| Carbon dioxide (CO2) is being considered as a replacement for conventional hydrofluorocarbon refrigerants (HFC's) in air-conditioning applications. As a natural refrigerant, CO2 is less likely to have negative impacts on the environment than HFC's. The main challenge with CO 2 cycles is their low energy efficiency, caused by high throttling losses. Replacing the isenthalpic expansion by a polytropic expansion in an expander reduces these losses. The expander performance depends on the machine type and design. In expander systems, new control strategies must be explored. Integrated compressor-expander devices require identical rotational speed of both components, which complicates the system control. For integrated machines, matching of gas forces is necessary for maximum efficiency.; A variety of displacement mechanisms have been analyzed in compressor and expander application. The geometry of each machine has been optimized preliminarily. The mechanisms have been compared using a single tool for the performance simulation. This tool, based on a general formulation of displacement mechanisms, is applicable to compressors and expanders. It takes advantages of common characteristics of displacement devices while efficiently accounting for the differences and is one of the major contributions of this thesis. The simulation tool provides for the calculation of gas forces.; The reciprocating piston and rotary piston mechanisms have the best performance as compressors and expanders. System simulation was conducted and indicates that the optimization of the expander displacement is the preferable control scheme. It is possible to design an optimized integrated machine with compressor and expander mounted on a common shaft, which performs close to maximum efficiency over a range of operating conditions. Two-stage cycles with the expander coupled to either of the compressor stages are discussed.; A semi-hermetic R134a scroll compressor has been modified and used as expander. The isentropic and volumetric efficiency was 42% and 68%, respectively, indicating that machining tolerances in R134a compressors are too large for CO2 expanders. The leak across the scroll wall offers a good opportunity for improvement of the performance. |
