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Project Title:

Heat and material flow at H2 storage in new pulverized materials and additives

Ref.No.: 131

Project Type and Category:

Basic research

Project Duration:

1.89 – 12.98

Project Participants:

Prof. Dr.-Ing. Erich Hahne, Inst. f. techn. Thermodynamik u. Wärmetechnik, Uni Stuttgart,
Others: SFB 270 (See ref.no. 73)

Sponsor:

See ref.no. 73

Project Budget and
Funding:

See ref.no. 73

Project Description and Objectives:

For the construction of hydrogen storages with zeolites the knowledge of the thermophysical properties of the zeolite powders is nescessary. Zeolites are highly porous crystalline alumosilicates. The hydrogen molecules are forced into the zeolitic cages under pressure (30 bar) and high temperature (600 K). After cooling down and depressurization the hydrogen stays encased in the intercristalline cavities. Decapsulation of the trapped hydrogen molecules can be achieved by heating the loaded zeolite. The encapsulation of hydrogen in zeolites also requires a heat supply. Therefore the effective thermal conductivity (lamda)eff. and the effective thermal diffusivity aeff of the zeolite powder are important thermophysical properties for the storage of hydrogen [1,2]. Measurements of eff. are carried out using a hot-wire method. The measured values are compared with calculated values based on a cellular-model.
Under normal conditions water is encased in the cavities of the zeolite. The polar water molecules are more strongly adsorbed than the unpolar hydrogen molecules. The storage capacity for hydrogen could be increased if the water is totally desorbed by heating. To investigate the decapsulation of the water molecules measurements of the specific heat are carried out.

Technical Goals:

In phases one and two of the project B4 experimental investigations of the thermal conductivity of metal hydrides have been carried out. Based on the cellular-model by Zehner, Bauer and Schlünder an extended model was developed to calculate the effective thermal conductivity of powdery materials with a reaction between fluid and solid. This extended model takes into account different methods of increasing effective thermal conductivity such as a built-in metalic matrix or the application of an external force. Measured and calculated values of the effective thermal conductivity of different metal hydrides show good agreement [3,4].
In the third phase of the project the heat conduction in zeolite powders is investigated. The experimental results are compared with calculations based on the model by Zehner, Bauer and Schlünder. Possibilities to improve the effective thermal conductivity of the zeolite powders such as the addition of copper or graphite particles are considered. The thermal contact resistance at the interface between a wall and the porous medium is measured with a laser-flash arrangement. The main advantage is, that it is not necessary to contact the sample for heating and measuring the temperature. For the calculation of the contact resistance between a wall and the porous medium a model is developed. The specific heat of zeolite powders is measured with a high-temperature DSC.
New synthetic zeolite powders, which are improved with regard to the storage capacity of hydrogen are only available in small quantities. For this reason a periodic hot-wire technique is developed. This technique enables measurements of the thermal conductivity and thermal diffusivity of samples with a volume smaller than 15 ml.

Project Status

Finalized

Preliminary or Final Results:

The effective thermal conductivity of zeolite powders is measured by means of the transient hot-wire method. This method is well suited for measuring the thermal conductivity of powdery materials because of its averaging of the local values over the whole length of the heating wire, its quickness and its high accuracy.
The transient hot-wire technique depends on the measurement of the time-dependent temperature field. For this a platinum wire is used simultaneously as a heat source and as a resistance thermometer [5]. The effective thermal conductivity of the powdery material depends primarily on the kind of gas in the voids and the pressure. The higher the thermal conductivity of the gas the higher is the effective thermal conductivity of the powdery material.
The effective thermal conductivity of zeolite powders under hydrogen pressure varies between 0.005 W/m K at 0.001 bar and 0.4 W/m K at 30 bar. The curves of eff. depending on the pressure show a typical S-shape. The effective thermal conducticity of zeolite NaA, CaA and KA is given in Fig. 1 for the pressure range of 0.001 bar to 30 bar. The effective thermal conductivity is less influenced by temperature. At 575 K the effective thermal conductivity eff. is about 25% higher than measured at 325 K.
For pressures smaller than 0.1 bar the heat conduction in the zeolite powder is dominated by the thermal conductivity of the pure solid material. In this pressure range the water molecules encased in the cavities of the zeolite improve the effective thermal conductivity of the powder up to 60%. Using metal-hydride powders as hydrogen storage material particle degradation limits the number of storing cycles. For zeolite powders the effective thermal conductivity was measured during 50 storing cycles. In contrast to the metal-hydride powders the effective thermal conductivity did not change (Fig. 2). Thus, storing hydrogen molecules in the zeolitic cages the particles did not decompose.
The experimental results of the effective thermal conductivity are compared with calculations based on the model by Zehner, Bauer and Schlünder. In this model the powdery material is represented by a unit cell. Assuming parallel lines of heat flow the heat transport is subdivided .

Related Reference Papers and Other Publications:

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Project Title: Heat and material flow at H2 storage in new pulverized materials and additives		Ref.No.: 131
Project Type and Category:	Basic research
Project Duration:	1.89 – 12.98
Project Participants:	Prof. Dr.-Ing. Erich Hahne, Inst. f. techn. Thermodynamik u. Wärmetechnik, Uni Stuttgart, Others: SFB 270 (See ref.no. 73)
Sponsor:	See ref.no. 73
Project Budget and Funding:	See ref.no. 73
Project Description and Objectives:	For the construction of hydrogen storages with zeolites the knowledge of the thermophysical properties of the zeolite powders is nescessary. Zeolites are highly porous crystalline alumosilicates. The hydrogen molecules are forced into the zeolitic cages under pressure (30 bar) and high temperature (600 K). After cooling down and depressurization the hydrogen stays encased in the intercristalline cavities. Decapsulation of the trapped hydrogen molecules can be achieved by heating the loaded zeolite. The encapsulation of hydrogen in zeolites also requires a heat supply. Therefore the effective thermal conductivity (lamda)eff. and the effective thermal diffusivity aeff of the zeolite powder are important thermophysical properties for the storage of hydrogen [1,2]. Measurements of eff. are carried out using a hot-wire method. The measured values are compared with calculated values based on a cellular-model. Under normal conditions water is encased in the cavities of the zeolite. The polar water molecules are more strongly adsorbed than the unpolar hydrogen molecules. The storage capacity for hydrogen could be increased if the water is totally desorbed by heating. To investigate the decapsulation of the water molecules measurements of the specific heat are carried out.
Technical Goals:	In phases one and two of the project B4 experimental investigations of the thermal conductivity of metal hydrides have been carried out. Based on the cellular-model by Zehner, Bauer and Schlünder an extended model was developed to calculate the effective thermal conductivity of powdery materials with a reaction between fluid and solid. This extended model takes into account different methods of increasing effective thermal conductivity such as a built-in metalic matrix or the application of an external force. Measured and calculated values of the effective thermal conductivity of different metal hydrides show good agreement [3,4]. In the third phase of the project the heat conduction in zeolite powders is investigated. The experimental results are compared with calculations based on the model by Zehner, Bauer and Schlünder. Possibilities to improve the effective thermal conductivity of the zeolite powders such as the addition of copper or graphite particles are considered. The thermal contact resistance at the interface between a wall and the porous medium is measured with a laser-flash arrangement. The main advantage is, that it is not necessary to contact the sample for heating and measuring the temperature. For the calculation of the contact resistance between a wall and the porous medium a model is developed. The specific heat of zeolite powders is measured with a high-temperature DSC. New synthetic zeolite powders, which are improved with regard to the storage capacity of hydrogen are only available in small quantities. For this reason a periodic hot-wire technique is developed. This technique enables measurements of the thermal conductivity and thermal diffusivity of samples with a volume smaller than 15 ml.
Project Status	Finalized
Preliminary or Final Results:	The effective thermal conductivity of zeolite powders is measured by means of the transient hot-wire method. This method is well suited for measuring the thermal conductivity of powdery materials because of its averaging of the local values over the whole length of the heating wire, its quickness and its high accuracy. The transient hot-wire technique depends on the measurement of the time-dependent temperature field. For this a platinum wire is used simultaneously as a heat source and as a resistance thermometer [5]. The effective thermal conductivity of the powdery material depends primarily on the kind of gas in the voids and the pressure. The higher the thermal conductivity of the gas the higher is the effective thermal conductivity of the powdery material. The effective thermal conductivity of zeolite powders under hydrogen pressure varies between 0.005 W/m K at 0.001 bar and 0.4 W/m K at 30 bar. The curves of eff. depending on the pressure show a typical S-shape. The effective thermal conducticity of zeolite NaA, CaA and KA is given in Fig. 1 for the pressure range of 0.001 bar to 30 bar. The effective thermal conductivity is less influenced by temperature. At 575 K the effective thermal conductivity eff. is about 25% higher than measured at 325 K. For pressures smaller than 0.1 bar the heat conduction in the zeolite powder is dominated by the thermal conductivity of the pure solid material. In this pressure range the water molecules encased in the cavities of the zeolite improve the effective thermal conductivity of the powder up to 60%. Using metal-hydride powders as hydrogen storage material particle degradation limits the number of storing cycles. For zeolite powders the effective thermal conductivity was measured during 50 storing cycles. In contrast to the metal-hydride powders the effective thermal conductivity did not change (Fig. 2). Thus, storing hydrogen molecules in the zeolitic cages the particles did not decompose. The experimental results of the effective thermal conductivity are compared with calculations based on the model by Zehner, Bauer and Schlünder. In this model the powdery material is represented by a unit cell. Assuming parallel lines of heat flow the heat transport is subdivided .
Related Reference Papers and Other Publications:	#