Comparison of pump cavitation mitigation techniques in micro scale organic Rankine cycle systems

The reduction of carbon dioxide emissions is becoming an ever increasing driving force in the development of innovative and technological solutions. The utilization of waste heat sources can help reduce carbon dioxide emissions. Various technological options exist, one of which is the Organic Rankine Cycle. Large scale commercial Organic Rankine cycles are already in operation, however, their transition into smaller scale systems is hindered by several technical challenges one of which is pump cavitation. Several cavitation mitigation techniques have been stated in the literature to reduce cavitation with three of the most practical being stored liquid column height, subcooling and pump bypass. The effects of all three of these techniques on the overall performance has been experimentally investigated on a micro scale ORC system designed for such purpose. Initially, the effects of 2.1 meters of liquid column height and by virtue system mass was investigated to determine its influence on the ORC cycle at; four differing pump speeds (600, 800, 1000 and 1200 rpm), three cold sink temperatures (10°C, 15°C and 20°C) and three expander loads. The effects of stored liquid column height had significant effect on the operation of the system. The maximum output power of 300W occurred at 1000 rpm with an optimum expander output trend present in most cases. At the highest tested 1200 rpm case the maximum boundaries of the system where insufficient to result in stable system operation, resulting in a maximum output power of only 240W. A reduction in cold side temperature results in no observable change in the stored liquid column height, with a general increase in expander output as a result of increased pressure ratio across the expander. The expander load had negligible effect on the low pressure side of the system both in terms of stored height and pressure. The experimental test rig was then modified to include a subcooler and pump bypass. The system was tested with increasing levels of system mass at various pump speeds with multiple degrees of subcooling and bypass valve angle. The combined influence of liquid column height with both subcooling and bypass effects was observed for each pump speed. The inclusion of the system bypass provided no observable benefit to the experimental test rig and only resulted in a deterioration in system performance. The combined interaction between subcooling and liquid column height is clearly observable in the data, with only the combined effect providing significant influence to allow the test rig to operate at its maximum pump speed and therefore with maximum output power of 370W. However, although the incorporation of subcooling extended the operational range of the test system. The system efficiency improvement was negligible, 3.7% compared with 3.6% for the liquid height only system. The added system components and required additional lower temperature cold sink, provide further additional barriers. The practical implications of these findings highlight the continual struggle to improve the viability of micro scale ORC systems, where the performance of the pump remains a significant barrier