Experimental investigation of thermal effects in a hydrogen cryo-adsorption storage system
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This thesis is submitted in partial fulfillment of the requirements for the PhD degree at the Norwegian University of Science and Technology. This work presents the outcome of experimental investigations of thermal effects in a hydrogen cryo-adsorption storage tank system. Cryoadsorption storage systems are considered a candidate technology for mobile applications. An extensive experimental analysis was conducted, including adsorbents characterization and investigations of the thermal behaviour during high-pressure filling and discharging. The results provide comprehensive datasets for verification of numerical tools for the prediction of thermal behavior of adsorptive hydrogen storage systems. The measurements of adsorption isotherms, heat capacities and thermal conductivity are also incorporated in this work. In its own right, the experimental data clearly demonstrates that the thermal processes dominate the operations of adsorption type storage systems. The content of the work is divided into two parts: • MATERIAL PROPERTIES. Characterization includes measurements of pressure-concentration-temperature (PCT) isotherms, effective thermal conductivities and heat capacities of two adsorbents • THERMAL BEHAVIOUR OF A CRYO-ADSORPTION TANK. Thermal behaviour of a cryo-adsorption tank during fast filling and discharging was studied through an experimental campaign. The material characterization has been conducted for two adsorbent classes (activated carbon and metal-organic-frameworks). The maximum measured hydrogen uptake at 77 K for the activated carbon adsorbent (NORIT R0.8) and metal-organic-frameworks (CuBTC-1,3,5) was 2.9 wt% and 2.4 wt%, respectively. Measured effective thermal conductiviy proved to be low (in the range of 0.05-0.25 W/mK) and can seriously limit the heat transfer within an adsorption type storage system. Specific heat capacity measurements were also performed, providing important input to modeling of dynamic thermal behaviour during fast-filling and discharging. Experimental investigations of charging and discharging were conducted with a 10 liter tank. Charging experiments showed that heat generated as a consequence of adsorption, gas compression and heat introduced from the inlet gas limits the storage rate. Discharging experiments have shown that without external energy supply, the penalties in the amount of the residual gas, caused by desorption and decompression, can be larger than 60 %. Suggestions for improved thermal management are provided in the final chapter of the thesis.