Experimental investigation of low-temperature fluidised bed thermochemical energy storage with salt-mesoporous silica composite materials

Low-temperature thermochemical energy storage (TCES) with composites ‘salt in porous matrix’ (CSPMs) is widely recognized as a sustainable and efficient solution for harnessing low-grade heat and off-peak electricity. However, many high-performance CSPMs described in the literature have been produced in powder form with particle sizes below 50 μm, which makes them unsuitable for direct use in conventional fixed-bed or fluidised-bed systems. Fluidised-bed systems, highly regarded for their rapid heat and mass transfer advantages, have been extensively used in high-temperature TCES. However, their potential for low-temperature TCES applications remains unexplored due to the lack of suitable thermochemical sorption materials. In this work, we aim to investigate the feasibility of fluidised-bed TCES systems for low-temperature TCES applications using the self-developed fluidisable and high-performance CSPMs. A series of CSPMs were prepared using a commercial mesoporous silica (CMS) as the host matrix and CaCl2, MgSO4 and MgBr2 as the salts, with the same salt loading level of 50 wt% and particle size range of 150–300 μm. A lab-scale fluidised-bed TCES system was constructed for a comprehensive assessment of the material properties, including minimum fluidisation velocity (umf), water adsorption capacity, temperature lift, and energy storage density (ESD). The results show that the salt/CMS composite powders can be easily fluidised with a umf of approximately 0.01 m/s and provide efficient solid mixing during bubbling fluidisation. Among the tested CSPMs, the CaCl2/CMS composite shows the best heat-discharging performance. Specifically, the CaCl2/CMS composite, when hydrated at 30 °C and 60% relative humidity, has a ESD of 1508 kJ/kg (equivalent to 264 kWh/m3) and provides a maximum bed temperature of 58 °C. In addition, it exhibits excellent stability for use in the fluidised-bed system, with similar fluidisation characteristics and ESDs after multiple cycles of heat charging and discharging processes. This work is believed to inspire future research on the development of CSPM powders.