Development of a small scale scroll expander

Graduation date: 2011The organic Rankine cycle (ORC) is a versatile approach in the utilization of waste heat in the production of electricity, clean drinking water, and cooling. The efficiency of these cycles must be maximized to make them economical. Parameters for the optimization of ORCs such as evaporator and condenser temperature have practical limits. This is why the expander has the greatest effect in increasing the efficiency of an ORC when recovering waste heat.\ud In this thesis the design, fabrication, and demonstration of a small scale scroll expander will be presented. Scroll expanders are positive displacement expanders that have consistently shown high efficiencies in literature. The fabricated expander was modified from a compliant scroll compressor. It was self-starting and had external control over the seal piston pressure. Spherical miter gears placed the power takeoff on the side of the expander which allowed the lubrication system designed by the compressor manufacturer to function properly. The gears had the unforeseen effect of acting as an oil mister which improved the lubrication of the scroll assembly. It was dynamically balanced to minimize vibration.\ud This expander was demonstrated with both R134a and R245fa as working fluids with varying rotational speeds, expansion ratios, and achieved isentropic efficiencies typically over 70%. It achieved a maximum isentropic efficiency of 87.2% and a maximum power output of 1 [kW]. The performance characteristics such as speed, torque, and power were measured\ud using two separate dynamometers. It was shown that the expander shows good off design performance and has an optimal pressure ratio of approximately 3.3. It was also shown that its performance is a weak function of rotational speed. It is important to note the significant impact that a larger diameter shaft and shaft seal can have on performance. Break-in is another important consideration when evaluating performance