# Comparative Assessment of Hygroscopic Properties and Thermal Performance of Activated Carbon-Based Physical Adsorbents and Advanced Composite Adsorbents

## TL;DR

This study compares the water adsorption and thermal performance of activated carbon and a composite material with calcium chloride for heat storage applications.

## Contribution

The study introduces a composite adsorbent with calcium chloride that significantly enhances water uptake and thermal output.

## Key findings

- CAC/Ca showed a 4~102% increase in water uptake capacity compared to CAC.
- The maximum discharge power of CAC/Ca increased from 2 kW/m3 to 20 kW/m3 with higher air velocity.
- CAC/Ca achieved a heat-release density of 547 kJ/kg at 2.5 m/s air velocity.

## Abstract

The water adsorption property was shown to be the critical process limiting the thermal output in the adsorption heat storage driven by the air humidity process, which was different for the physical adsorbent and the physical/chemical adsorbent. In this study, coconut shell-based activated carbon (CAC), a hierarchically porous material that is both low-cost and mass-producible, was utilized as a physical adsorbent and as a matrix for loading calcium chloride (CAC/Ca). The incorporation of calcium chloride in CAC, with a 24% content, resulted in a 4~102% increase in water uptake capacity. The water uptake dynamics of high-thickness adsorbents are inhibited, especially for CAC/Ca. In the context of the adsorption test conducted within a fixed-bed reactor, an increase in air velocity was observed to facilitate water vapor supply, thereby culminating in higher output temperatures for both CAC and CAC/Ca, indicating a higher hydration conversion. The maximum discharge powers of CAC/Ca increased from 2 kW/m3 to 20 kW/m3, with the air velocity increasing from 0.5 m/s to 2.5 m/s. The heat-release densities of CAC and CAC/Ca at the air velocity of 2.5 m/s were 156 kJ/kg and 547 kJ/kg, respectively.