Mixed Working Fluids Research Articles

Simulation study of R1234ze and its mixed working medium in TEG‐ORC combined cycle

AbstractThermoelectric generation (TEG) and organic Rankine cycle (ORC) technologies both have their respective limitations in recovering waste heat from ships. However, the combination of TEG and ORC is an effective approach to achieve cascaded waste heat recovery and improve heat utilization efficiency. There has been some progress in research on waste heat recovery in maritime applications based on TEG‐ORC combined cycles. The selection of a working medium is a crucial element that directly influences the design and performance of the entire system, but there is limited research on the impact of different working fluids on combined cycle systems. In this study, a simulation model of a TEG‐ORC combined cycle system was established to examine its output potential using two different working fluids R1234ze and a mixture of R245fa/R1234ze. The results demonstrate that compared to employing the R1234ze working medium, the combined cycle system with the mixed working medium achieved a 13% increase in maximum output power, a 102% improvement in optimum thermal efficiency, and a 53% reduction in optimum power production cost. Additionally, the power output distribution in the system utilizing the mixed working fluid was more uniform compared to the system employing a single working medium. This confirms the great potential of using a mixed working fluid in a combined cycle system.

Comparative and optimal study of energy, exergy, and exergoeconomic performance in supercritical CO2 recompression combined cycles with organic rankine, trans-critical CO2, and kalina cycle

The bottom cycle in supercritical carbon dioxide (sCO2) recompression Brayton combined cycle (RCBC) effectively recovers substantial waste heat for electricity generation, enhancing energy utilization. The recovery efficiency is significantly influenced by the bottom cycle configurations and specific working conditions. Besides, implementing these bottom cycles introduces challenges related to increased system dimensions and design complexity. The present study aims to understand and evaluate the characteristics and optimal trade-offs of various bottom cycles, including trans-critical, subcritical, and non-azeotropic mixed working fluids, by establishing 3E (energy, exergy and Exergoeconomic) models for sCO2/tCO2, sCO2/ORC, and sCO2/KC combined cycle systems. To analysis and enhance interpretability, statistical Global Sensitivity Analysis (GSA) alongside unsupervised learning-based Self-Organizing Map (SOM) techniques and parametric analysis are employed. These methods identify key parameters and discern system patterns. Subsequently, the Non-Dominated Sorting Whale Optimization Algorithm (NSWOA) and entropy weight-based Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) are applied to establish a Pareto-optimal decision space and identify compromised ideal design points for each combined cycle. The results quantitatively rank sensitivity for objective variables within the design space in different combined cycles and visually represent system nonlinearity and coupling relationships in 2D weight planes using SOM. The TOPSIS trade-off points based on the Pareto frontier for the three combined cycles are ηth = 45.08 %, cp,tot = 8.5199 $/GJ, ηth = 45.13 %, cp,tot = 8.3739 $/GJ, and ηth = 48.97 %, cp,tot = 8.4783 $/GJ, respectively. Integrating machine learning techniques provides a comprehensive understanding of system patterns, offering valuable insights for decision-makers in the design and optimization of combined cogeneration cycles.

Research on flammability of 3,3,3-trifluoropropene and its binary mixtures with flame retardants

3,3,3-trifluoropropene (R1243zf), which has good environmental and cycling characteristics, is a very promising fourth generation new working fluid. However, it is flammable, so studying its combustion characteristics and how to suppress its flammability are of great significance. Firstly, the flammability limits of three binary mixtures composed of a combustible substance (R1243zf) and three flame retardants (R245fa, R1216, and R13I1) were experimentally studied. The results showed that as the proportion of flame retardants increased, the lower flammability limit of the mixture increased, while the upper flammability limit decreased except for R245fa. The critical suppression ratios of R245fa, R1216, and R13I1 on R1243zf were 4, 0.538, and 0.538, respectively. Then, the improved group contribution method was used to predict the minimum inhibitory concentration of R1243zf, with an error of only 4.57 % compared to the experimental values. Finally, the influence of variable ambient temperature (25–100℃) on the flammability of R1243zf was studied, and the calculated adiabatic flame temperature method was used to predict the flammability limit at different temperatures. The error between the predicted values and the experimental values is only 1.7 %. The relevant results are of great significance for the safe application of R1243zf and new binary mixed working fluids.

Thermal-economic performance evaluation and bi-objective optimization of organic Rankine cycles using pure and mixed working fluids for waste heat recovery

To further enhance the recovery rate of low temperature waste heat, the low temperature flue gas in the sinter annular cooler is chosen as the heat source of organic Rankine cycle (ORC) system in the current study, and the system thermal-economic performance evaluation are conducted. The thermal-economic performance models of ORC system are established firstly, and the butane and isopentane are selected to constitute the mixed working fluid. Then the changes of system performance with the mass fraction of isopentane (MR601a) and ORC crucial parameters are evaluated comprehensively, and the bi-objective optimization is applied to ascertain the optimal ORC crucial parameters under various MR601a. The results indicate that the MR601a of 0.6 can obtain the maximum glide temperature. The higher the evaporating temperature is, and the lower the condensing temperature is, the larger the exergy efficiency is, while the smaller the levelized energy cost is. There is an optimal superheat degree to make the ORC system achieve the better overall performance under various MR601a. The exergy efficiency of butane is the maximum with a respectable waste heat recovery rate of 64.18 %, while the levelized energy cost of isopentane in the whole conditions is the minimum with the better economic performance.

Comparison of combustion and interaction mechanisms of mixed working fluids R152a and R1270: A theoretical and experimental study

In the background of global efforts to reduce carbon emissions, mixed working fluids could mitigate flammability concerns of low global warming potential (GWP) working fluids for organic Rankine cycles (ORCs) and compression refrigeration cycles. In this study, the combustion and interaction mechanisms of R152a/R1216 and R1270/R1216 were investigated. Density functional theory (DFT) and ReaxFF molecular dynamics (MD) methods were employed to analyze the combustion processes of the two binary working fluids and the formation pathways of the toxic substance HF. Additionally, the influence of R1216 on the combustion of R152a and R1270 was evaluated and compared at different reaction temperatures and component ratios. The results showed that, R1216 did not inhibit but rather promoted the consumption of R152a and R1270. Corresponding experiments demonstrate that the combustion of R152a and R1270 becomes more complete and occurs more quickly after the addition of low proportions of R1216. Subsequently, the flammability limits of the two binary working fluids were determined through experiments at different ambient temperatures. The experimental results indicated that the rise in ambient temperature expands the flammability limit ranges of both binary working fluids, and the higher the proportion of R1216, the more sensitive the flammability is to changes in ambient temperature.

Experimental study on solar wall by considering parametric sensitivity analysis to enhance heat transfer and energy grade using compound parabolic concentrator and pulsating heat pipe

Enhancing high-grade and rapid thermal storage for solar energy utilization is of paramount importance in reducing building energy consumption. Therefore, this study combines the Compound Parabolic Concentrator (CPC) and Pulsating Heat Pipe (PHP) to propose an integrated CPC-PHP solar wall (CPC-PHP-SW) that can enhance solar energy grade and rapid thermal storage. The mixed working fluid for CPC-PHP-SW is developed to reduce the dependence on solar radiation intensity and facilitate PHP startup. Furthermore, the operating characteristics and heat transfer performances of the CPC-PHP-SW are studied experimentally under various factors, including solar radiation intensity, working fluid, and cooling temperature. Sensitivity analyses of these three factors on heat transfer performances are conducted using Multi-factor analysis of variance. Results highlight that solar radiation intensity is the most influential factor on heat transfer performance, followed by cooling temperature, and working fluid. Moreover, employing a CPC with a concentration ratio of 2 reduces thermal resistance by up to 56.46 %, while the maximum effective thermal conductivity increases by up to 129.65 %. A recommended methanol to water ratio is 1:3, which effectively copes with a wide range of solar radiation intensity. These findings provide valuable references for the design and application of CPC-PHP-SW in building envelopes.

Evaluations of energy, exergy, and economic (3E) on a liquefied natural gas (LNG) cold energy utilization system

The LNG cold energy utilization system primarily integrates additional power cycles during the gasification process to harness the cold energy released in this process efficiently. This study introduces a combined cooling and power (CCP) system, which comprises a cascade organic Rankine cycle (ORC), an air conditioning refrigeration (ACR), and a direct expansion cycle (DEC). Furthermore, the thermodynamic and economic analyses of the CCP system were evaluated, and the optimization of each influence parameters was performed by the genetic algorithms, such as the composition of the quaternary mixed working fluid (Methane: Ethane: Propane: i-Butane), condensation pressure, evaporation pressure, and the first-stage expansion pressure within the system. According to the simulation results, the suggested overall exergy destruction of the CCP system proposed by this work is reduced by 13.18% under optimized conditions, with a maximum net power generation of 230.38 kW and an exergy efficiency of 45.58%. Economic analysis reveals that the system's investment cost, annual economic benefits, and levelized energy cost are 2.02 × 106 $, 2.75 × 105 $, and 0.10 $/kWh, respectively. With the system's official operation, the investment cost will be recovered within 8.61 years.

Effect of trans-1,3,3,3-tetrafluoropropene on flammability of difluoromethane

At present, mixed working fluids have received increasing attention due to their excellent comprehensive performance. Among them, R1234ze(E) (trans-1,3,3,3-tetrafluoropropene) and the R32 (difluoromethane) with excellent thermal properties can form a safe and environmentally friendly new mixture through advantageous complementarity. In this paper, the flammability of the binary mixture R1234ze(E)/R32 mentioned above was studied. Firstly, the flammability limits and critical concentration inhibition ratio of R1234ze(E)/R32 were tested at different ratios. Then, based on density functional theory (DFT), the interaction mechanism between the two components in the mixture was analyzed. The results show that with the increase of R1234ze(E), the flammable limit range becomes narrower. In the early stage of the reaction between R1234ze(E)/R32, R32 preferentially breaks the bond to generate H and F radicals, but the reactions between R1234ze(E) and free radicals are dominant. CF3 radical is an important free radical for R1234ze(E) to inhibit the flammability of R32. The research results will have guiding significance for the internal interaction mechanism of the flammability of new binary mixtures and their safe applications.

A new heat transfer correlation based on flow patterns for hydrocarbon refrigerants condensation flow

To accurately predict heat transfer coefficients for hydrocarbon refrigerants condensation flow in heat exchangers is imperative for designing efficient systems. While various studies have proposed numerous correlations for predicting heat transfer coefficients, some of these correlations are based on fitted experimental data and are limited to predicting specific conditions. The prediction accuracy and capability for different working fluids remain to be evaluated. Currently, most researches mainly focus on pure working fluids, with relatively little attention given to mixed working fluids. In this study, six commonly used heat transfer correlations are evaluated and a new heat transfer correlation based on flow patterns is proposed. The research results indicate that the existing correlations have poor universality for predicting hydrocarbon refrigerants, and the prediction accuracy needs to be improved. The correlation developed in this study demonstrates good prediction performance for condensation heat transfer coefficients of 413 sets of hydrocarbon refrigerants, including methane, propane, ethane/propane, and methane/propane. The overall mean relative deviation (MRD) is −1.77%, and mean absolute relative deviation (MARD) is 8.12%. The correlation developed in this study can provide theoretical guidance for predicting condensation heat transfer coefficient for pure and mixed hydrocarbon refrigerants, aiding in the design of efficient heat exchangers.

Theoretical and experimental study on the secondary heat recovery cycle of the mixed working fluid in ocean thermal energy conversion

The small ocean temperature difference causes the power generation efficiency to not be high while the power generation cost is high; this method has not been commercially applied. Therefore, it is particularly important to find a new thermal cycle to improve the cycle efficiency. In this paper, a new cycle of the ocean temperature difference using a non-azeotropic mixed as working fluid is proposed, and a secondary heat recovery device is added to the solution branch separated by the separator. Theoretical analysis and numerical calculation were used to analyze the relationships between cycle efficiency and working fluid concentration, turbine inlet pressure and cold seawater temperature were obtained. The results show that the maximum cycle efficiency can reach 5.25% by adding a secondary heat recovery device. A 10 kW OTEC experimental system was constructed and analyzed; the system performance was obtained. The efficiency of the experimental system was found to be as high as 3.8%, while the experimental value of the test system was lower than the theoretical calculation value. Finally, the heat transfer coefficients of an evaporator and condenser under these conditions were obtained, which provides basic data and technical support for the engineering application of an OTEC plant.

Wet-to-dry transition description in the mixture of working fluids

The organic rankine cycle performance and some similar processes depend on many factors. One of them is the category of the working fluid, influencing the performance through the phase/phases during and at the end of the expansion process. Droplet formation for wet fluids and superheated for dry fluids motivated the researchers to seek isentropic working fluids, where expansion could proceed and terminate in a saturated vapour state. The shape of the T-s diagram is a material property; it cannot be changed for real pure fluids, but small jumps can be initiated by replacing one working fluid with a chemically very similar one, like Propane (a wet one) with Butane (a dry one). Our study presents a much smoother transition, using mixed working fluids prepared from chemically similar materials to obtain almost ideal zeotropic mixtures. The main point of our study is to show the wet-to-dry transition for mixtures and prove or disprove the existence of compositions where the fluid can show T-s diagram resembling the ones for ideal isentropic working fluids. For this purpose, Propane was mixed with other alkanes, such as Butane, Pentane, and Hexane, in various compositions, and the thermophysical properties of fluids were calculated by using the REFPROP software program. Wet-to-dry transitions were shown for the Propane/Hexane mixture at (0.6584 + 0.3416 mass fraction), while (0.5823 + 0.4177 mass fraction) and (0.6436 + 0.3564 mass fraction) was the transition mixture for Propane/Butane and Propane/Pentane respectively. Consequently, when exceeding the mentioned composition range, the fluids switch from wet to dry without forming a composition showing ideal isentropic properties. Therefore, isentropic working fluid (showing an infinite slope for the saturated vapour branch in a finite, nonzero temperature range) was not found during this transition.

Effect of two halogenated olefins on combustion characteristics of Isobutane

To reduce the greenhouse effect, environmentally friendly working fluids such as unsaturated halogenated olefins and hydrocarbons have attracted much attention. This study focuses on the combustion characteristics of two binary mixtures of isobutane (R600a) and R1234ze(E) and R1233zd(E). The results indicate that the addition of R1234ze(E) and R1233zd(E) has different effects on the combustibility of R600a under rich and lean fuel conditions. From the perspective of flammability limit range, as the ratio of R1234ze(E) and R1233zd(E) increases, the flammability limit range of both mixtures first increases and then decreases. For R1233zd(E)/R600a containing chlorine (Cl), the maximum flammable limit range corresponds to (nF + nCl)/nH equal to 1/3, which is significantly smaller than the nF/nH ratio of 1 in R1234ze(E)/R600a. Secondly, through calculation and simulation, it was found that the flame retardancy of R1233zd(E) is stronger than that of R1234ze(E), as the Cl in the former has a stronger ability to capture active radicals. Finally, based on Chemical kinetics analysis, it is found that adding R1234ze(E) to R600a during oxygen enrichment will increase the concentration of active radicals H, O and OH during combustion. This leads to higher combustion temperature and smaller flame thickness, thereby promoting the combustion process. The results will have significant implications for the safe application of new mixed working fluids and the analysis of internal reaction mechanisms of the mixture flammability.

Experimental investigation on boiling heat transfer characteristics of R1234yf/R1336mzz(Z) in horizontal flow

Due to the inclusion of 18 HFCs, including R134a and R245fa, in the Kigali Amendment as controlled working fluids, the search for excellent alternatives to R134a and R245fa is urgent. R1234yf and R1336mzz(Z) are potential alternative working fluids to R134a and R245fa, but most studies have focused on the comparison of thermodynamic performance and boiling heat transfer capability of these pure working fluids. There is limited research on the boiling heat transfer performance of their mixed working fluids. Therefore, this study focuses on the flow boiling heat transfer characteristics of the mixed working fluid R134a/R245fa under non-azeotropic conditions. Experimental methods were employed to investigate the boiling heat transfer performance of the alternative working fluids R1234yf/R1336mzz(Z) (including R245fa/R1234yf and R1234yf/R1336mzz(Z) mixed working fluids) and evaluate the feasibility of R1234yf and R1336mzz(Z) as substitutes for R134a and R245fa. The results indicate that among the three mixed refrigerants, R134a/R245fa exhibits the highest heat transfer coefficient, while R1234yf/R1336mzz(Z) shows the lowest heat transfer coefficient. However, the differences in heat transfer coefficients among these three are relatively small, and their overall trends under different operating conditions are similar. It is evident that R134a/R245fa, with its good thermodynamic performance and stability, still holds potential as a transitional working fluid. R1234yf/R1336mzz(Z), with similar physical and thermodynamic properties, is an excellent alternative to R134a/R245fa.

Influence of composition of binary mixed working fluids on the performance of cascade heat pump

The optimized design of the cascade heat pump facilitates the recovery and reuse of waste heat in printing and dyeing wastewater. This paper constructs a cascade heat pump system with two-stage cycles sampling the same binary mixed working fluids. The effects of three kinds of combinations of working fluids, including R134a/R152a, R134a/R22, and R152a/R22, on the COP and heating power, are discussed through the thermodynamic cycle process simulation when the component ratio varies. It is found that, for the R134a/R152a mixed working fluids, the system performance can be adjusted approximately linearly by changing the component ratio. For the two mixed working fluids of R134a/R22 and R152a/R22, the performance of the cascade heat pump system is closer to the case of using pure R134a and R152a as corresponding working fluids. The system’s heating capacity is significantly reduced, compared with the case of pure R22 employed. Besides, with the component ratio of working fluid increasing, the COP and heating power change with opposite trends.

Hydrate-based composition separation of R32/R1234yf mixed working fluids applied in composition-adjustable organic Rankine cycle

Precise composition regulation of mixed working fluids is core to high performance composition-adjustable organic Rankine cycle (CA-ORC). However, most composition separation methods couldn't achieve precise regulation, limiting the application of CA-ORC. The hydrate-based composition separation proposed a promising avenue for precise regulation. In order to better apply to the CA-ORC, the separation characteristics of mixed working fluid hydrates should be clearly clarified, which remains to be a challenge. Facing the challenge, this study experimentally investigated hydrate-based composition regulation of R32/R1234yf mixed working fluid that possess both environmental and excellent thermodynamic properties. The separation characteristics of mixed working fluids were clarified, and the optimal separation operating conditions were analyzed covering a broad range of temperature (275.00 K–282.00 K) and pressure (0.30 MPa–0.54 MPa). The results show that hydrate-based composition regulation of R32/R1234yf can achieve excellent regulation performance, with composition change of R32 of 11.14–18.21 mol% in 90 min. Lower temperatures and higher pressures can lead to a better separation performance. Separation factor of R32 reach the maximum of 29.702, which is 4.12 times higher than the minimum. This study may provide a guidance to search energy-saving operating conditions for hydrate-based composition regulation and support its application in CA-ORC.

Improvement of Geothermal Power Plants by Utilizing Hydrogen Steam Superheating: A Review

The rapid growth of geothermal power plants is due to their great potential and environmentally friendly energy source. In its development, hydrogen is used as energy storage for several intermittent renewable energy sources. Hydrogen Steam Superheating utilizing hydrogen products can pressure steam as additional fuel to operate steam turbines. The electrolysis method with high-temperature water fluid produces more hydrogen content and is more efficient than ambient temperature water; therefore, it is suitable for producing superheating hydrogen steam by electrolysis using geothermal power plants. A combined cycle (flash & binary) geothermal power plant with additional hydrogen steam superheating can increase the power 8 - 9 MW using the mixed working fluid R-31-10 and RC-318 in a binary cycle power plant and improve the thermal efficiency of flash cycles by up to 12.3%. Thus, the application of adding hydrogen steam superheating is a method that can be utilized to increase the power capacity of geothermal power plants, replacing the current conventional method of drilling to open steam production wells.

Combustion and interaction mechanism of 2,3,3,3-tetrafluoropropene/1,1,1,2-tetrafluoroethane as an environmentally friendly mixed working fluid

Organic Rankine cycle (ORC) is an effective way to improve energy efficiency, and 2,3,3,3-tetrafluoropropene (R1234yf) is considered as a promising working fluid for ORC. However, the development of R1234yf in ORC is limited by its flammability, and the addition of 1,1,1,2-tetrafluoroethane (R134a) is an effective method to inhibit its flammability. In this paper, the combustion and interaction mechanism of R1234yf/R134a were investigated. Firstly, the combustion products of R1234yf/R134a were measured using an improved experimental device, the results showed a substantial reduction in the concentrations of CO, CO2, and HF with an increasing proportion of R134a. Then, the combustion mechanism of R1234yf/R134a was studied by ReaxFF-MD simulation. The initial decomposition time of R134a was found to be later than that of R1234yf, and a reduction in the production rate of combustion products was achieved with the addition of R134a. Furthermore, the combustion reaction was promoted by the increase in temperature. Finally, the main reaction pathways of working fluids were analyzed. The abstraction reaction of R134a with radicals and the scavenging of active radicals by flame retardant radicals were the key to inhibit the combustion reaction. The results have universal guiding significance for the safe application of R1234yf/R134a.

Pyrolysis mechanism of R601a/R245fa mixture: A ReaxFF-MD and DFT study

R601a/R245fa mixture is one kind of promising working fluid in ORC. In this study, the thermal decomposition mechanism of R601a/R245fa mixture was studied by reactive molecular dynamic simulation (ReaxFF-MD) and density function theory (DFT). In the R601a/R245fa mixed working fluid system, pyrolysis begins with the CC bond breaking of R601a, generating a large number of CH3 radicals, which further decompose to produce a certain amount of H radicals. Due to the presence of R245fa, the hydrogen extraction reactions between CH3, H radicals and R601a, which have low energy barriers were inhabited to a certain extent. The addition of R245fa reduces the generation of small molecule flammable gases such as H2 and CH4, however, it increases the risk of HF generation. When the molar ratio of R245fa exceeds 0.5, pyrolysis will cause more serious HF corrosion problems than pure R245fa.

Design and Internal Flow Characteristic Investigation of High-Temperature H2/Steam-Mixed Working Fluid Turbine

In this paper, an improved RSM-CFD method is used to optimally design a mixed turbine of non-equilibrium condensing steam (NECS) and hydrogen (H2), of which the response surface method (RSM) and computational fluid dynamics (CFD) are coupled to take into account the effects of the wet steam non-equilibrium condensation process of the multimixed working fluid. A single-stage H2/Steam (NEC)-mixed turbine was developed based on the improved RSM-CFD, and the effect mechanism of the H2 component ratio (ωH2) on the flow characteristics, internal power, and isentropic efficiency within the turbine stage were investigated. The results show that the isentropic efficiency (η) increases with the increase in the hydrogen component ratio (ωH2), since hydrogen, as a non-condensable component, can inhibit the nucleation and growth of steam, reducing the pressure pulsation on the blade surface; furthermore, it accelerates the transport and diffusion of liquid droplets, inhibits the flow separation, and reduces the flow loss in the flow channel. However, the internal power of the turbine (P) tends to decrease with increasing ωH2, since the increase in hydrogen reduces the pressure difference on the blade and lowers the torque of the fluid acting on the blade, and at the same time, the vortex and radial flow intensify, and the enthalpy drop inside the stage decreases. On this basis, the optimum operating conditions are found where the hydrogen component ratio (volume percent) ωH2 = 53%. Accordingly, the hydrogen component ratio should be maintained in the range of 38–68%, considering the work capacity and hydrogen yield of the mixed working fluid.

Effect of critical temperature and category of zeotropic mixture working fluids on the thermal performance in subcritical ORC

Several thermodynamic properties are crucial in selecting the optimal working fluids and significantly affect the performance of the Organic Rankine cycle (ORC); one of them is the liquid-vapor critical temperature. Four different thermodynamic models were used; they differed by the definition of maximal (TEva) and minimal (TCond) temperatures. Three different scenarios were investigated in all models by picking components from different working fluid classes to prepare zeotropic mixtures. The main focus of our study was on the investigation of the effect of the critical temperature (Tcr) of the mixture and the class of the components (wet/wet, wet/dry, and dry/dry) on thermal efficiency (ηth) and net-work output (Wnet). Results indicate that occasionally, the zeotropic mixtures with higher critical temperatures (but not always) outperform other mixtures under given conditions. Consequently, it cannot be asserted that the higher thermal efficiency and net- work output are monopolized by the highest critical temperature of all mixed working fluids. Selecting a certain mixing ratio and the appropriate model (evaporator and condenser temperatures set as on the bubble and dew points) that matches the ORC application can assist in obtaining the highest thermal efficiency and net-work output.