| Summary: | For CO2-free power generation in power plants or vehicles, hydrogen should be implemented as H2 possesses not only the highest energy density (120MJ/kg) but also can be generated from the environment and reactions. However, H2 is difficult to store due to its low density at ambient conditions. Hydrogen can be stored in the form of compressed gas (at > 700 bar and 298K), liquid phase at 20K, cryo-compression (at 100 bar and 77K), liquid organic H2 carrier or even ammonia. The storage of hydrogen in porous materials employing physisorption or chemisorption is an important research area. Cryo-adsorption systems in hydrogen storage are garnering attention due to their higher storage density as compared to compressed or liquefied systems. Metal-Organic Frameworks (MOFs) and Activated Carbons (ACs) provide large surface areas, pore sizes and high pore volume which are crucial for adsorption of hydrogen. Therefore, novel adsorbent materials with desirable properties must be designed and fabricated to improve the efficiency of cryo-adsorption systems. To understand the efficacy of hydrogen storage in porous materials including the randomness of hydrogen adsorption in terms of storage density, energy flow and dissipation, a thermodynamic study is required. Therefore, this thesis, at first, aspires to develop the thermodynamic frameworks of cryo-adsorption system for hydrogen storage in terms of density and entropy flow with generation, which provides a theoretical insight to understand the behavior of H2 molecules in porous materials as a function of pressure and temperature.
The newly formulated adsorbed phase density includes the effects of materials’ pore and skeletal volume with adsorbate uptakes as a function of pressure and temperature. Various MOFs and activated carbons such as Maxsorb-III, HKUST-1, MIL-101(Cr), MOF-5, DUT-117 (Cu), DUT-117 (Ni), IRMOF-10 and Maxsorb-III/ HKUST-1 composites are considered to capture the present research findings namely the adsorbed density and the entropy flow. In order to obtain these data, at first, the amount of hydrogen uptakes on these adsorbents are measured for the temperatures of 77K, 87K, 112K and 298K, and pressures up to 5 bar employing the volumetric apparatus equipped with open-loop-liquid Nitrogen (LN2) plus heating (in the bed) to maintain adsorption at 77 K, 87K and 112K. Secondly, a gravimetric apparatus is applied to measure H2 uptakes at 77 K under sub-atmospheric pressure conditions.
Hence the activated carbon such as Maxsorb-III shows gravimetric and volumetric uptake of 5.81wt% and 0.0174kg/L at 77K and 10 bar. MOF-177 shows the higher gravimetric and volumetric uptakes of 5.87wt% and 0.0205kg/L, respectively. IRMOF-10 provides relatively higher gravimetric uptake of 7.08wt% including the volumetric uptake of 0.015kg/L at 77K and 10 bar. These results meet the gravimetric uptake of U.S DOE requirements. However, it does not meet the volumetric uptake of 0.04 kg/L given by U.S DOE. Secondly, Maxsorb-III, MIL-101(Cr), AC-MIL-101 composites, MOF-177, UiO-66(Zr), MOF-5, DUT-117 and IRMOF-10 MOFs adsorb separately with hydrogen at 77K and 10 bar, and show the adsorbed phase densities of 36.3kg/m3, 29.3 kg/m3, 31.2 kg/m3, 36.6 kg/m3, 18.5 kg/m3, 23.7 kg/m3, 28.6 kg/m3 and 43.5 kg/m3. HKUST-1 and Max-HKUST-1 composites also provide the adsorbed phase densities of 12 kg/m3 and 11.6 kg/m3, respectively. The density of cryo-compressed hydrogen is obtained 3.2 kg/m3 at 77 K and 10 bar. This indicates that the proposed cryo-adsorption technology increases H2-storage density >10 times as compared to the density of gaseous hydrogen. The temperature-entropy diagram shows a close loop for the changing and discharging of hydrogen and ensures the storage of internal energy with less entropy dissipation under cryo-adsorption conditions.
The study of cryo-adsorption system shows promising results for hydrogen storage when compared against existing methods such as the cryo-compressed system and is evaluated in terms of adsorbed phase density, entropy, isosteric heat of adsorption and adsorption capacities.
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