Development of a capillary-assisted low-pressure evaporator for sorption heat transformers and thermal storage systems

Vapor compression refrigeration is the dominant technology used in air conditioning systems in almost all industrial, automotive, and residential applications. Vapor compression refrigeration is reliable, efficient, and low-cost; however, it consumes a significant amount of electrical energy, which is generated mostly by burning fossil fuels. It also uses refrigerants such as hydrofluorocarbons or hydrochlorofluorocarbons that have a high Ozone Depletion Potential and Global Warming Potential. These all contribute greatly to global warming. One promising technology is the waste heat-driven sorption heat transformers that run on low-grade waste-heat sources with temperatures of less than 90 degrees Celsius. Sorption heat transformers use alternative environmentally friendly refrigerants, such as water that have no environmental impact and no Global Warming Potential. However, sorption heat transformers have not penetrated the market as they are bulky and inefficient due to their low heat removal capacity, low coefficient of performance, and low operating pressure. This PhD program aims to address the first challenge, i.e., a low heat removal capacity, which is due to poor heat and mass transfer of the existing evaporator designs and leads to high weight and volume. A fundamental study is performed to assess, model, and optimize the performance of Capillary-Assisted Low-pressure Evaporator (CALPE) with water as a refrigerant. Capillary rise and heat transfer models are developed and experimentally validated by especially designed testbeds. A new modular design is proposed that maximizes the heat removal capacity of CALPE. This is done by establishing an in-depth understanding of the capillary mechanism in a capillary-assisted low-pressure evaporator, leading to a fin design with a maximum heat transfer area. Additionally, fundamental characterization of the capillary phenomenon is performed to design optimal fin geometries. An analytical model is developed that includes pertinent geometrical and operational parameters and thermophysical properties and accurately predicts the heat removal capacity of a CALPE. A novel CALPE is designed and fabricated using 3D-printing to provide a proof-of-concept demonstration for an optimal CALPE. Finally, experimental validation of the analytical models to predict the novel CALPE sample using custom-built test beds is given. The methodology of this PhD research project can be applied to various sorption heat transformers applications.