Adsorption Beds: The Significance of Thermodynamic Properties and Particle Size on System-Level Heat Pump Performance

The performance of an adsorption heat pump is a function of the equilibrium uptake and diffusion resistances of a particular system, which determines the refrigerant throughput during a cycle. Previous studies have sought improved sorption bed performance by increasing heat transfer; however, some of the proposed heat exchanger enhancements represent costly alterations to the system. This work instead investigates a method for optimizing sorption bed mass transfer, which can be implemented as a low-cost alternative to heat transfer enhancement or in addition to it. The objective is to balance the intra-particle diffusion resistance, which increases with adsorbent particle diameter, with the inter-particle pressure drop, which decreases with adsorbent particle diameter. A silica gel-water system model is used to show that the optimal particle geometry in a packed bed yields a 48% improvement in cooling duty and over 50% increase in coefficient of performance compared with larger particles (d(p) = 1.42 mm).