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Copyright of Low-Carbon Chemistry & Chemical Engineering is the property of Low-Carbon Chemistry & Chemical Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Energy Policy Journal / Polityka Energetyczna is the property of Mineral & Energy Economy Research Institute of the Polish Academy of Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Working fluid selection for closed-cycle turbo-refrigerators (CCTRs) for deep freezing, as a more fundamental topic, has not yet received careful consideration, leading to limitations in attempts to optimizing CCTR design in the early stages of cycle design. This paper proposed the working fluid selection criteria that to select the one leading to the lowest system weight and volume without compromising the system energy efficiency. Design criteria, such as specific speed and Reynolds number, and their dependence on fluid properties are introduced to ensure uncompromising-efficiency design of turbomachinery. Normalized heat transfer area of recuperator correlated with fluid properties is derived as the evaluation criterion for system weight and volume. Applying the proposed criteria, the comparison between potential pure gases shows the normalized heat transfer areas required for nitrogen, neon, and argon are 1, 1.54 and 2.18, respectively. Therefore, nitrogen among all pure gases has the greatest potential to reduce system weight and volume. The comparison among potential binary mixtures highlights the methane-nitrogen mixture with the molar mass of 24.1 g/mol which reduces the required heat transfer area by approximately 20 % compared to pure-nitrogen-based system and may reduce it by nearly up to 30 % for systems with larger cooling capacity levels.

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Maximizing the utilization of an available source serves as the ideal approach, provided that only technical factors are considered. For sources with low heat flux, however, cost‐effective solutions are more suitable due to the minimal net power generated, regardless of the effectiveness of the energy conversion. In such cases, utilizing low‐threshold technology may be the most fitting solution, the layout of these cycles should be simple and inexpensive. In the case of organic Rankine cycles (ORCs)‐based power cycles, this means the omission of superheaters or recuperative heat exchangers and the use of simple expanders and small heat exchangers. Simplifying the design, however, requires additional considerations about the elementary steps of the cycle. This work presents a procedure to select favorable working fluids for ORC while considering the expander's internal efficiency. The criteria for favourability is to have a nonideal expansion process starting and ending in (or very near) saturated vapor states to avoid problems related to wetness/dryness between the given maximal and minimal expansion temperatures. It is demonstrated that the design can be simplified under the simultaneous working fluid and expander selection method presented in this study, regardless of the type and isentropic efficiency of the expander. The resulting methodology applies the novel classification of working fluids using the sequences of their characteristic points on temperature‐entropy space. The proposed approach is illustrated with a case study finding optimal working fluid for an ORC system fitted to industrial waste heat, a low‐temperature geothermal, and a cryogenic heat source.

Selecting appropriate working fluids is important for improving the technology readiness level of electrochemical heat pumps (EHP), which is a promising emerging technology for space conditioning and thermal management applications. The major contribution of this paper is to develop and evaluate key metrics to identify electrochemically active working fluids based on heat pump cycle requirements. The key metrics include the possibility of phase change after a chemical reaction, reversible cell voltage and power consumption, cooling and heating capacity, and environmental and safety aspects. Such metrics are applied to conduct fluid screening to assess their suitability. Isopropanol/acetone showed the highest potential to deliver efficient cooling and heating when applied to an EHP system; the use of ethanol/acetaldehyde and methanol/formaldehyde would easily enable electrochemical fluid condensation over a wide range of operating temperatures. Although the approach is demonstrated for selected organic fluids, the screening framework can be readily employed for other chemical compounds expanding the design space of electrochemical heat pumps. • Developed a tool for thermodynamic evaluation of electro-active working fluids. • Evaluation metrics include thermophysical, reactions, GWP, and safety properties. • In cooling mode, system efficiency is improved by 15%–25% using isopropanol/acetone. • Ethanol/acetaldehyde system can operate down to −27.4 °C in heating mode. • Flammability and toxicity of the fluids should be addressed using secondary loops. [ABSTRACT FROM AUTHOR]

Renewable energy sources such as solar, wind, and geothermal energy have become increasingly popular due to their environmental benefits and long-term sustainability. This study proposes an innovative integration of a solar system and a geothermal system to optimize the performance of a geothermal power plant. The geothermal system is coupled with a solar system, which utilizes a direct and storage system to increase the efficiency of solar energy utilization and minimize energy loss. To recover waste heat, an electrolyzer is used in a triple-flash geothermal system that operates at high feed geofluid temperatures, producing hydrogen as a byproduct. The organic Rankine cycle with a zeotropic mixture working fluid is employed to generate additional power. After considering eight candidate working fluids, Butene/R123 is selected. The designed system exhibits an impressive 10.83 MW net power, 9.87 kg/h hydrogen, 18.86% energetic efficiency, 29.45% exergetic efficiency, and 4.88 years payback period with a 29.72 M$ net present value. Additionally, two energy-economic and exergy-economic scenarios are evaluated to determine the optimum state. The energy-economic scenario has a lower payback period, whereas the exergy-economic scenario offers greater profitability over the system's lifetime. [Display omitted] • Designing an integrated triple-flash geothermal and modified solar system. • Performing comprehensive economic and sensitivity analyses. • Utilizing and selecting zeotropic mixture in the ORC subsystem. • Achieving 30.02% exergetic efficiency with 4.82 years payback period. [ABSTRACT FROM AUTHOR]

Maximizing the utilization of an available source serves as the ideal approach, provided that only technical factors are considered. For sources with low heat flux, however, cost‐effective solutions are more suitable due to the minimal net power generated, regardless of the effectiveness of the energy conversion. In such cases, utilizing low‐threshold technology may be the most fitting solution, the layout of these cycles should be simple and inexpensive. In the case of organic Rankine cycles (ORCs)‐based power cycles, this means the omission of superheaters or recuperative heat exchangers and the use of simple expanders and small heat exchangers. Simplifying the design, however, requires additional considerations about the elementary steps of the cycle. This work presents a procedure to select favorable working fluids for ORC while considering the expander's internal efficiency. The criteria for favourability is to have a nonideal expansion process starting and ending in (or very near) saturated vapor states to avoid problems related to wetness/dryness between the given maximal and minimal expansion temperatures. It is demonstrated that the design can be simplified under the simultaneous working fluid and expander selection method presented in this study, regardless of the type and isentropic efficiency of the expander. The resulting methodology applies the novel classification of working fluids using the sequences of their characteristic points on temperature‐entropy space. The proposed approach is illustrated with a case study finding optimal working fluid for an ORC system fitted to industrial waste heat, a low‐temperature geothermal, and a cryogenic heat source. [ABSTRACT FROM AUTHOR]

A bi‐objective optimization with respect to coefficient of performance (COP) and volumetric heating capacity (VHC) is performed for a high temperature heat pump. For fixed temperature levels of the heat source and sink, the values for temperature in the evaporator and condenser as well as the degree of vapor super‐heating and condensate sub‐cooling are optimized. Constraints are placed on the minimal pressure in the evaporator, the maximal pressure in the condenser and the compressor discharge temperature. Results are presented for a one‐stage and a two‐stage heat pump cycle. Cyclobutane and several butene isomers are identified as working fluids with a good compromise between COP and VHC from a CoMT‐CAMD approach based on PC‐SAFT and a realistic model for the ideal gas heat capacity. [ABSTRACT FROM AUTHOR]

Geothermal heat mining simulations using supercritical CO2 (sCO2) were performed in this research. Working fluid selection criteria for power generation using sCO2 from a geothermal reservoir are then presented for subcritical, superheated and supercritical organic Rankine cycles (ORCs). Meanwhile, method of working fluid classification for ORC is proposed. To get the most feasible ORC design, this study introduces the concept of “turning point” for isentropic and dry working fluids, as well as minimum turbine inlet temperature for wet working fluids. A thermodynamic model was developed with capabilities to obtain the optimal working fluid mass flow rate, evaporation temperature, superheated temperature, and supercritical pressure, to evaluate the thermal performance of the three ORC approaches using hot produced sCO2. With this model, thirty potential working fluids with critical temperatures in the range from 50 to 225 °C were screened for utilizing hot produced sCO2 considering physical properties, environmental and safety impacts, and thermodynamic performances. Finally, the thermodynamic results were compared for all possible working fluids.

Geothermal heat mining simulations using supercritical CO2 (sCO2) were performed in this research. Working fluid selection criteria for power generation using sCO2 from a geothermal reservoir are then presented for subcritical, superheated and supercritical organic Rankine cycles (ORCs). Meanwhile, method of working fluid classification for ORC is proposed. To get the most feasible ORC design, this study introduces the concept of “turning point” for isentropic and dry working fluids, as well as minimum turbine inlet temperature for wet working fluids. A thermodynamic model was developed with capabilities to obtain the optimal working fluid mass flow rate, evaporation temperature, superheated temperature, and supercritical pressure, to evaluate the thermal performance of the three ORC approaches using hot produced sCO2. With this model, thirty potential working fluids with critical temperatures in the range from 50 to 225 °C were screened for utilizing hot produced sCO2 considering physical properties, environmental and safety impacts, and thermodynamic performances. Finally, the thermodynamic results were compared for all possible working fluids.

In this paper, for the low-temperature geothermal organic Rankine (ORC) power generation system, the subcritical saturated vapor cycle is selected as the key research object, the mathematical and physical model of the thermal process of the ORC system is established, and five organic working fluids are selected. With the net power produced per unit mass of geothermal water(PPH) as the optimization target, the optimal parameters of the ORC system are determined. Based on the selection of the best organic working fluid, various parameters that affect the performance and economic performance of the ORC system are analyzed. After simulation calculation, the results show that: when the system takes PPH as the optimization target, the working fluid R245fa can be selected as the best applicable working fluid.

Owing to different temperature rages of power generation and refrigeration in the cogeneration system, for the sake of selecting the working fluids that are suitable for both power generation and refrigeration simultaneously, 17 commonly used working fluids are evaluated in this paper, based on an organic Rankine cycle coupled with a two-stage vapor compression cycle system in different geothermal fluid temperatures. The performances of working fluids under different working conditions, and the maximum power generation as well as cooling capacity are analyzed. Additionally, the main parameters are analyzed to optimize the system performance. The results indicate that net power output has a local maximum where it corresponds to the optimal evaporation temperature. Besides, the lower the critical temperature, the greater the thermal conductance, and the pressure ratio decreases with evaporation temperature. Hydrocarbons all have higher total heat source recovery efficiency. R1234yf, propane and R1234ze, R152a have excellent maximum net power output when the geothermal fluid temperature is low and high, respectively. R134a always has better maximum net power output and cooling capacity. The net power output is used for cooling, and the COP is closed, therefore, maximum net power output results in the maximum cooling capacity. In addition, that of propane and R1234yf are excellent until the geothermal fluid temperature are 140 °C and 120 °C separately. R1234ze and R152a are good when the geothermal fluid temperatures are 140 °C and 150 °C, respectively. [ABSTRACT FROM AUTHOR]

The high-temperature gas flows into the environment from many industries. Utilizing the Organic Rankine cycle (ORC) to generate electricity from this energy is worthwhile. In this study, the best thermodynamic conditions of ORC with 27 working fluids are presented. The considered temperatures for the heat sources were divided into low, medium, and high, with values of 50–90 ℃, 95–190 ℃, and 195–280 ℃, respectively. The optimization was carried out to get the maximum power through a genetic algorithm in MATLAB software. The optimization parameters included the inlet temperature of the turbine (Ttur), the pressure of the (Pcond), the pressure of the boiler (Pboil), and the outlet temperature of hot air (Thf, out). The results indicated that in low-temperature heat sources, R170 (144.13–209.87 kW) was the best fluid, and in the medium- and high-temperature heat sources, R227ea (1372.31–1533.86 kW) and R141b (6640.97–7524.6 kW) were the optimal fluids. Moreover, considering environmental issues and flammability, R744 (low-temperature), R1234ze (medium-temperature), and R245ca (high-temperature) were the best working fluids. In the low- and medium-temperature heat sources, T cri T hf , in for optimal fluids were 0.46–0.6 and 0.55–0.88, respectively. However, in the high-temperature heat source, this ratio was 0.72–0.78. As working fluids were selected, if T hf , in - T cri was above 100 ℃ or lower than 20 ℃, these working fluids were left out. In the future scope, finding a temperature for the heat source above which water is the suitable fluid compared to organic fluids and investigating the effect of changing heat carrier from air to combustion gases are proposed. [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]

The present research presents an innovative thermal integration model for a biomass-based power plant to generate power, coolant, and liquefied hydrogen. This integrated arrangement comprises biomass gasification, gas turbine cycle, organic flash-bi-evaporator refrigeration cycle, multi-effect desalination, solid oxide electrolyzer, and Claude cycle. The selection of an eco-friendly fluid for the electricity-cooling production cycle is conduced relying on a comparative analysis. The subsequent examination evaluates the economic-sustainability performance criteria through sensitivity and contour analyses. The comparative assessment identifies R1234ze(Z) as the most suitable working fluid. Four objective functions are scrutinized, and two optimization scenarios are devised based on a machine learning approach using artificial neural networks and a multi-objective gray wolf optimization technique. In Scenario A, the focused objectives are the total destructed exergy and the cost of producing liquefied hydrogen. Meanwhile, Scenario B integrates the sustainability index and the overall system cost rate as primary criteria. Scenario A exhibits more favorable operational conditions and outcomes than scenario B, showing the objectives mentioned as 9955 kW and 3.25 $/kg, respectively. In this scenario, the sustainability index, the overall investment cost rate, and the net present value are obtained to be 1.74, 236 $/h, and 205 M$, correspondingly. • Innovative biomass-driven system, producing power, coolant, and liquefied hydrogen. • Working fluid selection procedure for better eco-friendly integrated process. • Sustainability and economic evaluation/optimization using ANN-MOGWO-TOPSIS. • Scenario A is the suitable route, showing E ˙ D , tot at 9955 kW and c LH at 3.25 $/kg. • SI tot , Z ˙ tot , and NPV are found at 1.74, 236 $/h, and 205 M$, respectively. [ABSTRACT FROM AUTHOR]

The heat pump assisted geothermal cascade heating system (GCHS) has a promising prospect in space heating. In this paper, considering thermodynamic, economic and environmental performance, the optimal working fluid from four hydrocarbon candidates and temperature-matching for the GCHS are determined simultaneously based on multi-objective optimization and grey relational analysis. The results show that according to the optimal compromise solutions, the GCHS with R600 has the highest thermodynamic performance (COPtot) of 22.58 and the least annual carbon emission (ACE) of 2.80 × 103 ton while R290 has the largest net present value (NPV) about 45.83 × 106 CNY. Additionally, all temperature variables considered generally tend to reach their extreme value except for the temperature lift of middle circulation water in the secondary plate heat exchanger. The optimal working fluid based on comprehensive performance is considered to be R600 with the largest grey relational degree of 0.752. Besides, the global sensitivity analysis on the GCHS with R600 indicates that the pinch point temperature difference of condenser is the most important influential parameter for COPtot with total Sobol index about 0.918 and the geothermal tail water temperature plays a significant role for both NPV and ACE with total Sobol index up to 0.806 and 0.992, respectively.

Hybrid solar thermal power plants using the Brayton cycle are currently of great interest as they have proven to be technically feasible. This study evaluates mechanisms to reduce fuel consumption and increase the power generated, improving plant efficiency. An energy and exergy model for the hybrid solar plant is developed using an estimation model for the solar resource to determine the plant operation under specific environmental conditions. The effect of using different working fluids in the Brayton cycle, such as air, and helium in transcritical conditions and carbon dioxide in subcritical and supercritical conditions, is evaluated. Additionally, the plant's exergy destruction and exergy efficiency are evaluated. In those, it can be highlighted that the helium cycle in the same operating conditions compared to other working fluids can increase the power by 160%, increasing fuel consumption by more than 390%. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The use of the organic Rankine cycle (ORC), organic flash cycle (OFC) and Kalina cycle (KC) is proposed to enhance the electricity generated by a supercritical CO2 recompression Brayton (SCRB) cycle. Novel comparisons of the SCRB/ORC, SCRB/OFC and SCRB/KC integrated plants from thermodynamic, exergoeconomic and sustainability perspectives are performed to choose the most appropriate bottoming cycle for waste heat recovery for the SCRB cycle. For comprehensiveness, the performance of the SCRB/OFC and SCRB/ORC layouts are examined using ten working fluids. The influence of design parameters such as pressure ratio in the supercritical CO2 (S-CO2) cycle, pinch point temperature difference in heater and pre-cooler 1, turbine inlet temperature and pressure ratio for the ORC/OFC/Kalina cycles are examined for the main system indicators including the net output power, energy and exergy efficiencies, and unit cost of power production. The order of the exergy efficiencies for the proposed systems from highest to lowest is: SCRB/ORC, SCRB/OFC and SCRB/KC. The minimum unit cost of power production for the SCRB/ORC system is lower than that for the SCRB/KC and SCRB/OFC systems, by 1.97% and 0.75%, respectively. Additionally, the highest exergy efficiencies for the SCRB/OFC and SCRB/ORC systems are achieved when n-nonane and R134a are employed as working fluids for the OFC and ORC, respectively. According to thermodynamic optimization design, the SCRB/ORC, SCRB/OFC and SCRB/KC systems exhibit sustainability indexes of 3.55, 3.47 and 3.39, respectively. [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]

This study aims to address the problem of removing excessive heat generated in electronic and microfluidic systems. For this purpose, a new design of a micro loop heat pipe is proposed. It is fabricated on a 2-inch silicon wafer, having a cross-section of 35 mm × 17 mm and an overall device depth of 150 μm. Experimental investigations showed that the proposed design has a low heat leak (undesirable heat conduction from the evaporator to the compensation chamber) compared to the various micro heat pipe designs presented in the literature. A temperature reduction of 28.6°C at 3.78 W heat load is recorded using methanol as a working fluid. This study also reveals that the present methodologies to select working fluids for conventional loop heat pipes cannot be applied to micro loop heat pipes. Hence, a new methodology is proposed for the same, based on four parameters of the working fluid: cooling capacity, dry-out limit, operation in anti-gravity regimes, and force exerted on the evaporator.

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]

Owing to different temperature rages of power generation and refrigeration in the cogeneration system, for the sake of selecting the working fluids that are suitable for both power generation and refrigeration simultaneously, 17 commonly used working fluids are evaluated in this paper, based on an organic Rankine cycle coupled with a two-stage vapor compression cycle system in different geothermal fluid temperatures. The performances of working fluids under different working conditions, and the maximum power generation as well as cooling capacity are analyzed. Additionally, the main parameters are analyzed to optimize the system performance. The results indicate that net power output has a local maximum where it corresponds to the optimal evaporation temperature. Besides, the lower the critical temperature, the greater the thermal conductance, and the pressure ratio decreases with evaporation temperature. Hydrocarbons all have higher total heat source recovery efficiency. R1234yf, propane and R1234ze, R152a have excellent maximum net power output when the geothermal fluid temperature is low and high, respectively. R134a always has better maximum net power output and cooling capacity. The net power output is used for cooling, and the COP is closed, therefore, maximum net power output results in the maximum cooling capacity. In addition, that of propane and R1234yf are excellent until the geothermal fluid temperature are 140 °C and 120 °C separately. R1234ze and R152a are good when the geothermal fluid temperatures are 140 °C and 150 °C, respectively.

In this work, organic Rankine cycle (ORC) is employed to expand the available temperature region of the conventional steam Rankine cycle (SRC) in severe cold territories. The feasibility of the proposed steam Rankine cycle -Organic Rankine cycle combined system is investigated and verified. Working fluids are compared and screened, the matching scheme of different working fluids suitable for different working conditions and objects (thermodynamic or economic performance) are obtained. The results show that the net power increment (NPI) of the combined system increases with the decrease of the condenser outlet temperature (CT). The NPI of ORC system can be effectively improved by adding a regenerator. For wet working fluids, when the evaporating pressure of ORC system is close to the critical pressure of working fluid, there exists an optimal superheat degree with maximum NPI of the system. The combined system using ammonia and R600 as working fluid has the largest NPI values. When the CT of SRC is higher than 90 °C, the Rankine cycle using ammonia as working fluid has better performance. When the CT of SRC is less than 90 °C, the regenerative ORC system using R600 as working fluid has a better performance.

One of the essential steps to design energy conversion-based systems is choosing an efficient working fluid under the design goals to access stable products with high efficiency and overcome environmental issues. In this regard, the current paper is motivated to devise and evaluate a novel geothermal-driven multigeneration system under the effect of various working fluids. The proposed system consists of a flash-binary geothermal power plant, an organic flash cycle (OFC), a power/cooling subsystem (an organic Rankine cycle (ORC) and a thermoelectric generator incorporated with a compression refrigeration cycle), and freshwater and hydrogen production units utilizing a humidification-dehumidification desalination unit and a low-temperature electrolyzer. Considering the design potential of the OFC and ORC, four different environmentally-friendly working fluids, i.e., R123 and R600 in the OFC and R1234yf and R1234ze(e) in the ORC are selected and classified in four groups to introduce the best one, under the energy, exergy, and economic (3E analysis) approaches. Also, the whole system is optimized through a genetic algorithm, respecting the optimal solution for the energy efficiency and unit exergy cost of the products. According to the results, R123/R1234ze(e) shows the highest cooling, hydrogen, freshwater production rates, and energy efficiency. Likewise, the maximum power generation and exergy efficiency belong to R600/R1234ze(e). Moreover, R600/R1234yf has the lowest unit exergy cost of products. • Design of a new geothermal system to produce power, cooling, hydrogen, and freshwater. • Integration on organic flash and organic Rankine cycles for multi-heat recovery. • Classifications of four different environmentally-friendly organic working fluids. • Comparison of thermodynamic and economic performances of selected classifications. • R123/R1234ze(e) acts its best among selected classifications. [ABSTRACT FROM AUTHOR]

This work addresses the multi-objective design of an organic Rankine cycle system for low-enthalpy applications. The design is generated from end-user demand variations. The model uses the reference equations of state of four working fluids commonly used in the literature: R1234yf, R1234ze (E), R134a and R245fa. This approach allows computing the optimal sizing of each unit of the equipment that composes the cycle, obtaining a suitable configuration for each working fluid. That is, the comparison between configurations is linked to the thermodynamic properties of the fluid. In addition, it addresses the phase changes present in each operation and the operational adjustments associated with changes in demand. Therefore, the optimal design captures partial load operation through a multi-period model. Given the off-grid operating conditions, the effects of coupling with energy storage systems are analyzed. The considered objective functions are the total annual cost and energy efficiency of the system. Results indicate that the sizing of the heat exchangers is significantly affected by phase change effects and that the R1234yf and R1234ze (E) fluids are promising since their thermodynamic properties allow for trade-offs between the economic and energy performance of the system. System efficiencies, considering part-load operation and demand variability, can be increased by up to 2 % with the inclusion of energy storage, significantly improving economic performance and reducing costs by up to 10 %. • A multi-objective NLP comprehensive model is used for sequential optimal design of ORC systems. • It is computed an optimal configuration according to the thermophysical properties of each fluid. • Size and pressure operation of turbine are identified as the core of efficiency performance. • Trade-offs for economic and energy efficiency performance are computed. • Working fluid selection should consider the configuration of the ORC. [ABSTRACT FROM AUTHOR]

To advance sustainable development and emission reduction efforts within the maritime industry, a novel combined cooling, heating, power, and storage (CCHPS) system is proposed. This system comprises a cogeneration system (ORC + ARC) and a thermal energy storage (TES) system. Initially, optimal working fluids are selected for the ORC and TES discharge systems. Subsequently, an analysis is conducted regarding the correlation between sensitivity parameters and system performance. Then, the cogeneration system's optimum design parameters are determined utilizing the multi-objective grey wolf optimization (MOGWO) algorithm. Finally, the TES system is analyzed and matched with the diesel generators to meet the ship's electricity demand. The CCHPS system achieves optimal thermal efficiency, payback period, annual CO 2 emission reduction, and annual fuel savings of 30.07%, 6.22 years, 8910.49 tons, and 2791.74 kL, respectively. In contrast to the TES-absent system, the CCHPS system exhibits a 12.88% increase in thermal efficiency, along with a 25.48% improvement in annual fuel savings and annual CO 2 emission reduction. [Display omitted] [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.

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.

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• Computer-aided molecular design and group contribution method are adopted. • 22,244 working fluid pairs are investigated and top 300 are further analyzed. • New HFOs and HCFOs are proposed and their molecular structures are obtained. • Development and selection of high-temperature working fluid are more critical. • R1252ye, R1261zf and R1251zd, R2351dzf should be prioritized for development. Cascade high-temperature heat pump is a promising system with an outstanding performance in large temperature lifting situations due to the adoption of different working fluids for low and high-temperature cycles. The properties of the working fluid pair profoundly influence the performance. However, studies on the working fluid pair selection for the cascade high-temperature heat pumps are limited. Therefore, computer-aided molecular design based on the group contribution method has been adopted to develop and select working fluid pairs on a relatively large scale. 352 and 1841 low-and high-temperature working fluids were generated and preliminarily screened. Finally, 22,244 working fluid pairs composed of 67 and 332 low and high working fluids were investigated. The top 300 pairs ranked by COP have been further analyzed with 36 low- and 15 high-temperature working fluids. In conclusion, existing working fluids of R1243, R1243yf, R1243zf, R143b, R281fa and R161 should be preferred as low-temperature working fluids. Based on large-scale screening, the COP values were proven to be more sensitive to the high-temperature working fluids, and their development and selection were more critical. Specifically, R263fa and R126zd showed the best performance and matching flexibility as high-temperature working fluids. For the in-existing working fluids, which were not developed or were uninformative, R1252ye, R1261zf, R1251zd and R2351dzf should be prioritized for future working fluid development. [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.

Highlights • LNG cold energy is recovered to generate electricity. • Power generation systems in series and parallel are compared. • System parameters and working fluids are optimized simultaneously. • Which combination method is better is discussed. • The optimal combination method and working fluid are related to LNG vaporization pressure. Abstract The irreversible loss of heat transfer between working fluid and LNG in the condensation process of the single-stage cycle which uses liquefied natural gas (LNG) cold energy to generate power is large, resulting in low power generation capacity. This can be improved effectively by combining separated cycles in series or in parallel. But which combination method (in series or in parallel) is more suitable for LNG cold energy power generation systems is not well understood. Therefore, in this paper, eight different systems are compared by combining single-stage condensation Rankine cycle and two-stage condensation Rankine cycle in series and in parallel respectively. Taking the net power output as the objective function, the key parameters and working fluids of the eight systems are simultaneously optimized at four different LNG vaporization pressures. The results show that with the increase of LNG vaporization pressure, the net power output of the system decreases. The vaporization pressure of LNG has an important influence on the system configuration (combination method) and working fluid selection. When the LNG vaporization pressure is 0.6 MPa or 2.5 MPa, the system S-T2B2 (Both top cycle and bottom cycle are two-stage condensation Rankine cycle in series) with the working fluid R32/R1150 (top cycle/bottom cycle) has the largest net power output. When the LNG vaporization pressure is 3.0 MPa or 7.0 MPa, the system P-L2R2 (Both left cycle and right cycle are two-stage condensation Rankine cycle in parallel) with the working fluid R41/R143a (left cycle/right cycle) has the highest net power output. From the economical point of view, when the LNG vaporization pressure is 0.6 MPa, the system S-T1B2 is recommended. When the LNG vaporization pressures are 2.5 MPa and 3.0 MPa, the system P-L1R1 is recommended. When the LNG vaporization pressures is 7.0 MPa, the system P-L2R2 is recommended. [ABSTRACT FROM AUTHOR]

Abstract The performance of thermodynamic cycle strongly depends on the properties of working fluids. A gas-fuelled hot water system based on coupling of an organic Rankine cycle and an air source heat pump cycle (ORC-ASHP) was investigated in this paper. This system is integrated with a gas burner to provide heat for ORC. The power generated in ORC is used to drive the air source heat pump cycle. Comparisons on the system performance are presented for seven different organic working fluids, with emphasis on the effect of thermodynamic properties of the working fluids. In terms of fuel-to-heat efficiency, benzene, toluene and cyclohexane have better performance for the high temperature heat source. The results indicate that working fluids presenting higher decomposition temperatures shows a higher fuel-to-heat-efficiency. When benzene is used as working fluids, the fuel-to-heat efficiency reaches 161.7% and 139.47% to provide 50 °C and 65 °C hot water respectively. [ABSTRACT FROM AUTHOR]

Highlights • An organic Rankine cycle with heat pump system is proposed for hydraulic systems. • The proposed system applied to a 260-ton hydraulic excavator is investigated. • The optimal working fluid for the proposed system is selected. • The earning of equilibrium temperature shows a maximum at certain oil temperatures. • The economic performance decreases with the increase in ambient temperature. Abstract The operating temperature of hydraulic oils should be controlled within a reasonable range to keep hydraulic systems in good working condition. However, due to the huge energy losses of hydraulic systems, the oil temperature will increase rapidly. In addition, some hydraulic equipment often operate in cold weather conditions. The conventional cooling with radiators and preheating with electric heaters will consume extra energy for hydraulic systems. Therefore, an organic Rankine cycle with heat pump system is proposed to achieve regenerative cooling and energy-efficient preheating. The system can operate in two modes: an organic Rankine cycle mode to produce electricity instead of consuming energy while cooling the hot oil, and a heat pump mode to preheat the oil with less energy before the hydraulic system starts up in a cold environment. The system is applied to a 260-ton hydraulic excavator to investigate the technical feasibility. On average, the calorific power of the excavator is 177 kW at 245 kW of consumed power in a typical cycle. Considering the cost of heat source, the earning of equilibrium temperature is proposed to evaluate the benefit of the organic Rankine cycle. The optimal working fluid is selected by comparing the system performance. Afterward, the peak points of earning of equilibrium temperature at different ambient temperatures are observed. Corresponding to the maximum earning of equilibrium temperature, the output power of the organic Rankine cycle ranges from 7.63 kW to 2.89 kW. These results show significant energy savings by applying the proposed system in the hydraulic system. [ABSTRACT FROM AUTHOR]

Abstract Liquefied Natural Gas (LNG) contains significant amounts of cold exergy, which is normally wasted during regasification at receiving terminals. This paper addresses working fluid screening for Organic Rankine Cycles (ORCs) utilizing LNG cold energy. Due to the cryogenic temperature of LNG, the condensation temperature of an ORC should be far below ambient temperature, and the working fluid should be totally different from conventional ORCs operating above ambient temperature. However, the working fluids also depend on the cycle configuration and the natural gas target pressure. In this study, a simulation-based optimization framework is proposed to compare the performance of 22 candidate working fluids. The Particle Swarm Optimization (PSO) algorithm is adopted to optimize 5 and 7 dimensional search spaces for the combined systems operated across and below ambient temperature. Promising working fluids for ORCs considering the effect of natural gas target pressure are suggested based on the optimization results. The most energy efficient working fluids are R125, R143a, R290 and R1270 for ORCs operated below ambient temperature without waste heat utilization. For ORCs operated across ambient temperature utilizing flue gas waste heat from a natural gas fired power plant, R170, R134a and R290 perform better than other working fluids. Highlights • This study investigates 22 working fluids for ORCs utilizing LNG cold energy. • A simulation-based optimization framework is applied to working fluid screening. • Optimal working fluids are suggested for CODS and CODW respectively. [ABSTRACT FROM AUTHOR]

Abstract Organic Rankine Cycle (ORC) is known as one of the best method amongst the other low-grade energy recovery methods. ORC can be applied for different applications such as geothermal, solar thermal and waste heat. However, the application of ORC in these applications requires the selection of suitable working fluid for better performance. So, this paper focuses on performance evaluation of ORCs for different working fluids. The modelling work was performed using MATLAB and, a thermo-physical database, REFPROP. The evaluation is performed based on the performance of the working fluid. The results reveal that MD2M & cyclopentane for temperature ranges 50 - 100 ⁰C, butane, neopentane & R245fa for 100 - 150 ⁰C, ethanol, methanol and propanone for 150-200 ⁰C and Water, m-Xylene and p-Xylene for 200 - 320 ⁰C are better working fluids for energy extraction. [ABSTRACT FROM AUTHOR]

Thermodynamic property prediction of working fluids is of great importance on the performance analysis of high-temperature organic Rankine cycles (ORCs) for utilizing low-grade heat sources. In this study, an intrinsic-group-contribution perturbed-chain statistical associating fluid theory (iGC-PC-SAFT) is proposed for the high-temperature ORC performance analysis with 8 siloxanes and 10 alkanes. By adjusting the initial values and corresponding value ranges of PC-SAFT parameters repeatedly according to the changes in molecular structures, the PC-SAFT parameters were fitted to determine the initial iGC-PC-SAFT parameters. Afterwards, the final iGC-PC-SAFT parameters for each working fluid were obtained based on a global optimization algorithm. Furthermore, the iGC-PC-SAFT equation of state (EoS) was used to calculate the thermodynamic properties of working fluids. Compared with the results from Coolprop database, maximum relative deviations for the saturated liquid density and the compressibility factor in the superheated region are 0.95% for MM, and 0.84% for MD 2 M, respectively. Besides, the average absolute relative deviations of ORC performance parameters calculated by the iGC-PC-SAFT EoS at different input pressures of turbine, including the thermal efficiency, turbine power output, pump power consumption, heat rejection, mass flow, and exergy efficiency are less than 2.04% for siloxanes (except for MM) and 6.09% for alkanes, respectively. • New method for developing intrinsic-group-contribution PC-SAFT was proposed. • Novel group-contribution parameters for siloxanes and alkanes were presented. • iGC-PC-SAFT exhibits negligible prediction accuracy loss compared with PC-SAFT. • ORC performance was evaluated and predicted by iGC-PC-SAFT. [ABSTRACT FROM AUTHOR]

Efficient power generation from low to medium grade heat is an important challenge to be addressed to ensure a sustainable energy future. Organic Rankine Cycles (ORCs) constitute an important enabling technology and their research and development has emerged as a very active research field over the past decade. Particular focus areas include working fluid selection and cycle design to achieve efficient heat to power conversions for diverse hot fluid streams associated with geothermal, solar or waste heat sources. Recently, a number of approaches have been developed that address the systematic selection of efficient working fluids as well as the design, integration and control of ORCs. This paper presents a review of emerging approaches with a particular emphasis on computer-aided design methods.

In China, the utilisation of low-temperature waste heat (especially at temperatures lower than 100 °C) plays a significant role in increasing the energy-consumption efficiency in the industry. The organic Rankine cycle (ORC) is considered as a promising method to recover the aforementioned part of the waste heat. In the study, six potential candidates, namely R141b, R142b, R245ca, R245fa, R600a, and R601a were screened from 12 dry or adiabatic organic working fluids based on their thermodynamic performances in the ORC. A multi-objective optimisation (MOO) was performed for the thermodynamic performance (exergy efficiency, EXE) and economic performance (levelised energy cost, LEC) by using non-dominated sorting genetic algorithm-II (NSGA-II). The Pareto frontiers were obtained for the six candidates with the algorithm, and each optimal compromise solution was accurately obtained with the fuzzy set theory. Based on the EXE and LEC of the optimal compromise solution, the total cost and power generation efficiency for the six candidates were determined. This was used to obtain an explicit evaluation index in economic performance, namely static investment payback period (SIPP), to identify that the R245ca corresponded to the most cost-effective working fluid with the shortest SIPP. This suggests R245ca was the fastest to cover the investment and cost of the ORC system. Furthermore, a fast decision-making method was introduced to select the optimal working fluid based on the grey relational analysis (GRA) by considering key physical property parameters of the working fluids. The results suggest that any potential working fluid to recover low-temperature waste heat in the ORC can be evaluated by the simplified grey relational degree (SGRD) proposed in the study. [ABSTRACT FROM AUTHOR]

The dual-loop Rankine cycle system is considered as the most effective way to recover the multi-grades waste heat of engine. However, the power consumption of mechanical pump is relative high, especially for that in the low-temperature loop of dual-loop Rankine cycle, which will influence the system efficiency and power output. Therefore, a thermofluidic pump driven by the engine coolant heat instead of electric energy is used to replace the mechanical pump of low-temperature loop in this paper. The effect of the design parameters on the performance of thermofluidic pump are analyzed firstly. There is always a reasonable height distribution among the components that makes the mass flow rate of thermofluidic pump achieve to maximum, which contributes to a smaller volume and less pressure fluctuation of pump. Then several working fluids are selected as the candidates for systems with thermofluidic pump and mechanical pump. Based on the comprehensive analysis (utilization rate of engine coolant, the energy and exergy efficiency, and output power), the R1234ze is regarded as the most suitable working fluid for both system with thermofluidic pump and system with mechanical pump. Afterwards, a comparison analysis between systems with thermofluidic pump and mechanical pump is implemented in the low-temperature loop. Results indicate that R1234ze based low-temperature loop with the thermofluidic pump has improvement of 7.6%, 9.1% and 7.7% on net power, thermal efficiency and exergy efficiency respectively compared with that of low-temperature loop with the mechanical pump. In addition, the mass flow rate will decrease by 8.6% in low-temperature loop, which conduces to reduce the size of component of cycle. All the results indicate that thermofluidic feed pump is an effective way to improve the performance of dual-loop system. [ABSTRACT FROM AUTHOR]

This study demonstrated an increase in the thermodynamic performance of a booster heat pump, which was achieved by choosing the working fluid among pure and mixed fluids. The booster heat pump was integrated in an ultra-low-temperature district heating network with a forward temperature of 40 °C to produce domestic hot water, by heating part of the forward stream to 60 °C, while cooling the remaining part to the return temperature of 25 °C. The screening of working fluids considered 18 pure working fluids and all possible binary mixtures of these fluids. The most promising solutions were analysed with respect to their performance under off-design conditions and their economic potential. The best-performing mixture showed a coefficient of performance (COP) of 9.0 and thereby outperformed R134a by 47%. Although the mixed working fluids resulted in higher investment cost, the economic performance was comparable to the pure fluids. The mixtures showed similar performance as the pure fluids at off-design conditions. It was concluded that the mixtures 50% Propylene/50% Butane and 50% R1234yf/50% R1233zd(E) could considerably improve the thermodynamic performance of the overall heat supply system while being economically competitive to pure fluids. [ABSTRACT FROM AUTHOR]

Power conversion systems based on Organic Rankine Cycles have been identified as a potential technology especially in converting low-grade waste heat into electricity as well as in small-scale biomass, solar, or geothermal power plants. One of the most important steps in designing such a system is the selection of the working fluid and the optimal fluids are highly dependent on the cycle heat source and condenser cooling fluid temperature levels as well as on the power scale. In this study, the use of different types of working fluids in subcritical organic Rankine cycles is investigated thoroughly by means of thermodynamic analysis. The studied fluid groups are hydrocarbons, fluorocarbons, and siloxanes. The impacts of the specific features related to different fluid groups as well as of the relation between the critical temperature and molar mass of the fluid on the cycle design and operational parameters are investigated. As a result of the study, guidelines and recommendations are generated that can be used in the preliminary evaluation of potential fluid candidates and cycle configurations. In general, the results of the study show that the higher the critical temperature of the fluid, the higher the obtainable cycle efficiency if the evaporation pressure slightly below the critical pressure of the fluid can be obtained. On the other hand, the high critical temperature of the fluid typically leads to significantly high expansion ratios over the process expander and to low condensing pressures. The high pressure ratios are especially present when considering the use of complex siloxanes and high critical temperature hydrocarbons as the working fluids. Based on the results, the studied fluorocarbons and low critical temperature hydrocarbons were evaluated as the most suitable fluids for low-temperature applications whereas siloxanes and hydrocarbons with a high critical temperature were evaluated as the most promising fluid candidates for high-temperature ORC applications. [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]