Toward Optimal Metal–Organic Frameworks for Adsorption Chillers: Insights from the Scale‐Up of MIL‐101(Cr) and NH₂‐MIL‐125

Abstract

The metal-organic frameworks (MOFs) MIL-101(Cr) and NH 2-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 (À19%) and power density (À66%) than Siogel. NH 2-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.

Water and Metal–Organic Frameworks: From Interaction toward Utilization

“…A comparison of such a packed bed performance of MIL-101(Cr) and NH 2 -MIL-53(Al) shows that mass and heat transport are slow in these systems, resulting in lower efficiency and power density than a silica gel. 336 Betke et al improved the heat transportation of CAU-10 (0.08 W m –1 K –1 for compacted pellet) by intergrowth with reticulated open-cellular alumina foams (0.9–1.78 W m –1 K –1 ). 349 Heat exchangers coated with the adsorbent are by far superior to packed beds, similar as for catalytic reactors, 28 in terms of power delivered per mass of adsorbent.…”

“… 396 , 397 The use of binders and coatings is used to alleviate this drawback. 280 , 336 Because not only conduction plays a role but also the heat capacity of the materials, a better parameter to characterize the thermal transport is the thermal diffusivity a = λ/(ρ· c p ) [m 2 s –1 ] that describes how fast temperature profiles move in the system (see Figure 2 ). The low heat capacity is positive in this sense, but particle and bed porosity and conductive support structures have an adverse effect.…”

“…Mass transport occurs by pore diffusion, 336 strongly coupled with adsorption and heat release, phenomena having a strong impact on the response time of the application. 17 , 73 , 280 As in heterogeneous catalytic processes, one has to combine intensive (scale independent) and extensive (scale dependent) variables.…”

Metal‐Organic Frameworks as Sorption Materials for Heat Transformation Processes

“…kW m –3 ) MOFs are only partly competitive to commercial silica gels due to the low specific density of MOFs. [ 140 ] Therefore, MOFs have to become more competitive in terms of cost per (volume) specific power. This may either be achieved by adjusting material characteristics in terms of higher uptake per volume of MOF or by improving heat and mass transfer by optimized shaping technologies.…”

Experimental Investigation of the Adsorption and Desorption Kinetics on an Open-Structured Asymmetric Plate Heat Exchanger; Matching Between Small-Scale and Full-Scale Results

“…For instance, some MOFs, such as MIL-100 and MIL-101 families, have recently attracted high attention due to their high capacity of water adsorption (Canivet et al, 2014). Graf et al (2020) have carried out experimental investigations on the dynamics of water adsorption in two different MOFs; namely, MIL-101(Cr) and NH2-MIL-125, and compared them with Siogel, which is characterized by a relatively lower capacity for water adsorption. The study showed that, for the common temperature set 10/30/80 °C, MIL-101(Cr) has demonstrated the highest adsorption capacity, but with significantly lower COP (−19%) and volumetric cooling power (−66%) than Siogel.…”