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The Organic Rankine Cycle (ORC) is a widely-used waste heat recovery approach, that converts low-grade heat into shaft work. This paper introduces a graphical methodology using Exergy-Enthalpy (Ex-H) diagrams, for evaluating and designing ORCs. ORC thermodynamics can be represented by approximate triangular exergy profiles without compromising accuracy. The exergy profile of the heat-absorbing process is crucial to ORC performance. Approximate triangular exergy profiles can be constructed for any ORC configuration to predict ORC performances, such as work output, exergy loss, and thermal efficiency. Furthermore, a systematic graphical approach is developed for integrating waste heat streams and ORCs, with step-by-step procedures outlined. The evolution procedure of the ORC exergy profile can determine working fluid selection and operating conditions. This visually intuitive yet scientifically rigorous tool clarifies the complex relationships between fluids, parameters, and system performance. Two case studies demonstrate the method's effectiveness, confirming it as a valuable tool for ORC design. • A visually intuitive and theoretically robust tool is developed for design of ORCs. • Triangular exergy profiles are constructed to predict key performance metrics. • Two cases validate the accuracy and effectiveness of the Exergy-Enthalpy Diagram. • Simplified ORC designs achieve better performance than complex literature designs. [ABSTRACT FROM AUTHOR]

With ever increasing fossil fuel price and growing demands in cutting emissions, waste heat recovery (WHR) appears as a promising pathway to improve propulsion system performance, which can be potentially achieved by incorporating a recuperator. This paper explores the advantages and potentiality of further recovering exhaust heat from recuperated turboshaft engine through the concept of organic Rankine cycle (ORC) to improve fuel economy and thermal efficiency. For the intended application, working fluid selection is conducted after systematic consideration of thermophysical properties, environmental impact, health and safety issues. Furthermore, parametric analysis is carried out from the viewpoint of thermodynamics, and the genetic algorithm is also employed to obtain the maximum net power output. Targeting the implicit coupling between recuperator effectiveness and ORC performance, the potential advantages of the combined engine-ORC system are comprehensively evaluated at different operational regimes. The influence of flight condition is also discussed through sensitivity simulations for different altitudes. Results reveal that the combination of the recuperated engine with ORC cycle operated using acetone generally offers the greatest benefits, significantly reducing specific fuel consumption by 46%–59 % relative to the baseline simple-cycle engine, depending on the availability of heat source. This analytical work contributes to provide valuable insights into WHR technologies in the aviation industry. • ORC is proposed to recover exhaust heat from turboshaft engine with a recuperator. • Working fluid selection procedure suitable for aeroengine is conceived and applied. • Parametric investigation is conducted and GA is employed for optimization. • System performance is comprehensively evaluated at different operational regimes. [ABSTRACT FROM AUTHOR]

A multi-objective optimization based on the non-dominated sorting genetic algorithm (NSGA-II) is carried out in the present work for the basic organic Rankine cycle (BORC) and regenerative ORC (RORC) systems. The selection of working fluids is integrated into multi-objective optimization by parameterizing the pure working fluids into a two-dimensional array. Two sets of decision indicators, exergy efficiency vs. thermal efficiency and exergy efficiency vs. levelized energy cost (LEC), are adopted and examined. Five decision variables including the turbine inlet temperature, vapor superheat degree, the evaporator and condenser pinch temperature differences, and the mass fraction of the mixture are optimized. It is found that the turbine inlet temperature is the most effective factor for both the BORC and RORC systems. Compared to the reverse variation of exergy efficiency and thermal efficiency, only a weak conflict exists between the exergy efficiency and LEC which tends to make the binary objective optimization be a single objective optimization. The RORC provides higher thermal efficiency than BORC at the same exergy efficiency while the LEC of RORC also becomes higher because the bare module cost of buying one more heat exchange is higher than the cost reduction due to the reduced heat transfer area. Under the heat source temperature of 423.15 K, the final obtained exergy and thermal efficiencies are 45.6% and 16.6% for BORC, and 38.6% and 20.7% for RORC, respectively. [ABSTRACT FROM AUTHOR]

Today, the study of thermal systems that take advantage of residual thermal sources in the power generation sector is of great importance to mitigate environmental impact and promote sustainable alternatives in this sector. Among these alternatives, the organic Rankine cycle (ORC) is of great relevance since it allows taking advantage of residual energy sources at low temperatures. This work presents a methodology to evaluate the feasibility of using a refrigerant as a working fluid in an organic Rankine cycle based on an exergetic viability index. As a case study, R134a, R600a, R245fa, and R123 refrigerants were considered. A residual thermal source was used that came from the Hybrid Cycle Plant of the Valley of Mexico. Thermodynamic analysis was performed to determine generated power, thermal efficiency, refrigerant mass flow, pinch point temperature difference, specific steam consumption, unused thermal exergy flow, exergy efficiency, and total heat transfer requirement. The weighted average of the differences between these indicators, the global warming index, and the ozone depletion potential relative to the most favorable indicator corresponded to the definition of the exergetic viability index of the refrigerant. The results indicate that the ORC operating at condensing temperatures of 25, 35, and 45 °C with R245fa shows the highest rate of exergetic viability despite not generating the greatest amount of power and being one of the refrigerants with the highest total heat transfer requirement. Finally, at condensing temperatures above 45 °C, it is observed that R600a is exergetically the most viable refrigerant used in the ORC. [ABSTRACT FROM AUTHOR]

A systematic multi-step method was developed for selection of the best working fluid(s) for the simple and regenerative Brayton cycles. The first step of the method involves development of the thermodynamic model of a cycle and development of theoretical expressions for its thermodynamic performance (thermal efficiency and net specific work output). In the second step, parametric calculations were performed for nine working fluids over a range of cycle operating conditions corresponding to the concentrated solar power plants (CSPs) to determine the effect of cycle operating conditions and thermo-physical properties of the working fluids on cycle performance. The statistical regression analysis is used in the third step to develop correlations for the cycle efficiency and net specific work of the same form as theoretical expressions developed in Step 1. The last step in the process involves construction of performance maps, where performance parameters, such as thermal efficiency and net specific work output are presented as functions of the cycle operating conditions. The performance map for thermal efficiency shows that, depending on the operating conditions, either N 2 or CO 2 are the best choices of the working fluids. However, considering the net specific work output, He is the best working fluid over the entire analyzed range of the cycle operating conditions. [ABSTRACT FROM AUTHOR]

• An automated scheme for the optimal configuration of organic Rankine cycle (ORC) system is proposed. • For ORC systems with the most common working fluids and heat sources are comprehensively simulated. • The influences of the ORC structure and working fluid on thermodynamic performance are explored. • The "double Pinch" rule for single-pressure and double-pressure evaporation ORC systems is verified. • A linear relationship is established to determine multi-stage evaporation potential of a working fluid. In organic Rankine cycle (ORC) system, the selection of the structure and the optimization of operating parameters are usually studied separately, which makes the existing design principles inadequate to describe the impacts of the structure on ORC performance, and thus greatly limits its practical applications. To this end, this paper addresses an automated scheme for the optimal configuration of the ORC system, aiming to simultaneously determine the working fluid, the structure and the operating parameters corresponding to the maximal system efficiency. In this work, a comprehensive simulation and optimization of four common ORC structures is first conducted with 18 working fluid candidates and the most common heat sources with the inlet temperature ranging at 200–120 °C. The influence mechanisms of the ORC structure on the thermodynamic performance of the ORC system are then explored to generalize the qualitative and quantitative principles for ORC configuration. Results show that, for the single-pressure evaporation ORC (SPC) with a given inlet temperature of the heat source, the Pinch point type formed by the working fluid and the heat source changes from vaporization Pinch point (VPP) to VPP&PPP (preheat Pinch point) and then to PPP as the critical temperature of the working fluid decreases. The system efficiency of SPC reaches its maximum when the double Pinch point of VPP and PPP is formed and double-pressure evaporation ORC (DPC) reaches its maximum system efficiency when the double Pinch points of HVPP and LVPP are formed. Besides, it is found that when compared with the basic ORC, the regenerative ORC enables to improve the thermal efficiency and system efficiency of the system at the same time, whereas the bleeding regenerative ORC increases the thermal efficiency while reduces the system efficiency. For the DPC, the type of Pinch point formed by the heat source and the working fluid in the optimal working condition is the key to determine whether a system has multiple evaporation potential. Specially, for the heat source with variable inlet temperature, a linear relationship between the inlet temperature of the heat source and the critical temperature of the working fluid can be established to determine the multiple evaporation potential of a certain working fluid. Based on these results, an automatic scheme for ORC configuration is developed and expected to provide an effective guidance for the selection of working fluid, configuration optimization and parameter determination of the actual ORC heat recovery process. [ABSTRACT FROM AUTHOR]

Cogeneration of heat and power systems based on the organic Rankine cycle (ORC-CHP) has been proven to be an effective way to utilize waste heat at medium and low temperatures. In this work, three ORC-CHP (combined heat and power based on organic Rankine cycle) systems are simulated and compared, including the SS (serial system), the CS (the condensation system), and the SS/CS. The multi-objective genetic algorithm (MOGA) is used to optimize the three systems respectively to achieve higher exergy efficiency and profit ratio of investment (PRI). The optimal thermal-economic performance is obtained. Twelve organic fluids are adopted to evaluate their performance as working fluids. The calculation results show that SS has the highest exergy efficiency, while SS/CS has the best economic performance. Compared with the highest exergy efficiency of SS and the best economic performance of SS/CS, CS will be the optimal solution considering these two objective functions. Under the optimal working conditions, SS has the highest thermal efficiency because it has the highest net power output. The components with the largest proportion of exergy destruction are the heat exchangers, which also has the highest cost. [ABSTRACT FROM AUTHOR]

• An aging based model of the ORC system is proposed. • The optimal working fluid considering fluid decomposition is selected. • The working fluid decomposition effects in design of the ORC systems is considered. • The proposed model manages decomposition rate by optimizing system's operating schedule. • Degradation rate and performance of two working fluids are evaluated. The Organic Rankine Cycle (ORC) is a power generation cycle similar to the conventional Rankine cycle which runs on a working fluid other than water. The selection of a working fluid is a critical part of designing an ORC system. Researchers have selected working fluids based on various performance criteria including thermal efficiency, safety, output power, and investment cost. It should be noted that working fluids are degraded over system operating lifetime, and this will affect the system performance. As a result, working fluid should be selected considering fluid degradation mechanism in long-term operation that is studied in this paper. The objective of this work is to develop a model which optimizes simultaneously the working fluid selection and operation schedule of ORC systems. Firstly, an aging based model of the ORC system is proposed which selects the optimal working fluid considering fluid decomposition over time. Then, an aging based optimization model is developed optimizing operating schedule of the system. The model is applied to a combined gas turbine-ORC system which toluene and cyclopentane are compared as two working fluids. The results revealed that by utilizing the toluene as a working fluid, the degradation rate and cost of electricity generation and maintenance is much higher (about 30%) and using cyclopentane will be more economical during the system lifetime. Finally, this study indicates that selection of working fluid just on the basis of the start point power production is not reliable and the design and optimization of the ORC systems should be evaluated through the long-term operation. [ABSTRACT FROM AUTHOR]

• A transcritical-subcritical parallel organic Rankine cycle system is proposed. • The proposed system shows better performance than others. • The optimization and design of proposed system is carried out. • R1233zd is the best candidate working fluid for proposed system. • The net power output of engine system is improved by 12.02%. In this study, a novel transcritical-subcritical parallel organic Rankine cycle system is proposed to recover the engine exhaust gas and coolant waste heat. Firstly, the proposed system is compared with the dual-loop organic Rankine cycle and parallel organic Rankine cycle systems. According to the results, the proposed system shows more net power output, higher thermal and exergy efficiency, and lower heat transfer requirement. Then the thermodynamic analysis including energy and exergy analysis are carried out, and the effects of design parameters including high-pressure turbine inlet temperature and pressure, low-pressure evaporation temperature on the performance of proposed system based on R601a are investigated. The results indicate that for a given high-pressure turbine inlet temperature, the net power output of system firstly increases and then decreases as high-pressure turbine inlet pressure increases and reaches to the maximum at turbine inlet temperature and pressure of 520 K and 8 MPa, and the net power output also shows a trend of firstly increasing and then decreasing with low-pressure evaporation temperature and reaches to the maximum at 330 K. Furthermore, five low global warming potential fluids are selected as working fluids of proposed system, and the results demonstrate that the R1233zd is the best candidate working fluid for proposed system. The maximum net power output of proposed system based on R1233zd reaches to 119.72 kW at the turbine inlet temperature and pressure of 530 K and 10 MPa, low-pressure evaporation temperature of 330 K, which achieves net power output increment of 12.02% for the engine system. Finally, an exergy destruction analysis demonstrates that the heat transfer processes have less exergy destruction for proposed system based on R1233zd compared with other working fluids. [ABSTRACT FROM AUTHOR]