may be from 150 to 250 MPa.

The mass of the mixture taken for the compacting of one pellet at the specified compacting pressure may be selected to provide the thickness of the pellet between 10 and 15 mm.

The loose powder from the second portion of the mixture may be divided by small portions whose number is equal to the number of the compacted pellets plus one.

After the pressing the last pellet with the fin in the containment the last portion of the loose powder may be added.

The pressure of the final compacting of the metal hydride bed in this case may be from 50 to 60 MPa.

The making of the metal hydride container may include the following steps: (a) . of installing the end cap with the hydrogen input / output pipeline;

(b) . of forming the metal hydride bed from the opposite side of the containment; and

(c) . of installing the second end cap. BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference to the accompanying drawings.

In the drawings there is shown in:

Figure 1 : Metal hydride bed according to the first embodiment of the invention;

Figure 2: Metal hydride bed according to the second embodiment of the invention;

Figure 3: Drawing of a metal hydride container with the metal hydride bed according to the first embodiment of the invention (Example 1 );

Figure 4: Drawing of a metal hydride container with the metal hydride bed according to the second embodiment of the invention (Example 2);

Figure 5: Components and tools for the forming of metal hydride bed according to the second embodiment of the invention (Example 2); and Figure 6: Pellets prepared at various compacting conditions after hydrogen absorption at P=40 bar and room temperature (Example 2).

DETAILED DESCRIPTION OF DRAWINGS

Referring to the drawings, the metal hydride container comprising the metal hydride bed in accordance to the invention is shown.

The container and the bed include the following components, as indicated by the corresponding reference numerals in Figures 1 to 5:

10 - mixture of metal hydride material with binder;

1 1 - mixture of metal hydride material with binder in the form of compacted pellets;

12 - heat conductive fins;

13 - porous member;

21 - cylindrical part of gas-tight containment;

22 - end cap of gas-tight containment equipped with hydrogen input / output pipeline;

23 - opposite end cap of gas-tight containment;

24 - hydrogen input / output pipeline;

25 - gas filter; 30 - compacting tools:

31 - matrix;

32 - die;

33 - accessories for pellet support and its pressing-out;

34 - dies for compacting the MH bed in the containment.

According to the invention, a metal hydride container comprises a metal hydride bed disposed in a gas-tight containment (21 -23) which is preferably made as a cylinder (21 ) with end caps (22, 23), one of which (22) is equipped with a hydrogen input / output pipeline (24). The metal hydride bed is formed by a mixture (10, 1 1 ) of a powdered hydride forming material and a binder.

The scope of this invention is related to metal hydride materials which form "unstable" hydrides, i.e. equilibrium of their interaction with gaseous hydrogen takes place at a pressure higher than the atmospheric pressure at the ambient temperature. The hydride forming material may include ABs-type intermetallide, for example, (La,Ce)Nis. Alternatively, the hydride forming material may include AB2-type intermetallide (e.g., (Ti,Zr)(Cr,Mn,Fe,Ni)2) and / or BCC solid solution alloy (e.g., on the basis of V). Since the materials belonging to the second group are characterised by higher sensitivity towards "poisoning" with gas impurities in hydrogen (e.g., oxygen and water vapours) and less easy activation than the ABs-type intermetallides possessing a strong catalytic effect in hydrogen transfer reaction, it is beneficial to take the materials based on the AB2-type and / or BCC alloys in form of mixture with an additive of the ABs-type alloy. The amount of the additive sufficient for the improvement of the activation performances and the "poisoning" tolerance is about 10 wt% as respect to the total weight of the hydride forming material. Further increase of the amount of the ABs-type alloy characterised by lower reversible hydrogen absorption capacity (<1.5 wt% H) than AB2-type intermetallides (1 .7-1 .9 wt% H) or BCC alloys (2-2.5 wt.% H) will result in the lowering hydrogen storage capacity of the metal hydride bed.

The mixture (10, 1 1 ) also includes a binder which combines high thermal conductivity, plasticity and high porosity under hydrostatic pressure. The best material possessing the required combination of these properties is expanded natural graphite (ENG).

The second component of the metal hydride bed is a plurality of heat conductive fins (12) which are disposed in the inner space of the containment (21 -23) filled with the mixture (10, 1 1 ) and have a firm thermal contact with the inner surface of the containment, e.g., its cylindrical part (21 ). Such a layout foresees that the cooling of the MH bed for removal the heat released during exothermic H2 absorption, and the heating of the MH bed for supply of the heat to provide endothermic H2 desorption is carried out from the external surface of the containment. This solution eliminates complication of the design of the containment by introducing inner heat exchangers known within prior art, and the optimisation of the metal hydride bed within this invention is mainly aimed at the improvement of the heat transfer inside metal hydride containers. Realisation of the present invention can be done in the course of the making the metal hydride container which has to include the steps of (a) installing the end cap with the hydrogen input / output pipeline, (b) forming the metal hydride bed to be placed in the containment from the opposite side, and (c) installing the second end cap.

The forming of the metal hydride bed according to the invention is carried out by filling the inner space of the containment (cylindrical part 21 with installed (e.g. welded) end cap 22) with the mixture (10, 1 1 ), as well as the installation of the fins 12. This procedure is carried out from the side of the opposite end cap (23) which is installed (e.g. welded) after the forming of the metal hydride bed.

The amount of metal hydride material loaded in the container according to the invention corresponds to the filling fraction from 0.50 to 0.60 of the real density of the hydride forming material in the hydrogenated state. Exceeding the upper limit is detrimental for the containment which could be deformed or broken due to expansion of the MH material during hydrogenation. Lowering the filling fraction below the lower limit will result in the decrease of hydrogen storage capacity of the metal hydride container, as well as in the lowering the effective heat conductivity of the MH bed. However, as it was discussed in the Background section, conventional solutions use the lower filling fractions to avoid local compaction of the MH material above the upper safety limit in the bottom part of the containment. Usage of the additive of ENG within this invention allows to mitigate this problem. Figure 1 shows the metal hydride bed according to the first embodiment of the invention when the mixture of the metal hydride material and the binder is taken as a loose powder. This embodiment is the easiest in the implementation and allows for high productivity and low cost even for big size metal hydride containers.

The powder of the mixture of the metal hydride material with expanded natural graphite (10) is loaded in a gas-tight containment consisting of cylindrical part (21 ) and end caps (22, 23). Before the loading, heat conductive fins (12) are uniformly installed (pressed) inside the containment to provide firm thermal contact with its inner surface. Such arrangement provides uniform temperature distribution within metal hydride bed during exothermic hydrogen charge and endothermic hydrogen discharge processes. In so doing, the rate of the corresponding heat removal and heat supply to / from MH will be limited by the rate of external cooling / heating the containment rather than by the internal thermal resistance of its content.

The end cap 22 is equipped with a hydrogen input / output pipeline (24) connected from its opposite side to a tubular gas filter (25) providing uniform distribution of hydrogen in the metal hydride bed and protecting gas manifolds connected to the pipeline 24 from their contamination with fine powder of the mixture (10).

The fins 12 may be perforated as shown in side view A. The central hole in the fin provides the axial filter 25 to penetrate throughout the length of the metal hydride bed while the periphery holes facilitate the loading of the powdered mixture 10 in the containment. The loading is carried out from the side opposite to the end cap 22 carrying the assembly of the hydrogen input - output pipeline 24 and filter 25. After the loading, the end cap 23 is installed (e.g. welded).

Despite the amount of the ENG additive within this embodiment is quite low, 1 to 2 wt.%, due to very low density (below 0.1 g/cm³) its volume fraction in the loose powder mixture will be quite high, approximately, 30 to 60%. Thus, if the mixing is good enough, the voids between the MH particles in the mixture will be filled with ENG. During the hydrogenation, the MH particles increase their volume, and stresses compacting the mixture appear. At the presence of easily compressible ENG the stresses will be absorbed by it thus lowering stresses in the wall of the containment. In addition, a network of the recompressed ENG is formed, and effective thermal conductivity of the MH bed increases.

Further improvement of the heat transfer performances of the MH bed can be achieved in the second embodiment of the invention when the mixture (10, 1 1 ) comprising higher amount of the ENG binder is taken as a compact. The second embodiment is schematically shown in Figure 2. The mixture is separated to two equal portions first of which is used for making compacted pellets (1 1 ) and the second portion is loaded in the containment as a loose powder (10). The containment is filled in an alternate manner in the sequence: (i) a portion of loose powder 10; (ii) a fin 12; (iii) a pellet 1 1 ; (iv) a fin 12. The outer diameter of the pellet should allow its firm contact with the inner surface of the cylindrical part of the containment 21 . In addition, the pellet should have an axial hole to allow hydrogen to flow along the whole metal hydride bed. To avoid blocking of the hydrogen flow by the mixture loaded in the form of loose powder, a porous member (13) is axially installed in the inner part of the containment (21 , 22) before its filling. The member 13 may be a porous pipe plugged from both ends or just a porous rod. To avoid contamination of gas manifolds with remaining powder of the MH material, an in-line filter 25 is installed in the hydrogen input / output pipeline 24.

The filling is carried out from the side of end cap 23 before its installation, by the loading of the loose powder followed by pressing-in the pellet and two fins. After the filling, the metal hydride bed in the containment (21 , 22) is finally compacted followed by the installation of the second end cap 23.

To avoid disintegration of the compacted mixture (10, 1 1 ) during cyclic hydrogen absorption / desorption, the content of the binder (ENG) should be not lower than 15 wt%, compacting pressure for making one pellet 1 1 should be not lower than 150 MPa, and the pressure of the post-compacting of the metal hydride bed should be not lower than 50 MPa. For the same reason, the mass of the mixture taken for the compacting of one pellet at the compacting pressure should be selected to provide the thickness of the pellet not higher than 15 mm. In doing so, the mass of the portion of the loose powder 10 loaded in between the pellets 1 1 should be approximately equal to the mass of the pellet. Finally, the first and the last portions of the mixture loaded in the containment should be taken in the form of the loose powder 10. When these conditions are met, the compacted metal hydride bed has a good uniformity and remains stable during cyclic hydrogen absorption / desorption. Further decrease of the portion of loose powder 10, or mass of one pellet 1 1 corresponding to the thickness of the pellet below 10 mm at the compacting conditions is related to too high labour efforts for the making the metal hydride bed and does not makes sense from the economic point of view.

The increase of amount of the ENG above 20 wt.%, compacting pressure for making one pellet above 250 MPa, as well as the post-compacting pressure above 60 MPa will result in the decrease of porosity of the mixture (10, 1 1 ) and, in turn, slowing hydrogen charge / discharge processes down due to mass transfer limitations. In addition, the increase of the post-compacting pressure above 60 MPa can result in the damage of the gas-tight containment (21 , 22).

During hydrogen absorption, the swelling of the metal hydride material results in further compacting of the portions of the mixture 10 taken as a loose powder thus avoiding too high stresses in the wall of the containment 21 . At the same time, the compacting force generated during hydrogenation assists in the formation of the uniformly compacted metal hydride bed as a whole that provides very good effective thermal conductivity and, in turn, fast dynamics of hydrogen charge and discharge.

Example 1

Example 1 illustrates the implementation of the first embodiment of the invention. Figure 3 shows the assembly drawing of the metal hydride container as a longitudinal cross-section (A) and a fragment of a 3D X-ray view (B). The cylindrical part of the containment (21 ) is made of 715 mm long stainless steel pipe, 51 mm in outer diameter, 3.2 mm in wall thickness. The containment also comprises end caps 22 and 23 one of which (22) carries the bored-through fitting 24 with a stainless steel tubular gas filter 25, 6 mm in outer diameter and 1 μ in pore size; one end of the filter 25 is connected to hydrogen input / output pipeline (25a, 6.35 mm in an outer diameter) penetrating through the fitting 24, and the opposite end of the filter 25 is plugged. 0.5 mm thick copper fins (12) are pressed in the containment. The fins 12 are perforated (see Figure 3B), so as the central hole, 8 mm in diameter, provides insertion of the filter 25, and the periphery holes facilitate the filling of the container with the powder (10) of MH material mixed with ENG binder.

The assembling of the metal hydride container which includes the forming of metal hydride bed is carried in the following sequence:

(a) . of installing (welding) the end cap 22 from one end of the pipe 21 ;

(b) . of installing the fitting 24 and the filter 25 with hydrogen input / output pipeline 25a;

(c) . of pressing the heat conductive fins into the pipe 21 from its open end, the distance between the adjacent fins is of 5 mm;

(d) . of preparation of mixture (10) of powders of: i. 2.88 kg (90 wt.%) of AB2-type hydrogen storage alloy

Ti0.55Zr0.45Cr0.84Mn0.39Fe0.55Ni0.22, ii. 0.32 kg (10 wt. %) of ABs-type hydrogen storage alloy iii. 32 g (1 wt.% of the total weight of the hydrogen storage alloys) of expanded natural graphite (ENG);

(e) . of filling the powdered mixture from the open end of the pipe 21 ;

(f) . of closing (welding) the open end of the pipe 21 with the second end cap 23.

The filling density of the MH material (total weight 3.2 kg) in the container (0.98 L in the void inner volume) is equal to 3.27 kg/L, or 55.5% of the density of the MH material in the hydrogenated state (the value calculated from XRD data for the AB2- and ABs-hydrides is about 5.89 kg/L).

After one activation cycle (vacuum heating to 150 °C during 1 hour followed by 2 hours long hydrogen absorption at hydrogen pressure of 80 bar and room temperature) the MH material absorbs 550 NL H2 that corresponds to the hydrogen storage capacity of 172 NL/kg. About 500 NL H2, or 90% of the stored hydrogen, can be released at the discharge pressure of 2 bar (absolute) and H2 output flow rate above 7.5 NL/min (or 2.34 NL/min per 1 kg of the hydrogen storage material) when the MH container is heated by an ambient air (To=20 °C), flow velocity about 3 m/s. During the H2 discharge the MH container is cooled down to -20 °C, but the temperatures measured by thermocouples located in different points of the metal hydride bed differ by not more than 5 degrees. So, the metal hydride bed is characterised by a good uniformity of spatial temperature distribution therein, and the heating of the external wall of the metal hydride container is the process limiting the heat supply to the MH material and, in turn, the H2 discharge flow rate.

The time of complete hydrogen charge of the container at hydrogen pressure of 80 bar and the cooling with ambient air (To=20 °C, v=3 m/s) varies within 1 .5- 2 hours; in doing so the MH container is heated up to 80-90 °C, but again the temperature differences between different points of the metal hydride bed do not exceed 5 degrees. Intensification of the external cooling (water circulation at To=20 °C) allows to shorten the hydrogen recharge time at the same charge pressure to 20-25 minutes.

Example 2

Example 2 illustrates the implementation of the second embodiment of the invention.

Figure 4 shows the assembly drawing of the metal hydride container as a longitudinal cross-section. The container is made of 288 mm long stainless steel pipe 21 , 32.5 mm in outer diameter and 2 mm in wall thickness. To provide efficient heating and cooling of the container with a flow of ambient air, the pipe 21 is equipped by external fins (21 a) formed by extrusion of a thick Aluminium pipe. The end cap 22 with ¼" NPT female thread for the installation of hydrogen input - output pipeline with in-line gas filter (not shown) is welded to the container before its filling which is carried out from the opposite side. The void volume of the container is 160 cm³ corresponding to 560 g of MH material (90 wt% of AB2-type alloy and 10 wt.% of ABs-type alloy, similarly to Example 1 ). In so doing, the MH filling density is 3.5 g/cm³, or 59.4% of the density of the material in the hydrogenated state.

The powder of the MH material is mixed with 84 g (15 wt.%) of the ENG powder. The mixture (total weight 644 g) is separated to two equal portions, 322 g each. The first portion is used for compacting fourteen pellets (1 1 ), 23 g in the weight each. The second portion is loaded in the container as a loose powder (10). In doing so, the MH bed is formed as an alternative arrangement of a portion of the loose powder (10), 0.5 mm thick aluminium fin (12), the pellet (1 1 ), and the fin (12).

Figure 5 shows components and tools for the forming of metal hydride bed after compacting of all the pellets 1 1 from the first portion of the mixture. The pellets 1 1 , fins 12 and the second portion of the loose powder 10 are pressed in the cylindrical part 21 of the containment from the side opposite to the end cap 22 with hydrogen input / output pipeline 24. The pressing is carried out with the help of long dies 34 while a pellet is compacted using matrix 31 , die 32 and support / pressing-out accessories 33. The inner diameter of the matrix 31 corresponds to an external diameter of the compacted pellet of 28.5 mm that is equal to the inner diameter of the cylindrical part 21 of the containment and the external diameter of the fin 12. In so doing, after the pressing-in both the pellet 1 1 and the fin 12 are in a firm contact with the inner surface of the containment 21 . The pellets 1 1 also have central axial hole, 8.3 mm in diameter, formed during the compacting with a help of a central rod of the accessories 33. Each pellet is compacted at the pressure of 150 MPa and room temperature, for 5 minutes. As it was experimentally shown, the decrease of the compacting pressure below this value, as well as the decrease of the content of ENG in the mixture below 15 wt% result in the disintegration of the pellet even after the first hydrogen absorption (Figure 6A) while the pellets with the higher content of ENG compacted at the higher pressures (Figure 6B) keep their original shape, though some delamination takes place.

Exceeding the content of ENG above 20 wt% and the compacting pressure above 20 MPa result in the decrease of porosity of the compacted pellets that hinders their hydrogenation due to insufficient hydrogen diffusion rate.

The forming of the MH bed in the cylindrical part of the containment 21 with installed end cap 22 (see Figure 5) is carried out in the following sequence:

(a) . of axial installing the porous member (13; assembly of tubular filters, 6 mm in the external diameter and 1 μ in the pore size, plugged from both ends) in the containment;

(b) . of alternate pressing the material and the heat conductive fins in the gas-tight containment from the opened side of the containment, using the dies 34, in the following sequence: i. a portion of the loose powder 10; ii. the fin 12; iii. the pellet 1 1 ; iv. the fin 12;

(c) . of final compacting the metal hydride bed; and

(d) . of installing (welding) the second end cap.

In doing so, the portion of the loose powder is about 21 .5 g that corresponds to fifteen portions in total. In turn, the first and the last portions of the mixture loaded in the containment are in the form of loose powder. Such arrangement facilitates the final compacting which is done at the pressure of 55 MPa and the room temperature, for 5 minutes.

According to the results of tests of the metal hydride container where the MH bed was formed in accordance with the procedure described above, about 90% of the stored H2 (90 NL) can be released at the discharge pressure of 2 bar (absolute) and H2 output flow rate above 2.5 NL/min (or 4.46 NL/min per 1 kg of the hydrogen storage material) when the MH container is heated by an ambient air (To=20 °C), flow velocity about 2 m/s. The container cooled by the ambient air at the same flow velocity can be completely recharged during 15 minutes at hydrogen pressure of 40 bar. The charge / discharge dynamic performances of the metal hydride container do not deteriorate during at least one hundred charge / discharge cycles. At the same time, the reference tests of the MH container with the same containment where the MH bed was formed using the procedure known from the prior art⁷ (10 wt% ENG, compacting of all the mixture in the containment at 100 MPa) showed that similar good dynamic performances were exhibited only on the first charge cycle followed by their sharp deterioration (maximum H2 discharge flow rate below 0.5 NL/min at the conditions specified above). After opening the container, it was found that the deterioration had its origin in the complete disintegration of the compact to powder.

The metal hydride bed formed according to the present invention is characterised by high rates of hydrogen absorption and desorption due to high effective thermal conductivity achieved. It also allows to use maximum allowed filling density of the MH material in the containment without compromising safe operation. At the same time, it requires quite simple layout of hydrogen storage containment with external heating and cooling which can be manufactured with minimal labour efforts and for low costs. The implementation of the first embodiment of the invention where the mixture of the MH material and the binder (preferably, ENG) is used in a powdered form is especially cost efficient. The second embodiment which uses the compacted mixture prepared according to the procedure described above allows to reach very good dynamics of hydrogen charge and discharge but requires additional tools and labour for the compacting.

Claims

PATENT CLAIMS

1 . A metal hydride bed which is disposed in a metal hydride container, which includes:

(a) a mixture of a powdered hydride forming material which is able to form hydride when interacting with gaseous hydrogen at pressure higher than the atmospheric pressure at the ambient temperature, and a binder which combines high thermal conductivity, plasticity and high porosity under hydrostatic pressure; and

(b) a plurality of heat conductive fins which are disposed in the inner space of a gas-tight containment of the metal hydride container filled with the mixture and have a firm thermal contact with the inner surface of the said space.

2. A metal hydride bed as claimed in claim 1 , in which the gas-tight containment of the metal hydride container is a cylinder with two end caps, one of which is equipped with a hydrogen input/output pipeline.

3. A metal hydride bed as claimed in claims 1 and 2, in which the heat conductive fins are perforated, disposed inside the containment transversally, and uniformly distributed along the cylinder axis in the mixture of the hydride forming material with the binder.

4. A metal hydride bed as claimed in claim 3, in which the hydride forming material includes ABs-type intermetallide.

5. A metal hydride bed as claimed in claim 3, in which the hydride forming material includes AB2-type intermetallide and/or BCC solid solution alloy additionally comprising ABs-type intermetallide as claimed in claim 4.

6. A metal hydride bed as claimed in claim 5, in which the ABs-type intermetallide is taken in the amount of 10% as respect to the total weight of the hydride forming material.

7. A metal hydride bed as claimed in any one of the preceding claims, in which the binder is expanded natural graphite.

8. A metal hydride bed as claimed in claim 7, in which the ratio of weight of the hydride forming material to its total volume in the metal hydride bed is from 0.50 to 0.60 of the real density of the hydride forming material in the hydrogenated state.

9. A metal hydride bed as claimed in claim 8, in which the mixture is a loose powder where the expanded natural graphite is taken in the amount from 1 to 2% as respect to the weight of the hydride forming material.

10. A metal hydride bed as claimed in claim 9, in which the hydrogen input - output pipeline is connected with a tubular filter longitudinally disposed in the metal hydride bed.

1 1 . A metal hydride bed as claimed in claim 8, in which the mixture is taken as a compact where the expanded natural graphite is taken in the amount from 15 to 20% as respect to the weight of the hydride forming material.

12. A metal hydride bed as claimed in claim 1 1 , in which the compact has an axial hole throughout the length of the metal hydride bed, and the hydrogen input - output pipeline is equipped with an in-line gas filter.

13. A metal hydride bed as claimed in claim 12, in which the axial hole additionally comprises a porous member.

14. A method of forming the metal hydride bed as claimed in claim 9, which includes the steps:

(a) of pressing the heat conductive fins in the gas-tight containment;

(b) of weighting and mixing the powders of the hydride forming material and the expanded natural graphite; and

(c) of filling the containment with the powdered mixture;

15. A method of forming the metal hydride bed as claimed in claim 1 1 , which includes the steps:

(a) of weighting and mixing the powders of the hydride forming material and the expanded natural graphite;

(b) of separating the mixture to two equal portions;

(c) of compacting the pellets from the first portion of the mixture;

(d) of installing the porous member on the axis of the containment; (e) of alternate pressing the material and the heat conductive fins in the gas-tight containment in the following sequence: i. the loose powder from the second portion of the mixture; ii. the heat conductive fin; iii. the pellet; iv. the heat conductive fin; and

(f) of final compacting the metal hydride bed.

16. A method as claimed in claim 15, in which the compacting pressure for making one pellet as claimed in claim 15(c) is from 150 to 250 MPa.

17. A method as claimed in claim 15, in which the mass of the mixture taken for the compacting of one pellet as claimed in claim 15(c) at the compacting pressure as claimed in claim 16 is selected to provide the thickness of the pellet between 10 and 15 mm.

18. A method as claimed in claims 15 to 17, in which the loose powder from the second portion of the mixture is divided by small portions whose number is equal to the number of the compacted pellets plus one.

19. A method as claimed in claim 18, in which the first and the last portions of the metal hydride material loaded in the gas-tight containment are the loose powder from the second portion of the mixture, as claimed in claim 15(e)i.

20. A method as claimed in claim 19, in which the pressure of the final compacting the metal hydride bed is from 50 to 60 MPa.

21 . A method of making the metal hydride container comprising the metal hydride bed as claimed in any one of claims 9 and 10, which includes the steps:

(a) of installing the end cap with the hydrogen input / output pipeline;

(b) of forming the metal hydride bed as claimed in claim 14 from the opposite side of the containment; and

(c) of installing the second end cap.

22. A method of making the metal hydride bed as claimed in any one in claims 12 and 13, which includes the steps:

(a) of installing the end cap with the hydrogen input / output pipeline with the in-line gas filter;

(b) of forming the metal hydride bed as claimed in any one of claims 15 to 20 from the opposite side of the containment;

(c) of installing the second end cap.

23. A metal hydride bed which is disposed in a metal hydride container substantially as hereinbefore described with reference to the accompanying drawings.

24. A method of forming the metal hydride bed substantially as hereinbefore described with reference to the accompanying drawings.

25. A metal hydride container substantially as hereinbefore described with reference to the accompanying drawings.