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10:00   Parallel Session: Simulation and Design Tools
Chair: Stefano Rebay
20 mins
Stefano Clemente, Jonathan Demierre, Daniel Favrat
Abstract: The aim of this work is to present a design method for a CHP (Combined Heat and Power) system based on Organic Rankine Cycle. The procedure is based on the Energy Integration of the thermal streams and leads to a multi-objective optimization of the system. The chosen case study is a micro cogenerator coupled with a biomass boiler sized to satisfy the heat and power demands of a residential building. A real woody biomass composition has been chosen and analyzed, in order to define reliable flue gases composition and cooling profile. Moreover, four different heat load scenarios for an average-sized house have been taken into account: concurrent functioning of heating system and hot water production system (1), stand-alone operation of only one of the previous systems (2 and 3) and switch-off of both of them (4). The aim of the procedure is to integrate an ORC in this system, in order to satisfy also a certain part of the electricity demand of the house. The main variables of the cycle (i.e. the maximum and the minimum temperatures and the output mechanical power) was the optimization variables: the two chosen objectives was the maximization of the first law efficiency of the system and of its exergetic performances. An air condenser has been added to the system, in order to permit the dissipation of a certain heat load (in some cases this could increase the exergetic efficiency). The Energy Integration method permits to couple all the streams of the system (i.e. to simulate the ideal heat exchanger network) in order to achieve the Minimum Energy Requirement (MER) from the hot source (in the present case, the biomass boiler) for the given values of the output streams: in such a way the internal regeneration of the ORC is not more a configuration chosen a priori, but becomes one of the optimized parameters. The Pareto curves for different working fluids at each considered scenario have been compared, in order to find the best ones suitable for the studied application. Finally, the best system configurations in each scenario have been compared and a trade-off between them has been found, in order to determine the definite layout of a real system: in particular, the multi-objective optimization presented in this work allowed to take decisions on the working fluid, on the Organic Rankine Cycle size and parameters and on the heat exchanger network.
20 mins
John Harinck, David Pasquale, Rene Pecnik, Piero Colonna
Abstract: There is a growing interest in organic Rankine cycle (ORC) turbogenerators because of their ability to efficiently utilize external heat sources at low to medium temperature in the small to medium power range. ORC turbines typically feature very high pressure ratios and expand the organic working fluid in the dense-gas thermodynamic region. Performance assessment and design by means of fluid dynamic analysis thus requires CFD solvers coupled with accurate thermodynamic models of the working fluid [1]. As a result of these additional technical challenges and the fact that commercial interest in ORCs has gained only in recent decades, the fluid dynamic design of ORC turbines using CFD has not yet reached the same level of maturity and sophistication as that of steam and gas turbines. We present a steady-state three-dimensional CFD study of the Tri-O-Gen ORC radial turbine stage operating with toluene. The radial turbine stage consists of the high-expansion radial nozzle, the turbine rotor and the diffuser. They are coupled in the CFD model by adopting a mixing plane interface. The simulation is performed with a commercial Reynolds-averaged Navier-Stokes solver. In order to account for real gas behavior of the expanding working fluid, thermodynamic properties are calculated with the FluidProp library [2], implementing an accurate multiparameter equation of state [3]. A look-up table approach allows the CFD solver to obtain values for thermodynamic and transport properties in a computationally efficient way. Results are analyzed with the aim of providing guidelines for further improving the turbine performance and future work is also briefly discussed.
20 mins
Pietro Marco Congedo, Christophe Corre, Jean-Paul Thibault, Gianluca Iaccarino
Abstract: Organic Rankine Cycles (ORCs) are of key-importance when exploiting energy systems, such as power plants, with a high efficiency. Flexibility with respect to the characteristics of the heat source requires a design fitted to maximize the overall performance. The variability of renewable heat sources makes more complex the global performance prediction of a cycle. The thermodynamic properties of the complex fluids used in the process are another source of uncertainty. The need for a predictive and robust simulation tool of ORCs remains strong. A finite-volume solver has been recently developed for efficiently computing a turbine stage in ORC applications [1]; it includes advanced equations of state in order to properly take into account the complex fluids classically used in ORCs. The performance of the turbine stage is evaluated by using three criteria computed from the numerical steady solution : the turbine isentropic efficiency, the enthalpy jump and the relative temperature variation. Based on this experience, it is now planned to insert the local approach devoted to the sole turbine stage into a more global analysis of the whole cycle. This integration of the finely computed turbine stage into a more global cycle analysis will lead us to derive performance indices such as efficiencies based on the first and second laws as well as the net specific work of the cycle. The elements included in this analysis combine fluid properties with cycle working conditions, namely : high and low temperatures and pressures. In this cycle analysis the turbine is the only simulated component but its computed performances depend on the whole cycle conditions (super-heating or not, multistage expansion, etc). Because of the strong existing sources of uncertainty in ORC cycles, a second objective of this work is to take into account uncertainty quantification (UQ) to increase the reliability of the coupled local/global approach and the robustness of the proposed designs. Several UQ strategies, already used for robust optimization of dense gas airfoils [2] and shock-tube [3], will be taken into account. For the turbine stage design, a parametrization and optimization loop similar to those proposed in [2] will be used. The flexibility of the analysis and its potential to contribute to innovative designs will be assessed on several cycles and working fluids among a selection of those used in the literature.