Conceptualization, Thermodynamics, Kinetics, and Prototyping of Continuous Electrochemical Refrigeration
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Rajan, Aravindh
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Abstract
The rise of the average global temperature, the growing global population and rapid urbanization are expected to lead to a quadrupling of the global cooling demand. Countries with rapidly growing GDPs like India and Nigeria are poised to indulge in the rampant purchase of air conditioners. The state of the art in modern refrigeration and heat pumping is based on the vapor compression cycle. The refrigerants used in these units have a global warming potential (GWP) 1-4 orders of magnitude larger than that of carbon dioxide. One method of mitigating the effects of climate change brought about by the refrigeration sector, is the invention and development of zero GWP, or not-in-kind (NIK) technologies. Drawing inspiration from the vast recent progress made in flow batteries, this dissertation proposes and demonstrates an electrochemical refrigerator. First it conceptualizes the Brayton Electrochemical Refrigerator (BECR) and the Stirling Electrochemical Refrigerator (SECR). After developing low order models to describe the electrochemical cells in both these incarnations, the theoretical analyses are presented, revealing the key thermodynamic, kinetic, and operational parameters. Whenever possible, dimensionless figures-of-merit are introduced to guide future research. Then, the key material properties and tradeoffs pertaining to the half-cells that constitute an electrochemical refrigerator (and by extension, an electrochemical heat engine) are discussed. Based on these tradeoffs, the dissertation proposes using a specific class of half-cell reactions, that afford an entropy change different from that of conventional half-cell reactions. Half-cell reactions are screened, and the appropriate ones are chosen for the proof-of-concept electrochemical refrigerator. Then, the dissertation discusses the setup and performance of the BECR proof-of-concept experiment. This first demonstration of continuous electrochemical refrigeration achieves a peak COP of ~8, and a peak cooling load of ~1 W. Finally, this dissertation introduces the concept of Electrochemically Assisted Advective Cooling (EAAC), a cooling scheme that has the potential to achieve high cooling fluxes. At its core, it leverages the synergy of increasing mass and heat transfer coefficients with increasing Reynolds number to add electrochemically generated entropic heat absorption cooling as a topping cooling load to advective cooling. The proof-of-concept experiment is introduced, and its results and avenues for future improvement are discussed.
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2021-12-08
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Dissertation