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14:00   Working fluids II
Chair: Prof. Piero Colonna
20 mins
Matthias Lampe, Marina Stavrou, Joachim Gross, André Bardow
Abstract: A simultaneous optimization of working fluid and process enables an optimal design of an ORC system. A new method that allows for the simultaneous optimization and thus a holistic design is introduced. In the presented framework, the PC-SAFT equation of state is employed as model for the working fluid properties. The simultaneous optimization is achieved by the continuous-molecular targeting computer-aided molecular design (CoMT-CAMD).
20 mins
Athanasios Papadopoulos, Mirko Stijepovic, Patrick Linke, Panos Seferlis, Spyros Voutetakis
Abstract: Organic Rankine Cycle (ORC) systems commonly utilize single working fluids to support power generation, although reports [1] indicate that mixtures enable a significant increase of the ORC performance. Despite the enormous importance of both single and mixed working fluids for ORCs, their selection is largely based on trial-and-error approaches applied in repositories containing conventional options. The authors have been the first to address the above limitation through the use of a Computer-Aided Molecular Design (CAMD) method applied in the design and selection of single ORC working fluids [2, 3]. The considerable advantages of the designed fluids compared to conventional choices constitute a significant motivation for the development and implementation of a CAMD-based method for the design of ORC mixtures. The challenges associated with such an effort are due to the need to simultaneously determine the optimum mixture composition (chemical structure of all participating working fluids) and concentration (amount of each fluid in the mixture). Furthermore, uncertainty associated with the predictive capabilities of the employed models may impact both on the type of the designed fluids and their operating features. While CAMD-based mixture design has yet to be considered in ORC research, it has been reported for few other applications. This work presents a systemic method for the simultaneous determination of the composition and concentration of binary ORC working fluid mixtures using CAMD-based optimization, implemented in three interacting stages. The first stage aims to explore and identify the highest possible economic, operating, environmental and safety performance limits of a wide set of mixtures in an ORC system. This is approached by searching for chemically feasible molecular structures only for one of the two components of a binary mixture, while emulating the mixture behaviour of the remaining component within a much wider structural design space. The second stage serves to determine the optimum and chemically feasible structure of the second component for each one of the molecules already obtained in the first stage, together with the optimum mixture composition. Subsequently, a non-linear sensitivity analysis method is presented to address model-related uncertainties in the mixture selection procedure. The identification of multiple optimum mixture candidates is accomplished through a multi-objective formulation of the CAMD-optimization problem The merits of the proposed approach are illustrated through a case study on ORC systems. The considered design indices employed in CAMD reflect important ORC performance measures such as thermodynamic efficiency and exergy through an ORC mathematical model utilized in the course of CAMD-optimization. Important mixture properties such as flammability, toxicity and azeotropic mixture behaviour are also considered. [1] Angelino, G., Di Paliano, P. C., Multicomponent working fluids for organic rankine cycles (ORCs), Energy 1998; 23(6): 449. [2] Papadopoulos A.I., Stijepovic M and Linke P., On the systematic design and selection of optimal working fluids for Organic Rankine Cycles, Applied Thermal Engineering 2010, 30, 760. [3] Stijepovic M.Z., Linke P., Papadopoulos A.I., Grujic A.S., On the role of working fluid properties in Organic Rankine Cycle performance, Applied Thermal Engineering 2012, 36, 406-413
20 mins
Matteo Pini, Andrea Spinelli, Giacomo Persico, Stefano Rebay
Abstract: The reliable modeling of real-gases is nowadays of great importance in many industrial applications, especially in the energy field. The prediction of real-gas thermodynamic properties based on the direct use of an equation of state (EoS) and of its derivatives, implies a high computational cost in case of numerical studies, when a set of governing equations is iteratively solved (e.g. detailed CFD calculations, dynamic plant simulations). A different approach is represented by the use of look-up tables. In the thermodynamic regions of interest, a grid of nodal points (storing all properties) is preliminary built. Within the discretized domain, the properties in any point are computed using fast interpolation methods, with a dramatic reduction in computational time. However, a proper technique has to be applied to guarantee the thermodynamic consistency, which is not automatically satisfied as in the case of direct EoS application. Finally the desired accuracy can be addressed by selecting the number of nodes and the interpolation scheme. This paper presents a novel interpolation method for property calculation of real gases using look-up tables. Herein, any grid has been built using accurate EoS implemented in the software FluidProp. The method assigns a selected functional form to the internal energy e as a function of the specific volume v and of the specific entropy per unit mass s (e=e(v,s)). Within any cell of the thermodynamic domain, the coefficients of the functional form are calculated referring to the local grid data; therefore, a fundamental relation is locally established, in such a way that any thermodynamic property of any internal point is intrinsically consistent. The method is here presented for the siloxane MDM and for the carbon dioxide. Both single and two-phase regions close to vapor saturation line have been explored, for reduced temperature ranging between reduced temperature of 0.6 and 1.05. The accuracy and the computational cost of the method have been assessed in comparison with those resulting from direct EoS computation. As an example of application, the throughflow calculation of a centrifugal turbine operating with MDM is also presented.
20 mins
Marco Astolfi, Silvia Lasala, Carlo De Servi, Ennio Macchi
Abstract: Nowadays, the Organic Rankine Cycles technology is well known and widely used in small solar thermal power plants, for the exploitation of hot geothermal brines and for waste heat recovery from industrial processes like steel and iron furnaces. In Europe, biomass combustion is another field where ORCs are commonly used [5]. To promote the installation of Organic Rankine several counties in Europe are reducing ORCs’ LCOE by applying feed-in-tariff mechanisms, but where these incentives are not present biomass fuelled ORCs show considerably higher LCOE The main limits in LCOE reduction are strictly related to the biomass furnace cost and to the efficiency of Organic Rankine Cycles. Nowadays, as regards biomass applications, ORCs commonly use pure working fluids (i.e. siloxanes or long chain alkanes) in a saturated recuperative configuration. In particular, the selected evaporation temperature is the result of the cycle efficiency optimization through the evaluation of the trade-off between the maximization of both the power production and the heat recovery from the synthetic oil used as Heat Transfer Fluid (HTF) in the biomass burner. The performance of the ORC can be improved by the adoption of multicomponents blends as working fluids[6]. However complex analytical tools are needed to design such ORC units and, in particular, the selection of a mixture characterized by a proper temperature glide is not trivial because requires the evaluation of the miscibility of mixture components, its thermal stability and the study of the heat transfer coefficients in phase transition. In this paper the design of a saturated recuperative ORC coupled with a small capacity biomass fired boiler is investigated using different working fluids. Pure toluene and toluene/ethanol mixtures are considered and both thermodynamic and techno-economic aspects will be here analyzed.