Thermodynamic study of isosteric heat at zero coverage : applications to adsorption cooling and desalination
Date of Issue2017
School of Mechanical and Aerospace Engineering
The production of cooling and drinkable water represents major and essential needs for the mankind. Vapor compression cooling and desalination systems are mainly driven by electricity, produced by burning fossil fuels, which contributes to the global warming (GW) because of the encountered direct CO2-emissions. The refrigerants used in vapor compression systems possess high global warming potential (GWP), resulting in an indirect contribution to the GW and, consequently, to the severe consequences on the worldwide climate. Recovering the waste heat generated upon burning fossil fuels to provide both cooling and fresh water demands through the application of adsorption assisted cooling and desalination cycle may remarkably contribute in mitigating the bad consequences of the GW. At present, the bulky size of adsorption chiller and desalination systems obstructs its practical application. The size of an adsorption bed depends on the quality of the porous adsorbents. Therefore, the present thesis aims to develop a thermodynamic framework for calculating the interaction potentials between the adsorbate and the adsorbent pore structures, from which the isosteric heat at zero coverage is formulated as a function of pore width and volume. The proposed modelling provides necessary information for the design of adsorbent materials. The proposed interaction potential model includes the Lennard Jones (LJ) Potential with the inclusion of electrostatic and induction potential. The model is first applied to the graphite surface, in which the electrostatic and induction potentials are obtained to be very small compared to the LJ potential. The maximum isosteric heat is found in the super micro-pore regions. On the other hand, the Reverse Monte Carlo (RMC) and molecular dynamics methods are applied to understand the silica structure for water adsorption. For water interaction with silica gel, the electrostatic interaction is found to be high. For water adsorption on CHA and AFI types zeolites, the most favourable adsorptive sites are identified at the positions of channels inside the zeolite frameworks. A detailed experimental investigation is conducted for measuring (i) the amount of CO2 uptakes on various activated carbons, and (ii) water uptakes on various types of silica gels and zeolites under static conditions. These isotherms data are fitted with isotherm models for their use in adsorption cooling and desalination modelling. Employing the knowledge of isosteric heats, pore structures and isotherm data, a thermodynamic model is developed for calculating the energetic performances of adsorption cooling and desalination systems. From the present analysis, it is noticed that both graphite structure or activated carbon and silica gels are optimised with respect to pore sizes and volumes for given evaporating and heat source temperatures. The best performances of adsorption cooling and desalination cycles are obtained from a compromise between the greatest pore volume and the optimal pore size. From the present analysis, it is observed that there is no cooling and water production performance for pore widths varying from 0 to 0.3 nm. For better efficiency, the silica gel should be designed with a pore size from 1 nm to 2 nm for cooling application due to its higher coefficient of performance (COP). The average pore size of silica gel should be designed with 1 nm for the production of more water. On the other hand, for refrigeration purposes, the activated carbon should be designed with the average pore size ranging from 7 to 15 Å and higher micro-pore volumes. This thesis thus provides some important information regarding the knowledge of isosteric heat at zero coverage for various adsorbent – adsorbate pairs, which can help in the design of new carbonaceous materials or carbon–silica or zeolites composites for adsorption assisted cooling and desalination purposes.
DRNTU::Science::Physics::Heat and thermodynamics