Subcritical Organic Rankine Cycle System Research Articles

Organic rankine cycle systems with mixture of pure fluids: On infeasible fluid’s fractions due to the interaction between the mixture glide and the hexs pinchs

The Organic Rankine Cycle (ORC) is a promising technology for power generation from low-grade heat. The selection of working fluids is one of the important key points to improve the performance of an ORC system. Zeotropic mixtures show promising performances as working fluids. In fact, their temperature glide during phase change enables better match between the working fluid and the heat source/sink temperatures. In order to reveal the performance of mixture in ORC system, this paper deals with the thermodynamic model of the subcritical Organic Rankine Cycle (ORC) systems driven by low grade heat source while using zeotropic mixture working fluids with a special consideration to the interaction between phase change glides and the pinch value and their location in both the evaporator and the condenser (HEXs). Zeotropic mixtures of seven pure fluids are evaluated as working fluids for a subcritical ORC system. The mass fraction effects of mixtures on the thermal efficiency are analyzed. For given working conditions (working fluid mass flow, pressure and bubble temperature) the results show that for each considered zeotropic mixture there exist mass fraction ranges that are not consistent with the pinch values constraint in the HEXs and leads to so-called ‘infeasible zones’ with unreal HEXs dimensions. Results shows also that, out of these “infeasible fractions” zone, keeping unchanged the working conditions, the thermal performances of ORC system using zeotropic mixture are always better than the thermal performances of the same systems using the correspondent pure fluids. In addition, out of these highlighted “unfeasible zones” it was found that mixture with high temperature glide improve the thermal efficiency of ORC system.

Modelling investigation on the influence of measurement error on performance evaluation of organic Rankine cycle system

The evaluation of the performance of organic Rankine cycle (ORC) depends on the measured data of the operating parameters. The influences of the measurement errors during the evaluation of the system’s performance, which are derived from the measuring instruments, are rarely studied in the literature. In order to explore the effects of measuring instruments’ accuracy on the evaluation of system performance, the error propagation model of a subcritical ORC system was established. The working fluids adapting to different operating temperature range, including R134a, R245fa and R123, were selected. Temperature, pressure and flow rate of fluid were adopted as the operating parameters that were measured. The net output power and the efficiency were used as the performance parameters that were evaluated. Sensitivity analyses of the measurement errors of single parameter and multiple parameters were conducted under five designed conditions. The results show that the key positions of the measurement errors affecting the performance evaluation were at the expander’s inlet, followed by the expander’s outlet, and the inlet and outlet of the fluid pump. To keep the errors of net output power and thermal efficiency within ± 10%, the measurement errors of the temperature at the inlet and outlet of the expander and fluid pump were less than ± 2%, ± 5%, ± 9% and ± 9% in all cases, respectively. To meet the same requirements, the measurement errors of the pressure at the inlet and outlet of the expander were less than ± 10% and ± 100% in all cases, respectively. The fluid adapting to higher operating parameters allowed higher measurement errors for both the temperature and the pressure. To keep the errors of net output power and thermal efficiency within ± 10%, the allowable measurement error of the expander’s inlet temperature increased from ± 0.8% for R134a to ± 2.0% for R123. The allowable measurement errors at the expander’s inlet decreased with the increase in the operating parameters for the same fluid. In most of the operating parameters, the allowable measurement errors of R245fa were larger than R134a and less than R123 under the same operating temperature range. The importance of the accuracy of temperature’s measurement outweighed those of pressure and flow rate. Finally, recommendations were made for the selection of accuracy of the measuring instruments.

Evaluation of Subcritical Organic Rankine Cycle by Pure and Zeotropic of Binary and Ternary Refrigerants

The simulation configuration and process analysis of the Subcritical Organic Rankine Cycle (SORC) system are carried out for the potential comparison between pure, binary, and ternary zeotropic mixtures of R1234ze(E), R1234yf, and R134a as refrigerant working fluids based on applying the flue gas as a heat source with medium temperature. The compression pressure was selected as an optimized variable input parameter of SORC with the lower limit of boundary condition (1.4 MPa); to mitigate air ingress and sub-atmospheric pressure that led to approach optimum net power output generated. Increasing the compression pressure has a positive relationship with the superheated temperature and the mass enthalpy change in the evaporation and, therefore, in the expansion process. In parallel, the enthalpy and entropy changes in the flue gas and cold water positively correlate with exergy efficiency. So, R1234ze(E)/ R1234yf/R134a with 68.35% and R1234yf/ R134a with 69.29% as the lowest and highest exergy efficiency in the highest compression pressure; furthermore, the SIC consequences of increasing the cost of each component of the SORC system that has a direct relationship with the PPC and the required exchanger area of evaporation and condensation process and generating a net power output of the turbine. As a result, the maximum to the minimum value of specific investment cost (SIC) achieves R134a with 5807402.18-22455670.61 $.kW-1 and R1234yf with 16.82-17.38% reduction, respectively. To sum up, the lowest payback period (PBP) was R1234yf with 302 days.

Fuel-saving performance and main losses of an organic-Rankine-cycle-based exhaust heat recovery system in heavy truck application scenarios

• Fuel-saving effect of ORC-WHR facilities on trucks are comprehensively evaluated. • Fuel-saving effect degradation due to existing components’ limitations is examined. • Fuel-saving effect degradation due to interactions in system integration is studied. • Effects of system dynamics and control strategies are explored. • It is suggested the system power density should be greater than 70 W/kg. Recovering and converting the waste heat within the exhaust gas of high-density diesel engines to mechanical or electrical energy using an organic Rankine cycle is a hotspot in recent years, and many optimistic views towards the fuel-saving capacity of this technology have been reported. But considering the off-the-shelf components’ limitations, system integration, dynamics, and other attached penalties, there lacks a comprehensive evaluation yet, especially in driving cycle scenarios. For this, a heavy truck co-simulation platform is built, embedded with organic-Rankine-cycle-based exhaust heat recovery models. Based on that the fuel-saving performance of organic Rankine cycle facilities are evaluated and main losses due to limiting factors are also discussed. It is found that an ideal subcritical organic Rankine cycle system improves the truck’s fuel economy by 4.48–7.52% when it is fully loaded. But due to the performance limitations of existing components and system interactions, the fuel-saving effect is reduced from 7.52% to 3.5%. Further considering organic Rankine cycle system dynamics and control operations, this value degrades to 2.07–2.24%, and it continues to decline with the attached weight. It is suggested that the system power density should be greater than 70 W/kg at least. At this point, its fuel-saving effect is just 1.75%.

Optimization and Analyzing of Subcritical Organic Rankine Cycle Using R1234ze(E) for Low and Medium Temperature Heat Source

A model of Subcritical Organic Rankine Cycle (SORC) driven by trans-1, 3, 3, 3-tetrafluoroprop-1-ene, R1234ze(E) as a new-typed environmental friendly refrigerant working fluid, was developed and analyzed in the view of the 180-220°C wet fluegas combustion and burning from natural gas, as a low to medium temperature heat source without the outlet temperature limit to generate optimum power by a turbine. The net power output, total efficiency, fluegas outlet temperature, and exergy efficiency of current SORC system were studied based on variation of vapor generator outlet temperature and turbine outlet pressure as optimized parameters in terms of per mass flow rates of working fluid, heat source, and heat sink. Results show that the maximum net power output is 5.2675 MW and with increasing of mass enthalpy change and temperature in expansion process causes to increase of net power output, steadily. And the maximum net power output was 53.1% increased. Furthermore, by decreasing the turbine outlet pressure, the total efficiency of the current SORC system is increased to 0.08107 because of the increase of net power output. Likewise, the exergy efficiency of the SORC system increasing significantly, and received to the 0.328, because of increasing the enthalpy change and decreasing the entropy change of fluegas in the evaporator system.

Zeotropic Mixture Selection for an Organic Rankine Cycle Using a Single Screw Expander

The organic Rankine cycle (ORC) is a popular and promising technology that has been widely studied and adopted in renewable and sustainable energy utilization and low-grade waste heat recovery. The use of zeotropic mixtures in ORC has been attracting more and more attention because of the possibility to match the temperature profile of the heat source by non-isothermal phase change, which reduces the irreversibility in the evaporator and the condenser. The selection of working fluid and expander is strongly interconnected. As a novel expander, a single screw expander was selected and used in this paper for efficient utilization of the wet zeotropic mixtures listed in REFPROP 9.1 in a low-temperature subcritical ORC system. Five indicators, namely net work, thermal efficiency, heat exchange load of condenser, temperature glide in evaporator, and temperature glide in condenser, were used to analyze the performance of an ORC system with wet and isentropic zeotropic mixtures as working fluids. The calculation and analysis results indicate that R441A with an expander outlet temperature of 320 K may be the suitable zeotropic mixture used for both open and close type heat source. R436B may be selected with an expander outlet temperature of 315 K. R432A may be selected with an expander outlet temperature from 295 K to 310 K.

Simultaneous optimization of multiparameters on a subcritical organic Rankine cycle system for low-grade waste heat recovery

This study aims to improve the thermo-economic performance of the subcritical organic Rankine cycle (ORC). Accordingly, an economical evaluation and the simultaneous multiparametric optimization on the subcritical ORC system for recovering low-grade waste heat of flue gas were conducted using electricity production cost (EPC) as the evaluation indicator. Results show that the optimum evaporating and condensing temperatures are mainly influenced by the inlet temperatures of flue gas and cooling fluid, respectively. High critical temperature of working fluids indicates high thermal-economic performance of the ORC system for 19 kinds of selected working fluids. Furthermore, the optimum pinch point temperature difference of evaporator decreases and that of condenser increases with the variation in the critical temperature of working fluids, but the optimum condensing temperature is unchanged. On the basis of a large number of calculation data, the correlations among the optimum cycle and external operating parameters of the subcritical ORC are proposed. These correlations can guide the engineering design of the subcritical ORC system.

A Generalised Assessment of Working Fluids and Radial Turbines for Non-Recuperated Subcritical Organic Rankine Cycles

The aim of this paper is to conduct a generalised assessment of both optimal working fluids and radial turbine designs for small-scale organic Rankine cycle (ORC) systems across a range of heat-source temperatures. The former has been achieved by coupling a thermodynamic model of subcritical, non-recperated cycles with the Peng–Robinson equation of state, and optimising the working-fluid and cycle parameters for heat-source temperatures ranging between 80 ° C and 360 ° C . The critical temperature of the working fluid is found to be an important parameter governing working-fluid selection. Moreover, a linear correlation between heat-source temperature and the optimal critical temperature that achieves maximum power output has been found for heat-source temperatures below 300 ° C ( T cr = 0.830 T hi + 41.27 ). This correlation has been validated against cycle calculations completed for nine predefined working fluids using both the Peng–Robinson equation of state and using the REFPROP program. Ultimately, this simple correlation can be used to identify working-fluid candidates for a specific heat-source temperature. In the second half of this paper, the effect of the heat-source temperature on the optimal design of a radial-inflow turbine rotor for a 25 kW subcritical ORC system has been studied. As the heat-source temperature increases, the optimal blade-loading coefficient increases, whilst the optimal flow coefficient reduces. Furthermore, passage losses are dominant in turbines intended for low-temperature applications. However, at higher heat-source temperatures, clearance losses become more dominant owing to the reduced blade heights. This information can be used to identify the most direct route to efficiency improvements in these machines. Finally, it is observed that the transition from a conventional converging stator to a converging-diverging stator occurs at heat-source temperatures of approximately 165 ° C , whilst radially-fibered turbines seem unsuitable as the heat-source temperature exceeds 250 ° C ; these conclusions can be used to inform expander design and selection at an early stage.

The Exergy Loss Distribution and the Heat Transfer Capability in Subcritical Organic Rankine Cycle

Taking net power output as the optimization objective, the exergy loss distribution of the subcritical Organic Rankine Cycle (ORC) system by using R245fa as the working fluid was calculated under the optimal conditions. The influences of heat source temperature, the evaporator pinch point temperature difference, the expander isentropic efficiency and the cooling water temperature rise on the exergy loss distribution of subcritical ORC system are comprehensively discussed. It is found that there exists a critical value of expander isentropic efficiency and cooling water temperature rise, respectively, under certain conditions. The magnitude of critical value will affect the relative distribution of exergy loss in the expander, the evaporator and the condenser. The research results will help to better understand the characteristics of the exergy loss distribution in an ORC system.

Case Study of an Organic Rankine Cycle (ORC) for Waste Heat Recovery from an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology for the conversion of waste heat to electricity. Although many energy intensive industries could benefit significantly from the integration of ORC technology, its current adoption rate is limited. One important reason for this arises from the difficulty of prospective investors and end-users to recognize and, ultimately, realise the potential energy savings from such deployment. In recent years, electric arc furnaces (EAF) have been identified as particularly interesting candidates for the implementation of waste heat recovery projects. Therefore, in this work, the integration of an ORC system into a 100 MWe EAF is investigated. The effect of evaluations based on averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC system is found capable of generating a net electrical output of 752 kWe with a steam buffer working at 25 bar. If combined heating is considered, the ORC system can be optimized to generate 521 kWe of electricity, while also delivering 4.52 MW of heat. Finally, an increased power output (by 26% with combined heating, and by 39% without combined heating) can be achieved by using high temperature thermal oil for buffering instead of a steam loop; however, the use of thermal oil in these applications has been until now typically discouraged due to flammability concerns.

Exergy analysis of a coupled transcritical and subcritical organic Rankine cycle system for waste heat recovery

This paper presents a detailed exergy analysis for a novel coupled organic Rankine cycle (CORC) system driven by the waste heat of the flue gas, which combined a transcritical organic Rankine cycle (TORC) with a subcritical organic Rankine cycle (SORC). The results show that the exergy efficiency of the CORC increases with the increase of the inlet pressure of the TORC expander and the decrease of the pinch point temperature difference of the internal heat exchanger. However, the increase of the flue gas temperatures at the inlet and outlet of the evaporator has negative effects. The CORC using isentropic working fluids exhibits better performance than that using dry working fluids. Furthermore, high critical temperature of the working fluid can improve the exergy efficiency. The exergy loss is mainly distributed in the heat exchangers and the expanders, which is different from the result in the traditional TORC.

Thermodynamic performance analysis of a coupled transcritical and subcritical organic Rankine cycle system for waste heat recovery

We present a novel coupled organic Rankine cycle (CORC) system driven by the low-grade waste heat, which couples a transcritical organic Rankine cycle with a subcritical organic Rankine cycle. Based on classical thermodynamic theory, a detailed performance analysis on the novel CORC system was performed. The results show that the pressure ratio of the expander is decreased in the CORC and the selection of the working fluids becomes more flexible and abundant. With the increase of the pinch point temperature difference of the internal heat exchanger, the net power output and thermal efficiency of the CORC all decrease. With the increase of the critical temperature of the working fluid, the system performance of the CORC is improved. The net power output and thermal efficiency of the CORC with isentropic working fluids are higher than those with dry working fluids.

Selection of organic Rankine cycle working fluids in the low-temperature waste heat utilization

In the current study, simulations based on the engineering equation solver (EES) software are performed to determine the suitable working fluid for the simple organic Rankine cycle system in different temperature ranges. Under the condition of various temperatures and a constant thermal power of the flue gas, the influence of different organic working fluids on the efficiency of the subcritical organic Rankine cycle power generation system is studied, and its efficiency and other parameters are compared with those of the regenerator system. It is shown that the efficiency of the subcritical organic Rankine cycle system is the best when the parameters of the working fluid in the expander inlet are in the saturation state. And for the organic Rankine cycle, the R245fa is better than other working fluids and the efficiency of the system reaches up to 10.2% when the flammability, the toxicity, the ozone depletion, the greenhouse effect and other factors of the working fluids are considered. The R601a working fluid can be used for the high-temperature heat source, however, because of its high flammability, new working fluid should be investigated. Under the same condition, the efficiency of the organic Rankine cycle power generation system with an internal heat exchanger is higher than that of the simple system without the internal heat exchanger, but the efficiency is related to the properties of the working fluid and the temperature of the heat source.

The determination and matching analysis of pinch point temperature difference in evaporator and condenser of organic rankine cycle for mixed working fluid

ABSTRACTExergo-economic analysis of the pinch point temperature difference (PPTD) in both evaporator and condenser of sub-critical organic Rankine cycle system (ORCs) are performed based on the first and second laws of thermodynamics. Taking mixture R13I1/R601a as a working fluid and the annual total cost per net output power Z as exergo-economic performance evaluation criterion, the effects of PPTD in evaporator ΔTe, and the PPTD ratio of condenser to evaporator y, on the exergo-economic performance of ORCs are analyzed. Moreover, how some other parameters influence the optimal PPTD in evaporator ΔTe,opt and the optimal PPTD ratio of condenser to evaporator yopt are also discussed. It has been found that the exergo-economic performance of ORCs is remarkably influenced by ΔTe and y, and there exists ΔTe,opt and yopt. In addition, ΔTe,opt and yopt are affected by heat transfer coefficient ratio of condenser to evaporator ß, the temperature of working fluid at dew point in condenser T1a, and composition of R13I1/R601a: larger ß and T1a lead to lower ΔTe,opt and yopt; by contraries, larger mass fraction of R13I1 makes ΔTe,opt and yopt increase, and yopt increases linearly. The effects of the temperature of working fluid at bubble point in evaporator T3a, mass flow rate of exhaust flue gas mg, and inlet temperature of exhaust flue gas Tgi on ΔTe,opt and yopt are very slight. For comparison, three additional working fluids, namely R601a, R245fa, and 0.32R245fa/0.68R601a, are also taken into account.

Economical evaluation and optimization of subcritical organic Rankine cycle based on temperature matching analysis

This paper presents an economical evaluation and parametric optimization on subcritical ORC (organic Rankine cycle) system for recovering the low-temperature waste heat of flue gas which is released from industrial boilers. First, the effects of the PPTD (pinch point temperature difference) in evaporator and condenser, and evaporating temperature on the system performance are examined with the EPC (electricity production cost) as the evaluation criterion. The optimal evaporating temperature, PPTDs in evaporator and condenser, as well as condensing temperature corresponding to the minimum EPC are provided simultaneously for the subcritical ORC system under different heat source temperatures. Then, the optimal parameter values are compared for nine potential organic working fluids with the critical point temperature from 124 °C to 235 °C. Results show that there exists a possible relationship between the critical temperature of working fluids and the economical performance of the system. Furthermore, it is suggested that the working fluids including R123, n-pentane, R11 and R141b, whose critical temperature range from 180 °C to 210 °C, are preferable for recovering the low-temperature waste heat of flue gas.

Selection of Working Fluids for Different Temperature Heat Source Organic Rankine Cycle

In the current paper, under the condition of different flue gas temperatures and constant flue gas thermal power, the influence of different organic working fluids on the efficiency of sub-critical organic Rankine cycle system were studied. The efficiency and other parameters of the simple system were calculated. The results show that the efficiency of sub-critical organic Rankine cycle system could reach maximum when the parameters of the working fluids in the expander inlet are dry-saturation. Flammability, toxicity, ozone depletion and other factors of the working fluids should be considered in the organic Rankine cycles. R245fa is considered a better choice for low-temperature heat source power generation, and the efficiency of the system is about 10.2%; for the high-temperature heat source, R601a can be considered; however, due to its high flammability, novel working fluids should be further discovered for power generation.

Performance comparison and parametric optimization of subcritical Organic Rankine Cycle (ORC) and transcritical power cycle system for low-temperature geothermal power generation

Organic Rankine Cycle (ORC) is a promising technology for converting the low-grade energy to electricity. This paper presents an investigation on the parameter optimization and performance comparison of the fluids in subcritical ORC and transcritical power cycle in low-temperature (i.e. 80–100°C) binary geothermal power system. The optimization procedure was conducted with a simulation program written in Matlab using five indicators: thermal efficiency, exergy efficiency, recovery efficiency, heat exchanger area per unit power output (APR) and the levelized energy cost (LEC). With the given heat source and heat sink conditions, performances of the working fluids were evaluated and compared under their optimized internal operation parameters. The optimum cycle design and the corresponding operation parameters were provided simultaneously. The results indicate that the choice of working fluid varies the objective function and the value of the optimized operation parameters are not all the same for different indicators. R123 in subcritical ORC system yields the highest thermal efficiency and exergy efficiency of 11.1% and 54.1%, respectively. Although the thermal efficiency and exergy efficiency of R125 in transcritical cycle is 46.4% and 20% lower than that of R123 in subcritical ORC, it provides 20.7% larger recovery efficiency. And the LEC value is relatively low. Moreover, 22032L petroleum is saved and 74,019kg CO2 is reduced per year when the LEC value is used as the objective function. In conclusion, R125 in transcritical power cycle shows excellent economic and environmental performance and can maximize utilization of the geothermal. It is preferable for the low-temperature geothermal ORC system. R41 also exhibits favorable performance except for its flammability.