(66g) Computational Design of Metal-Organic Frameworks for Energy-Efficient Adsorption-Based Refrigeration Systems

Energy used for refrigeration, air conditioning and heat pump processes accounted for 17% of the global electricity consumption in 2015 and is expected to further increase. Adsorption-driven refrigeration has emerged as a promising and more energy-efficient alternative to traditional vapor-compression systems, since it can utilize lower-grade thermal energy such as solar energy and industrial waste heat. The choice of working fluid/adsorbent working pair is critical to the performance of an adsorption refrigeration system. Metal-organic frameworks (MOFs), due to their large surface areas and tunable chemical environments, have the potential to be excellent adsorbents for a variety of working fluids. Here, I will present my recent work on computational modeling and optimization of MOF-based adsorption refrigeration systems. In the first part, we tried to establish a structure-property relationship between the MOFs’ structures and their working capacities, using ethanol as the working fluid. We focused on a set of zirconium-based MOFs with the same building blocks but different topologies and pore sizes. Using grand canonical Monte Carlo (GCMC) simulations, we were able to predict the working capacity of each MOF/ethanol working pair by simulating the ethanol adsorption isotherms in those MOFs. Our results suggest that working conditions with lower temperatures (such as ice-making) favor MOFs with smaller pores, and vice versa. Semi-quantitative relationships between the pore sizes and the working capacities were established and verified by experiments. In a follow-up study with propane and isobutane as working fluid (which have been recommended by EPA as freon substitutes), we were able to further validate our structure-property relationships. The working capacity of the NU-1003/isobutane working pair, one of the best pairs we discovered in this study, is 3 times larger than the MIL-101/isobutane pair which was previously the best one reported in the literature. We hope that our results can guide the design of materials with superior performance for adsorption-based refrigeration, which is a promising solution to reduce global energy cost and carbon emissions in the future.