Development of a novel adsorption calorimeter : applications to silica gels bonded to flat and finned metal heat exchange surfaces

The primary objective of this thesis was to develop a novel adsorption calorimeter to assess the equilibrium and dynamic behaviour of adsorbent-bond- metal sub-system, a constituent of an adsorption chiller. In this research, a novel calorimeter consisting of an adsorber and an evaporator section has been built. Thermoelectric modules were used to control the temperature and measure the instantaneous heat flow at the adsorber and evaporator sections. This thermoelectric module system was tested against an inert aluminium sample. The aluminium sample temperatures were controlled to constant, sinusoidal and triangular set point to within 0.1 K (1 standard deviation). The effective Seebeck coefficient and thermal conductance of the module were extracted from the calibration experiment. The inferred heat flow had a typical accuracy of 10 %. Further tests with a sinusoidal temperature variation yielded the root mean square error of cumulative heat flow as - 8 %. Three and one mm nominal diameter Type A silica gel beads were chosen as the adsorbent, and water as the refrigerant. In the adsorption-evaporation experiment, the plot of heat of adsorption versus time was well fitted to exponential recovery (r2 > 99%). Cumulative net heat flow at the adsorber at time equal to infinity gave the adsorptive capacity, while the time to reach 63.2 % of this value gave the apparent Linear Driving Force constant (Kldf). Adsorptive capacities of the silica gels were fitted to Henry's law and were within 20 % of previously published data. Specific heats of adsorption (obtained by Arrhenius plots of the Henry's law constants) were 2495 kJ/kg and 2634 kJ/kg for 3 and 1 mm diameter gel beads respectively, which were within 9 % of the published data (2710 kJ/kg). Specific heat of adsorption for both samples, calculated from energy balance, were within 8.4 % of values inferred from Arrhenius plot of the Henry Law coefficient. The apparent Kldf for 1 mm diameter gel-beads was 3 times higher compared to the one for 3 mm diameter beads. Activation energy (1261 kJ/kg for 3 mm diameter beads and 1537 kJ/kg for 1 mm diameter beads) was obtained from a further Arrhenius plot, and was between 50 and 58% of the specific heat of adsorption. This suggests the surface diffusion is the governing mechanism for the water adsorption onto silica gel. Tests with de sorption- condensation and temperature varied sinusoidally versus time revealed the ineffectiveness of the water-containing section when acting as a condenser. The adsorption behaviour of coated fins was studied experimentally. The apparent K1df for silica gel coated to a stainless steel fin bonded to an isothermal aluminium plate was ~ 50 % less than that of silica gel bonded directly to flat plate. For an aluminium fin that was cut from a single piece of metal, the apparent Kldf was reduced by 10 %. A numerical model predicted the apparent K1df of the coated fin with an accuracy better than 22 %. The model was developed using the rate constant for silica gel bonded onto a flat plate, the thermal contact resistance at the. root of the fin (if applicable) and the temperature gradient along the fin. Other fin configurations that were not tested experimentally were simulated in the model. In an adsorption chiller, it is estimated that a specific cooling power as high as 275 W/kg and 750 W/kg could be obtained for the 3 and 1 mm diameter gel beads bonded to the flat plate heat exchange surface. For the silica gels coated onto 20 mm high aluminium fins (1 mm thickness), the specific cooling power could be reduced by 30 and 33 % for 3 and 1 mm diameter gel beads respectively. The specific cooling power is sensitive to the adsorber configuration and cycle time.EThOS - Electronic Theses Online ServiceGBUnited Kingdo