Dynamics of Vapor Compression Cycle with Thermal Inertia

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The use of heating, ventilation, air conditioning, and refrigeration (HVACR) systems is always increasing. Reducing energy consumption has become necessary in modern times for environmental, economic and legislative reasons. Thus, there is ongoing research to improve the performance and reduce the negative environmental impact of these systems. HVACR systems are normally sized for peak load conditions. As a result, these systems operate under off-peak conditions most of the time by on-off cycling. The average efficiency of the system during cycling is lower due to transient losses caused by refrigerant migration and redistribution. This motivates a detailed understanding of the dynamics of vapor compression systems (VCS) for their improved design and performance.

The dissertation contributes towards reducing energy consumption from HVACR by exploring both sides: improving the performance of current systems and developing highly energy efficient personal conditioning systems (PCS) which reduce the load of HVACR systems altogether. PCS reduce the energy consumption of building HVACR by up to 30%. Multi-physics modeling for thermo-fluid, electricity and mechanical domains is conducted to compare performance of four PCS employing different thermal storage options. The dissertation then focuses on vapor compression system based version of PCS called Roving Comforter, operating cyclically between its cooling and recharge mode. Exhaustive study of design space including optimization of thermal storage, operation with a natural refrigerant and alternate recharge modes is conducted to improve its overall coefficient of performance.

The dissertation then presents comprehensive dynamic validated modeling of air-conditioning systems operating in cyclic operations to characterize cyclic losses. Parametric study with different operating conditions is carried out to provide guidelines for reduction of these cyclic losses. Secondly, a physically based model of the test setup for quantifying the cyclic losses of air-conditioning systems is developed and used to understand its influence on the cyclic losses. A new term called “Thermal Inertia Factor” is defined to enable more uniform rating of equipment from various test centers and help selection of actual energy efficient air-conditioners.