Exploring the potential of MgCl2 hydrates in fluidized bed reactors for large-scale thermal energy storage

Thermal energy storage can play a crucial role in the energy transition by decoupling supply and demand, allowing for increased variable renewable energy integration. Thermochemical energy storage using salt hydrates is especially promising due to their high theoretical storage capacity and low cost, but the focus has mostly been on small-scale residential applications. The present study consequently explores the potential of large-scale thermal energy storage with salt hydrates in scalable fluidized bed reactors. MgCl 2 supported on alumina particles was prepared and, along with unsupported MgCl 2 , tested in a lab-scale fluidized bed reactor. A map of feasible operating conditions was established based on the finding that particle agglomeration occurs upon hydration to MgCl 2 hexahydrate, combined with other material and process integration limitations. Subsequently, water hydration/dehydration (discharging/charging) cycles were successfully demonstrated at different temperatures and humidities with the supported salt, as well as with the unsupported salt. Finally, a techno-economic assessment was conducted for seasonal and medium-term heat storage from a combined heat and power plant using MgCl 2 in fluidized bed reactors. The supported salt was uneconomical in all scenarios due to the lower storage capacity and the more expensive support material used. The unsupported salt, however, returned competitive levelized costs of heat of 49.3 €/MWh for storage during passing weather fronts and 92.4 €/MWh for seasonal storage. Further research is therefore recommended to demonstrate the long-term stability of unsupported MgCl 2 in fluidized bed reactors or, alternatively, develop stable materials with a high active material fraction on low-cost supports.