16:00
Session 5B: System design (2)
Chair: Florian Heberle
16:00
20 mins
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Working Fluid Distribution of a 315 kW Organic Rankine Cycle System in the off-design Conditions
Zhe Wu, Puyao Wang, Qinyang Han, Jianzhao Li
Abstract: The research in this paper is based on a 315kW organic Rankine cycle (ORC) system using brazed plate heat exchangers and a radial turbine. In order to study the law of distribution and change of the working fluid in the off-design conditions, this paper takes the working fluid distribution as the research target and uses the moving boundary method to establish the heat exchanger simulation model. 315kW ORC power generation tests working with R134a were performed, and the test data is used for calculating the level of the brazed plate heat exchangers which is difficult to measure directly, and the error is within ± 5%. The working fluid distribution were obtained under different conditions of evaporator outlet superheat degree (13~21.5°C), condenser outlet supercooling degree (4~11.5°C) and working fluid pump flow (2~20kg/s). Combined with the temperature distribution of the heat exchanger and the power generation of the system, the performance characteristics of the system under off-design conditions are analysed. The research results show that the change of working fluid flow rate, superheat degree and supercooling degree are all have effects on the distribution of working fluid in the system. And the influence of working fluid flow rate is the most significant. The working fluid distribution can directly reflect the working state of the heat exchanger, and the stability of the system operation. The research results provide reference for the heat exchanger design margin of the ORC power generation system, and provide guidance for the stable and safe operation of the control system.
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16:20
20 mins
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Robust Optimization of an Organic Rankine Cycle for Geothermal Application
Aldo Serafino, Benoit Obert, Léa Vergé, Paola Cinnella
Abstract: A high level of know-how has been reached about Organic Rankine Cycles (ORCs). However, improving and optimizing their design is still a fundamental issue. Traditionally, thermodynamic and techno-economic ORC optimization have been performed only at the point identified by plant nominal working conditions. Just recently, some optimizations have started to consider also part-load performance. In any case, the approach used in all these works is always deterministic, as all model hypothesis and operating conditions are considered as a priori perfectly known. In practice, due to the manifold sources of uncertainty, the determistic approach is not thermodynamically and economically efficient and it is not always optimal for part-load operation. To overcome these difficulties, in this work we adopt instead a Robust Design (RD) for the design of ORCs under uncertainty. Specifically, an innovative RD optimization (RDO) technique relying on two nested Bayesian Kriging surrogates is employed to perform a thermodynamic cycle optimization by means of Taguchi’s RD criteria. The objective is to design an efficient ORC for geothermal application, affected both by epistemic uncertainty, mainly generated by the unknown properties of the thermal source, and aleatory one, given by the variable condensing temperature. The proposed RDO approach is used to select the best values for the design parameters avoiding an excessive sensitivity of the system to changes in the nominal operating conditions. The optimal solution obtained with the proposed approach is compared with the results from the deterministic optimization of the problem.
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16:40
20 mins
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Influence of the Fluid-dynamic Properties of Organic Fluids on Pump Performance
Giovanna Cavazzini, Francesco Giacomel, Alberto Benato, Serena Bari, Guido Ardizzon
Abstract: In the industrial sector, medium, low and ultra-low temperature waste heat represents a significant source of energy loss as well as constitutes a harmful environmental effect which must be avoided. In this context, the Organic Rankine Cycle (ORC) technology is a proven solution because, being the working fluid an organic substance with low boiling temperature, it is better than water when low grade heat needs to be recovered. However, the recovery process presents several challenges when dealing with low and ultra-low temperature (<150°C) heat sources and numerous studies have been already carried out with the aim of, from the one hand, improving the ORC performance from a global point of view and, on the other hand, focusing on the performance of key components like heat exchangers and turbines.
Only few works focused on ORC pumps performance because it is considered an established component. The result is that ORC models available in literature completely disregarded the influence of different properties (i.e. density, viscosity, …) of the considered working fluid on the pump performance by assuming a constant value in the range from 65% to 85%. This simplified approach to the pump models generally brings, on one side, to an overestimation of the achievable ORC efficiency and, on the other side, to an unfair comparison between different working fluids behavior.
This paper presents an in-depth analysis of the influence of the organic substances fluid-dynamic properties on the pump performance. The performance of a multi-stage centrifugal pump, designed to serve with water but suitable for ORC applications, is experimentally and numerically investigated by means of the commercial CFD code, Ansys CFX. Flow fields and performance of the pump operating with eight organic fluids typically used in ORC applications (R134a, R141b, R245fa, R152a, R142b, Acetone, Benzene and Toluene) are investigated by properly combining the CFD code with CoolProp. All the fluids are assumed to be sucked by the pump at a condensing temperature of 30°C in pure liquid condition. A negligible heat transfer from the machine to the environment is considered during the numerical simulation.
Besides the expected density factor, which modifies the best efficiency point in terms of design mass flow rate and head, the comparison of the pump dimensionless performance, within the frame work of the similarity laws, highlighted a clear influence of the Reynolds number that is greatly affected by the different fluid properties (density and viscosity). Differences in efficiency of approximately 2%, being the flow coefficient equal, are detected due to different viscous losses.
This finding influences the ORC system efficiency, stuck around 10-12% in case of low-grade waste heat recovery applications. Possible developments of prediction methods as well as of new design strategies, based on the different working fluid properties and, hence, on the expected flow regime, will be foreseen.
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17:00
20 mins
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Design and Operation of a Centrifugal Compressor in a High Temperature Heat Pump
Benoit Obert, Giacomo Persico, Paolo Gaetani
Abstract: This article presents the design, simulation and operation of a single stage 800 kWe centrifugal compressor operating with HFO R1234ZE(E). The compressor is integrated inside a high temperature heat pump that converts heat from saturated steam at 50°C into 3 MWth of heat at 75°C for a district heating network. The compressor specifications require a high pressure ratio (above 3), a high isentropic efficiency and a wide operation range.
The compressor design process is discussed in detail, starting from the preliminary optimization based on the mean-line model and then focusing on the aerodynamic design of each component of the compressor. In particular, the design of the inlet guide vanes (IGV), of the impeller, of the vaneless and vaned diffusers, and finally of the exhaust volute are presented in detail. Nominal and off design performances are then assessed by means of computational fluid dynamics (CFD), performed by applying the commercial solver ANSYS CFX. The simulations are based on the two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations supplemented by a k- turbulence model; the thermodynamic properties of the working fluid are modelled by an equation of state based on a Helmoltz free energy approach, implemented as look-up tables in both the design and simulation steps. Preliminary experimental results of a test campaign aimed at partially mapping the compressor performance in terms of pressure ratio, electrical power and rotation speed are presented, and a thorough comparison between experimental data and simulation results concludes the paper.
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