THERMODYNAMIC SIMULATION AND EXPERIMENTAL VALDIATION OF A CASCADED TWO-STAGE ORGANIC RANKINE CYCLE

asme-orc2015 Tracking Number 197

Employing a zeotropic mixture as a working fluid in ORC allows for an exergetically favourable heat transfer to the evaporator due to the temperature glide. However, during heat discharge via the condenser, the temperature glide becomes a disadvantage. Therefore, a cascaded combination of a two-staged ORC, where the high temperature (HT) cycle is operated with a zeotropic mixture and the low temperature (LT) cycle is operated with a pure fluid in supercritical mode, facilitates both favourable heat uptake from the source as well as heat discharge to the environment [1]. As a test rig for according two-stage cycle innovations, an electrically heated CORC cycle was designed and commissioned at the University of Paderborn. To achieve a high efficiency in each cycle, the design strongly depends on the temperature level of the heat source. The integration of four electrical heating rods as a primary heat source into the HT cycle – each with 50 kW and one of them adjustable – the design enables for the specification of different temperature levels and the LT cycle is supplied with the unused thermal energy of the HT cycle. After successful commissioning of the two-stage CORC, experimental results are used to evaluate cycle and component performance in comparison to the intended design. For this purpose, a detailed cycle simulation is performed using EBSILON®Professional, which can be fed with the operating parameters. The aim is to complement the flexible test rig with a suitable thermodynamic model, which allows for the study of cycle variations, such as fluid changes, hardware design improvements, etc. First results on modeling and experimental validation are presented for a combination of two pure fluids that exemplify heat the integration between the HT and LT cycles. With a validated simulation tool based on energy and mass balances as well as suitable equations of state, the optimization of individual components of the CORC-test rig, such as heat exchangers, pumps, condensators, turbines, as well as working fluids can be carried out rapidly and at low cost. The long-term goal of the present project is to put a two-stage CORC system into practice. REFERENCES [1] B. Liu, P. Rivière, C. Coquelet, R. Gicquel, F. David, “Investigation of a two stage Rankine cycle for electric power plants”, Applied Energy, 100 (2012) 285–294.