Optimal thermal compressors for miniaturized ammonia-water absorption systems
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The use of conventional vapor-compression refrigeration and cooling systems is accompanied by a significant demand for high grade energy, typically in the form of electricity. Heat driven chillers, on the other hand, can use a variety of low-grade thermal energy sources such as waste heat and solar thermal, and their widespread application can significantly improve overall global energy utilization efficiency. Recent advances in microscale heat and mass exchangers have attracted interest in the development of small-capacity vapor-absorption systems. In the first part of this study, a state-of-the-art small capacity ammonia-water absorption chiller is developed. The results of this development provide guidance for more detailed investigation into specific components and subsystems. While thermal compressor optimization emerges as one of the key focus areas, the core work in this study addresses the development of optimal vapor generation and purification components, i.e., the desorber and rectifier. Optimal component designs are obtained based on a thermodynamic framework for quantitative thermal compressor characterization. Two novel design concepts are proposed and their feasibility is established with a hydrodynamic investigation. Non-equilibrium heat and mass transfer models are then developed for the design of full-scale components. An experimental evaluation of both design concepts is conducted, whereby performance at the sub-component, component, and thermal compressor levels was evaluated. At the sub-component level, local temperature, flow and pressure measurements are used to determine heat and mass transfer coefficients. Component-level results enable refinement of the developed heat and mass transfer model. Finally, assessment of the thermal compressor as a single entity is validated, and a control algorithm is developed for optimal system operation over a wide range of operating conditions. Therefore, results from this study guide the design of optimized thermal compressors and development of their control systems for application in small-capacity absorption heat pumps.