14:00
Session 4B: System design (1)
Chair: Andreas Schuster
14:00
20 mins
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Feasibility of an On-board Micro-ORC System for Small Satellites
Fiona Leverone, Matteo Pini, Angelo Cervone, Eberhard Gill
Abstract: Small satellites are receiving increased recognition in the space domain due to their reduced associated launch costs and their shorter lead time when compared to larger satellites [1, 2]. However, this advantage is often at the expense of mission capability, such as available electrical power. A possible solution is to change from the conventional solar photovoltaic and battery configuration to a micro-Organic Rankine Cycle (ORC) and thermal energy storage system. This unique approach has the potential to offer higher system efficiency and power density.
However, limited literature is available on micro-ORC systems, which are capable of producing a few hundred Watts of electrical power, especially for small satellite applications. A feasibility study of these systems and a fluid selection study were conducted. This was done by using a multi-objective genetic algorithm to optimise an on-board micro-ORC system for various working fluids such as Toluene (C7H8), Hexamethyldisiloxane (MM), and Octamethylcyclotetra-siloxane (D4). The two objective functions were to minimise the total volume and maximise the thermal energy storage capacity. This paper describes the proposed system layout and model, including case study validations to increase model fidelity. The specific objectives of this study are: i) the working fluid selection, and ii) the optimisation of the proposed system incorporating the design of the thermodynamic cycle and the sizing of the turbine and heat exchangers.
Results show the design of the micro-ORC system is dependent on the mission designer, and various design configurations are provided from the Pareto frontier. It was also found that when using high energy density materials, such as silicon, for the thermal energy storage system, the evaporator operates in the inverted annular film boiling regime which reduces the heat transfer coefficient. Additional challenges include high micro-turbine rotational speeds, large thermal cycling, small blade heights, and large condensers. Finally, the storage configuration of the concentrator was identified as crucial for the feasibility of the system on-board small satellites. Future work will focus on improving the fidelity of the model, especially the heat transfer coupling of the evaporator by more in-depth modelling or experimental investigations.
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14:20
20 mins
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Irreversibility in the Organic Rankine Cycle for Low-grade Thermal Energy Conversion System
Takeshi Yasunaga, Yasuyuki Ikegami
Abstract: Organic Rankine cycles (ORCs) are used in power generation applications for low-temperature heat sources such as waste heat recovery from plants, geothermal binary, hot springs, and ocean thermals. Meanwhile, ORC equivalent technologies are applied in ocean thermal energy conversion (OTEC), which uses the temperature difference between the surface and the depths. ORCs with such low-grade thermals must consider various forms of irreversibility, not just the performance of the turbine/generator and heat exchangers. Typical ORCs are mainly composed of a turbine/generator, a working fluid circulation pump, and heat exchangers. A new performance evaluation concept for low-grade thermal energy conversion (LTEC) is proposed that normalizes the thermal efficiency considering heat leak and equilibrium state as a dead state based on finite-time thermodynamics. In addition, considering the trade-off between the heat transfer performance and the pressure drop due to the flow, a unique performance evaluation method based on the maximization of the net obtainable energy per heat transfer is initiated on a plate-type heat exchanger in LTEC. Here, to keep the cost of systems down, ease of examination, and proper design of the equipment, standardization and calculation tools are useful. In this research, theoretical maximization of the available power employing the irreversibility of LTEC system is conducted employing the parameter analysis of simple Rankine cycle to arise the coefficient of irreversibility. In the results, the coefficient of irreversibility is clarified on working fluids (HCFC245fa and HCFO1224yd(z)), turbine efficiency, and working fluid circulation pump efficiency, respectively.
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14:40
20 mins
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Design Optimization of a Heat Recovery ORC for a Novel Biomass to Methanol Plant
Cristina Elsido, Emanuele Martelli, Marco Astolfi, Giulio Guandalini, Matteo Romano
Abstract: This work presents the optimization of ORCs for recovering waste heat from a novel process for the co-production of methanol and electricity from biomass. The process uses a flexible sorption enhanced gasification reactor, developed in the framework of the H2020 EU project FLEDGED. The heat integration study is particularly interesting because the process features multiple hot and cold streams at different temperatures (i.e., the calciner flue gases, the syngas coolers, etc.) making it necessary to optimize not only the ORC configuration (i.e., single vs. multiple pressure levels, regenerative vs. simple cycle, combined heat and power arrangement, etc.) but also the heat exchanger network.
For two candidate working fluids, hexane and R1233zde, the cycle and heat integration are optimized using the p-h ORC superstructure and a systematic synthesis methodology recently proposed by Martelli et al. (2017). The optimization aims at maximizing the economic performance of the system taking into account the trade-off between efficiency and costs.
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15:00
20 mins
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Exergetic and Economic Analysis of a Solar Driven Small Scale ORC
Tryfon Roumpedakis, George Loumpardis, Sotirios Karellas
Abstract: The continuous increase in the energy demand, the reduction on the fossil fuels reservoirs as well as their malevolent impact on the environment has turned interest towards more “green” alternatives for decentralized power production. In this study, the performance of a small scale low temperature solar driven ORC is investigated for application in South-East Mediterranean region. The study includes the assessment of multiple scenarios, in terms of the working fluid, the site of installation and the solar collector’s type. For each scenario a multi-objective genetic algorithm has been developed and executed in order to optimize the payback period and the mean exergy efficiency for each solar driven ORC for an annual operation using an hourly step. The results indicate that the optimum was located for all scenarios in the minimum heat storage tank capacity. On the other hand, the correlation between the solar field area and the optimization parameters is more complicated and directly connected to the climatic conditions of each considered location. The maximum exergy efficiency among the considered scenarios is in the range of 6.2% for a flat plate collectors’ driven ORC using R245fa as working fluid. The minimum payback period was reported for the case of Larnaca, using parabolic trough collectors and R152a as the ORC’s fluid. Finally, for a more broad comparison of the system’s results, the annual energy production of the ORC was translated in primary energy savings. For this analysis, it was observed that in all cities R152a was achieving the highest savings for the maximum area of the solar field.
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15:20
20 mins
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Design of ORC Systems under Variable Input Parameters: a Multi-scenario Approach
Mario Petrollese, Daniele Cocco
Abstract: Organic Rankine cycle (ORC) powered by solar energy is a viable and effective option for a high efficiency conversion of solar thermal energy into electricity at a distributed scale. However, the fluctuations of the thermal energy produced by the solar collectors often force solar-based ORC systems to operate at part-load conditions. Consequently, the intrinsic uncertainty of solar irradiation requires the development of novel approaches able to give robustness to the design phase of power generation systems fed by solar energy. A novel methodology for the preliminary design of solar ORC systems, based on the minimization of the expected Levelized Cost of Energy (LCOE) under variable input conditions is therefore proposed and analyzed in this paper. The expected variations of the solar irradiation together with the fluctuation in the ambient temperature that affects the condenser pressure, are considered during the design phase by adopting a multi-scenario approach. The proposed methodology has been tested by referring to a medium-scale ORC unit and by considering different working fluids. As case studies, the direct coupling of the ORC unit with a solar field and the integration of a Thermal Energy Storage (TES) system have been investigated. In all the cases, the results obtained by using a multi-scenario approach have been compared with those obtained by a single-scenario approach, achieving a lower value of the actual LCOE. In fact, the ORC configuration obtained by adopting a multi-scenario approach is characterized by lower performance under design conditions, but it is less sensitive to the variation of the main inputs. This fact is particularly evident for the case with the direct coupling of the solar field, where important fluctuations in the heat source mass flow rate are expected, while it becomes more and more marginal with the rise in the TES storage capacity.
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