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11:10   Parallel Session: Simulation and Design Tools
20 mins
Francesco Casella, Tiemo Mathijssen, Piero Colonna, Jos van Buijtenen
Abstract: Several applications of ORC power systems demand for stringent control requirements. Examples are solar conversion by means of a direct vapour generator [1], heat recovery from automotive engines [2] and operation in island mode, connected to a local grid and local loads, as it would be the case for instance for installations in remote areas of developing countries. In all these cases normal operation is intrinsically dynamic and a suitable control strategy must be implemented. Dynamic models are an indispensable tool to study and optimize the design the control system [3][4]. A dynamic model suitable for ORC power systems and flexible in terms of plant configuration and working fluids is presented. As an example, the model of the Tri-o-gen power unit is obtained by assembling components and modules of the Modelica ThermoPower library linked to the FluidProp package for the calculation of fluid thermophysical properties. Simulation results are validated by comparison with field data obtained from ad hoc experimental runs. A discussion about possible control strategies is briefly outlined.
20 mins
Vincent Lemort, Sylvain Quoilin, S├ębastien Declaye, Assaad Zoughaib
Abstract: The behaviour of the ORC has been extensively studied theoretically and experimentally in the past few years. Most of the proposed ORC models are steady-state models, accounting for stabilized working conditions. However, some ORC applications are used with inherently variable heat source inputs. The case of heat recovery on internal combustion engines is a good example of varying heat source flow rate and temperature in very short periods of time. ORC cycles coupled to solar applications can also be highly dynamic in the absence of storage since direct radiation can fluctuate quickly depending on the climatic conditions. Steady-state models are not able to predict the transient behaviour of the cycle with such heat sources, nor can they simulate a proper cycle control strategy during part-load operation or start & stop procedures. This paper proposes a dynamic model of an ORC by focusing specifically on the dynamic behaviour of the heat exchangers, the dynamics of the other components being of minor importance. The model is developed under the Modelica environment using the TILMedia library for the computation of the working fluid thermodynamic properties. This model is specifically designed to ensure the robustness and the speed of the simulation algorithms: - Initialization strategies are developed by simplifying the problem at time zero in order to avoid too complex non-linear initial systems of equations. - Simplified heat transfer laws are proposed based on heat transfer correlations available in the scientific literature. - Numerical issues such as division by zero are avoided by linearising or limiting to a finite value some thermophysical equations or properties. The studied system is a small-scale waste heat recovery system using a scroll machine as expansion device. The heat exchangers are plate heat exchangers and the pump is a diaphragm pump. In order to control the system under transient conditions, the set point of the evaporating temperature is optimized using a steady state model and implemented into the control unit, where it is continuously re-adjusted to the optimal value. The two control variables are the superheating at the expander inlet and the evaporating temperature. Two types of controllers are implemented and compared in terms of performance (seasonal performance of the system), safety (achievement of set points), and robustness: a feedback control strategy based on two PID controllers and a model predictive control strategy (MPC). The potential of the later controller over classical PID is discussed.
20 mins
David Pasquale, Antonio Ghidoni, Stefano Rebay
Abstract: During the last decade, organic Rankine cycle (ORC) turbogenerators have become very attractive for the conversion of low-temperature heat sources in the small to medium power range. ORCs usually operate in thermodynamic regions characterized by high pressure ratios and strong real-gas effects in the flow expansion, therefore requiring a non-standard turbomachinery design. In this context, due the lack of experience, a promising approach for the design can be based on the intensive use of computational fluid dynamics (CFD) and optimization procedures to investigate a wide range of possible configurations. In this work an optimization strategy which aims to increase the performance of ORC turbines is presented. The capability of this strategy is demonstrated by analyzing an existing turbine, which is an impulse one-stage radial turbine where a strong shock appears in the stator channel due to the high expansion ratio. The goal of the optimization is to minimize the total pressure losses produced by the shock and to obtain a uniform anular flow at the stator discharge section, in terms of magnitude and direction of the flow velocity. To achieve this purpose, a global optimization method and a computational fluid dynamic solver are adopted. In particular, the optimization strategy is based on the coupling of a Genetic Algorithm with a surrogate-model (Kriging). The numerical solutions of the two-dimensional Euler equations are computed with the in-house built code zFlow [1]. The working fluid is toluene, whose thermodynamics properties are evaluated by an accurate equation of state, available in FluidProp [2]. The computational grids created during the optimization process have been generated through a fully automated 2D unstructured mesh algorithm based on the advancing-Delaunnay strategy [3].