EXPERIMENTAL INVESTIGATION OF A SMALL-SCALE TWO STAGE ORGANIC RANKINE CYCLE ENGINE OPERATING AT LOW TEMPERATURE

asme-orc2015 Tracking Number 87

A prototype two-stage heat-to-power engine based on the Organic Rankine Cycle (ORC) has been developed for operation at a wide thermal load input range, coming from variable thermal sources, such as evacuated tube solar collectors [1]. The system is used to produce electrical energy through the expansion of a refrigerant (R245fa) in two scroll expanders which are connected in series [2]. The intense fluctuation of the temperature and heat input dictates the use of a two-stage engine, for flexible and efficient operation even at low thermal load, thus these expansion machines can operate within a narrow pressure ratio range, showing high expansion efficiency up to 70%. When operating at high heat input both expanders operate, while for low heat input, the first expander is completely by-passed. The net capacity of the ORC engine is 10 kWe, when supplied with 100 kW of heat at a temperature of 140 °C. This engine has been tested in an appropriate test-rig at the laboratory, using an electrical heater. The power produced by the two hermetic scroll expanders is driven to an electric panel through inverters. The present study examines the ORC engine testing, when the heat transfer fluid is water with temperature of 95 °C. The engine is tested under both single- and two-stage configuration. The measurements have shown that even at such low-temperature the thermal efficiency is adequate (up to 7% for single- and two- stage operation), and that the second expander can operate with increased isentropic efficiency up to 66%, while for the first one this value is much lower, due to its under-expansion (pressure ratio 1.7-2.7 for the first and 3-7 for the second expander). The maximum thermal efficiency is observed for low pump speed, while the highest power production of 3.5 kW was noticed at single-stage operation, insignificantly higher than the 3.3 kW produced at the two-stage operation. Acknowledgement: The present work is conducted within the framework of the project with contract No. 09SYN-32-982, partly funded by the Greek General Secretary of Research and Technology (GSRT). REFERENCES [1] G. Kosmadakis, D. Manolakos, E. Davou and G. Papadakis, “Implementation of a two-stage Organic Rankine Cycle using scroll expanders, operating under variable heat input”, Presented at the 2nd Int. Seminar on ORC Power Systems (ASME-ORC2013), Rotterdam, The Netherlands, 7-8 October 2013. [2] V.M. Nguyen, P.S. Doherty and S.B. Riffat, “Development of a prototype low-temperature Rankine cycle electricity generation system”, Appl. Therm. Eng., Vol. 21, no. 2, pp. 169–181, (2001).