(506b) Enhancing Building Thermal Management with Thermochemical Materials-Based Thermal Energy Storage

Buildings globally are significant contributors to energy consumption, with the United States playing a prominent role in this regard. Among the various energy demands, thermal loads stand out for their substantial contribution to CO2 emissions. Addressing these loads is pivotal for enabling deeper integration of renewable energy sources. On-site Thermal Energy Storage (TES) emerges as a crucial solution for managing peak loads and mitigating the intermittency of renewable energy sources. Thermochemical Materials (TCMs), comprising reactive pairs of inorganic salts and water vapor, offer promising advantages. Notably, they boast a higher theoretical energy density of approximately 500 kWh/m3 and demonstrate minimal self-discharge due to energy storage within chemical bonds. These characteristics make them ideally suited for compact, stand-alone solutions catering to daily-seasonal energy storage within buildings. However, TCMs encounter challenges, particularly regarding material instabilities at both the particle and reactor levels (packed beds of salt), resulting in suboptimal multi-cycle efficiency and elevated levelized costs of storage.

In conventional salt hydrates, water can comprise approximately 1/4–1/2 of the mass of the salt hydrate itself. For instance, in commonly investigated salts like MgSO4·7H2O, water occupies approximately 51% of the salt hydrate's mass, leading to significant volume and porosity changes during dehydration (approximately 71.8% volume reduction).

Previous investigations into the transitions of TCM salt hydrates between various hydrate phases have revealed different mechanisms during dehydration and hydration. While hydration may follow different pathways, such as direct solid–solid transition or dissolution and recrystallization processes, salt dehydration for most salts typically involves the diffusion of water molecules out of the salt hydrate crystal. Consequently, this water removal induces solid-state transformation, causing mechanical stress and strain on the crystal, leading to defects such as dislocations and cracks, ultimately resulting in salt pulverization. While both hydration and dehydration may contribute to the deterioration of salt hydrates over cycling, dehydration is considered the primary contributor to salt hydrate pulverization.

We have developed a model to predict the pulverization limit (i.e., critical size, Rcrit) of salt hydrates and validated it for various salt hydrates. Additionally, we demonstrated the effect of Rcrit on dehydration and hydration kinetics, providing insights into the long-term stability of salt hydrates and their composites. To determine the extent of salt hydrate pulverization due to expansion during cycling, we developed a theoretical model for a single spherical particle by solving coupled mechanical stress and mass diffusion equations. While this model assumes a perfectly spherical, defect-free particle with isotropic material properties, the final predictions are minimally affected by these assumptions, focusing instead on the rate of dehydration.

Based on our model, we optimized the synthesis recipe for TCM composites, preconditioning pristine salts to Rcrit before mixing them with the host matrix. TCM composites made using preconditioned salt hydrates exhibited mechanical integrity and stability for over 2000 cycles, compared to 20 cycles with as-received salt hydrates. Preconditioning salts minimize crack formation, slipping, and changes in expansion/contraction behavior within the host matrix, resulting in more predictable mechanical behavior during cycling and potentially better long-term performance of composite TCMs. Furthermore, we are developing correlations between material properties and reactor performance to identify pathways for enhancing heat and mass transport efficiency at the reactor level.