Abstract
To date, ORC is the most efficient and economical approach for the recovery of low-to-medium heat to power. In general, a volumetric-type screw-expander is selected as the ORC’s engine core for power capacity less than 300 kW due to its superior performance and competitive cost. This thesis theoretically and experimentally studies on the system characteristics and performance behaviors of screw-expander ORCs. Steady-state models of the ORC’s components are theoretically developed and experimentally validated. Then, the model of the ORC system is established to predict its performance. In view of practical and economical applications of an ORC, a trade-offs analysis between cycle efficiency and amount of power output is performed for system optimization under limited heat source for subcritical ORCs and trans-critical ORCs. Two sets of ORCs are designed and developed, and a series of performance tests are done to explore the characteristics of these two ORC systems. (1)20kW screw-expander ORC: using R134a as working fluid, converted the heat of 60~85°C hot water into power. (2) 50kW screw-expander ORC: using R245fa as working fluid, converted the heat of 90~105°C hot water into power. A theoretical expansion model of screw expander is developed and compared with experimental data. The achieved performance of these two ORCs are promising, with expander efficiency of 72.5% and cycle efficiencies higher than the typical efficiencies reported for the considered temperature range. The ORCs can be used to exploit the low temperature heat, as low as 60°C, with high performance which predict their wide application and potential energy saving.