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14:20   Session 2A: System Optimization (1) Chair: Davide Ziviani 14:20 20 mins Influence of the pinch-point-temperature difference on the performance of the “preheat-parallel” configuration for a low-temperature geothermally-fed CHP Sarah Van Erdeweghe, Johan Van Bael, Ben Laenen, William D'haeseleer Abstract: In this work, we investigate the performance of the so-called “Preheat-parallel” CHP configuration, for the connection to a thermal network (TN). A low-temperature geothermal source (130°C), and the connection to a 75°C/50°C and a 75°C/35°C thermal network are considered. For a pure parallel CHP configuration, the brine delivers heat to the ORC and the thermal network in parallel. However, after having delivered heat to the ORC, the brine in the ORC branch still contains some energy which is not used. The “Preheat-parallel” configuration utilizes this heat to preheat the TN water before it enters the parallel branch, where the TN water is heated to the required supply temperature. The “Preheat-parallel” configuration is especially favorable when connected to a thermal network with a low return temperature, a large temperature difference between supply and return temperatures – thereby exploiting the preheating effect – and for high heat demands. In this paper, we focus on the effect of the pinch-point-temperature difference (ΔTpinch) on the plant performance. ΔTpinch is directly related with the size and cost of the heat exchangers and strongly influences the preheating-effect, which is the most characteristic feature of the “Preheat-parallel” configuration. First, we present the results of a detailed sensitivity analysis of ΔTpinch. A higher ΔTpinch results in a lower preheating-effect, a lower net power output and, correspondingly, lower plant efficiency. Furthermore, we compare the performance of the “Preheat-parallel” configuration with the convenient parallel and series CHP configurations. For all three configurations, the performance decreases with an increase of ΔTpinch. For the considered thermal network requirements, the net power generation is the highest for the “Preheat-parallel” configuration. With respect to the parallel configuration, the gain in net power generation stays approximately constant (75°C/35°C TN) or decreases (75°C/50°C TN) with the imposed pinch-point-temperature difference. With respect to the series configuration, the gain in net power generation increases for a higher value of ΔTpinch. This means that the impact of ΔTpinch is the biggest for the series configuration, followed by the “Preheat-parallel” configuration, and that the impact on the performance of the parallel configuration is the smallest. 14:40 20 mins Iterative approach for the design of an Organic Rankine Cycle based on thermodynamic process simulations and a small-scale test rig Sebastian Kuboth, Marc Neubert, Markus Preißinger, Dieter Brüggemann Abstract: Waste heat recovery from industrial processes may be a door opener for market penetration of Organic Rankine Cycle (ORC) systems. Within this study, an ORC for industrial waste heat recovery is designed by adopting an iterative approach. Therefore, experiments are performed in a thermal oil heated 1 kW test rig with internal recuperator and a maximum thermal efficiency of 10.6%. The results are iteratively implemented in thermodynamic process simulation. Thus, the simulation results of different stationary operating points can be compared to experimental measurements for different steps of the iterative design process. Results outline the importance of experimental results for the design of ORC systems. The simulation accuracy can be significantly improved with a single reimplementation of experimental data, which enables accurate sensitivity analysis on ORC waste heat recovery system performance. For two different representative operating points, the mean deviations between experiment and simulation decrease from 8.4% to 1.0% and 4.1% to 1.7% respectively, considering the enthalpy and pressure of all thermodynamic state points. 15:00 20 mins A systematic methodology for the techno-economic optimization of Organic Rankine Cycles Cristina Elsido, Alberto Mian, Emanuele Martelli Abstract: This work presents a general and systematic methodology for the techno-economic optimization of Rankine cycles. The proposed superstructure for Rankine cycles allows to reproduce a wide range of cycle configurations, such as cycles with/without regenerator, cycles with single or multiple pressure levels, and cycles integrated with multiple heat sources. The model is integrated with a recently developed methodology capable of optimizing also the arrangement and sizing of the heat exchangers of the plant (heat exchanger network synthesis). This allows to perform a full techno-economic optimization of the entire system. The resulting problem is a challenging Mixed Integer Non Linear Problem (MINLP) which is solved with an ad hoc algorithm. The methodology is applied to two case studies for power cycles with single and multiple heat sources. This work can help engineers identify the right thermodynamic cycle to integrate with an industrial process and design techno-economically optimal Rankine cycles for waste heat recovery from single or multiple heat sources, by considering heat integration and cycle design optimization simultaneously. 15:20 20 mins Neural networks for small scale ORC optimization Alessandro Massimiani, Laura Palagi, Enrico Sciubba, Lorenzo Tocci Abstract: Organic Rankine Cycle (ORC) technology is considered as a cost effective technology to produce electricity out of low grade thermal energy sources. Despite of the potential market available, ORCs for small scale applications still find it difficult to create their own market. One of the reasons is undoubtedly their excessive specific market price [$/kW] which leads to a high payback period. Another reason is that SMEs (Small - Medium Enterprises), which are by far the customers that may profit the most from small scale ORCs, are not sufficiently aware of the potential savings this technology could lead to. As it is often the case, at the smaller scales additional design issues arise which raise the specific price $/kW even futher, thus limiting the market potential of this technology. Small scale ORCs represent a viable method to retrofit Diesel generators in stationary applications. The ORC power plant receives as an input the thermal energy of internal combustion engine exhaust gas and converts it into electricity by means of a thermodynamic cycle. The inherently low-temperature source leads inevitably to low recovery efficiencies, and therefore it is important for designer to optimize both the cycle parameters and the working fluid, the two issues being of course intimately connected. Machine learning techniques (MLT) are receiving interest in the optimization of ORC plants. The reason is twofold: firstly, the thermodynamic problem is highly non-linear, which makes it impervious the design of efficient optimization algorithms. Secondly, MLTs allow the user to perform feature selection and eliminate from the optimization problem all these variables which do not affect significantly the objective function. Among MLTs, artificial neural networks (ANN) methods have proven to perform well in energy optimization problems. This paper presents a case study of a 20 kW ORC system which converts the sensible heat of the exhaust gas of a Diesel engine into electrical energy. A thermodynamic model of the ORC system and a detailed 1-D model of the expander have been developed using the software MATLAB. Hence, an in-house code has been written which implements Neural Networks, using various classes of activation functions, to evaluate and compare their performance. First, the procedure used for the optimization process is described. Subsequently, both the cost and size of the system have been minimized to meet reasonable specific cost for the plant. This study underlines the need for an integrated approach in which thermodynamic, technical and economic criteria are considered simultaneously in order to design an efficient and cost-effective system. 15:40 20 mins Cryogenic ORC to enhance the efficiency of LNG regasification terminals Marco Astolfi, Anton Marco Fantolini, Gianluca Valenti, Salvatore De Rinaldis, Luca Davide Inglese, Ennio Macchi Abstract: Liquefied Natural Gas (LNG) is transported worldwide by ship in cryogenic vessels where LNG is stored at ambient pressure and temperature of -160°C. LNG is then vaporized offshore or onshore in regasification plants and pumped into gas pipelines for distribution. Several technologies for LNG regasification are available on the market; among them, the most important are the Open Rack Vaporizer (ORV), the Submerged Combustion Vaporizer (SCV) and the Intermediate Fluid Vaporizer (IFV) [1]. In the first one, LNG flows upwards in tubes while sea water flows outside in counter flow. This technology is widely used because of its low operational costs, but it is a capital intensive solution because of the large heat transfer surfaces, the need of water treatment and, additionally, it is critical because of vibrational issues in part load. The second regasification technology consists in a pool of water heated by the flue gases obtained by the combustion of gas derived by the regasification line. This technology is compact but it entails a high consumption of primary energy. The IFV consists of a shell and tube heat exchanger in which an intermediate fluid vaporises by absorbing heat from the seawater and condenses by releasing heat to the LNG. All these technologies require large investments and involve the consumption of electrical energy and/or fuel. One of the most promising options to increase the regasification plants efficiency is the introduction of a power cycle working between the seawater and the vaporizing LNG. This is an interesting solution from both theoretical (LNG consists in a unique industrial-scale example of “cold exergy”) and technological points of view (because a non-conventional design of each component must be addressed to face difficulties related to cryogenic temperatures). Organic fluids are the only reliable option for this field thanks to the possibility to reach cryogenic temperatures avoiding vacuum condition in the condenser: they have been studied for this application since 1980, some pilot plants have been installed in Japan [2,3] and recently ORMAT presented a patent focused on the topic [4]. This paper reports the results of an extensive thermodynamic optimization of ORC for LNG regasification plants that are presented highlighting the large benefits attainable in terms of primary energy savings compared to both ORV and SCV technologies. Different cycle configurations are investigated including the option of multilevel condensation plants. The adoption of several working fluids candidate is analysed and different design constraints are included to obtain a more reliable solution. The optimal combinations of cycle configuration and working fluid are eventually presented with a preliminary design of the main components.