Subcritical Organic Rankine Cycle Research Articles

Influence of axial turbine efficiency maps on the performance of subcritical and supercritical Organic Rankine Cycle systems

The utilization of low-to-medium enthalpy geothermal resources for electricity generation is commonly done using the binary cycle power plant technology. The low temperature of the geothermal fluid necessarily results in low thermal efficiencies but it can be profitably used as a driver to reduce the equipment cost. In particular, single stage axial flow turbines, a cheaper option compared to multistage solutions, can be used due to the moderate enthalpy drop in the expansion process. In this work the design performance of ORCs (Organic Rankine Cycle systems) equipped with single stage axial flow turbines is optimized for utilization of a 150 °C geothermal fluid. Accurate predictions of turbine efficiency are included in the optimization procedure to take into account the influence of the thermodynamic cycle parameters, fluid properties and size on the maximum achievable efficiency. A set of six fluids is considered to examine a wide spectrum of design conditions. Results show that supercritical ORCs outperform subcritical ORCs even taking into account the detrimental effect of high expansion ratios on turbine efficiency. Hydrofluoroolefins are found to exhibit a net power output approximately equal or even higher than the best hydrofluorocarbons.

Chemical kinetics method for evaluating the thermal stability of Organic Rankine Cycle working fluids

Organic Rankine Cycle (ORC) systems are widely used to generate electricity with industrial waste heat and renewable energy. Transcritical ORCs with high temperature heat sources are more attractive than subcritical ORCs due to their lower exergy losses, higher thermal efficiencies and higher work outputs. The working fluid thermal stability is the primary consideration in the working fluid selection due to decomposition at high temperatures. This paper presents a chemical kinetics method for evaluating the thermal stability of ORC working fluids. A chemical kinetics experimental system was built with n-pentane as the test working fluid. The influences of pressure and temperature were analyzed experimentally and theoretically. An apparent chemical kinetics model was established to predict the thermal stability with the apparent kinetic parameters of n-pentane measured experimentally. This apparent chemical kinetics model gives significant guidance for the working fluid selection and ORC system design.

Analysis of the quantitative correlation between the heat source temperature and the critical temperature of the optimal pure working fluid for subcritical organic Rankine cycles

Organic Rankine cycles can use various working fluids. Different heat sources and working conditions lead to very different optimal working fluids. This paper presents a new quantitative criterion for selecting the optimal working fluid for open type heat sources with temperatures from 150 to 350 °C. The heat source recovery exergetic efficiency is used as the evaluation indicator. Over 30 working fluids with critical temperature from 71.87 to 357.65 °C are under evaluation. The working fluid critical temperature's influence on the heat source utilization, cycle efficiency and exergetic loss in the evaporator and condenser is analyzed to show that the critical temperature is the key parameter for selecting the optimal working fluid. The critical temperature of the optimal working fluid is determined by the heat source temperature, the pinch point temperature difference and the condensation temperature. When the cooling fluid outlet temperature is between 30 and 60 °C, which covers the most common natural cooling condition and the requirement of combined heat and power system, the correlation between the critical temperature of the optimal working fluid and the heat source temperature with different pinch point temperature difference is given. This correlation provides concise guidance for the optimal pure working fluids selection and mixture design.

Investigation on the fluid selection and evaporation parametric optimization for sub- and supercritical organic Rankine cycle

ORC (Organic Rankine cycle) is a promising technology for recovery of low-grade heat. In the previous studies, different conclusions for working fluid selection criterion can be found and relatively few work for supercritical ORC has been made. Therefore, this paper investigates the net power output of ORC utilizing waste flue gas with various evaporation parameters and 12 working fluids in both subcritical and supercritical condition. The results indicate that the variation of the net power output with evaporation pressure is related to the heat source temperature, and the maximum net power output appears at supercritical condition rather than subcritical condition if the heat source temperature is about 25–40 °C higher than the working fluid's critical temperature. Besides, the parametric optimization is performed, and the most suitable working fluids for various flue gas inlet temperature of 150–250 °C have been found. It can be found that the most suitable working fluids have a critical temperature about 40–65 °C lower than flue gas inlet temperature, and the optimum condition is always supercritical. For subcritical ORC, it is better to adopt the working fluids with low evaporation latent heat and high liquid specific heat to pursue a high net power output.

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.

Comparative Investigation of the Performances of Subcritical and Transcritical Biomass-Fired ORC Systems for Micro-scale CHP Applications

The paper aims to analyse the energetic performances of Organic Rankine Cycles (ORCs) for biomass micro-scale CHP applications. Subcritical and transcritical cycles have been compared and the influence of internal regeneration on system behaviour has been evaluated. The investigation illustrates the noticeable influence of the operating conditions and ORC configuration on the main CHP performances. Furthermore, results reveal that the proper choice of the organic working fluid is fundamental to assure proper operations and optimise the system behaviour.The analysis shows that the transcritical cycle with internal regeneration offers the maximum electric performances, whereas the saturated cycle in simple configuration guarantees the highest thermal efficiency. Furthermore, all the investigated systems guarantee positive energy saving capabilities.

Optimal Heat Source Temperature for thermodynamic optimization of sub-critical Organic Rankine Cycles

Based on a sub-critical ORC (Organic Rankine Cycle) process, this study introduces the term OHST (Optimal Heat Source Temperature) with consideration of a suitable thermal match between heat source and working fluid. A theoretical formula is developed for predicting the OHST, which shows that OHST only depends on evaporation pressure and pinch point in the preheater and evaporator. A comparative study between the predicted OHSTs and those obtained from cycle simulations is performed, showing that the proposed formula is reliable, provided that HTF (Heat Transfer Fluid) is homogeneous and has good consistency in terms of heat capacity for different temperatures. To demonstrate the application of the proposed OHST-theory for thermodynamic optimization of ORC systems, a case study is presented based on a simple ORC coupled with thermal water at 140 °C. Consequently, using R227ea leads to the highest system efficiency of 10.38%, due to a better thermal match in the preheater and evaporator. In order to increase the exploitation of the thermal potential from the heat source, a dual-fluid-ORC is proposed, where R245fa and R227ea are considered for the high and low temperature ORC processes, respectively. Finally, this combination leads to the highest system efficiency of 11.07%.

Multi-criteria Design Optimization and Screening of Heat Exchangers for a Subcritical ORC

Evaporator and condenser are two main components of ORC and they are predominant in exergy destruction, cost and material consumption. The optimization and screening of evaporator and condenser is important to achieve the optimal performance of ORC. In this paper, the popular used shell and tube heat exchangers and plate heat exchangers are selected as candidate heat exchangers for ORC. The detailed parametric and structural models of heat exchangers are formulated. The candidate heat exchangers are optimized and screened under the objective of maximum thermal efficiency (THE), minimum specific cost (SIC) and minimum heat exchanger area per unit power output (APR). Genetic algorithm (GA) is used to solve the model. Then two candidates of ORC schemes with different heat exchangers are screened by fuzzy multi-criteria decision making process. Single objective and fuzzy multi-criteria design optimization and screening cases are demonstrated to testify the proposed methodology.

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.

Payback period estimation and parameter optimization of subcritical organic Rankine cycle system for waste heat recovery

This paper presents a theoretical model on the payback period of a subcritical ORC (organic Rankine cycle) system for recovering low-grade waste heat of flue gas. Based on the minimum payback period principle, a detailed internal parameter optimization is carried out. Furthermore, the influences of external parameters on the payback period are analyzed and a new criterion of screening working fluids is proposed. Results show that the payback period of the ORC system decreases before increasing with the increase of evaporation temperature, condensation temperature, and the pinch point temperature differences in the evaporator and condenser. The minimum payback period of the ORC system decreases monotonously with the increase of the inlet temperature, mass flow rate of the flue gas, and the electricity price. When the minimum payback period and the net power output are used as indicators of selecting working fluids of the ORC system, this work identifies a specific working fluid for a given inlet flue gas temperature. We demonstrate this process by showing that R236a, R245fa, and R113 should be selected as the working fluids when the inlet temperatures of the flue gas are around 150 °C, 200 °C and 250 °C, respectively.

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.

Methodical thermodynamic analysis and regression models of organic Rankine cycle architectures for waste heat recovery

The ORC (organic Rankine cycle) is an established technology for converting low temperature heat to electricity. Knowing that most of the commercially available ORCs are of the subcritical type, there is potential for improvement by implementing new cycle architectures. The cycles under consideration are: the SCORC (subcritical ORC), the TCORC (transcritical ORC) and the PEORC (partial evaporation ORC). Care is taken to develop an optimization strategy considering various boundary conditions. The analysis and comparison is based on an exergy approach. Initially 67 possible working fluids are investigated. In successive stages design constraints are added. First, only environmentally friendly working fluids are retained. Next, the turbine outlet is constrained to a superheated state. Finally, the heat carrier exit temperature is restricted and addition of a recuperator is considered. Regression models with low computational cost are provided to quickly evaluate each design implications. The results indicate that the PEORC clearly outperforms the TCORC by up to 25.6% in second law efficiency, while the TCORC outperforms the SCORC by up to 10.8%. For high waste heat carrier inlet temperatures the performance gain becomes small. Additionally, a high performing environmentally friendly working fluid for the TCORC is missing at low heat carrier temperatures (100 °C).

Low grade waste heat recovery with subcritical and supercritical Organic Rankine Cycle based on natural refrigerants and their binary mixtures

The Organic Rankine Cycle (ORC) is a promising technology for low temperature waste heat-to-power conversion. This work investigates the waste heat recovery potential of the ORC and some of its innovative variations, such as the supercritical cycle and the use of binary zeotropic mixtures. Focus is given to natural hydrocarbons having low ozone depletion and global warming potential. By performing simulations for heat source temperatures ranging from 150 to 300 °C, the optimal operation mode and working fluids are identified. Meanwhile, several technical parameters (such as the turbine size parameter and rotational speed) are calculated. The system exergetic efficiency is affected by the critical temperature of the working fluids and in the case of mixtures by their temperature glide in the condenser. The maximum efficiency ranges from 15 to 40% for heat source temperatures between 150 and 300 °C, respectively. For temperatures above 170 °C, the optimal fluids are mixtures combined with supercritical operation. Compared to the subcritical ORC with R245fa, the achievable efficiency improvement ranges between 10 and 45%. Although smaller turbines can be used due to the lower size parameter values of supercritical mixtures, R245fa has lower volume flow ratio, rotational speed and UA values.

Multi-Objective Thermo-Economic Optimization Strategy for ORCs Applied to Subcritical and Transcritical Cycles for Waste Heat Recovery

Organic Rankine cycles (ORCs) are an established technology to convert waste heat to electricity. Although several commercial implementations exist, there is still considerable potential for thermo-economic optimization. As such, a novel framework for designing optimized ORC systems is proposed based on a multi-objective optimization scheme in combination with financial appraisal in a post-processing step. The suggested methodology provides the flexibility to quickly assess several economic scenarios and this without the need of knowing the complex design procedure. This novel way of optimizing and interpreting results is applied to a waste heat recovery case. Both the transcritical ORC and subcritical ORC are investigated and compared using the suggested optimization strategy.

Multi-objective optimization of evaporation and condensation temperatures for subcritical organic Rankine cycle

A multi-objective function F(X) by the incorporation of single-objective functions, i.e., net power output Wnet, exergy drop of the exhaust gas from inlet to outlet ΔEg, total exergy destruction rate I and system total cost C2013, has been put forward as the subcritical organic Rankine cycle (ORC) performance indicator and solved with the method of linear weighted evaluation function. The optimization of the evaporation temperature Te and condensation temperature Tc of subcritical ORC has been carried out by the use of pure working fluids R600a, R245fa, R601a, R601, R123, and non-azeotropic mixed working fluids R600a/R601a, R245fa/R601a, R245fa/R601, R600a/R245fa. The results reveal that there exist optimal evaporation temperature Te,opt and condensation temperature Tc,opt minimizing the multi-objective function F(X); while Te,opt and Tc,opt can not exactly exist when choosing single-objective functions, i.e., Wnet, I, C2013 and ΔEg as the performance indicators. Although Te,opt and Tc,opt vary with different working fluids, the ORC performance of mixed working fluids is not always better than that of pure working fluids. The multi-objective optimization of ORC shows superiority to the single-objective optimization, owning to its pursuit of the best comprehensive performance.

Thermodynamic and economic optimizations of a waste heat to power plant driven by a subcritical ORC (Organic Rankine Cycle) using pure or zeotropic working fluid

This paper carried out the thermodynamic and economic optimizations of a subcritical ORC (Organic Rankine Cycle) using a pure or a zeotropic mixture working fluid. Two pure organic compounds, i.e. n-pentane and R245fa, and their mixtures with various concentrations were used as ORC working fluid for this study. Two optimizations, i.e. exergy efficiency maximization and LCOE (Levelized Cost of Electricity) minimization, were performed to find out the optimum operating conditions of the system and to determine the best working fluid from the studied media. Hot water at temperature of 150 °C and pressure of 5 bars was used to simulate the heat source medium. Whereas, cooling water at temperature of 20 °C was considered to be the heat sink medium. The mass flow rate of heat source is fixed at 50 kg/s for the optimizations. According to the results, the n-pentane-based ORC showed the highest maximized exergy efficiency (53.2%) and the lowest minimized LCOE (0.0863 $/kWh). Regarding ORCs using zeotropic working fluids, 0.05 and 0.1 R245fa mass fraction mixtures present the comparable economic features and thermodynamic performances to the system using n-pentane at minimum LCOE. The ORC using R245fa represents the least profitable system.

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.

Discussion of the internal heat exchanger's effect on the Organic Rankine Cycle

This paper explores the performances of IHE (Internal Heat Exchanger) in ORC (Organic Rankine Cycle) systems. Although previous studies hold multitudinous opinions, this study gives clear statements of IHE in both subcritical and supercritical ORC systems by setting a new model taking pressure drop in loops and acid dew point into consideration. Commonly used working fluids R123 and R600 are chosen for subcritical and supercritical cases separately. The temperature of the heat source applied is 200 °C and the mass flow rate of it is 1 kg/s. The analysis is accomplished by program Engineering Equation Solver. A modified method of calculating maximum heat exchange in IHE is given when modeling a supercritical cycle, because of the momentously changing specific heat near the critical point. Besides, a new approach is put forward to calculate the outlet temperature of the heat source and find the location of pinch point in supercritical cases. The results provide that IHE is beneficial to a subcritical case, but it improves system performance only in part of the low pressure stage in a supercritical case. Moreover, after the acid dew point Tad is taken into account, it is found that IHE is able to enlarge euphemistically the maximum system net output in a subcritical case. And in a supercritical case, the original evaporation pressure which does not conform to the rule Th,out > Tad is available now. It is revealed that the utilization of IHE will strengthen the applicability of the system.

A comparative study on thermo-economic performance between subcritical and transcritical Organic Rankine Cycles under different heat source temperatures

In this paper, the net power output, exergy efficiency and levelized energy cost of system were selected as performance indicators for assessing Organic Rankine Cycle (ORC). Firstly, the turbine inlet temperature and pressure meeting the requirement of pinch point temperature difference of evaporator in transcritical ORC (trans-ORC) were determined based on performance optimization. Subsequently, the thermo-economic performance of a subcritical ORC (sub-ORC) using R601 as working fluid and a trans-ORC using R134a as working fluid were compared under different heat source temperatures and a fixed outlet temperature of flue gas. Results show that for trans-ORC, when the pinch point temperature difference of evaporator lies between the inlet and outlet of evaporator, a lower inlet pressure of turbine is favorable; when the pinch point temperature difference of evaporator is located at the outlet of evaporator, there exists an optimal inlet pressure of turbine. Either for sub-ORC or trans-ORC, the net power output increases and levelized energy cost decreases with the increase in heat source temperature. For sub-ORC, exergy efficiency of system increases monotonously with heat source temperature, while for trans-ORC, exergy efficiency of system grows up firstly and then reduces (or keeps constant) with the increasing of heat source temperature. Moreover, for net power output and exergy efficiency of system, there exist a range of heat source temperatures making trans-ORC better than sub-ORC, and the heat source temperature region extends with the increase in pinch point temperature difference of evaporator. For levelized energy cost of system, the sub-ORC is always superior to trans-ORC.

An Integrated Comparison for Working Fluids Selection in Both Subcritical and Supercritical ORC

A comparison focusing on the properties and the system performances in working fluids selection for both subcritical and supercritical ORC is proposed. 16 candidates including wet, isentropic and dry fluids are considered. Thermal efficiency, exergy efficiency, Specific net power output and UA value are set as the system performance indicators. Under the premise of working fluids safety and comprehensive consideration of all aspects of system performances, R152a by supercritical ORC is determined by integrated comparison.