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Session: System design and optimization V
Session starts: Tuesday 08 October, 14:00
Presentation starts: 14:40
Room: Ruys & Rijckenvorsel Zaal

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Daniël Walraven (KU Leuven)
Ben Laenen (VITO)
William D'haeseleer (KU Leuven)

Abstract:
An ORC can be split into two levels. The first level contains the cycle parameters (temperatures and pressures) and the second level contains the components (heat exchangers, turbine, etc.). Often, both levels are optimized seperately and the interconnection between these two levels is chosen, based on experience. Pinch point temperature differences, pressure drops, etc. are assumed. This design process will in general not lead to the optimum system configuration. In a system optimization, the two levels of the ORC are optimized together and no assumptions have to be made about the interconnecting parameters. In this presentation, a simple version of a system optimization is explained. For a given heating and cooling source and for a fixed pump and turbine efficiency, the configuration of the heat exchangers is optimized together with the cycle parameters. Plate heat exchangers with chevron-type corrugations and shell-and-tube heat exchangers are modeled and can be used in the cycle. Single- and double-pressure cycles are investigated and both cycles can be a standard or a recuperated one. The developed model is based on a previous developed code and this code is extended with heat exchanger models which are taken from the literature. The Bell-Delaware model is used for the shell-and-tube heat exchangers. For single-phase plate heat exchanger the method of Martin is used while the models of Han, Lee and Kim are used for plate-type condensers and evaporators. The results show that plate-type heat exchangers are, as expected, more efficient than shell-and-tube heat exchangers. It is also shown that adding a second pressure level is useful, but the effect is less than in the case with ideal components. This is because adding an extra pressure level improves the fit between the heat source cooling curve and the working fluid heating curve, but this will also increase the heat transfer surface. So, the pinch point temperature difference has to increase to keep the heat transfer surface constant. A similar effect is seen in the recuperator. The average temperature difference in the desuperheater is often relatively high, so replacing it partly with a recuperator, in which the average temperature difference is mostly lower, will also increase the heat transfer surface. This model will be further extended with a turbine model, a model for air cooled condensers and a model for mechanical draft water cooling towers. Economics will also be taken into account.