09:00
Session 6B: System Design - Cycle configurations
Chair: Christos Markides
09:00
20 mins
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Thermo-economic Analysis of Dual-pressure Evaporation Organic Rankine Cycle System Using R245fa
Jian Li, Yuanyuan Duan, Zhen Yang
Abstract: Dual-pressure evaporation cycle is an emerging cycle type in the organic Rankine cycle (ORC) field, which remarkably increases the conversion efficiency, and the adaptability to various heat sources is better compared with the conventional single-pressure evaporation cycle. However, the published studies on the dual-pressure evaporation cycle are mainly limited to the thermodynamic performance. The studies on the thermo-economic performance are insufficient to date. This study analyzed the thermo-economic performance of dual-pressure evaporation ORC system using R245fa for heat sources of 100–200°C. The heat exchangers are shell-and-tube type and the working fluid flows in the tubes. The effects of heat source temperature, mass flow rate of heat source fluid, and pinch point temperature differences (PPTDs) on the system thermo-economic performance were quantitatively analyzed. The thermo-economic performance of single-pressure and dual-pressure evaporation cycles was compared. Results show that the specific investment cost (SIC) of dual-pressure evaporation ORC system decreases as the heat source temperature increases. Increasing the mass flow rate of heat source fluid will substantially reduce the SIC. Increasing the PPTDs is beneficial to reduce the SIC at a high heat source temperature. The purchased costs of heat absorbers and condenser are two largest terms in the system, and their ratios to the total purchased equipment cost are 30.9%–49.5% and 32.9%–43.7%, respectively. The SIC of dual-pressure evaporation cycle increases by 5.7%–14.2% compared to the single-pressure evaporation cycle, and that is mainly ascribed to the remarkable increase in the purchased cost of heat absorbers.
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09:20
20 mins
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Structural Uncertainty Estimation of Turbulence Models in Organic Rankine Cycle Applications
Giulio Gori, Nassim Razaaly, Gianluca Iaccarino, Pietro Marco Congedo
Abstract: The investigation of the complex non-ideal fluid flows of interest for Organic Rankine Cycle (ORC) turbomachines largely relies on numerical tools. In Computational Fluid Dynamics (CFD) simulations, a popular strategy to predict the flow behavior is to rely on the Reynolds- Averaged Navier-Stokes equations (RANS). In RANS computations, turbulence models must be employed, to reconstruct the Reynolds stress term arising from the time-averaged decomposition of the Navier-Stokes equations. Generally, the accuracy of RANS simulations is questionable for flow configurations involving adverse pressure gradients, inhomogeneous flow directions, flow separation or strong stream lines curvature (as in the case of turbomachinery applications). For such flows, the inherent model-form assumptions in the RANS approach introduce potential accuracy limitations which affect the credibility of CFD predictions [1]. Moreover, turbulence models often consist in empirical or semi-empirical closures that depend on a set of coefficients optimized for a specific fluid flow. Tough literature is teemed with works reporting on the estimation of turbulence coefficients for flows of fluids of common interest (air, water and many other), little if nothing can be found regarding complex non-ideal fluid flows of interest for ORC applications. Indeed, the scarce amount of experimental data regarding non-ideal flows prevents the empirical estimation of turbulence coefficients. Moreover, due to the complexity of the task, the direct quantification of the errors introduced by RANS closure models is intractable in general. Recently, formal uncertainty quantification techniques have been developed to provide a probabilistic characterization of the corresponding confidence levels. Here, we apply the Eigenspace Perturbation Method (EPM) [2] to a set of exemplary flow configurations of interest for ORC applications. Namely, a non-ideal flow expanding through a converging-diverging nozzle, a non-ideal supersonic stream over a backward facing step and the flow around a typical ORC turbine stator blade. Numerical results show that a systematic and comprehensive treatment of the RANS inherent uncertainties is fundamental for the further improvement and optimization of ORC power production systems.
[1] Karthik Duraisamy, Gianluca Iaccarino and Heng Xiao, Turbulence Modeling in the Age of Data, Annual Review of Fluid Mechanics, Vol 51:357:377, 2018
[2] Michael Emory, Johan Larsson and Gianluca Iaccarino, Modeling of Structural Uncertainties in Reynolds- Averaged Navier-Stokes closures, Physics of Fluids 25 (110822) 2013
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09:40
20 mins
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A Comparison between Cascaded and Single-stage ORC Systems Taken from the Component Perspective
Martin White, Matthew Read, Abdulnaser Sayma
Abstract: Compared to single-stage ORC systems, cascaded ORC systems could have benefits for relatively high-temperature waste-heat recovery applications, which include the potential for higher expander isentropic efficiencies owing to lower expansion ratios, the removal of sub-atmospheric condensation pressures and the possibility to utilise two-phase expansion. Previous investigations suggest that cascaded systems could produce up to 5% more power than an equivalent single-stage system. The aim of this paper is to compare the different systems in terms of exergy destruction within the system and the heat-transfer area requirements. Firstly, the exergy analysis reveals that cascaded systems reduce the total exergy destruction related to the expansion process, but this is offset by the exergy destruction within the additional heat exchange process. However, cascaded cycles also lead to less exergy destruction within the heat-addition process. To assess the heat-transfer area requirements, a discretised double-pipe heat-exchanger model is developed for the condenser and intermediate heat exchanger that transfers heat from the topping cycle into the bottoming cycle, whilst a discretised finned-tube cross-flow heat-exchanger model is developed for the evaporator. The geometry of each heat exchanger is optimised to minimise the heat-transfer area subject to imposed pressure drop constraints. The results reveal that cascaded cycles require larger heat-transfer areas, which is due to the additional heat-transfer process and reduced temperature differences within the evaporator. Ultimately, the best performing cascaded cycles, which produce 4.0% and 5.9% more power than their single-stage counterparts, require 22.7% and 23.2% more heat-transfer area. Future investigations should investigate how this trade-off impacts economic performance.
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10:00
20 mins
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Enhanced Cascade Cycle
Roberto Bini, Davide Colombo, Claudio Pietra
Abstract: Two level pressure ORC cycles (also known as cascade cycles) has been realized for 30 years as a solution to increase the conversion efficiency of large geothermal plants or, in other words, to increase the power output of a certain geothermal resource, by realizing a thermodynamic cycle that better couples with the cooling of the resource.
An improvement of this well-known cycle is proposed which allows to further increase the conversion efficiency by introducing an additional intermediate pressure level.
Two alternatives are presented to exploit the additional vapour flow, showing the efficiency increase achievable for a given typical application and also reporting some considerations about the cost effectiveness of this solution.
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10:20
20 mins
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Effect of Working Fluid Type on Low Temperature Rankine Cycle Optimization
Mina Shahrooz, Per Lundqvist, Petter Nekså
Abstract: The characteristics of the working fluid play an important role on the performance of low temperature Rankine cycles. One important criterion categorizes fluids based on the slope sign of the saturated vapor curve in T-s diagram to distinguish between so-called wet and dry expansion. Wet expansion is preferably avoided due to the damage inflicted on turbine blades by liquid droplets. On the other hand, superheat in expander inlet and outlet increases the loads on heat exchangers and increases cost of the system. Therefore, in this paper a methodology is presented to minimize expander superheat, while maximizing net power and taking the isentropic efficiency of the expander into account. Results indicate that so-called wet fluids do not necessarily need to be excluded from the fluid candidate selection. Depending on the evaporating and condensing temperature levels, by using an expander with a specific isentropic efficiency, it is possible to decrease the superheat in the expander and in some cases close to zero. However, comparison should be between the performances of various fluids in a cycle with all external parameters and fluids should not be excluded from the screening list only based on their types.
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