Toward Optimal Metal–Organic Frameworks for Adsorption Chillers

Abstract

The metal–organic frameworks (MOFs) MIL-101(Cr) and NH₂-MIL-125 offer high adsorption capacities and have therefore been suggested for sustainable energy conversion in adsorption chillers. Herein, these MOFs are benchmarked to commercial Siogel. The evaluation method combines small-scale experiments with dynamic modeling of full-scale adsorption chillers. For the common temperature set 10/30/80 °C, it is found that MIL-101(Cr) has the highest adsorption capacity, but considerably lower efficiency (−19%) and power density (−66%) than Siogel. NH₂-MIL-125 increases efficiency by 18% compared with Siogel, but reduces the practically important power density by 28%. From the results, guidelines for MOF development are derived: High efficiencies are achieved by matching the shape of the isotherms to the specific operating temperatures. By only adapting shape, efficiencies are 1.5 times higher. Also, higher power density requires matching the shape of the isotherms to create high driving forces for heat and mass transfer. Second, if MOFs’ heat and mass transfer coefficients could reach the level of Siogel, their maximum power density would double. Thus, development of MOFs should go beyond adsorption capacity, and tune the structure to the application requirements. As a result, MOFs could to serve as optimal adsorbents for sustainable energy conversion.