INTEGRATED MATERIAL AND PROCESS FOR INTEGRATED OPERATION OF A HYDRIDE STORAGE SYSTEM

Abstract

The present invention relates to a composite material for hydrogen storage based on metal hydrides and to a method of operating a hydrogen storage system based on metal hydrides capable of releasing and absorbing hydrogen. Such hydrogen storage systems based on metal hydrides may be applicable as a fuel source for a fuel cell. The composite material for hydrogen storage comprises a powder or pellets of a hydride and a phase changing material (PCM), wherein the PCM is an encapsulated phase changing material (EPCM) which is homogeneously dispersed within the powder or pellets of the hydride.

Claims

1. A composite material comprising a powder or pellets of a hydride and a phase changing material (PCM), wherein the phase changing material is an encapsulated phase changing material (EPCM) which is homogeneously dispersed within the powder or pellets of the hydride.

2. The composite material of claim 1, wherein the phase changing material is a microencapsulated phase changing material (MEPCM) which is homogenously dispersed within the powder or pellets of the hydride.

3. The composite material of claim 1, wherein the phase changing material is in the form of particles having a diameter in the range of from 1 μμm to 50 mm.

4. The composite material of claim 1, wherein the phase changing material is selected to have a peak Tf of the melting area ΔT.sub.f, as determined by differential scanning calorimetry (DSC), above the hydrogen desorption temperature T.sub.1 at the operational desorption pressure of the metal hydride.

5. The composite material of claim 4, wherein the phase changing material is selected to have a peak T.sub.f of the melting area ΔT.sub.f, as determined by DSC, above the hydrogen desorption temperature T.sub.1 at the operational desorption pressure and below the hydrogen absorption temperature T.sub.2 at the operational desorption pressure of the metal hydride.

6. The composite material of claim 4, wherein the phase changing material is selected to have a peak Tf of the melting area ΔT.sub.f, as determined by DSC, close as possible to (T.sub.1+T.sub.2)/2.

7. The composite material of claim 1, wherein the minimum amount of phase changing material in the powder or pellets of the hydride is selected according to the following equation: $m_{\mathrm{PCM}}=\frac{\Delta H_r\left(\mathrm{hyd}\right).Math.m_{\mathrm{HYD}}}{\Delta H_m\left(\mathrm{PCM}\right)}$ wherein m.sub.PCM is the minimum amount of phase changing material, ΔH.sub.r (HYD) is the enthalpy of the hydrogen absorption reaction, m.sub.PCM is the stored hydrogen mass in the hydride, and ΔH.sub.m (PCM) is the latent heat of melting of phase changing material.

8. The composite material of claim 1, which is present in the form of a compacted material.

9. The composite material of claim 1, further comprising a heat transfer enhancement agent incorporated therein.

10. The composite material of claim 9, wherein the heat transfer enhancement agent is an expanded natural graphite (ENG).

11. A hydrogen storage tank comprising the composite material of claim 1, the hydrogen storage tank being coupled to a fuel cell.

12. Method of operating a hydrogen storage system based on metal hydrides capable of releasing and absorbing hydrogen wherein the composite material according to claim 1 is sequentially charged with hydrogen and discharged again.

13. The method of claim 12, wherein hydrogen discharge takes place at a pressure between 10 kPa and 2,000 kPa.

14. The method of claim 12, wherein hydrogen charging takes place at a pressure between 150 kPa and 70,000 kPa.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0033] As stated, in an embodiment of the invention, the phase changing material (PCM) is selected so as to have a melting area ΔT.sub.f, as determined by differential scanning calorimetry (DSC), between the hydrogen desorption temperature (T.sub.1) at the operating desorption pressure and the hydrogen absorption temperature (T.sub.2) at the operating absorption pressure of the selected metal hydride. Also the shell material shall be selected to withstand the required temperatures and the contact with hydrogen without harm.

[0034] In general, the pressure for the hydrogen desorption is preferably selected to be between 10 kPa and 2,000 kPa, and the pressure for the hydrogen absorption, the charging pressure, is preferably selected to be between 150 kPa and 10,000 or even 30,000 kPa or 70,000 kPa, depending on the hydride.

[0035] While PCM encapsulated with a polymer might be useful for being dispersed within a powder or pellets of a medium-temperature hydride or of a low-temperature hydride, it is preferred that PCM encapsulated with a metal is dispersed within a powder or pellets of a high-temperature hydride, so as to avoid degradation of the shell.

[0036] The minimum amount of phase changing material may be selected by dividing the energy stored in the hydride's chemical bonds by the latent heat of melting, ΔH.sub.m (PCM), of the PCM material. The total energy, E.sub.tot (HYD), that is stored in a hydride's chemical bonds is determined by multiplying the enthalpy of the absorption reaction, ΔH.sub.r (HYD), of the hydride with the stored mass of hydrogen, m.sub.HYD, in the hydride, where the stored mass of hydrogen in the hydride is the product of its capacity and the total mass of the hydride.

[00002]$m_{\mathrm{PCM}}=\frac{E_{\mathrm{tot}}\left(\mathrm{HYD}\right)}{\Delta H_m\left(\mathrm{PCM}\right)}=\frac{\Delta H_r\left(\mathrm{hyd}\right).Math.m_{\mathrm{HYD}}}{\Delta H_m\left(\mathrm{PCM}\right)}$

[0037] As an example, the minimum amount of Crodalherm™ 53, a PCM melting at about 53° C. and having a latent heat of melting, ΔH.sub.m(PCM), of 226 kJ/kg, in Hydralloy® C5, Ti.sub.0.95Zr.sub.0.05Mn1.46V0.45Fe0.09, a hydride storage material having a ΔH.sub.r=25 kJ/mol H.sub.2, and a capacity of 1.5 wt % (this means that for each 100 kg of storage material, 1.5 kg of H.sub.2 can be stored) can be calculated by first determining the stored energy (E) in 100 kg of the hydride (disregarding any heat losses) as follows:

[00003]$E=\frac{1500g}{2g\text{/}\mathrm{mol}}.Math.25=18750\mathrm{kJ}$

[0038] On the basis of stored energy (E) in 100 kg of the hydride, then the minimum amount of PCM for each 100 kg of hydride can be calculated by dividing the energy by the heat capacity of the PCM:

[00004]$m_{\mathrm{PCM}}=\frac{18750}{226}=82.97\mathrm{kg}\mathrm{of}\mathrm{PCM}$

[0039] Therefore, for every 100 kg of Hydralloy® C5, at least 82.97 kg of active CrodaTherm™ 53 PCM material are required. This calculation is provided as an illustrative example only. It has to be borne in mind that CrodaTherm™ 53 is not an encapsulated phase changing material, however according to the invention it will have to be provided as such. The encapsulation or matrix embedding will add some weight (although scarcely any volume at all) to this calculation.

[0040] In an embodiment of the invention, the metal hydride and the encapsulated phase changing material may be in the form of a compacted material. The term compacted material as used below means a material whose density is significantly higher than that of the raw materials in the powder state. This material is obtained in particular by compressing a mixture of raw materials in powder or granular form, either with or without the addition of a compacting agent, thus reducing the porosity.

[0041] In still another embodiment, the composite material comprising a powder or pellets of a hydride and a phase changing material (PCM) may further contain a heat transfer enhancement agent incorporated therein. Such heat transfer enhancement agent may be chosen, e.g. from graphite such as expanded natural graphite (ENG) and other known materials.

[0042] In still another embodiment, the composite material is incorporated in a hydrogen storage tank which is preferably coupled to a fuel cell.

[0043] In a still further embodiment, the present invention relates to a method of operating a hydrogen storage system based on metal hydrides capable of releasing and absorbing hydrogen wherein a composite material comprising a powder or pellets of a metal hydride and a phase changing material (PCM) is provided, wherein the PCM is an encapsulated phase changing material (EPCM) or matrix embedded phase changing material (MPCM) which is homogeneously dispersed within the powder or pellets of the hydride, and wherein the composite material is sequentially charged with hydrogen and discharged again. In an embodiment, hydrogen discharge takes place at a pressure between 10 kPa and 2,000 kPa. In another embodiment hydrogen charging takes place at pressure between 150 kPa and 70,000 kPa, preferably between 200 kPa and 30,000 kPa.