Parametric study of dual-bed adsorption refrigeration system with heat and mass recovery

Abstract

The demand for refrigeration and cooling systems has been increased rapidly in past few decades. Increase in emissions of harmful refrigerants used in vapor compression cooling systems has been a major reason in the ozone layer depletion and global warming. Recent agreements like Kyoto protocol, Montreal protocol and latest Paris agreement also emphasize the actions need to be made toward installation of cooling systems with natural refrigerants having very low or zero ODP. Adsorption cooling systems have recently gained the interest due to its capacity to utilize low temperature waste heat and provide cooling without using harmful refrigerants. These systems run on consecutive cycles of adsorption and desorption of water in two beds of silica gel. Adsorption chillers can be integrated into combined cycle power plants, datacenter thermal management systems, and concentrated photovoltaic/thermal systems.

However, their commercialization at the large scale has been hampered due to their low power density and efficiency, which leads to high system cost. This thesis work analyses the performance of a dual bed adsorption refrigeration system using lumped parameter approach based on the linear driving force model, which presents a precise description of the adsorption cooling cycle for the simulation. Previous studies have established that the variation between lumped-parameter models and distributed parameter models is only about 10%. Thus, the lumped parameter approach is favored because it simplifies the system and saves computational time. This model is based on the fundamental equations of heat and mass transfer. Performance of the two-bed silica-gel water adsorption chiller has been analyzed here. The model is used to investigate the effect of physical and operating conditions including heat transfer coefficient, hot water inlet temperature, cold water inlet temperature, cycle time, fluid flow rate etc. on the chiller performance.

This work demonstrates the importance of switching time in between the cycles. It noticeably shows that immediate switching of the beds after the half cycle has adverse effects on the performance of the adsorption chiller. The effect of heat and mass recovery on system performance is studied. A significant improvement of around 50% increase in COP has been achieved by applying heat recovery case. Internal recirculation as well as external recirculation in heat recovery scheme is also studied.

A comparison in the performance parameters in the case of Silica-gel, SAPO-34 and Al Fumarate as adsorbents is studied. The results of this work have been verified with the results of the literature as well as experimental data. Experiment on this adsorption chiller is undergoing at IBM Research Zurich.