Optimal Working Fluid Research Articles

Organic Rankine Cycle with Pure and Mixed Working Fluids for LNG Cold Energy Recovery

Liquefied Natural Gas (LNG) contains a large amount of cold exergy, which is normally wasted during the regasification process. Organic Rankine Cycles (ORCs), which have been widely employed for low-temperature waste heat utilization above ambient temperature, can also exploit LNG cold energy efficiently. Due to the low regasification temperature of LNG, the ORC for LNG cold energy recovery is quite different from the conventional ORCs recovering low-temperature waste heat. In this study, we investigate the working fluids for such an ORC recovering LNG cold energy. Both pure and mixed working fluids are studied and compared. According to open literature, mixed working fluids perform better than pure working fluids because the mixed working fluid can achieve a better match with the heat source. Therefore, the main motivation for mixed working fluids is to obtain gliding temperature for a better match with sensible heat sources. However, a few studies show that pure working fluid outperform mixed working fluids when the evaporating condition is close to the critical point. In addition, if the evaporation pressure is greater than the critical pressure, pure working fluids can also match well with a sensible heat source. Therefore, it is too general to claim that mixed working fluids will perform better than pure working fluids. For cases where the mixed working fluids perform better than pure working fluids, the optimal composition of the working fluid has to be determined. In this study, the performance of ORCs with pure and mixed working fluids are investigated for different types of ORCs. The optimal working fluid and composition for an ORC recovering LNG cold energy can be determined based on a simulation-based framework. The results show that mixed working fluids do not show significant improvement in net power output of the integrated system.

A comparative analysis of real-time power optimization for organic Rankine cycle waste heat recovery systems

Organic Rankine cycle waste heat recovery technology has been gaining more and more attention in recent years. Real-time power optimization is crucial to the system performance. Even though individual real-time optimization methods exist, literature rarely investigate comparison of different real-time power optimization methods. This paper first time compares three real-time implementable power optimization methods for an organic Rankine cycle waste heat recovery system. Three optimization methods include Proportional-Integral-Derivative rule-based method, nonlinear model predictive control and dynamic programming. In the Proportional-Integral-Derivative rule-based method, rule-based method defines the optimal working fluid vapor temperature trajectory and Proportional-Integral-Derivative controller manipulates the pump speed to follow that trajectory. In the nonlinear model predictive control method, reference vapor temperature is defined close to the saturation temperature and then the model predictive control minimizes the vapor temperature tracking error by controlling the pump speed. In the Dynamic Programming method, the net power produced by the organic Rankine cycle system is defined in the cost function, which is maximized by controlling the pump speed. A Random Forest machine learning model is utilized to extract the rules for Dynamic Programming and then implemented in real-time. All the three methods are implemented in the same experimentally validated plant model for the comparison analysis. The comparison results show that the Dynamic Programming Random Forest method has similar performance with nonlinear model predictive control method and outperforms the Proportional-Integral-Derivative rule-based method by 9.9% in net power production. Dynamic Programming Random Forest method can be an alternative to the nonlinear model predictive control for its low computation cost and high net power production.

Aging based design and operation optimization of organic rankine cycle systems

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

Waste heat recovery through organic flash cycle (OFC) using R245fa–R600 mixture as the working fluid

R245fa and R600 are two working fluids with which organic flash cycle (OFC) yields acceptable thermodynamic and economic performance to recover waste heat from low-temperature flue gas free from SO2. However, very high flammability of R600 and higher global warming potential (GWP) of R245fa are two issues of serious concern. Mixture with suitable mole fraction of these two fluids as the optimum working fluid for OFC is explored in this study. A mixture of R245fa and R600 with a mole fraction ratio (0.6/0.4) is selected conservatively as the working fluid of the OFC to suppress the flammability of R600. The corresponding value of GWP of the mixture comes below 800 kg of CO2 per kg of working fluid. It is observed that OFC using this mixture may yield higher work output per kg of flue gas flow and lower bare module cost/kW compared to that of OFC using either of the pure working fluid. OFC yields the highest work output per kg of flue gas for an R245fa/R600 mixture with (0.4/0.6) mole fraction ratio. Bare module cost per unit power output becomes a minimum when R245fa mole fraction is 0.37 in the mixture.

Methodology of exergy-based economic analysis incorporating safety investment cost for comparative evaluation in process plant design

This study proposes a new methodology of exergy-based economic analysis incorporating safety investment cost (SIC) for the comparative evaluation of process design alternatives for plants in terms of efficiency and economics while considering safety. Exergy-based economic analysis is employed to synthetically evaluate the design factors within the same framework. The SIC is estimated as the cost required to ensure the safety of the design by reducing accident risks to a level that is ‘as low as reasonably practicable’ (ALARP), which presents a rational approach to converting the risks associated with design alternatives into monetary values for economic analysis. The proposed method is applied to select the optimal working fluid in the Rankine cycle used in the recovery of both waste heat from a gas turbine and cold energy from liquefied natural gas (LNG) of the regasification process. The Rankine cycle, using ammonia, propane, and carbon dioxide as the working fluid, produces electric power of 11.7 MW, 8.9 MW, and 7.4 MW with specific exergy costs of 45.0 $/GJ, 61.9 $/GJ, and 77.8 $/GJ, respectively. Ammonia can be selected as the best alternative. The results are limited to quantitative risk assessment that does not consider domino effect and environmental damage.

Thermodynamic Selection of the Optimal Working Fluid for Organic Rankine Cycles

A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum temperature). The basis of this method is the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The most suitable existing working fluid can be selected, where an ideal adiabatic (isentropic) expansion step between a given upper and lower temperature is possible in a way, that the initial and final states are both saturated vapour states and the ideal (isentropic) expansion line runs in the superheated (dry) vapour region all along the expansion. Problems related to the presence of droplets or superheated dry steam in the final expansion state can be avoided or minimized by using the working fluid chosen with this method. Results obtained with real materials are compared with those gained with model (van der Waals) fluids; based on the results obtained with model fluids, erroneous experimental data-sets can be pinpointed. Since most of the known working fluids have optimal expansion routes at low temperatures, presently the method is most suitable to choose working fluids for cryogenic cycles, applied for example for heat recovery during LNG-regasification. Some of the materials, however, can be applied in ranges located at relatively higher temperatures, therefore the method can also be applied in some limited manner for the utilization of other low temperature heat sources (like geothermal or waste heat) as well.

Multi-objective thermo-economic optimization of organic Rankine cycle (ORC) power systems in waste-heat recovery applications using computer-aided molecular design techniques

In this paper, we develop a framework for designing optimal organic Rankine cycle (ORC) power systems that simultaneously considers both thermodynamic and economic objectives. This methodology relies on computer-aided molecular design (CAMD) techniques that allow the identification of an optimal working fluid during the thermo-economic optimization of the system. The SAFT-γ Mie equation of state is used to determine the necessary thermodynamic properties of the designed working fluids, with critical and transport properties estimated using empirical group-contribution methods. The framework is then applied to the design of sub-critical and non-recuperated ORC systems in different applications spanning a range of heat-source temperatures. When minimizing the specific investment cost (SIC) of these systems, it is found that the optimal molecular size of the working fluid is linked to the heat-source temperature, as expected, but also that the introduction of a minimum pinch point constraint that is commonly employed to account for inherent trade-offs between system performance and cost is not necessary. The optimal SICs of waste-heat ORC systems with heat-source temperatures of 150 °C, 250 °C and 350 °C are £10120, £4040 and £2910 per kW, when employing propane, 2-butane and 2-heptene as the working fluids, respectively. During a set of MINLP optimizations of the ORC systems with heat-source temperatures of 150 °C and 250 °C, it is found that 1,3-butadiene and 4-methyl-2-pentene are the best-performing working fluids, respectively, with SICs of £9640 per kW and £4000 per kW. These substances represent novel working fluids for ORC systems that cannot be determined a priori by specifying any working-fluid family or by following traditional methods of testing multiple fluids. Interestingly, the same molecules are identified in a multi-objective optimization considering both the total investment cost and net power output. These findings highlight the power of this approach as it enables the selection of novel working fluids while optimizing ORC systems using single or multiple thermo-economic performance indicators.

Performance analysis on novel thermodynamic cycle under the guidance of 3D construction method

The efficiency improvement of organic Rankine cycle, which is an efficient way to recover the waste heat of internal combustion engines, is an eternal topic. The 3D construction method of thermodynamic cycle provides a possible way to construct efficient thermodynamic cycles based on zeotropic working fluid, whose core is to improve the thermodynamic performance by using different working fluids in different thermodynamic processes. Based on the 3D construction method of thermodynamic cycle, a composition adjustment organic Rankine cycle was proposed in this paper to recover the waste heat of internal combustion engines. The optimum working fluid used in trunk and branch processes were selected according to the requirements of different thermodynamic processes. The effects of key parameters were analyzed, and the optimized operation conditions and optimal performance were obtained. The results shown that the composition adjustment organic Rankine cycle performs best when using R123/toluene (0.9/0.1) in trunk process and R123/toluene (0.96/0.04), R123/toluene (0.66/0.34) in low-pressure and high-pressure branch processes, respectively. Under the optimal condition, the net output power, thermal efficiency and exergy efficiency are 50.07 kW, 10.96% and 45.79%, respectively. The irreversible losses in evaporator I and evaporator II account for 35.01% and 30.54% of the total irreversible losses in composition adjustment organic Rankine cycle respectively. This work provided an advanced power cycle for the waste heat recovery of internal combustion engines. What’s more, composition adjustment organic Rankine cycle proves the feasibility of the 3D construction method and opens up a novel way to construct efficient thermodynamic cycles.

Energy production during regasification of liquefied natural gas: Identification of optimal working fluids in low temperature Rankine cycles

In this paper, the objective is to develop new graphical tools that allow straightforward identification of the optimal design of a Rankine cycle that uses LNG being regasified as a heat sink. To achieve that objective, a numerical model is developed in order to predict the value of the specific work output produced by the system. Then, numerical optimizations are performed and the results are reported in a graphical form. These new figures provide the values of the maximal specific work (objective function) for a wide range of operating conditions. These figures also provide the corresponding values of the four optimized design variables. The main result is that ethylene is the best working fluid for most of the operating conditions considered. Furthermore, a sub-optimal analysis is performed for each fluid investigated and the resulting figures allow to identify the fluids that can provide a specific work output close to that provided by the best working fluids. Finally, optimal designs identified in this paper are compared with optimal designs of the direct expansion (DE) and of the double direct expansion (D2E) systems.

A novel combined cooling-heating and power (CCHP) system integrated organic Rankine cycle for waste heat recovery of bottom slag in coal-fired plants

The extensive use of organic Rankine cycle (ORC) in waste heat recovery motivates the employment of ORC technique to recover the waste heat in coal-fired plants. A novel combined cooling-heating and power (CCHP) system integrated Organic Rankine Cycle (CCHP-ORC) for recovering the waste heat of bottom slag in coal-fired plant is first proposed in this paper. A MATLAB procedure with REFPROP database is developed for the proposed system based on the mass, energy and exergy balances of each component. With the application of Analytic Hierarchy Process (AHP) method, R1234ze(E) and heptane/R601a are adopted as the optimal working fluids for the Sing Fluid CCHP-ORC and Dual Fluid CCHP-ORC systems, respectively. Parametric studies are conducted to investigate the effect of superheat degree of turbine inlet temperature, chilled water mass flow rate and condenser temperature on the coefficient of performance, thermal efficiency, heat exergy, cooling exergy, total exergy production and exergy production rate. The results show that the superheat degree of turbine inlet temperature benefits the enhancement of the exergy production rate. The enhancement of chilled water mass flow rate leads to an increase in coefficient of performance and refrigerating capacity while the cooling exergy, total exergy production and exergy production rate decrease. Except for exergy production rate in Dual Fluid CCHP-ORC system, the rise of condenser temperature leads to a decrease of performance parameters. As the condenser temperature rises from 25 °C to 40 °C, the thermal efficiency of Cycle2 declines 19.4% and 18.3% respectively in the Sing Fluid CCHP-ORC system and Dual Fluid CCHP-ORC system.

Thermodynamic Analysis of a Hybrid Power System Combining Kalina Cycle with Liquid Air Energy Storage.

Liquid air energy storage (LAES) is a promising energy storage technology in consuming renewable energy and electricity grid management. In the baseline LAES (B-LAES), the compression heat is only utilized in heating the inlet air of turbines, and a large amount of compression heat is surplus, leading to a low round-trip efficiency (RTE). In this paper, an integrated energy system based on LAES and the Kalina cycle (KC), called KC-LAES, is proposed and analyzed. In the proposed system, the surplus compression heat is utilized to drive a KC system to generate additional electricity in the discharging process. An energetic model is developed to evaluate the performance of the KC and the KC-LAES. In the analysis of the KC subsystem, the calculation results show that the evaporating temperature has less influence on the performance of the KC-LAES system than the B-LAES system, and the optimal working fluid concentration and operating pressure are 85% and 12 MPa, respectively. For the KC-LAES, the calculation results indicate that the introduction of the KC notably improves the compression heat utilization ratio of the LAES, thereby improving the RTE. With a liquefaction pressure value of eight MPa and an expansion pressure value of four MPa, the RTE of the KC-LAES is 57.18%, while that of the B-LAES is 52.16%.

Robust Design of Multicomponent Working Fluid for Organic Rankine Cycle

A method for designing the optimal working fluid mixture of organic Rankine cycles (ORC), which is robust against uncertainty in its composition, is developed. The objective function for designing ...

Comparative analysis of an organic Rankine cycle with different turbine efficiency models based on multi-objective optimization

The radial-inflow turbine is a key component of the organic Rankine cycle (ORC) system, and its efficiency is related to working fluid properties and working conditions. In this paper, multi-objective algorithm was employed to conduct a comparative analysis of an ORC system with different turbine efficiency models (constant and variable). The system thermal efficiency and multi-objective optimization results between the constant turbine efficiency ORC system (CTORC) and the variable turbine efficiency ORC system (VTORC) were compared, and the reasons for the difference between the CTORC and VTORC system were analyzed. Sensitivity analysis was performed to compare the difference between the CTORC system and the VTORC system at different waste flue gas inlet temperature. The results show that the system thermal efficiency of both the CTORC and the VTORC increases with the increasing evaporation temperature and decreases with the increasing condensation temperature, while the variation rate of system thermal efficiency is different between CTORC system and VTORC system. The predicted turbine efficiency is significantly different for different working fluids and different working conditions. Considering the system comprehensive performance, R236ea is the optimal working fluid for the CTORC system, while R365mfc is for the VTORC system. For different working fluids, the error caused by using constant turbine efficiency model is different. With the increasing waste flue gas inlet temperature, the error caused by using constant turbine efficiency increases for R236ea, while the error caused by using constant turbine efficiency increases first and then decreases for R365mfc.

Performance prediction and working fluids selection for organic Rankine cycle under reduced temperature

This paper presents the approach for selection of optimal working fluids and cycle performance prediction for organic Rankine cycle (ORC) based on a theoretical exergy efficiency model (EEM) under reduced temperature. 18 working fluids with critical temperature from 100 to 200 °C are under evaluation. When the condensing temperature is 40 °C and reduced evaporating temperature is 0.85, the maximum overall exergy efficiency will be obtained when the heat source temperature approaches to the critical temperature of working fluid. Then, the optimal working fluids are selected by overall exergy efficiency. When the heat source inlet temperatures are 130 °C, 150 °C, 170 °C and 190 °C, the optimal working fluids are R236ea, R245fa, R245ca and R365mfc, respectively. In addition, under the same entire reduced temperatures the exergy efficiencies of different working fluids tend to be approximately equal and obey the circular distributions. The quantitative correlation between the optimal reduced evaporating temperature and reduced heat source temperature is given. The correlation provides a non-dimensional method to calculate the optimal evaporating temperature for different working fluids on the target of exergy efficiency.

Comparison on Thermodynamic Performances between Basic and Regenerative Sub-Critical Organic Rankine Cycles Coupled with Low Grade Heat Source

The coupling effects of low grade heat source on thermodynamic performances of both the basic and regenerative sub-critical Organic Rankine cycle (ORC) systems are investigated and compared. Turbine inlet pressure and turbine inlet temperature are treated as independent variables. The system exergy efficiency is selected as the thermodynamic performance criterion. The waste hot water with temperature range of 373.15K to 423.15K is selected as the low grade heat source. Optimization studies are carried out by using eight common organic working fluids to recover heat energy, and relevant operating conditions are obtained respectively. The results indicate that both hot fluid inlet temperature and the allowable minimum hot fluid outlet temperature influence the optimal working fluid, relevant turbine inlet condition and system exergy efficiency. Optimal system exergy efficiency increases monotonously with increasing of hot fluid temperature for both ORC systems above. In comparison to the basic cycle configuration, optimal system exergy efficiency for the regenerative system using the same working fluid is not changed if relevant turbine inlet vapor is saturated, but is significantly improved if relevant turbine inlet vapor is overheating. Besides, the optimal working fluid is dependent on the heat source temperature and specific cycle configuration.

Thermodynamic and economic analysis of a hybrid ocean thermal energy conversion/photovoltaic system with hydrogen-based energy storage system

The purpose of this study is to define and assess a new, renewable and sustainable energy supply system for islands and remote area where ocean thermal energy conversion (OTEC)/photovoltaic with hydrogen storage system is proposed. Components of this system are a turbine, generator, evaporator, condenser, pumps, photovoltaic panels, electrolyzer, hydrogen tanks, fuel cell and converter. To evaluate the proposed hybrid system, energy, exergy and economic analysis are employed. For OTEC, an optimization algorithm is applied to find the optimum working fluid (R134A, R407C, R410A, R717, R404A, and R423A), evaporation and condensation temperatures, and cold and warm seawater temperature differences between the inlet and outlet of evaporator/condenser. The results demonstrate that the maximum specific power of OTEC was achieved to be 0.3622 kW/m2 for R717 and 0.3294 kW/m2 for R423A working fluids. The overall energy efficiency for the hybrid renewable energy system was obtained 3.318%. The maximum energy loss was occurred by the turbine. The exergy efficiency of the hybrid system was obtained 18.35% and the payback period of the proposed system was obtained around 8 years. The unit electricity cost for the system was achieved as 0.168 $/kWh which is valuable compared to 0.28 $/kWh of the previous system.

Multi-objective optimization and improved analysis of an organic Rankine cycle coupled with the dynamic turbine efficiency model

Considering the effect of variable turbine efficiency on organic Rankine cycle (ORC) system working fluid selection and parameters determination, a multi-objective optimization model which considered the thermodynamic performance and economic factors simultaneously was coupled with a dynamic turbine efficiency model. The Pareto optimal solutions of optimization with constant turbine efficiency and with variable turbine efficiency were compared, specific quantitative analysis of turbine efficiency on an ORC system is presented and sensitivity analysis of the heat source temperature was conducted. The results reveal that the variation of net power output between constant turbine efficiency and variable turbine efficiency is different, and the distribution of Pareto optimal solutions with constant turbine efficiency and with variable turbine efficiency is also different. With constant turbine efficiency, R245ca is the optimal working fluid. However, with variable turbine efficiency, R365mfc is the optimal working fluid. In specific quantitative analysis, it can be found that the difference in the optimization results is related to the working fluid properties. For R245fa, the variation of turbine efficiency significantly affects the distribution of the Pareto optimal solutions. However, for R365mfc, the variation tendency of turbine efficiency is gentle, which only leads to a small deviation in the optimization results.

Energy, exergy, and economic analysis of a new triple-cycle power generation configuration and selection of the optimal working fluid

The present study investigated energy, exergy and economic analyses on a new triple-cycle power generation configuration. In this configuration, the energy of the exhaust gas and the wasted energy in the condenser of the steam cycle is recovered in the heat recovery steam generator (HRSG) and the evaporator of organic Rankine cycle (ORC), respectively. A computer code was written in MATLAB to analyze the triple-cycle configuration. Validation through this program showed that the highest errors were 5.6 and 7.1%, which occurred in gas and steam cycles, respectively. The results revealed that the highest generated entropy was associated with the combustion chamber and the evaporator in the steam cycle. The first and second laws of thermodynamics efficiencies were improved by roughly 270 and 8%, respectively, through adding each of the steam and organic Rankine cycles. The entropy generated by the cycle increased by roughly 400 and 4% by adding the steam and organic Rankine cycles, respectively. The price of the produced electricity was also reduced by roughly 60 and 70%, respectively, for these two cycles.

Determination of the optimal working fluid for the turbine recovering combustion engine exhaust gases heat

Microsteam turbine implementation for combustion engine exhaust gases heat recovery and subsequent acquisition of additional power is being investigated in many developed countries of the world. The results of such studies have already found application in some trucks. But this type of turbines is very weak in the Russian market. Turbine installation behind the combustion engine works under conditions of low volumetric flow of work fluid. This leads to a decrease in the height of the blade and vane wheels flow passage and an increase of the relative values of the gaps in the seals which are the reasons for the growth of the working fluid leakages. High degree of pressure reduction when selecting single-stage turbine leads to a supersonic velocity in the flow passage and an increase of the losses due to powerful shock waves. The efficiency of the turbine installation under these operating conditions is low and requires additional investigations. In this work, the working fluids which can give the greatest efficiency of the turbine installation were investigated. It was shown that not only thermodynamic but also hazardous and economic parameters must be taken into consideration. Working fluid with the high thermodynamic efficiency was compared with the one that profitable from economic point of view. The most appropriate substance was chosen and implemented in the microsteam turbine. The turbine stage which allows increasing economy and ecological compatibility of the combustion engine was developed and optimized by analytical methods.

Multi-objective optimization of ORC parameters and selection of working fluid using preliminary radial inflow turbine design

Organic Rankine cycles (ORCs) are used to generate power from low temperature heat sources. In this study, 28 hydro carbon (HC), hydro fluorocarbon (HFC) and hydro fluoroolefin (HFO) working fluids are used for three different heat source temperatures (90, 120 and 150 °C). A 1-D radial-inflow turbine (RIT) design with loss calculations is incorporated into the model to add dynamic turbine stage efficiency. The turbine model is run together with the ORC, and real gas properties are used for the entire system. A genetic algorithm multi-objective optimization is used to design an ORC with minimum thermal conductance per net power output and maximum performance factor (PF). The decision variables are specific speed, condensation pressure, pressure ratio at the turbine stage, degree of superheating and pinch point temperature difference (PPTD) in the evaporator. Pareto frontiers are obtained at the end of the study, and a decision-making method (TOPSIS) is used to select an optimum solution for each working fluid. R1234yf, R1234ze(e) and isobutane are found to be the optimum working fluids at 90, 120 and 150 °C heat source temperatures respectively. For each solution, turbine geometry, fluid velocities and impeller rotational speed are calculated.