The effect of cycle boundary conditions and adsorbent grain size on the water sorption dynamics in adsorption chillers

Abstract

The aim of this work was an experimental study of the temporal evolution of isobaric adsorption uptake (release) for simplest configuration of an adsorbent-heat exchanger unit, namely, a monolayer of loose adsorbent grains located on a metal plate. The study was performed by a large temperature jump method at four various boundary conditions of an adsorptive heat transformation cycle typical for air-conditioning application driven by low temperature heat: T e = 5 and 10 °C, T c = 30 and 35 °C and T HS = 80 °C. The size of the Fuji silica grains was varied from 0.2 to 1.8 mm to investigate its effect on water sorption dynamics. For each boundary set and grain size the experimental kinetic curve could be described by an exponential function up to 80–90% of the equilibrium conversion. Desorption runs are found to be faster than appropriate adsorption runs by a factor of 2.2–3.5, hence, for optimal durations of the isobaric ad- and desorption phases of the chilling cycle should be selected accordingly. The size R of the adsorbent grains was found to be a powerful tool to manage the dynamics of isobaric water ad-/desorption. For large grains the characteristic time was strongly dependent on the grain size and proportional to R 2. Much less important appeared to be an impact of the boundary conditions which variation just weakly affected the dimensionless kinetic curves for the four tested cycles. The maximal specific cooling/heating power was proportional to the maximal temperature difference Δ T and the contact area S between the layer and the metal plate, and can exceed 10 kW/kg. The heat transfer coefficient α estimated from this power was as large as 100–250 W/(m 2 K) that much exceeds the value commonly used to describe the cycle dynamics.