Optimal Working Fluid Research Articles

EXPERIMENTAL AND NUMERICAL COMPARATIVE INVESTIGATION OF OPTIMUM WORKING FLUID RATIO FOR THERMOSYPHON HEAT PIPES

EXPERIMENTAL AND NUMERICAL COMPARATIVE INVESTIGATION OF OPTIMUM WORKING FLUID RATIO FOR THERMOSYPHON HEAT PIPES

Superstructure-based mixed-integer nonlinear programming framework for hybrid heat sources driven organic Rankine cycle optimization

Organic Rankine cycle (ORC) is a promising technology capable of harnessing low-grade energy, e.g. renewable energy and waste heat, and converting it into electricity. Conventional ORC often operates with single heat source, which can be unfavorable to its efficiency due to the poor matching between its working fluid and heat source. Implementing hybrid heat sources is generally more energy-efficient as multiple heat sources can effectively match with the working fluid, however, the design can be a challenging task. This study proposes a superstructure-based method for the ORC design that simultaneously synthesizes heat exchanger network for hybrid heat sources and working fluid, and optimizes the ORC and heat sources' parameters. A mixed-integer nonlinear programming model is formulated to achieve the simultaneous optimization. We also develop a tailored multi-step initialization algorithm to facilitate the optimization. The model is applied to the design and analysis of ORCs driven by solar energy and waste heat with four different working fluids. Results demonstrate that the hybrid heat source-driven ORC improves the system performance. Cyclohexane is found to be the optimal working fluid for the proposed ORC system with a 48.19% increase in net power output compared with the single heat source-driven ORCs running separately.

Economic and environmental evaluation of heat pump-assisted pressure-swing distillation of maximum-boiling azeotropic mixture water-ethylenediamine

Different options to reduce the energy demand of a pressure-swing distillation process for separating the maximum-boiling azeotropic mixture water (A)-ethylenediamine (B) are studied by rigorous simulation. The total annualised cost (TAC) without any energy demand reduction option is minimised by a genetic algorithm coupled to a flow-sheet simulator, then the options partial (PHI) and full heat integration (FHI), and vapour recompression heat pumps (VRC) were studied. Heat pumps are applied either for only one (the high- or the low-pressure) or both columns. By optimising the flow rate of the working fluid of heat pumps, the compressor work, thus the energy demand and capital cost of the heat pump are considerably reduced. For the steam and electricity prices used, the optimised PHI is the most economical, reducing the TAC by 23.7%. The influence of these prices on the TAC of each configuration is also studied. The environmental evaluation is performed by calculating CO2 emissions and Eco-indicator 99 values. Heat pump-assisted processes have lower values compared to the other configurations, especially with optimal working fluid flow rate. PHI or FHI leads to a reduction of both above indicators, but they are still higher than those of heat pump-assisted processes.

Synthesis and simultaneous optimization of multi-heat source multi-pressure evaporation organic Rankine cycle with mixed working fluid

The organic Rankine cycle (ORC)’s performance relies on heat source properties, cycle structures, and working fluids. Influences of these factors, however, are often interdependent, making it difficult to determine the optimal design of ORC systems. To address this issue, we propose a mixed-integer nonlinear programming model for the multi-pressure evaporation ORCs driven by multiple heat sources that not only optimizes operating conditions, but also identifies the best working fluid composition and cycle structure. Results demonstrate that the proposed model is accurate and effective. The dual-pressure evaporation ORC driven by multi-heat source shows a 10.87% rise in the net power output (NPO) compared to the one with single pressure evaporation. In general, 0.9/0.1 n-pentane/n-heptane is the optimal mixed working fluid generating 2.71–12.11% more power than the pure working fluids for the numerical studies. Dual-pressure evaporation ORC is found to be a balanced design for the multi-heat source driven ORC in terms of its NPO and cycle complexity.

Thermodynamic analysis of working fluids: What is the highest performance of the sub- and trans-critical organic Rankine cycles?

The thermodynamic performance limits of power cycles are governed not only by the first and second law of thermodynamics, but also by the cycle configuration and working fluid properties. The organic Rankine cycle (ORC) is a promising technology for low-and-medium temperature heat utilization. This work further develops the thermodynamic performance limits analysis framework of sub- and trans-critical ORCs under realistic heat source conditions, and investigates the impact mechanism of key property parameters on system performance. Working fluid thermodynamic properties are characterized using a corresponding state model with 5 property parameters that are usually available in the early stages of system design. The property and system parameters are optimized simultaneously using a multi-objective genetic algorithm. The thermodynamic performance limits, and the optimal working fluid properties and system operation conditions are identified. The obtained limits represent the highest performance of ORC. The obtained optimal property and system parameters represent the desired working fluid and system operation characteristics. The impact mechanism of key property parameters on system performance is discussed in detail in a sensitivity analysis. It is found that the critical temperature is the most important property parameter, and that it changes the preference over efficiency and compactness objectives. The obtained thermodynamic performance limits, optimal parameter sets, and impact mechanism provide insights for working fluid selection in ORC research and implementation.

Performance investigation of a reverse osmosis desalination system powered by solar dish-Stirling engine

Solar desalination is a promising technology to reduce the costs and environmental impact associated with the use of fossil fuel resources. Currently, almost all reverse osmosis (RO) desalination technologies powered by solar thermal energy are based on organic Rankine cycle (ORC) systems. Considering the significant advantages of the Stirling cycle in the utilization of solar energy, in this paper we present a RO desalination system powered by a solar dish-Stirling (DS) engine; a detailed parametric analysis based on finite-time thermodynamics is carried out to evaluate the water productivity as well as the energy and exergy efficiency of the system. The water productivity monotonically increases with increasing absorber temperature, while the optimal absorber temperature for maximum energy and exergy efficiency ranges from 1100 to 1300 K. Linear correlations are found between the optimal working fluid temperature of the source side and the maximized water productivity, energy/exergy efficiency, and average absorber temperature. The water productivity and energy/exergy efficiencies show an initial increase followed by a decrease with increasing sink side temperature. Among these properties, the stagnation point for maximum water productivity is found at a higher sink side temperature.

Optimal hydrocarbon based working fluid selection for a simple supercritical Organic Rankine Cycle

In this paper, several hydrocarbons' performance as a working fluid for a supercritical Organic Rankine Cycle are studied. Energy, exergy, and exergoeconomic analysis are conducted for the cycle operating with a low-grade waste heat source. Initially, fifteen different working fluids, primarily hydrocarbons, were considered based on their environmental characteristics, global warming potential, ozone depletion potential, flammability, and toxicity. The cycle efficiency for each working fluid was analyzed for different cycle operating pressures and different inlet heat source temperatures from 400 K to 500 K for a constant turbine work output. Three different trends of cycle performance characteristics were observed for various working fluids with the cycle operating at different evaporator pressures and source inlet temperatures. The working fluids that exhibited similar characteristic trends were then categorized into families for further analysis. Three hydrocarbons, i.e., Dimethyl Ether, R1234yf, and Diethyl Ether, were each selected as the representative of their respective family, along with Isobutane as the reference working fluid for comparison. After detailed energy and exergy analyses, Dimethyl Ether and Isobutane were found to be promising working fluids for the range of pressure and source inlet temperatures considered. Finally, the exergoeconomic analysis showed Dimethyl Ether to be the optimal working fluid based on the unit cost of net work produced and the total cost of exergy destruction, which were calculated to be (59.18 ± 0.49) $/MWh and (8.92 ± 0.48) $/h, respectively, when operating at 450 K.

Optimal working fluids and economic estimation for both double stage organic Rankine cycle and added double stage organic Rankine cycle used for waste heat recovery from liquefied natural gas fueled ships

Double and added double stage organic Rankine cycle systems are configured to recover exhaust gas waste heat of dual fuel engines. To evaluate the performance of the models proposed here, energy, exergy and economic analyses are performed. Several working fluids are evaluated for recommendation for double and added double stage organic Rankine cycle systems. In the double stage organic Rankine cycle, cycle 1 and cycle 2 are connected in parallel. Working fluids R123, R141b, and R601 are used in cycle 1, and R245fa, R236ea, and R1233zd in cycle 2. In the double stage organic Rankine cycle, the working fluid combinations of R601-R1233zd, R601-R245fa and R123-R245fa show better performance when considering power, heat transfer area and payback period, which are 1760 kW, 2108.9 m2 and 4.21 year, respectively for R601-R245fa. In the added double stage organic Rankine cycle, cycle 1 and cycle 2 are connected in two stages and cycle 1 and cycle 3 in parallel. The net power of the working fluid combinations of R123-R245fa and R123-R1233zd are 1799 kW and 1782 kW, respectively, which are higher than those of the others. Further, for R123-R245fa, the heat transfer area and payback period are 3352 m2 and 6.20 year, respectively, which is better compared to those of other working fluid combinations.

Thermodynamic analysis of a hydrogen fuel cell waste heat recovery system based on a zeotropic organic Rankine cycle

In this study, a thermodynamic model of a hybrid system consisting of a proton-exchange membrane fuel cell (PEMFC) system and an organic Rankine cycle (ORC) system is established using Aspen Plus software. The optimal working fluid is selected by comparing the performance of pure working fluids and zeotropic mixtures of working fluids in an ORC system. The zeotropic working fluid consisting of R245fa/R123 with a mixing ratio of 0.6/0.4 shows the best performance. The effects of some key parameters, including the current density and operating temperature, on the performance of the PEMFC–ORC system with zeotropic mixing fluids are discussed. The results indicate that the power and efficiency of the PEMFC–ORC system with the zeotropic mixing fluid increase with increasing operating temperature. When the R245fa/R123 zeotropic working fluid with a mixing ratio of 0.6/0.4 is used, the net power and efficiency of the hybrid system reach optimum values under the studied boundaries, with improvements of 12.87% and 4.84%, respectively. The influence of the maximum allowable evaporation pressure and stack operating temperature on the selection of optimum working fluid for the hybrid system is discussed in addition.

Multi-criteria assessment and optimization of waste heat recovery for large marine diesel engines

The energy crisis combined with increasingly severe environment pollution have highlighted the significance of the energy conservation and emission reduction in shipping industry. To improve the energy utilization rate and reduce pollution, a dual-pressure organic Rankine cycle system is proposed to reuse the heat of exhaust gas in marine engines. Many researches focused on the thermodynamics and thermos-economics of the waste heat recovery system, and optimized it from these two goals to recover more energy and decrease cost. However, the impact of the system itself on the environment, which was significant to assess the greenness, was often neglected in these studies. The system with high energy recovery rate may also cause serious environment pollution in the manufacturing or operation process, hence the environmental factor should be taken into account in the optimization. To solve the problems, an evaluation method combined thermodynamics, economics and environment is proposed. The parametric study of the dual-pressure organic Rankine cycle system with six common used working fluids is assessed by this method. Finally, taking the indicators of evaluation method as objective function, the multi-objective optimization method is conducted to figure out the optimum operation conditions for the system. Results show cyclopentane is the optimal working fluid considering thermodynamic, economic and environmental properties. Under the conditions of the evaporation pressure in high-pressure circuit 2.79 MPa, the evaporation pressure in low-pressure circuit 1.97 MPa, the condensation temperature 35 °C, the superheating temperature 1.2 °C, the system obtains a good performance with exergy efficiency 60.24%, electricity production cost 0.167 $/kWh and the environmental impact of net power output 11831.9 mPts/GJ.

Comparative analysis of working fluids for heat pump-type flue-gas cooling–reheating system

• A novel flue-gas cooling-reheating system for coal-fired power plants is proposed. • The thermodynamic system is optimised under fixed operating parameters. • The rank sum ratio method is applied to working-fluid selection for the first time. • The trade-off among environment, health, safety, and costs is assessed. An upgraded flue-gas cooling–reheating system called an organic gas–gas heater, which builds a heat-pump cycle downstream of the wet desulfurisation process, is proposed for coal-fired power plants. Compared with the existing system, it can improve the particulate matter 2.5 emissions and alleviate sulfuric-acid corrosion. Adjustability of operating parameters is restricted by the standards in power industry and the automatic control habit of users. In view of this, multi-objective optimisation was conducted for the organic gas–gas heater system, on the premise of unalterable heat-source and heat-sink parameters and fixed working-fluid evaporating and condensing temperatures. According to comparison results for 31 selected working fluids under six operating conditions, it is feasible to realise a trade-off among thermodynamic, economic, and environmental objectives, even though the sole alterable parameter is the type of working fluid. The rank sum ratio method was applied to the field of working-fluid selection for the first time. As a comprehensive assessment method, it was confirmed in this study to be simple and effective. Moreover, R245ca is the optimum working fluid in this study, but the results indicate that the priority of the working fluid must be reconsidered if the operating conditions or assessment criteria change.

A high-efficiency scheme for waste heat harvesting of solid oxide fuel cell integrated homogeneous charge compression ignition engine

Facing the energy shortage and environmental pollution problems in the world, a high-efficiency fuel cell scheme combined with waste heat recovery is regarded as an optimal selection of auxiliary power generation equipment for ships. In this study, a novel system, comprising solid-oxide-fuel-cell-preheating subsystem, homogeneous charge compression ignition engine and organic Rankine cycle, is designed and analysed. First, the mathematical model is established and verified to ensure the reliability of simulation results. Then, the effect of relevant parameters on homogeneous charge compression ignition engine performance is observed, and the optimal zeotropic working fluid of organic Rankine cycle is determined according to the exhaust temperature. Finally, the thermodynamic and economic analysis are carried out. The results indicate that the performance of the new system is better than that of the traditional combined system using gas turbine. Thermal efficiency and exergy efficiency of the system are more than 63.6% and 61.3%, which is about 18.76% and 17.95% higher than that of simple cell. In addition, the payback time is about 2.98 years, which is about half of the traditional combined system and can fully meet the economic requirements. This novel system can be a new selection of marine auxiliary power generation equipment.

Optimization of an Organic Rankine Cycle-Based Waste Heat Recovery System Using a Novel Target-Temperature-Line Approach

Abstract Organic Rankine cycle (ORC)-based waste heat recovery (WHR) systems are simple, flexible, economical, and environment-friendly. Many working fluids and cycle configurations are available for WHR systems, and the diversity of working fluid properties complicates the synergistic integration of the efficient heat exchange in the evaporator and net output work. Unique guidelines to select a proper working fluid, cycle configuration and optimum operating parameters are not readily available. In the present study, a simple target-temperature-line approach is introduced to get the optimum operating parameters for the subcritical ORC system. The target-line is the locus of temperatures satisfying the pinch-point temperature difference along the length of the heat exchanger. Employing the approach, study is carried out with 38 pre-selected working fluids to get the optimum operating parameters and suitable fluid for heat source temperatures ranging from 100 °C to 300 °C. Results obtained are analyzed to get cross-correlations between key operating and performance parameters using a heat-map diagram. At the optimum condition, optimal working fluid’s critical temperature and pressure, evaporator saturation temperature, effectivenesses of the heat exchange in the evaporator, cycle, and overall WHR system exhibit strong linear correlations with the heat source temperature.

Possibility of optimal efficiency prediction of an organic Rankine cycle based on molecular property method for high-temperature exhaust gases

Working fluid selection is critical for the system design of an organic Rankine cycle (ORC). Computer-aided molecular design (CAMD) provides a new approach to search novel working fluids. In this study, the possibility of using molecular properties to predict the optimal system performance of an ORC is investigated. First, for a heat source of high-temperature exhaust gases, the influences of the molecular properties including the critical temperature and the specific heat capacity on the exergy efficiency of an ORC using 10 alkanes as working fluid are estimated. Then, the possibility of using the parameters of PC-SAFT to assess the optimal efficiency of the ORC is explored. The relation of the optimal temperature difference between the heat source and the molecular properties is determined. The prediction precision of PC-SAFT method is compared with the other four different molecular property methods. The results indicate that the parameters of PC-SAFT can be used to predict the optimal exergy efficiency of ORC system and the corresponding optimal heat source temperature. The average deviations of the exergy efficiency predicted is only 1.44% for the PC-SAFT method. The search range of the optimal working fluid can be decreased and the computation load is reduced significantly.

Thermo-economic analysis of Phosphoric Acid Fuel-Cell (PAFC) integrated with Organic Ranking Cycle (ORC)

Hydrogen produced from renewable resources has been gaining attention and its use is encouraged, as an effort of emission control. Consequently, the development of hydrogen production methods allows for fuel cell technologies to be developed in parallel. As a clean system for generating electricity, the reaction between hydrogen and oxygen occurring in a fuel cell produces only water and heat. By recovering heat release, thermal energy can be converted to mechanical work or electric power, improving performance and profitability of the system. This work studies the process integration of a 300 kW Phosphoric Acid Fuel-Cell (PAFC) and the Organic Ranking Cycle (ORC); a combined heat recovery process for generating additional electricity. A thermo-economic analysis is performed to determine the optimal working fluid with minimum payback time and net profit of PAFC/ORC integration. Among 15 working fluids examined, utilizing ammonia for ORC increased additional electricity produced by 6.64%, showing the greatest net profit, approximately 149 K€, and the payback time is reached after 3.3 years. Examined fluids with high critical temperature were water, benzene, and toluene, and obtained great efficiency improvement (about 9–11%) but they were unable to results to profit during 10 years of fuel cell lifetime.

Selection of optimal fluid for refrigeration cycles

Refrigeration and air-conditioning play an important role in our life and industrial applications. They have great impact on our life. They have also contributed to the world’s major environmental issues like ozone layer depletion and global warming. Common refrigerants such as CFCs and HCFCs which are working as fluid in refrigeration cycles have unfavorable environmental impacts and this has brought about concerns and regulations prohibiting their production and use as refrigerants by the year 2030. The development of different refrigerants over time took place based on safety and environmental impact issues. This paper, presents the selection of optimal working fluids for Vapor-Compression Refrigeration Cycle (VCRs) based on computer aided molecular design (CAMD) and process optimization techniques. The resulting methodology utilizes from CAMD for the generation of optimum working fluid candidates. Candidates were evaluated as alternative refrigerants for the R134a refrigerating system through simulation using Aspen Hysys V8.0, with restricted by priority coefficient of performance COP, environmental and safety criteria. Ethyl trifluoromethyl ether 1,1,1-TrifluoroButane (new refrigerant) shows a good environmental and toxicity data also have high COP, 4.5, 2.7 respectively and were favored amongst the studied refrigerants as the choice alternative refrigerants to replace R134a. The methodology systematically identified conventional molecular structures that enable optimum VCRs process performance.

Development of a novel cogeneration system by combing organic rankine cycle and heat pump cycle for waste heat recovery

This paper presents a competitive cogeneration system, which combines the organic Rankine cycle with the heat pump cycle through a“built-in”evaporator. With this innovative structure, the input waste heat temperature in both the sub-cycles can simultaneously be maintained at a high level. The optimal working fluid is screened. Energy and economic advantages of the proposed system are proven. Moreover, the influence of main thermodynamic parameters on system performance is investigated. The results indicate that, compared with the traditional system, the thermal efficiency and waste heat recovery efficiency of the proposed system using the “built-in” evaporator increased by 6.4% and 1.2%, respectively. Meanwhile, it can be found that the thermal efficiency of the proposed system increased linearly, while the waste heat recovery efficiency showed a variation that followed a quadratic function with the increase of evaporation temperature. The peak value of the waste heat recovery efficiency was 9.9% for the evaporation temperature of 335 K. Moreover, the exergy performance of the system was very sensitive to the variation of evaporation temperature. An increase of 24 K in the evaporation temperature resulted in a drop in total exergy losses of nearly 76%, which resulted in an increase in the system exergy efficiency by about 27%.

Binary-objective optimization of latest low-GWP alternatives to R245fa for organic Rankine cycle application

Organic Rankine cycle (ORC) enables to harness waste heat resource to generate electrical power. R245fa is assumed as an appropriate working fluid for low-temperature waste heat recovery. However, strict regulations of CO2 emissions stimulate scientists to search for potential alternatives to R245fa. This paper presents a comparative analysis of various latest low-GWP refrigerants for ORC application. An energetic-economic-environmental evaluation model is established and further integrated into the non-sorted genetic algorithm (NSGA-II) for multi-objective optimization (MOO). A classic decision-making method is employed to screen out the optimal refrigerant for current study. Apart from that, the whole-year ambient conditions in Sydney are taken into consideration to screen refrigerants. Comparing the cycle thermal efficiency, R1224yd(Z) leads to 3.3% higher than R245fa. Comparing the economic cost, it saves 10.4% more than R245fa. Comparing the whole year’s performance, R1224yd(Z) exhibits 11.2% higher cycle thermal efficiency than R245fa while R1234ze(Z) saves 9.85% more economic cost than R245fa. R1224yd(Z) is considered as the optimal working fluid for current ORC system. R1224yd(Z) leads to 16% higher LECTs than R245fa. Comparing the maximum LECTs, R1224yd(Z), R1234ze(Z) and R1233zd(E) is 16%, 9.2% and 13.5% higher than R245fa, respectively.

Key parameter influence mechanism and optimal working fluid screening correlation for trans-critical organic Rankine cycle with open type heat sources

Trans-critical organic Rankine cycles (ORC) are promising for low to medium temperature heat recovery. To achieve a better cycle performance, the screening of optimal working fluids is quite significant. The optimal working fluids vary with the heat sources, working conditions and performance criterion, and it is difficult to build a general optimal working fluid screening criterion for the trans-critical ORC. Finding the relationship between the working fluid thermodynamic properties and the cycle performance is an effective way to establish general method for optimal working fluid screening. This paper provides theoretical analysis of how the key working fluid thermodynamic properties influence on the cycle performance under a wide temperature range of open type heat sources using various working fluids. Especially the influence of ‘uniformity of cp’ on the heat source utilization is originally analysed. In addition, a relatively general quantitative correlation for selecting the optimal working fluid for trans-critical ORC with heat source temperatures from 150 to 350 °C is presented.

Performance Analysis of Solar Organic Rankine Cycle with Stable Output Using Cyclopentane/Cyclohexane Mixtures▴

AbstractKeeping output stable is necessary for solar organic rankine cycle (SORC) considering variable solar irradiation conditions. In this study, zeotropic mixtures are introduced into SORC with stable output. Evaporation pressure and turbine inlet temperature are optimized with the overall system efficiency as optimization objective. Comparisons are drawn between the basic SORC and regenerative SORC. The influences of zeotropic mixture composition on the overall system efficiency, ORC efficiency, heat source outlet temperature, mass flow rate of heat source and working fluids and temperature rise of internal heat exchanger (IHE) are analyzed. The results show that zeotropic mixtures can slightly increase the overall system efficiency of the basic SORC system compared with the optimal pure working fluid system, but for the regenerative SORC system, the optimal overall system efficiency is higher when pure cyclohexane is used. When cyclopentane mole fraction is 0.4, the overall system efficiency of the basic SORC system reaches maximal value of 16.08%, which is 0.12% higher than for the optimal pure working fluids system, relatively. Under the optimum conditions, the optimal overall system efficiency of the regenerative SORC system is 52.11% higher than that of the basic SORC system.