Experimental Investigation of a Transcritical Organic Rankine Cycle with Scroll Expander for Low—Temperature Waste Heat Recovery

Organic Rankine cycle (ORC) is a common solution to recover low-grade waste heat. Transcritical cycle has been identified through thermodynamics analysis as a way to improve cycle efficiency and recover more heat from waste sources. However, few experimental tests were conducted to confirm the interest of transcritical ORC and investigate operational behaviours. In this study, experiments on a transcritical organic Rankine cycle are presented. The ORC includes a diaphragm pump, two plate heat exchangers (PHE) for heating, a modified 10 kWe scroll compressor used as an expander and a PHE condenser. A PHE economizer can be connected to compare standard and regenerative configuration. The working fluid used is the R-134a refrigerant with a critical pressure and temperature of 40.5 bar and 101.2 °C. The prototype is tested at different conditions, with/without expander, in regenerative or standard configuration, with a total of 70 steady-points. The heat source temperature ranges from 55 °C to 120 °C with a maximum power of 160 kWth. The scroll expander operates with an inlet pressure up to 44 bar and a pressure ratio around 2, producing a maximum power of 6 kWe with a 66 % expander efficiency at 1600 rpm. Due to the high pump consumption, the maximum net power measured is 1.5 kWe, resulting in a maximum energetic and exergetic net efficiency of 1 % and 4.5 % respectively. The subcritical and transcritical operations have been compared in this work. The expander shows a higher output power and efficiency at transcritical conditions. However, the ORC performances are comparable for the subcritical and transcritical conditions, mainly because of the increased pump consumption. Improvements opportunities are identified through energetic and exergetic analysis of the losses. Better management of the condensation pressure and higher pump performances could substantially increase the global performances of the ORC.