Understanding the Transport Properties of 4th Generation Refrigerants in Metal-Organic Frameworks by Molecular Simulations

Abstract

Upon the high climate change risks, a global effort of phasing out high Global-Warming Potential (GWP) refrigerants has been taking place from 2016 onwards, when the Kigali's agreement was signed by several countries, officially implemented since 2018. The phase out plan according to the Kigali amendment of the Montreal protocol includes the widely used R-134a refrigerant along with many other Hydroflurocarbons (HFCs). Meanwhile, the demand for cooling and air conditioning is on the rise since the conventional air conditioning technology is becoming much more economically affordable. Consequently, we face a steady annual increase in global energy consumption for cooling, accounting for one fifth of the energy consumption worldwide. New proposed fourth generation refrigerants, including R-1234(yf) and R-1234ze(E) and their blends, have been widely studied and proven to sufficiently perform as drop-in replacement for phased out R-134a. Conversely, an adsorption-based cooling process is considered a potential alternative for the widely used, energy-intensive, conventional vapor compression cycle. This MSc work belongs to a long term project in our research group in order to identify the most suitable Metal-Organic Frameworks (MOFs) for adsorption of fourth generation refrigerants along with the investigation of their adsorption behavior for sustainable cooling applications. Transport properties are key to understand the adsorption mechanism of the refrigerants in the MOFs and they are essential to evaluate the kinetics of the adsorption/desorption process where refrigerants transport in and out the MOFs in an adsorption-based thermal energy storage unit. Hence, the goal of this MSC Thesis is to investigate, for the first time, the transport properties of fourth generation refrigerants in the confinement of promising MOFs for cooling applications. For this purpose , Molecular Dynamics simulations have been applied to calculate the transport and structural properties of three refrigerants, namely R-134a (as a benchmark), R-1234yf and R-1234ze(E), and two of their blends, namely R-513A and R-450A, upon adsorption in six MOFs, namely Mg- and Zn-MOF-74, Cu-BTC, MOF-177, MOF-200 and NU-100, which were identified as promising for thermal energy storage. For all possible 30 combinations of working pairs, the self-diffusivity coefficient, the shear-viscosity coefficient, Radial distribution function and probability distribution function have been reported. Main findings showed R-1234yf to have the highest self-diffusivity coefficient and the lowest shear-viscosity coefficient in all six MOFs while R-1234ze(E) showed the lowest self-diffusivity coefficient and the highest shear-viscosity coefficient all at 298.15 K. Similarly, R-513A showed a higher self-diffusivity coefficient and a lower shear-viscosity coefficient in comparison with R-450A. Looking at the MOFs, all refrigerants have the lowest self-diffusivity coefficient and the highest shear-viscosity coefficient in Mg-MOF-74. n. With similar transport properties, the behavior in the five remaining MOFs are ordered in direction of increasing refrigerant self-diffusivity coefficient, Zn-MOF-74 < Cu-BTC < Nu-100 < MOF-177 < MOF-200. Moreover, the effect of metal nodes and pore structure on the refrigerant transport properties, cluster forming, biased adsorption and molecules' orientation are reported and discussed enlightening the effect of the microscopic behavior on the macroscopic performance of the systems and complementing studies solely based on adsorption.

Date of Award	Jul 2021
Original language	American English