CAREER: Nonequilibrium effects in thermochemical energy storage: linking microstructure to thermal transport

Decarbonization of the buildings sector, which consumes over a third of the primary energy in the United States, is essential to meet climate and sustainability goals. Thermal loads (e.g., space heating and hot water) account for a large portion of building energy use, and this can be satisfied using a thermal battery that stores heat to match demand with supply. Among the different storage materials, thermochemical salts are promising for storing renewable energy as heat as they undergo reversible dehydration-hydration reactions with a higher energy density compared to phase change or sensible storage. However, these salt hydrates experience structural changes (i.e., mechanical stress) and hygrothermal instabilities (e.g., melting and dissolution) that reduce their energy density during cycling of the battery (charge-discharge). To this end, the overall research goal of this project is to provide a mechanistic understanding of the key factors governing thermochemical phase transitions and its impact on coupled heat-and-mass transport. This fundamental knowledge will enable the development of reversible thermal batteries with long-term stability, while training graduate and undergraduate students at the intersection of materials and thermal science. The educational goal of this project is to provide interdisciplinary and experiential learning opportunities for traditionally underrepresented students in STEM, as well as curriculum development for teachers to increase literacy about energy storage broadly. To bridge our understanding of thermochemical reactions across different length (molecular, micro, and macro) and timescales (chemical reaction vs. diffusion), three research objectives will be pursued: (i) Elucidating the kinetic limitations on transport using in situ thermal analysis at different temperatures and vapor pressures, (ii) Correlating reaction reversibility with structural stability, and designing composite materials that facilitate thermal cycling, and (iii) Developing a transient and multiscale model for coupled thermal-fluid transport in porous media to predict energy and power density. These objectives will be pursued in tandem with two education and outreach efforts targeted at broadening and deepening participation in STEM: (iv) recruiting a diverse pool of high school students and establishing engineering education for teachers, and (v) engaging first generation college students in interdisciplinary research. Overall, this approach will reveal structure-property relationships under realistic conditions, which will help address long-standing questions about the energy density and cyclability of salt hydrates for thermal energy storage. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.