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Study Indicates Engineered Pore Environment Enhances Attachment of Refrigerants

Research findings may support development of more efficient cooling systems

July 2017
Study Indicates Engineered Pore Environment Enhances Attachment of Refrigerants

Posted as “Just Accepted” by the Journal of the American Chemical Society (JACS), the article will include cover artwork being developed at PNNL. Metal—organic frameworks (MOFs) are constructed of metal ions/clusters connected by organic linkers, or bridging-ligands, whose geometry and connectivity dictate the structure of the MOFs. By adjusting linker geometry and other characteristics, the size, shape, and internal surface properties of MOFs can be tuned for targeted applications, such as cooling systems.

Recent research findings at PNNL may ultimately help lower energy consumption in homes and buildings across the nation. A study—“Pore-engineered metal-organic frameworks with excellent adsorption of water and fluorocarbon refrigerant for cooling applications”—investigated the effect of the pore environment on the adsorption of refrigerants, specifically water and fluorocarbon R134a, for potential use in adsorption cooling applications.

Heating and cooling systems account for almost half of the electricity used in the typical U.S. home. New more efficient, sorption-based cooling systems could significantly reduce the amount of energy needed to heat and cool homes or buildings. The technology works by taking advantage of the adsorption and desorption—the attachment and release—of vapor on and from the surface of a solid. As the vapor absorbs and desorbs, heat is generated and consumed, a process that for decades has been widely used in cooling applications. Unlike a mechanical compressor that requires a great deal of electrical power, heat transformation in an adsorption cooling system can be driven by waste heat or solar thermal energy.

Findings Reveal “Cool” Possibilities

The efficiency of adsorption cooling systems depends on identifying the best adsorbate-adsorbent working pairs. In this case, PNNL researchers examined water and fluorocarbon R134a vapors and their ability to attach to the surface of and fill the pores of a pore-engineered metal-organic frameworks (MOFs), in particular Ni-MOF-74. Among fluorocarbon refrigerants, R134a—a colorless, odorless gas derived from ethane—is the most promising for household, commercial, and automotive air conditioning because of the gas’s low boiling temperature and zero ozone depletion potential. MOFs are versatile nanoporous materials whose characteristics offer many possibilities for controlling their properties, including their surface pores.

“Highly modular MOFs, designed to provide thermal and chemical stability, high surface area, and large pore volumes, exceeded expectations for refrigerant sorption. Such MOFs provided enhanced loading capacities for applications like novel adsorption chiller systems,” said Radha Motkuri, who leads the materials development team.

The research team found that fluorocarbon R134a refrigerants exhibited exceptionally high gas uptake, or loading, and reversible loading in these porous MOFs. “This means fluorocarbon/MOF working pairs can be exploited for eco-friendly adsorption cooling applications because higher sorbent working capacity directly translates to smaller size cooling system designs,” said Pete McGrail, who leads the development of adsorption cooling technologies at PNNL.

Understanding the effects of pore structure and framework topology on the adsorption of guest molecules in pores is crucial for designing more effective MOFs. By determining the effects of pore-engineered MOF-74 on the sorption properties of water and fluorocarbon refrigerants, the team identified a fluorocarbon/MOF combination that has the potential to be one of the most promising working pairs for use in adsorption cooling applications. “To our knowledge, our article is the first to report adsorption isotherms of fluorocarbon R134a in MOFs and one of the highest water sorption materials at high relative humidity,” Motkuri added.

Results suggest pore engineering is an excellent strategy for enhancing the loading of guest molecules—and their associated cooling capacity—in the pores of MOFs used in sorption-based cooling systems. In addition, the authors found that these MOF-74 materials are highly infrared sensitive even at extremely low concentrations (water at 0.00001 mbar and R134a at 0.001 mbar), which may open up a new area of refrigerant leak detection, a major problem in the adsorption cooling/refrigeration industry. Generally leaks lead to a reduction in cooling efficiency and increased power consumption.

The reported research was funded by DOE’s Geothermal Technologies Office (GTO). GTO also funded PNNL’s multilayered geothermal Harmonic Adsorption Recuperative Power (HARP) project, which includes “materials development.” The latter requires better understanding the host-guest chemistry to find the best host material—in the case of this research, pore-engineering coupled with adsorption of water and R134a. The novel HARP system uses low-grade heat to generate electric power and may help industry reduce costs and increase efficiency.

PNNL Research Team: Jian Zheng, Rama S. Vemuri, Luis Estevez, Phillip K. Koech, Tamas Varga, Donald M. Camaioni, Thomas A. Blake, B. Peter McGrail, and Radha Kishan Motkuri.

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