Subcritical Cycle Research Articles

Experimental energetic analysis of CO2/R41 blends in automobile air-conditioning and heat pump systems

This study primarily aims to evaluate the cooling and heating characteristics and provide a performance comparison of CO2, R41, and several CO2/R41 blends for applications in automobile air-conditioning and heat pump systems. Experiments were conducted to reveal the effects of the refrigerant charge, compositions, ambient temperatures, and compressor speeds among the trans-critical and subcritical cycles. The performance merits of the environmental friendliness, energy efficiency, and safety of using CO2/R41 blends are emphasized. The results demonstrate that, at an optimum charge amount, the coefficient of performance of pure CO2 in the heating and cooling modes can be improved up to a maximum of 14.5% and 25.7%, respectively, by increasing the R41 mass fraction. The heating and cooling capacities were both decreased with an increasing R41 mass fraction, as the system mass flow rates decreased owing to the reduction in suction density. The overall system operation pressure, gas cooler outlet temperature, and high- and low-side pressure drops in both the heating and cooling modes were reduced significantly when the mass fraction of R41 increased from 0% to 100%. Furthermore, the effect of the CO2 mass fraction on the flammability of CO2/R41 blends was studied to discuss the advantages of its safety application. Finally, based on the experimental data, a correlation of the optimum high-side pressure for the trans-critical CO2/R41 cycle was developed, showing a deviation of ±5% from the data.

Working fluid selection for organic Rankine cycle power generation using hot produced supercritical CO2 from a geothermal reservoir

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

Exergy analysis on a heat pump working between a heat sink and a heat source of finite heat capacity rate

The optimum performance of a pure subcritical refrigeration cycle depends significantly on the temperature lift of the heat source and sink. Therefore, the maximization of the system efficiency has to be linked to them. This paper shows an exergy analysis of each heat pump component (condenser, evaporator, expansion valve and compressor) considering that the heat source and sink are not at constant temperature. The performed study shows the components with more possibilities for improvement. Based on this analysis, the optimization of cycle parameters like subcooling and superheat as a function of the external conditions have been done. In addition, this work has demonstrated that the components having a higher influence in the system irreversibility's depends significantly on the temperature lift of the secondary fluids. Finally, the obtained results show potentials improvements of the efficiency up to 23% if the system is able to operate in the optimal subcooling and superheat.

Design and performance analyses for a novel organic Rankine cycle with supercritical-subcritical heat absorption process coupling

Organic Rankine cycle (ORC) is a promising heat-power conversion technology for renewable energy development and waste heat recovery. This study proposes a novel ORC that can couple supercritical and subcritical heat absorption processes. The proposed dual-pressure (supercritical + subcritical) heat absorption cycle can potentially better enhance the heat-power conversion efficiency compared with the existing cycle types. For the dual-pressure (supercritical + subcritical) heat absorption cycle, the heat absorption pressures and vapor generator outlet temperature were optimized to achieve the maximum net power output, and its thermodynamic and exergy performance was studied for various heat source temperatures. The thermodynamic performance superiorities of the proposed novel cycle over those of other cycle types were also quantitatively analyzed. Results show that the dual-pressure (supercritical + subcritical) heat absorption cycle has advantages of substantially increasing the system efficiency compared to the conventional subcritical and dual-pressure evaporation cycles, and remarkably increasing the heat absorption capacity compared to the transcritical cycle. For R1234ze(E), the maximum net power output of the dual-pressure (supercritical + subcritical) heat absorption cycle is the largest for heat source temperatures above approximately 135 °C, and it can increase by 19.9%, 49.8%, and 20.4% at most compared with those of the conventional subcritical, transcritical, and dual-pressure evaporation cycles, respectively. The external exergy efficiency is 92.4–94.9% for heat source temperatures of 150–200 °C which indicates that the heat source fluid and working fluid can achieve an excellent temperature match. For other five working fluids, the maximum net power outputs of the dual-pressure (supercritical + subcritical) heat absorption cycle are also the largest.

Optimization Simulation, Using Steel Plant Off-Gas for Power Generation: A Life-Cycle Cost Analysis Approach

This research aims to contribute to the international desires of greenhouse gas mitigation through a study on the optimization of 100 MW off-gas power plants’ subcritical steam cycle, operating in conjunction with steel plants. The performance and fiscal efficiency of the off-gas power plants are based on data collected from Krakatau Steel plants in Indonesia, simulated and analyzed through thermodynamic simulation model (PEPSE). The three scenarios simulated for comparison are: (1) 100 MW off-gas power plant as-is operation, (2) 100 MW off-gas power plant optimized, and (3) 100 MW cogeneration off-gas power plant. The optimization of the off-gas power plant and use of steam extraction was found to have a 45 M and 26 M USD savings in comparison to the base power plant, assuming a 5.5% discount rate and 400 Million USD capital expenditure. The theoretical findings are of use to academia and industry in providing proof of potential cost-savings of integrating off-gas into existing power generation operations.

Optimal Performance Characteristics of Subcritical Simple Irreversible Organic Rankine Cycle

Based on the theory of finite time thermodynamics, a subcritical simple irreversible organic Rankine cycle (SSIORC) model considering heat transfer loss and internal irreversible losses is established in this paper. The total heat transfer surface area is taken as a constraint, and R245fa is adopted as working fluid of the cycle in the performance optimization. The evaporator heat transfer surface area and mass flow rate of the working fluid are optimized to obtain the maximum power output and thermal efficiency of the SSIORC, respectively. In addition, the influences of the internal irreversibilities on the optimal performances are also investigated. The results show that when the evaporator heat transfer surface area is varied, the relationship between power output and thermal efficiency is a loop-shaped curve, and there exist maximum power output and thermal efficiency points, respectively. However, the two maximum points are very close to each other. When the mass flow rate of the working fluid is varied, the relationship between power output and thermal efficiency is a parabolic-like curve. With the decreases of expander and pump irreversible losses, the performances of the irreversible SSORC are close to those of the endoreversible SSORC with the only loss of heat transfer loss.

Modeling and theoretical analysis of a CO2-propane autocascade heat pump for electrical vehicle heating

Heat pump and its efficiency are of great significance to the energy saving of electric vehicle in cold climate. This paper proposes a modified autocascade heat pump (ACHP) system using natural CO2-propane mixture. The main purpose is to enhance the heating performance compared with that of conventional heat pump. Using a thermodynamic model of the proposed ACHP cycle, the transcritical and subcritical cycle performance under various conditions are carried out from the perspective of first and second law of thermodynamics, and the transcritical operating characteristics of ACHP system is introduced for the first time. Additionally, an exergy performance comparison has been carried out. Results indicate that the ACHP system can produce more heating capacity, lower compression pressure ratio, lower heat rejection temperature for low ambient temperature conditions and it consumes less compressor work for all the temperature range researched. For transcritical ACHP cycle, the best COP is 2.7 obtained when CO2 mass fraction is 0.8 at −20 °C, which is 12.3% higher than that of CO2 single stage heat pump. For subcritical ACHP cycle, the best COP is 3.97 while heating capacity is much lower than that of transcritical cycle in the same condition. Finally, the exergy analysis confirms the feasibility of the ACHP system and points out exactly the further optimization locations in the ACHP system.

Numerical Continuation of Limit Cycle Oscillations and Bifurcations in High-Aspect-Ratio Wings

This paper applies numerical continuation techniques to a nonlinear aeroelastic model of a highly flexible, high-aspect-ratio wing. Using continuation, it is shown that subcritical limit cycle oscillations, which are highly undesirable phenomena previously observed in numerical and experimental studies, can exist due to geometric nonlinearity alone, without need for nonlinear or even unsteady aerodynamics. A fully nonlinear, reduced-order beam model is combined with strip theory and one-parameter continuation is used to directly obtain equilibria and periodic solutions for varying airspeeds. The two-parameter continuation of specific bifurcations (i.e., Hopf points and periodic folds) reveals the sensitivity of these complex dynamics to variations in out-of-plane, in-plane and torsional stiffness and a ‘wash out’ stiffness coupling parameter. Overall, this paper demonstrates the applicability of continuation to nonlinear aeroelastic analysis and shows that complex dynamical phenomena, which cannot be obtained by linear methods or numerical integration, readily exist in this type of system due to geometric nonlinearity.

Annual performance of subcritical Rankine cycle coupled to an innovative particle receiver solar power plant

Concentrated solar power plants using molten salts as heat transfer and storage fluid have emerged as the preferred commercial solution for solar thermal electricity in central receiver technology. Despite their ability to store large amounts of thermal energy and efficient receiver designs, further efficiency improvements are constrained by tight temperature restrictions when using molten salts (290 °C–565 °C). In this work, a novel heat transfer fluid based on a dense particle suspension (DPS) is used due to its excellent thermophysical properties that extend the operating temperature of solar receiver and allow its coupling with higher-efficiency power cycles. In this paper, the design of a DPS solar receiver working at 650 °C has been optimized for two commercial sizes (50 MWth and 290 MWth) coupled to an optimized subcritical Rankine cycle. The results showed that a five-extraction reheated Rankine cycle operating at 610 °C and 180 bar maximizes power plant efficiency when coupled with a DPS central receiver, giving 41% power block efficiency and 23% sun-to-electricity efficiency. For optimization purposes at design point conditions, in-house code programmed into MATLAB platform was used while TRNSYS software was employed for annual plant performance analysis.

High-grade energy compensation ratio in a conventional subcritical vapour-compression heat-pump cycle

The concept of high-grade energy compensation (HGEC) has been developed and derived based on thermodynamic analysis for better understanding and assessing the high-grade energy-use efficiency in a conventional subcritical vapour-compression heat-pump (VCHP) cycle. The derivation proposes a lower limit of HGEC ratio in a conventional subcritical VCHP cycle, meaning that the COP and/or EER that we get used to employing for assessing energy efficiency tend to overestimate the high-grade energy-use efficiency.

Fuzzy Nonlinear Dynamic Evaporator Model in Supercritical Organic Rankine Cycle Waste Heat Recovery Systems

The organic Rankine cycle (ORC)-based waste heat recovery (WHR) system operating under a supercritical condition has a higher potential of thermal efficiency and work output than a traditional subcritical cycle. However, the operation of supercritical cycles is more challenging due to the high pressure in the system and transient behavior of waste heat sources from industrial and automotive engines that affect the performance of the system and the evaporator, which is the most crucial component of the ORC. To take the transient behavior into account, the dynamic model of the evaporator using renowned finite volume (FV) technique is developed in this paper. Although the FV model can capture the transient effects accurately, the model has a limitation for real-time control applications due to its time-intensive computation. To capture the transient effects and reduce the simulation time, a novel fuzzy-based nonlinear dynamic evaporator model is also developed and presented in this paper. The results show that the fuzzy-based model was able to capture the transient effects at a data fitness of over 90%, while it has potential to complete the simulation 700 times faster than the FV model. By integrating with other subcomponent models of the system, such as pump, expander, and condenser, the predicted system output and pressure have a mean average percentage error of 3.11% and 0.001%, respectively. These results suggest that the developed fuzzy-based evaporator and the overall ORC-WHR system can be used for transient simulations and to develop control strategies for real-time applications.

Optimizations of the organic Rankine cycle-based domestic CHP using biomass fuel

The purpose of this study is to determine the optimal operating conditions and performance for the design of ORC based biomass compact CHP for 2 kW of electric, 25 kW of thermal power productions and 60 °C warm water supply. Eight organic working fluids were selected based on thermo-physical properties and related environmental regulations: cyclopentane, isopentane, n-pentane, diethyl ether, HFO-1233zd, HFC-245fa, HFE-7000 and HFE-7100. The selected organic fluids were classified into three groups considering latent heat and boiling point. The group A fluids contained cyclopentane, isopentane, n-pentane and diethyl ether. The group B fluids contained HFO-1233zd and HFC-245fa. The group C fluids contained HFE-7000 and HFE-7100. A micro CHP system composed of a biomass boiler (200 °C heat source), an ORC power cycle and a cooling water line (20 °C cooling water supply) was modeled in four variants depending on whether post-heater and IHE were applied or not. The subcritical ORC cycle and saturated vapor state at the inlet of the expander were considered for the analysis. As a result of thermodynamic analyses and optimizations, the group A fluids have the best CHP performance because of the greatest latent heat amount. The systems using the group A fluids have the lowest mass flow rates from 0.053 kg/s to 0.081 kg/s, the lowest required heat supplies from 31.64 kW to 34.61 kW, the highest ORC efficiencies from 5.95% to 7.29% and the CHP efficiencies from 71.83% to 72.32%. The group B fluids have the mass flow rates from 0.157 to 0.215 kg/s, the highest required heat supplies from 36.98 kW to 46.41 kW, the lowest system efficiencies from 4.59% to 6.05% and the highest CHP efficiencies from 72.05% to 73.41%. The group C fluids have the highest mass flow rates from 0.213 kg/s to 0.230 kg/s, the required heat supplies from 32.30 kW to 40.54 kW, the system efficiencies from 5.07% to 6.36% and the lowest CHP efficiencies from 71.31% to 72.33%. In addition, ORC systems using the group A or group C fluids can operate at low pressure and can meet system requirements with low cooling water mass flow rate because of the high boiling points. For the group A fluids, both post-heater and IHE are very effective for the system, and the system using the group B fluids can highly improve the system through the application of the post-heater. For the group C fluids, application of the IHE significantly improves system performance.

Effects of Post-Weld Heat Treatment on the Microstructure and Toughness of Flash Butt Welded High-Strength Low-Alloy Steel

Effect of post-weld heat treatment on the weld microstructure, texture, and its correlation to the toughness of flash butt welded joints were investigated. Upon flash butt welding, the α and γ-fiber in the parent material converted to Goss (110)[001], rotated Goss (110)[1\( \bar{1} \)0], and rotated cube (001)[1\( \bar{1} \)0], (001)[\( \overline{11} \)0] textures along the fracture plane. Formation of these detrimental texture components was a result of shear deformation and recrystallization of austenite at temperatures above Tnr resulting in a drop of toughness at the weld zone. Inter-critical and sub-critical annealing cycles proved to be less effective in reducing the Goss (110)[001], rotated Goss (110)[1\( \bar{1} \)0], and rotated cube (001)[1\( \bar{1} \)0], (001)[\( \overline{11} \)0] texture components, and therefore, toughness values remained unaffected. Post-weld heat treatment in the austenite phase field at 1000 °C for 5 seconds resulted in the formation of new grains with different orientations leading to a reduction in the texture intensities of both Goss and rotated Goss components and therefore improved weld zone toughness. Prolonged annealing time was found to be ineffective in improving the toughness due to grain growth.

A Novel Pinch Point Design Methodology Based Energy and Economic Analyses of Organic Rankine Cycle

A generalized methodology for pinch point design and optimization of subcritical and transcritical organic Rankine cycles (ORCs) using both wet and dry fluids is adopted in this study. The presented algorithm can predict the pinch point location in evaporator and condenser simultaneously and optimize the evaporator pressure for best performance with various heat source and sink conditions. Effects of pinch point temperature difference (PPTD), isentropic efficiency, subcooling, superheating and regenerator on the energy and economic performances are discussed for selected working fluids. System yields similar optimum design for both maximum power generation and minimum capital cost per unit power. At optimum condition, ammonia is best in terms of higher thermal efficiency and lower component size, R152a is best in terms of higher net power output and heat recovery efficiency (11.1%), and toluene is best in terms of lower capital cost and cost per unit power output (7060 $/kW). Effect of heat source and sink parameters on both energy and economic performances is significant. Contour plots are presented to select the best ORC design parameters for available heat source condition. PPTD and expander isentropic efficiency have significant effect on performances. However, the effect of subcooling, superheating and regenerator depends on working fluid.

Performance assessment of shell and tube heat exchanger based subcritical and supercritical organic Rankine cycles

In this study, thermal models for subcritical and supercritical geothermal powered organic Rankine cycles are developed to compare the performance of these cycle configurations. Both of these models consist of a detailed model for the shell and tube heat exchanger integrating the geothermal and organic Rankine cycles sides and basic thermodynamic models for the rest of the components of the cycle. In the modeling of the heat exchanger, this component was divided into sever?al zones and the outlet conditions of each zone were found applying logarithmic mean temperature difference method. Different Nusselt correlations according to the relevant phase (single, two-phase, and supercritical) were also included in this model. Using the system-level model, the effect of the source temperature on the performances of the heat exchanger and the organic Rankine cycle was assessed. These performance parameters are heat transfer surface area and pressure drop of tube side fluid for the heat exchanger, and electrical and exergetic efficiencies of the integrated organic Rankine cycles system. It was found that 44.12% more net power is generated when the supercritical organic Rankine cycle is used compared to subcritical organic Rankine cycle.

Exergetic performance assessment of solar driven combined CO<SUB align="right">2 power and refrigeration system

The scope of this study is to assess a solar driven combined power-refrigeration system working with carbon dioxide (CO2) through energy and exergy. The system consists of a supercritical Brayton cycle, a transcritical organic Rankine cycle, and a subcritical vapour compression refrigeration cycle. All three subsystems operate with carbon dioxide as the working fluid thanks to its zero-ozone depleting potential and negligible global warming potential. In addition, it is a sustainable working fluid. The combined system's heat energy demand is supplied by using parabolic trough solar collector system. Heat energy demand of organic Rankine cycle is supplied by the rejected heat of supercritical Brayton cycle; while the refrigeration cycle is driven by the power generated from the organic Rankine cycle. The results show that the energy efficiencies of the Brayton and organic Rankine cycles are found to be 12.9% and 4.47%, respectively, while the performance coefficient of the refrigeration cycle is determined to be 3.35. The energy efficiency of the overall system is determined to be 20.89%; while the exergy efficiency of the overall system is determined to be 12.95%. The exergy destruction rate of the whole system is calculated as 4,891 kW.

Subcritical organic ranking cycle based geothermal power plant thermodynamic and economic analysis

This paper is focused both on the thermodynamic and economic analysis of an organic Rankine cycle (ORC) based geothermal power plant. The analysis is applied to a case study of the geothermal field Recica near the city of Karlovac. Simple cycle configuration of the ORC was applied. Thermodynamic and economic performance of an ORC geothermal system using 8 working fluids: R134a, isobutane, R245fa, R601, R601a, R290, R1234yf, and R1234ze(E)], with different critical temperatures are analyzed. The thermodynamic analysis is performed on the basis of the analysis of influence of the operation conditions, such as evaporation and condensation temperatures and pressures, and evaporator and con-denser pinch point temperature difference, on the cycle characteristics such as net power output, and plant irreversibility. The economic analysis is performed on the basis of relationship between the net power output and the total cost of equipment used in the ORC. Mathematical models are defined for proposed organic Rankine geothermal power plant, and the analysis is performed by using the software package engineering equation solver. The analysis reveals that the working fluids, n-pentane and isopentane, show the best economic performances, regardless the evaporation temperatures, while the working fluid R1234yf and R290 have the best thermodynamic performances. In addition, each analyzed working fluid has its corresponding economically optimal condensation temperature (and condensation pressure). Economically optimal pinch point temperature difference of evaporator has different values, depending on the working fluid, while pinch point temperature difference of condenser has similar values for all analyzed working fluids. Analysis results demonstrate that the subcritical ORC geothermal power plant represents a promising option for electricity production application.

Technical and economic optimization of subcritical, wet expansion and transcritical Organic Rankine Cycle (ORC) systems coupled with a biogas power plant

Generally, >40% of the useful energy (cooling engine and exhaust gases) are wasted by a biogas power plant through the cooling radiator and the exhaust gases. An efficient way to convert this waste heat into work and eventually electricity is the use of an organic Rankine cycle (ORC) power system. Over the last few years, different architectures have been widely investigated (subcritical, wet expansion and trans-critical). Despite the promising performances, realistic economic and technical constraints, also related to the application, are required for a meaningful comparison between ORC technologies and architectures. Starting from the limited literature available, the aim of the present paper is to provide a methodology to compare sub-critical, trans-critical and wet expansion cycles and different types of expanders (both volumetric and turbomachinery) from both technical and economic point of view, which represent one of the main novel aspects of the present work. In particular, the paper focuses on the thermo-economic optimization of an ORC waste heat recovery unit for a 500 kWe biogas power plant located in a detailed regional market, which was not investigated yet. By means of a genetic algorithm, the adopted methodology optimizes a given economic criteria (Pay-Back Period, Net Present Value, Profitability Index and Internal Rate of Return) while respecting technical constraints (expander limitations) and thermodynamic constraints (positive pinch points in heat exchangers, etc.).The results show that optimal ORC solutions with a potential of energy savings up to 600 MWh a year and with a pay-back period lower than 3 years are achievable in the regional market analysed.

Generalized pinch point design method of subcritical-supercritical organic Rankine cycle for maximum heat recovery

Novel methodology for pinch point design and optimization of subcritical and supercritical organic Rankine cycles is proposed for maximum heat recovery. The proposed method is able to predict pinch point locations in both evaporator and condenser simultaneously. As a main advantage, both evaporator and condenser pressures can be optimized simultaneously by optimizing only working fluid mass flow rate to get maximum net work output or heat recovery efficiency for given heating fluid and cooling fluid inlet conditions using selected working fluids. Working fluids have been selected based on thermodynamic and environmental criteria and compared based on various performance parameters (net work output, thermal efficiency, heat recovery efficiency, irreversibility, exergetic efficiency, turbine size parameter and heat transfer requirement). The present method seems to be better than previous pinch point design methods as it optimize the cycle by considering both source and sink. At optimum operation, ammonia is best in terms of lower mass flow rate requirement, higher exergetic efficiency, lower turbine staging and turbine size, whereas, isopentane is best in terms of higher power output and heat recovery efficiency. Novel contour plots are presented as well to select optimum ORC design parameters for available heat source and sink.

Performance simulation and exergy analysis of a hybrid source heat pump system with low GWP refrigerants

Abstract Low global warming potential R1234ze series refrigerants have been widely accepted as the working fluids for high temperature heat pump system in industrial applications. To improve the system performance of a single source R1234ze(Z) heat pump system with waste heat recovery, a hybrid source heat pump system combined with a CO2 transcritical heat pump cycle and a R1234ze(Z) subcritical heat pump cycle is proposed in this paper. To compare the system performance of proposed hybrid source heat pump and single source heat pump systems, thermal characteristic is investigated for each system. Exergy analysis and economic analysis are also conducted to compare the system efficiencies of hybrid and single source heat pump system. The study indicates that total power consumption and heating capacities of the proposed system increase with CO2 discharge pressure increasing. There also exists an optimal CO2 discharge pressure for hybrid source heat pump system to achieve the best system COP. The system exergy efficiency increases with CO2 discharge pressure increasing before it reaches to a relative stable level. Compared with the single source heat pump, both the system COP and exergy efficiency are improved by 24.8% and 27.2% at 100 °C condensing temperature.