Impact of the expander lubricant oil on the performance of the plate heat exchangers and the scroll expander in a micro-scale organic Rankine cycle system

Abstract

Organic Rankine Cycles (ORCs) are recognized as suitable systems to convert the thermal energy from low-grade heat sources to electricity. However, their performance and reliability are subjected to several deficiencies especially at micro-to-small scales. In this work, the impact of the expander lubricant oil on the performance of the heat exchangers and the scroll expander of a non-regenerative, micro-scale ORC unit is investigated. In particular, the oil circulation rate (OCR) is theoretically assessed for each experimental data set based on the thermal balance between the hot and cold streams of the condenser. Then, the performance of the other components is assessed using the lubricant-R134a mixture properties, assuming the same OCR. Results have shown that the presence of the lubricant oil leads to 5–15% capacity loss of the evaporator and the condenser of the studied ORC system. The calculated mass charge of the evaporator can also be under estimated up to 6.5% approximately if the oil is neglected. In addition, neglecting the lubricant oil may lead to over-estimation of the expander mechanical efficiency and under-estimation of its volumetric efficiency up to about 50% in very low shaft speeds. Hence, despite it is usually neglected in the literature, the results of the present analysis show that the impact of expander oil is relevant in micro-scale ORC systems.