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[en] This paper reports on a stochastic simulation model which is developed for design and analysis of steam power plants (Rankine Cycles). Stochastic input variables turbine inlet temperature and pressure, turbine efficiency, pump efficiency, etc. are randomly introduced into the system model via their statistical distributions. Monte Carlo method is used for stochastic simulation. Output design variables (steam generation rate, heat duty, thermodynamic efficiency and carnot cycle efficiency) are characterized by their respective mean, standard deviation, normal distribution probability density function and normal distribution cumulative probability function. The application of the stochastic simulation is described by a demonstration problem
[en] Highlights: •An open-access database of ORC experiments is presented and released. •ORC main parameters are discriminated and harmonized in the database frame. •A selection of simplified ORC thermodynamic performance criteria is proposed. •Database analysis shows actual Organic Rankine Cycle state-of-the-art performances. •A statistical method for ORC performances analysis and comparison is introduced. -- Abstract: The Organic Rankine Cycle (ORC) is a technology commonly used for low-grade thermal energy conversion in electricity. This technology is mature for large power scale and last research focused on small scale units for domestic or onboard applications. This paper presents an extensive open-access database of more than 100 ORC experiments collected from about 175 scientific literature references. Data harmonization and database frame are presented. Clear and consistent components and ORC performance criteria are proposed and applied to the data set of various ORC. An overview of the ORC experimental state-of-the-art is displayed and major trends are drawn. Efficiency of key components such as expanders and pumps are analyzed and used for ORC parametric optimization case study. Correlations of some parameters with ORC performances are statistically investigated, performance improvement of novel fluid or cycles is evaluated.
[en] Steam/water Rankine cycles are mainly used as energy conversion systems (ECS) for large power plants. Due to the low density of steam at low pressure, the turbine size is generally very big. Furthermore water cooling is almost always preferred as cooling option since it provides the best energy efficiency as compared to air cooling. Nevertheless, water use can become difficult in the future due to environmental constraints (withdrawal or heat release limitations). In the present work, a two-range Rankine cycle (TSRC) is considered, combining a steam/water cycle and organic Rankine cycle (ORC) cooled by an air cooled condenser (ACC). The back-pressure of the steam cycle is limited and the heat which remains in the steam is transferred to an ORC through a condenser-boiler. Due to organic fluid high density, it is possible to reduce the installation size. Furthermore, the organic fluid is likely to provide additional power when ambient temperatures are low. With those two advantages it is expected for dry cooling to become more effective. The paper reports the methodology that has been used in order to optimize a design in terms of fluid selection, component sizing and efficiency. Preliminary cost analyses related to the different working fluids and local ambient conditions are given. (authors)
[en] Highlights: • n-Heptane HCCI combustion under air and oxygen intake was compared. • n-Heptane auto-ignition postponed due to higher specific heat capacity as oxygen increase. • The increment of heat release fraction during low temperature reaction is studied. • Oxygen enrichment lead to suppressed negative temperature coefficient. • The mechanism of low temperature reaction enhancement as oxygen increase is investigated. - Abstract: To take maximum advantage of the high efficiency of homogeneous charge compression ignition combustion mode and internal combustion Rankine cycle concept, in this study, the n-heptane auto-ignition characteristics have been investigated using a compression ignition internal combustion Rankine cycle engine test bench and a zero-dimensional thermodynamic model coupled with a detailed kinetic model. The n-heptane auto-ignition process shows that under both air and oxygen intake, a typical two-stage combustion in which oxygen enrichment has very minor effects on the n-heptane high temperature reaction. The higher specific heat capacity of oxygen compared with nitrogen leads to an overall increased specific heat capacity, which lowers the in-cylinder temperature during compression stroke, thereby delaying the low temperature reaction initial timing. The higher oxygen content also improves the H-atom abstraction, first O_2 addition, second O_2 addition and peroxyalkylhydroperoxide isomerization, thereby improving the overall reaction rate and the heat release fraction of low temperature reaction. As a result, the in-cylinder temperature at the end of low temperature reaction also increases, thereby shortening significantly the negative temperature coefficient duration compared with a combustion cycle using air as oxidizer.
[en] The objective of this paper is to model and optimise solar organic rankine cycle (ORC) engines for reverse osmosis (RO) desalination using currently available solar thermal collectors. The proposed systems are intended to be potentially attractive for remote areas without (or with very high cost) access to the public electricity grid. In this study the ORC system is modelled using the Aspen Plus process simulator, with the required inputs from other programmes to model Reverse osmosis and thermal solar plants in the same modelling platform. The first part of this paper analyzes a comprehensive list of candidate working fluids for the ORC desalination application, and a selection is made of the most interesting fluids according to the type of solar collectors used in each case. The optimal operating temperature is calculated for the solar ORC integrated with the RO plant that optimises the global ORC-thermal solar plant efficiency. The second part of the paper deals with the applicability of the system obtained. Two case studies were examined from Almeria and Barcelona that can be considered representative of two different levels of solar radiation characteristic of the Mediterranean area of Spain. In these case studies the area of the solar field collectors was calculated, considering both brackish and sea water desalting applications for a handling capacity of 15 m3/day. An economic estimation is also reported comparing the present results with those of an equivalent photovoltaic-RO plant. The technical-economic results obtained for the two locations suggested the adequate thermal solar technology to be that represented by the PTC collector system. The use of an equivalent photovoltaic system to generate electricity to drive the RO desalination system had a higher cost than the optimised solar ORC-RO system specially when using the best solar thermal technology
[en] Graphical abstract: Comparison between water and CO_2 technologies footprints. Display Omitted - Highlights: • CO_2 gas cycles applied to nuclear power stations using real plant data. • Supercritical, regenerative, recompression reheated SRRR-CO_2 cycle used for analysis. • CO_2 net cycle efficiency 34.04% versus 33.51% H_2O net cycle efficiency. • Balance of plant mass 40% reduced with CO_2 (as compared with H_2O). • Balance of plant footprint 10 times reduced with CO_2 (as compared with H_2O). - Abstract: In this study, closed CO_2 cycles are investigated for potential application in existing nuclear power stations, referring in particular to Mochovce power station currently under construction in Slovak Republic. Three different CO_2 cycles layouts are explored in the range of temperatures offered by the nuclear source and of the existing cooling towers. The investigation shows that the common opinion that S-CO_2 cycles are well suited in the medium to a high temperature range only (higher than about 450 °C) seems unjustified. For a primary heat source with a maximum temperature of 299 °C and a heat sink with a minimum temperature of 19 °C and reasonable assumptions about advanced turbomachines and heat exchanger performances, the supercritical recompressed reheated regenerative CO_2 cycle would yield a net efficiency of 34.04%, which compares well with the 33.51% net efficiency of the existing Rankine cycle. The estimated length of the complete turboset (2 turbines, 1 pump and 1 compressor) would be less than 11 m (versus two wet steam turbines of 22 m each for the same power), resulting in a factor of 10 reduction in the footprint of the balance of plant. The total CO_2 cycle equipment and main pipelines would have a combined weight of 3957 tons, while in the Mochovce 3 NPP existing Rankine cycle, the main components and connecting piping weigh nearly 7377 tons, thus a 40% reduction. These results suggest that the adoption of CO_2 in nuclear power stations would not penalize the plant efficiency and would yield significant savings on installation costs and construction times from the much more compact balance of plant.
[en] The efficiency of a Rankine cycle is primarily determined by the temperatures of heat addition and rejection. However, no working fluid has been identified which will operate in a Rankine cycle over an extremely wide temperature range. Multiple Rankine topping cycles offer a technique for achieving high thermal efficiencies in power plants by allowing the use of several working fluids. This paper gives a history of Rankine topping cycles, presents an analysis for the calculation of the overall efficiency of a three-module multiple Rankine cycle, and presents results from a case study for a sodium-mercury-water cycle
[en] The performance of ORC systems strongly depends on working fluid properties. We explore the relationships between working fluid properties and ORC common economic and thermodynamic performance criteria from a theoretical and an analytical point of view. The mapping of individual properties and performance criteria presented in this paper will provide a basis for the development of efficient and systematic strategies and approaches for ORC working fluid selection in future. - Highlights: ► We analyse the relationships between working fluid properties and ORC performance. ► Thermodynamic and economic ORC performance criteria are considered. ► Individual working fluid properties are mapped against ORC performance criteria. ► The mapping provides a basis for development of working fluid selection approaches.
[en] Techniques are described for determining optimum plant configuration and operating conditions for complex binary cycles employing feedwater heaters, moisture separators, multi-stage compressors, and a regenerator. A model describing the efficiency of binary cycles is developed in a form suitable for optimization. As part of the optimization procedure, the system is partitioned into Brayton and Rankine portions by fixing coupling variables, and then for fixed values of coupling variables the two cycles are optimized separately. Dynamic programming is used in the Rankine portion of the binary cycle and a modified search technique in the Brayton cycle. For the range of reactor outlet and boiler saturation temperatures examined, and for boiler exit temperature up to 5360C, binary cycle efficiencies become optimum at 3710C boiler saturation temperature, 1100C to 1700C superheat, no regeneration for reactor exit temperatures below 1170K and appreciable amount of regeneration above 1170K. Optimum binary cycle efficiencies are 0.506, 0.533 and 0.560 for reactor exit temperatures of 1060K, 1170K and 1280K, respectively. (author)