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Session: Poster Session and Sponsors Exhibition
Session starts: Thursday 22 September, 14:00

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Johann Fischer (Universitaet fuer Bodenkultur Wien, Austria)
Martin Wendland (Universitaet fuer Bodenkultur Wien, Austria )
Ngoc Anh Lai (University of Science and Technology Hanoi, Vietnam)

Abstract:
For conversion of medium or low temperature heat to power one may think of using besides organic Rankine cycles (ORC) also trilateral cycles (TLC). In the TLC the liquid working fluid is pressurized and heated to its boiling point. Then it undergoes a flash expansion into the wet vapour region whereby it delivers work. Finally, the fluid is condensed. The advantage of the TLC is a very good match between the heating up curve of the working fluid and the cooling down curve of the heat carrier. The advantage of the ORC is that its cycle is closer to the Carnot cycle. Hence one would expect that the exergy efficiencies of both cycle types are similar. Recent papers, however, claimed that the efficiency of the TLC is 1.5 to 3 times higher than that of the ORC and hence we decided to reinvestigate that question. We consider optimized TLC- and ORC-systems which include the heat transfer from the heat carrier to the working fluid, the cycle process, and the heat transfer from the working fluid to the cooling agent. Optimization criterion is the exergy efficiency of the system for power production p being the ratio of the net power output to the incoming exergy flow of the heat carrier. Model calculations were made for five cases I to V specified by the inlet temperature of the heat carrier and the inlet temperature of the cooling agent. The inlet temperature pairs are for I (350°C, 62°C), II (280°C, 62°C), III (280°C, 15°C), IV (220°C, 15°C), V (150°C, 15°C). For TLC we use water throughout as working fluid and hence the only parameter for optimization is the boiling temperature. For the ORC we use different working fluids depending on the temperature interval. Their thermodynamic properties are obtained from the molecular based BACKONE and PC-SAFT equations of state. First, we searched for optimal ORC working fluids for the cases I and II considering alkanes, aromates and linear siloxanes in subcritical cycles (p/pc = 0.9) with and without superheating and supercritical cycles (p/pc = 1.2), all with internal heat exchange. Rankings based on the exergy efficiency p, the cycle thermal efficiency th and on the volume and the heat flow rates show cyclopentane to be the best working fluid for all studies of cases I and II which is caused by its only slightly overhanging dew line in the T,s-diagram. Moreover its autoigintion temperature is more than 100 K higher than the maximum cycle temperature considered. Hence, we used in the comparison between TLC and ORC as ORC working fluid for cases I to III cyclopentane, for case IV n-butane and for case V propane. It is found that the exergy efficiency p is larger for the TLC than for the ORC betwen 14% and 20% for cases I to IV and by 29% for case V. On the other hand, the outgoing volume flows from the expander are larger for the TLC than for the ORC by a factor ranging from 2.8 for case I to 70 for case V which is caused by the low vapour pressure of water for the low temperatures.