Organic Rankine Cycle Thermal Efficiency Research Articles

Thermal performance enhancement of novel integrated vapor compression and organic rankine cycle for electricity production: a full-condensing vs. desuperheating approach comparison in ultra-low-grade waste heat recovery

This paper presents the thermodynamic analysis, performance assessment and working fluid selection for a novel integrated vapor compression cycle and the organic Rankine cycle system, which recovers ultra-low-temperature waste heat (50 °C) rejected by the condenser of a vapor compression cycle (VCC). The study is conducted on a vapor compression system of a 35 kW cooling capacity, followed by the component sizing of the organic Rankine cycle (ORC) based on the available waste heat in the condenser of the refrigeration system. The effect of the operational parameters of the vapor compression cycle on the overall system performance are investigated. The system performance of the integrated system is estimated in terms of coefficient of performance, cycle thermal efficiency and exergy efficiency. The novel integrated systems has been analyzed thermodynamically for 36 pairs of pure refrigerants. Results show that the working fluid pair R600a/R141b with combined coefficient of performance (3.54), ORC thermal efficiency (3.05 %), electricity saving ratio (13.34 %) and total exergy destruction (6.94 kW) is the most suitable VCC/ORC working fluid pair in full condensing method. R407c-R141b is found to be the best working fluid pair in desuperheating method for the same integrated system. The performance analysis for full condensation and desuperheating methods are also investigated. The results indicate a 12.5 % improvement in the coefficient of performance of the integrated system using the full condensation method and a 2.5 % improvement using the desuperheating method compared to the standalone air conditioning system. Additionally, the sensitivity analysis results show that there exists optimum operating condition that maximize the system performance of the integrated system.

Numerical simulation and low speed experiment of a low partially admitted rate axial turbine for small scale organic Rankine cycle

High rotating speed is a prominent feature of turbine used for small-scale organic Rankine cycle (ORC) system, which can bring the problems of safety, reliability, sealing, vibration, economics, etc. Partial admission method has potential to reduce the rotating speed of small turbine. In this study, an axial turbine with low partially admitted rate of 0.033 was tested in ORC loop using R1233zde as working fluid, and numerically researched using Ansys CFX software with k-omega turbulence model. Results showed that the specific powers of turbine resulting from CFD should be corrected with losses to match well with the experimental results. Losses could reduce the optimal rotating speed of turbine. The optimal rotating speed, mass flow rate, power and efficiency of turbine under rated condition were 8000 RPM, 115.8 g/s, 611.8 W and 35.8 %, respectively. The corresponding power and thermal efficiency of ORC were 551.0 W and 2.15 %. The influences of inlet and back pressures on the performance of turbine were different. With constant inlet pressure and temperature, the mass flow rate kept almost unchanged after chocking, but the power continued to increase. Under allowable conditions, reducing back pressure of turbine played an important role in improving turbine performance.

Development of 180 kW Organic Rankine Cycle (ORC) with a High-Efficiency Two-Stage Axial Turbine

The design of turbines used to convert thermal energy into electrical energy in an organic Rankine cycle (ORC) is crucial. A high-speed turbine requires high-performance bearings, which increases turbine manufacturing costs. In this study, a high-efficiency two-stage axial turbine at a low rotational speed was developed, and the ORC performance was presented. We designed a 180-kW axial turbine of 12,000 rpm. To increase turbine efficiency, the number of turbine stages was set to two, and turbine blades were designed to reduce pressure losses. One-dimensional design parameters of blades that minimized the total pressure loss were selected using an in-house code based on a generalized reduced gradient (GRG) nonlinear algorithm. Three-dimensional turbine blade modeling and numerical analysis were performed using commercial software. The total-to-static isentropic efficiency and output of the two-stage axial turbine were predicted to be 85.1% and 176 kW, respectively. ORC performance was assessed using the predicted turbine performance results. Assuming the temperature of the condenser outlet working fluid to be 25 °C, the ORC thermal efficiency and exergy efficiency were found to be 7.40% and 34.49%, respectively. Our findings highlight the applicability of various rotational speeds and number of stages for an axial turbine in an ORC.

Numerical investigation of organic fluid flow boiling for proton exchange membrane fuel cell cooling and waste heat recovery

High power density is one of the competitive edges of proton exchange membrane fuel cell (PEMFC), making it a promising technology for many applications. However, its intensive heat generation leads to a high cooling burden, and a large amount of low-grade waste heat is difficult to utilize, which brings technical difficulty to the system's thermal management. In this study, the organic fluid (R245fa) with low boiling temperature is selected for the PEMFC stack's phase change cooling, and the organic Rankine cycle (ORC) is employed to recover waste heat. Four cooling channels are designed: serpentine straight, spiral straight, serpentine thin and serpentine zigzag channels. The boiling flow is simulated to reveal the effect of channel layout, sectional size and channel shape on the cooling and heat recovery performance. The results show that the cooling plates can be cooled to a reasonable temperature by the organic fluid, and the maximum surface temperature difference is within 15 K, which demonstrates the good cooling performance of flow boiling. The highest ORC thermal efficiency of 7% is achieved by the serpentine straight channel and the spiral straight channel, while the serpentine zigzag channel has the best temperature uniformity contribution resulting from its secondary circulation flow. This study demonstrates the potential of flow boiling on PEMFC thermal management and gives guidance on the cooling channel design.

Nonlinear modeling and multi-scale influence characteristics analysis of organic Rankine cycle (ORC) system considering variable driving cycles

The reasonable construction of the organic Rankine cycle (ORC) system model under road conditions is the key to analyze, evaluate, and optimize the performance of the ORC system. However, due to the variability of high-temperature waste heat source and the strong coupling correlation of operating parameters, the operation characteristics of the ORC system show evident time-varying characteristics. Based on the coupling correlation and redundancy characteristics of the ORC system in complex environment, this paper presents a nonlinear modeling framework for the multi-scale influence analysis of the ORC system under road conditions. The nonlinear modeling framework can improve the prediction accuracy of ORC model by at least 68.36% and reduce the time cost by 53.37%. Based on the nonlinear model, the synergistic effects of multiple variables on ORC system performance at different scales are studied. The frequent fluctuation of vehicle speed enhances the coupling correlation of operating parameters, resulting in nonlinear, hysteretic dynamic characteristics of ORC thermal efficiency. The maximum thermal efficiency of ORC is only 3.28%. The nonlinear modeling framework proposed in this paper can provide a practical solution for constructing the intelligent analysis, design, and optimization models of ORC systems under complex road conditions.

Multi-Objective Optimization of Organic Rankine Cycle (ORC) for Tractor Waste Heat Recovery Based on Particle Swarm Optimization

Diesel engines are widely used in agricultural tractors. During field operations, the tractors operate at low speed and high load for a long time, the fuel efficiency is only about 15% to 35%, and the exhaust waste heat accounts for 38% to 45% of the energy released from the fuel. The use of tractor exhaust waste heat can effectively reduce fuel consumption and pollutant emissions, of which the organic Rankine cycle (ORC)-based waste heat recovery conversion efficiency is the highest. First, the diesel engine map is achieved through the test rig, a plate-fin evaporator is trial-produced based on the tractor size, and the thermodynamic and economic performance model of the ORC are established. Then, taking the thermal efficiency of ORC and the specific investment cost (SIC) as the objective function, the particle swarm optimization (PSO) algorithm and the technique for order of preference by similarity to ideal solution (TOPSIS) decision method were used to obtain the optimal operating parameter set under all working conditions. Finally, the results showed that the ORC thermal efficiency could reach a maximum of 12.76% and the corresponding SIC value was 8539.66 $/kW; the ORC net output power could be up to 8.31 kW compared with the system without ORC; and the maximum brake specific fuel consumption (BSFC) could be reduced by 8.3%. The improvement in the thermodynamic performance will lead to a sacrifice in economic performance, and at high speeds, the economic benefits and thermal efficiency reach a balance and show a better thermal economic performance. Recovering exhaust heat energy through ORC can reduce tractor fuel consumption and pollution emissions, which is one of the effective technical means to achieve “carbon neutrality” in agricultural production. At the same time, through the PSO algorithm, the optimal combination of ORC operating parameters is obtained, which ensures that the exhaust heat energy can be effectively recovered during the tractor field operation, and provides a basis for the adjustment of real-time work strategies for future research.

An effective and simplified method to select the working fluid for waste heat recovery based Organic Rankine Cycle

Organic Rankine Cycle (ORC) is an attractive option to utilize the low-grade waste heat for power generation. The selection of working fluid for ORC is a challenging task because of environmental constraints as most of the organic fluids has the capacity to damage the environment. In this research, a method for the selection of an optimum working fluid for the operation low grade waste heat is determined. The selection of the optimum working fluids depends upon the thermal efficiency, Global Warming Potential (GWP), Ozone Depletion Potential (ODP) and Atmospheric Lifetime of the fluid. Twelve different organic fluids including R134a, butene, R22, R152a, R245fa, R290, R161a, isobutene, isobutane, dimethyl ether, R600 and R124 are selected for the study. The ORC is analyzed by using EES at 2 MPa, 2.5 MPa and 3 MPa. The thermal efficiency of ORC is determined and is found that high operating pressure is favorable for the operation of ORC. At 2.5 MPa, the top three working fluids are R-245fa, R-600 and Iso-butene with an efficiency of 12.7%, 12% and 11.3% respectively. On the basis of thermal efficiency, R-245fa is the best but it has the highest GWP and atmospheric life of 1050 and 7.7 years. R-600 has GWP and atmospheric life of just 20 and 0.018 years. On the basis of environmental constraints, R-600 is found to be more beneficial than R-245fa. It is concluded that R-600 is the optimum working fluid for the operation of low-grade waste heat ORC.

Performance Analysis of Organic Rankine Cycle with the Turbine Embedded in a Generator (TEG)

The organic Rankine cycle (ORC) is a thermodynamic cycle in which electrical power is generated using an organic refrigerant as a working fluid at low temperatures with low-grade enthalpy. We propose a turbine embedded in a generator (TEG), wherein the turbine rotor is embedded inside the generator rotor, thus simplifying turbine generator structure using only one bearing. The absence of tip clearance between the turbine rotor blade and casing wall in the TEG eliminates tip clearance loss, enhancing turbine efficiency. A single-stage axial-flow turbine was designed using mean-line analysis based on physical properties, and we conducted a parametric study of turbine performance, and predicted turbine efficiency and power using the tip clearance loss coefficient. When the tip clearance loss coefficient was applied, turbine isentropic efficiency and power were 0.89 and 20.42 kW, respectively, and ORC thermal efficiency was 4.81%. Conversely, the isentropic efficiency and power of the turbine without the tip clearance loss coefficient were 0.94 and 22.03 kW, respectively, and the thermal efficiency of the ORC was 5.08%. Therefore, applying the proposed TEG to the ORC system simplifies the turbine generator, while improving ORC thermal efficiency. A 3D turbine generator assembly with proposed TEG structure was also proposed.

Performance Assessment and Working Fluid Selection for Novel Integrated Vapor Compression Cycle and Organic Rankine Cycle for Ultra Low Grade Waste Heat Recovery

This paper presents the performance assessment and working fluid selection for a novel integrated vapor compression cycle-organic Rankine cycle system (i-VCC-ORC), which recovers ultra-low-temperature waste heat rejected (50 °C) by the condenser of a vapor compression cycle (VCC). The analyses are carried out for a vapor compression cycle of a refrigeration capacity (heat input) of 35kW along with the component sizing of the organic Rankine cycle (ORC). The effects of the operational parameters on integrated system performance were investigated. The integrated system performance is estimated in terms of net COP, cycle thermal efficiency and exergy efficiency by completely utilizing and recovering the heat rejected by the condenser of the VCC system. R600a-R141b with COPnet (3.54) and ORC thermal efficiency (3.05%) is found to be the most suitable VCC-ORC working fluid pair. The integration of the vapor compression refrigeration cycle with the organic Rankine cycle increases the COP of the system by 12.5% as compared to the standalone COP of the vapor compression system. Moreover, the sensitivity analysis results show that there exists an optimum operating condition that maximizes the thermal performance of the integrated system.

Advanced waste heat recovery system based on organic rankine cycle

In this research, Organic Rankine Cycle (ORC) is used to recover heat from exhaust gas of a four-stroke diesel engine. After retrofitting ORC to the engine, Brake power increased from 10473.91 kW to combined – cycle Brake power of 10736.00kW, thermal efficiency increased from 36.01% to combined – cycle thermal efficiency of 51.32% and Exhaust gas temperature decrease from 358oC to 120oC at the exit of the turbocharger. ORC with R12, R22, R134a and R290 as working fluids at saturation and superheated temperatures, pressures and condenser pressures at different ranges were used to compare refrigerants performance in converting low grade exhaust gas waste heat into useful work. This research presents theoretical analysis on four different refrigerants. Applying the above-mentioned refrigerants as working fluid superheated vapour temperature for R12 is 131.72oC, R134a is 129.37oC, R22 is 113.40oC and R290 is 116.95oC. ORC Power generated by turbine gives 94.98kW, 95.56kW. 130.32kW. 262.64kW respectively, ORC Thermal efficiency gives 36%, 29%, 37% and 38% for R12, R22, R134a, and R290 respectively. Combined – cycle power for each of the refrigerant gives 10568.89kW, 10604.23kW, 10569.47kW and 10736.00kW respectively, combined – cycle thermal efficiency for each refrigerant gives 51.14%, 51.18%, 51.14% and 51.32% for R12, R134a, R22 and R290 respectively. R290 offers optimal performance compared to other refrigerants used in this research. The retrofitting of the ORC has saved some supposedly waste exhaust heat energy and has increased both combined cycle power output and thermal efficiency of the engine cycle.

Identifying the key system parameters of the organic Rankine cycle using the principal component analysis based on an experimental database

The organic Rankine cycle (ORC) is a promising technology for medium-and-low temperature heat utilization. However, the mechanism of how system parameters affect output have been investigated very little in the experimental aspect. Experimental investigation on the impact of each system parameter on system performance requires decoupling these system parameters. In this work, a series of experiments are conducted on a 10 kW scale ORC experiment setup. Statistical analysis is performed to identify a key parameter subset based on an experimental database. 6 system parameters, including temperature (Te) and pressure (pe) at the evaporator outlet, temperature (Tc) and pressure (pc) at the condenser inlet, expander shaft efficiency (ηSSE), and working fluid pump efficiency (ηP) are obtained. Combined with the ORC net power output and thermal efficiency, an experimental database of system operation conditions is constructed. Subsequently, the principal component analysis (PCA) of ORC is conducted based on the experimental database. Prediction models are developed using multi-linear regression (MLR), back propagation artificial neural network (BP-ANN), and support vector regression (SVR). Finally, accounting for the prediction performance of models and system parameter inter-correlation behavior, the key parameter subset is determined with the exhaustive feature selection method. The results imply that the key parameter subset is (pe, ηP, pc, ηSSE). Further removing or including more system parameters would reduce the accuracy of prediction models. In addition, the MLR models are slightly less accurate than the more sophisticated BP-ANN and SVR models.

Thermodynamic analysis and optimization of organic Rankine cycles based on radial-inflow turbine design

In recent years, high-temperature organic Rankine cycle (ORC) with a heat source temperature over 300 °C has attracted interests, which has wide range of applications from industrial point of view. However, few researchers have focused on the coupled design and optimization of the high-temperature ORC system and radial-inflow turbine (RIT). In this work, a coupled design of the ORC system and RIT with the heat source inlet temperatures of 300, 350 and 400 °C is developed using non-dominated sorting genetic algorithm. The ORC thermal efficiency and turbine performance factor (PF) are introduced as optimization objectives. Two mixtures of siloxanes, seven pure siloxanes and nine pure hydrocarbons are considered as working fluid candidates. In addition, the thermodynamic comparison between mixtures of siloxanes and pure working fluids is presented. Moreover, the influences of the turbine inlet total temperature, total-to-static expansion ratio, and specific speed on the ORC thermal efficiency, turbine performance, and turbine size are investigated. The results show that the turbine power output and cycle thermal efficiency are more sensitive to the expansion ratio while turbine size is more sensitive to the turbine inlet total temperature and specific speed. In addition, PF increases with the rise of the turbine inlet total temperature, expansion ratio and specific speed. Moreover, compared with pure working fluids, mixtures of siloxanes have higher efficiencies, but larger turbine and heat exchanger size. Cyclohexane, toluene, m-xylene and hexane with higher PF and smaller size of turbine and heat exchanger are promising working fluids in applications where the compact turbine and heat exchanger is preferred, such as vehicle waste heat recovery.

Performance and Sustainability Analysis of an Organic Rankine Cycle System in Subcritical and Supercritical Conditions for Waste Heat Recovery

This study addresses the performance analysis of a subcritical and supercritical Organic Rankine Cycle (ORC) with the addition of a preheater or superheater integrated with a turbofan engine. This analysis will try to explore the heat transfer throughout the evaporator for the purpose of determining the ORC output power and thermal efficiency. A simplified numerical model of the ORC for waste heat recovery is presented. The model depicts the evaporator by using a distributed model, and includes parameters such as the effectiveness, heat capacity and inlet temperature of the waste heat and the organic fluid. For a given set of initial parameter values, the output power and thermal efficiency, as well as the mass flow rate of the working fluid are acquired by solving the system’s thermodynamic cycle with the aid of MATLAB software. The model is then verified by using data from an industrial waste heat recovery system. The connection between the turbofan engine and the ORC system was established and evaluated by means of Thrust-Specific Fuel Consumption (TSFC) as well as fuel burn. It was found that the supercritical ORC with a preheater and superheater exhibits lower TSFC than the subcritical ORC, whereas the impact of the ORC in terms of waste heat recovery in relation to the environment and sustainability indices is quite small, but still considerable depending on the engine’s weight.

Multi-objective optimization of organic Rankine cycle system for the waste heat recovery in the heat pump assisted reactive dividing wall column

The application of heat pump (HP) technique to reactive dividing wall column (RDWC) achieving energy-saving has received more and more attention, however, massive low-temperature (<100 °C) waste heat would be hereby produced. Therefore, in this work, the organic Rankine cycle (ORC) is adopted to effectively convert the produced waste heat of the compressed stream to clean energy (i.e., electricity). The HP assisted RDWC (HP-RDWC) of diethyl carbonate process is taken as an example, the ORC system with five working fluids candidates are explored. The operating parameters of the ORC system (e.g., flow rate of working fluid and inlet pressure of evaporator) are optimized based upon the maximum net revenue and ORC thermal efficiency through the improved multi-objective genetic algorithm. The optimal ORC system is determined by considering the economic (i.e., net revenue) and thermodynamic efficiency (i.e., ORC efficiency) performances. The results illustrated that the net revenue of the ORC system with R123 and R600a could achieve 175,807.2 US$ and 133,665.5 US$ with ORC efficiency of 15.57 and 16.19%. In addition, total annual cost of the HP-RDWC integrated ORC processes with working fluids R123 and R600a could be reduced by 11.78% and 10.30%, respectively.

Optimization of cyclic parameters for ORC system using response surface methodology (RSM)

ABSTRACT The present work focuses on response surface methodology-backed thermodynamic analysis of organic Rankine cycle (ORC) driven by low-temperature heat sources (temperatures less than 150°C). The zeotropic mixture of Butane and Pentane is employed as a working fluid. Both pure fluids have zero ozone depletion potential and lower global warming potential. The second order nonlinear polynomial mathematical model was developed by RSM methodology to get the relationship between thermal efficiency of ORC and decision variables. Furthermore, the analysis of variance (ANOVA) technique was applied to get apprehend of most significant decision variables in terms of p-values and F-values. In addition, contour plots were drawn to analyze a range of decision variables in which thermal efficiency of ORC is found to be higher. The results indicate that the thermal efficiency is mostly affected by heat sink temperature, followed by isentropic efficiency of the turbine and mass fraction of zeotropic mixture.

Energy and Exergy Analysis of Different Exhaust Waste Heat Recovery Systems for Natural Gas Engine Based on ORC

Waste heat recovery (WHR) from exhaust gases in natural gas engines improves the overall conversion efficiency. The organic Rankine cycle (ORC) has emerged as a promising technology to convert medium and low-grade waste heat into mechanical power and electricity. This paper presents the energy and exergy analyses of three ORC–WHR configurations that use a coupling thermal oil circuit. A simple ORC (SORC), an ORC with a recuperator (RORC), and an ORC with double-pressure (DORC) configuration are considered; cyclohexane, toluene, and acetone are simulated as ORC working fluids. Energy and exergy thermodynamic balances are employed to evaluate each configuration performance, while the available exhaust thermal energy variation under different engine loads is determined through an experimentally validated mathematical model. In addition, the effect of evaporating pressure on the net power output, thermal efficiency increase, specific fuel consumption, overall energy conversion efficiency, and exergy destruction is also investigated. The comparative analysis of natural gas engine performance indicators integrated with ORC configurations present evidence that RORC with toluene improves the operational performance by achieving a net power output of 146.25 kW, an overall conversion efficiency of 11.58%, an ORC thermal efficiency of 28.4%, and a specific fuel consumption reduction of 7.67% at a 1482 rpm engine speed, a 120.2 L/min natural gas flow, 1.784 lambda, and 1758.77 kW of mechanical engine power.

Experimental study of the organic rankine cycle under different heat and cooling conditions

A small-scaled organic Rankine cycle (ORC) system using R123 as working fluid was experimentally investigated. For ORC and regenerative organic Rankine cycle (RORC), the impacts of the evaporating and condensing temperatures on the performances of main components (i.e., the expander, the pump, and heat exchangers) and the system were tested. The comparison between these two systems under identical working conditions was also carried out. The results showed that the expander shaft power of ORC is greater than that of RORC under the identical evaporating temperature and condensing temperature. When the evaporating temperature is relatively low, the expander shaft power is more sensitive to the condensing temperature, and the thermal efficiency of ORC is higher than that of RORC. With the increasing of the evaporating temperature, the thermal efficiency of RORC exceeded that of ORC. Therefore, ORC is recommended for the low temperature heat source, while for the high temperature heat source, RORC is recommended for its higher thermal efficiency.

Design space exploration for waste heat recovery system in automotive application under driving cycle

Organic Rankine Cycle (ORC) systems have been recently considered for the promising application to Waste Heat Recovery (WHR) in automotive application. However, the design of ORC systems for highly transient operations typical of heavy-duty vehicles faces several challenges. This work extends the method of Design Space Exploration (DSE) to optimize the design of an ORC system for heavy-duty vehicle applications. The goal is to match the size of ORC system with the feature areas of driving cycle. Starting from drive cycle data, the feature areas are identified to represent the driving cycle. Then, a Weighted Least Square method is adopted to determine the initial design condition from feature areas, and based on the design condition, the initial ORC model is built for the further optimization. Particle Swarm Optimization (PSO) with parallel computation is adopted to optimize the performance of nonlinear ORC system with coupled constraints. The result shows that the optimal system can output 6.87% more power than the initial system and 20.08% more power than the inferior system over the feature areas of WHTC test. The analysis shown in this work also includes general design guidelines for optimizing the system architecture and optimally sizing the heat exchangers to improve the ORC thermal efficiency.

A Distributed Generation Hybrid System for Electric Energy Boosting Fueled with Olive Industry Wastes

This paper presents the theoretical model and the simulation of a cutting edge hybrid power system composed of an externally-fired gas turbine (EFGT) coupled with an organic Rankine cycle (ORC) as a bottoming unit for the maximization of electrical power. The power plant is fed with different biomass sources from olive industry wastes (pruning, dry pomace, stones, leaves and twigs), providing more than 550 kW of electric power and a net electrical efficiency of 26.0%. These wastes were burnt directly at atmospheric pressure in an EFGT, producing 400 kW of electric power and exhaust gases at 300 °C. Ten suitable ORC working fluids have been studied to maximize the electric power generation: cyclohexane, isohexane, pentane, isopentane, neopentane, R113, R245fa, R365mfc, R1233zd and methanol. The best fluid was R1233zd, reaching 152.4 kW and 22.1% of ORC thermal efficiency; as drawback, however, R1233zd was not suitable for Combined Heat and Power CHP applications due its lower condensation temperature. Thus, despite R113 gave minor electricity production (137.5 kW) this allowed to generate additional thermal power (506.8 kW) in the way of hot water at 45 °C.

Second-law-based screening of working fluids for medium-low temperature organic Rankine cycles (ORCs): Effects of physical and chemical properties

Abstract The diversity of both organic fluids and performance criteria makes the selection and evaluation of working fluids for organic Rankine cycles (ORCs) full of challenge. To overcome this challenge, this paper first reports on second-law-based analytical expressions for thermal efficiency and a turbine size parameter, which reveal key dimensionless parameters relating to operating temperatures and thermodynamic properties of organic fluids. This paper systematically screens more than 70 working fluids involving a wide range of physical and chemical properties. This study shows that specific turbine size generally increases with critical temperature, reduced ideal gas heat capacity, and the atom number of working fluids. Different-family fluids may lead to ten-fold relative differences in specific turbine size even though they have the same atom numbers. The thermal efficiency of ORCs appears to be independent of critical temperature, or at most, a weak function of critical temperature for low boiling point wet fluids and other halocarbons. This is a result of Jacob number being independent of or dependent weakly on the critical temperature of organic fluids such as alkanes, alkenes, and benzenes.