Understanding the Adsorption of the 4th Generation Refrigerants in the Metal-Organic Framework for Energy Storage

Abstract

The global increase in energy demand caused problems related to pollution and climate change to occupy bigger attention. Therefore, scientists are searching for sustainable and greener solutions to help preserve the environment and natural resources and at the same time maintain the required energy demand. Refrigeration is an energy intensive process, yet it is considered a necessity, particularly in hot and arid countries. Current refrigeration processes are based on vapor compression cycle, where waste heat generated from the condenser is rejected to the environment. A possible solution to make this process more energy efficient is to include an energy storage unit that works by adsorption/desorption, where this will act as a modulator between supply and demand of energy. Previously reported adsorbate-adsorbent pairs suffer from low energy density and this is attributed to the limitations related to the thermophysical properties of the adsorbate-adsorbent pairs. In this MSc Thesis, fourth generation refrigerants, i.e. hydrofluoroolefins - R1234yf and R1234ze(E), were studied as adsorbates since they are candidates to replace the currently used, high global warming potential refrigerant, hydrofluorocarbon - R134a, as agreed globally in various protocols. Amongst the various adsorbents, metal-organic frameworks (MOFs) appear to be promising candidates for this application, thanks to their high tunability, stability and porosity. A combined experimental and simulation approach has been used to study the adsorption of R134a, R1234yf and R1234ze(E) refrigerants in M-MOF-74 (M=Ni, Mg, Zn), Cu-BTC, MOF-177 and MOF-200 to have a deeper understanding of the adsorbate-adsorbent interactions and the role that the metals and the geometry of the MOFs play in the final adsorption behavior. For the first time, experimental results on the adsorption of R1234yf on any MOF have been reported. Additionally, the adsorption of R134a on the studied MOFs have not been experimentally tested, to the best of our knowledge. The main findings are: i) Cu-BTC has the highest binding enthalpy upon interacting with any of the refrigerants, ii) compared to its family of analogues, Mg-MOF-74 showed the highest heats of adsorption and uptakes for all three pure refrigerants, iii) MOFs' metal-sites represent the preferred adsorption sites for fluorine atoms of the refrigerants due to the strong F----M+ interactions, and iv) comparing the three refrigerants studied, R1234ze(E) has the highest heat of adsorption when interacting with any of the MOFs.

Date of Award	Dec 2020
Original language	American English