Fluid In Organic Rankine Cycle Research Articles

Comparative study of combined solid oxide fuel cell-gas turbine-Organic Rankine cycle for different working fluid in bottoming cycle

Worldwide efforts are being made for evolving viable electricity generation alternative to the traditional fossil fuel based power generation systems and also to enhance the efficiency of existing power generation systems. Studies have been made for integrating fuel cell with gas turbine cycle and Organic Rankine cycle, but there exist possibility of further investigation by incorporating the measures to increase effective energy utilization in the cycle along with consideration of different working fluids in Organic Rankine cycle. In the present paper, a combined cycle system consisting of technologies namely solid oxide fuel cell, gas turbine and Organic Rankine cycle is studied. The considered combined cycle system employs Heat Recovery Steam Generator followed by Organic Rankine cycle as bottoming cycle for recovering the waste heat coming out from hybrid SOFC-GT system through a Heat Recovery Vapour Generator. The considered SOFC-GT system has methane gas being used as fuel which is reformed by both external and internal reformers with anode gas recycling. Here three different working fluids namely R141b, R245fa and R236fa are used in bottoming Organic Rankine cycle for comparing their effects on the cycle performance of proposed SOFC-GT-ORC combined cycle system based on first law of the thermodynamics. Results have been obtained from the computer simulation based on thermodynamic modeling of SOFC-GT-ORC combined cycle and the effects of gas turbine inlet temperature, cycle pressure ratio, fuel utilization factor and ORC turbine inlet temperature are investigated on the cycle performance. The study indicates that the efficiency is increased about 8%-12% by recovering SOFC-GT waste heat through ORC and R236fa is found to be the best in terms of power generation capacity and efficiency of SOFC-GT-ORC system. Outcome from this paper gives insight to the power sector professionals and research community working for evolving efficient and robust power generation alternatives based on the combination of direct energy conversion system and indirect energy conversion system.

Thermodynamic analysis of organic Rankine cycle with Hydrofluoroethers as working fluids

This paper presents the analysis of organic Rankine cycle (ORC) using hydrofluoroethers (HFEs) such as HFE7000, HFE7100 and HFE7500 as working fluids under external conditions. HFE's has been chosen over chlorofluorocarbons (CFC's) and hydro chlorofluorocarbons (HCFC's) as it is environmentally friendly. Both the CFC's and the HCFC's possess ozone depletion potential (ODP), while the hydrofluorocarbons (HFC's) have relatively significant global warming potential (GWP). The HFE's that have excellent thermophysical properties and low toxicity can be recommended as a long term solution to the environmental issues. The HFE's have zero ODP and very low GWP compared to the CFC's, HCFC's and HFC's. A thermodynamic model has been developed using Engineering Equation Solver (EES) software to simulate the system under steady state conditions. Parametric analysis is conducted to examine the effects of some thermodynamic parameters on the system performance using different working fluids. When turbine inlet temperature was varied from 70°C to 110 °C keeping condensation temperature fixed at 28 °C, HFE7000 produces the maximum thermodynamic efficiency and performs better in view of the net work output under the given working conditions. However, when evaporation pressure was kept constant at 1.2 bar and condensation temperature was varied from 20°C to 30 °C, HFE7500 produced the maximum efficiency of 12.3% in comparison with 7.6% for HFE7100 and 4.1% for HFE7000. The work demonstrates the use of hydrofluoroethers as working fluid in ORC.

Fluid-to-fluid scaling of heat transfer to mixed convection flow of supercritical pressure fluids

Fluid-to-fluid scaling for supercritical heat transfer can effectively reduce the difficulty and cost of heat transfer experiments in supercritical boilers and supercritical water reactors and can reduce the number of experiments by converting experimental data of the model fluid to the prototype fluid in organic Rankine cycles. Currently, most existing scaling methods are only suitable for forced convection, while few are developed for mixed convection where buoyancy significantly affects the heat transfer. This paper attempts to extend the applicability of scaling method to mixed convection with the aid of computational fluid dynamic simulations. The scaling parameters were analyzed first and then the shear-stress transport k-ω model was used to analyze the supercritical heat transfer characteristics of water and R134a to provide further information for developing a dimensionless number. The results show that significant variations of properties and flow parameters occur in the layer of y+ = 5 to 100 and the axial velocity gradient in this layer changes in quite a similar manner to the wall temperature. Based on numerical results, the axial velocity gradient was used with a thermal resistance analogy to derive a new dimensionless number, Re−0.9πA, to scale the mass flux. Then, a set of fluid-to-fluid scaling laws were developed to predict the heat transfer to supercritical fluids. To validate the newly proposed scaling laws, well-developed correlations were used for forced convection flow and a direct validation method was developed for buoyancy-influenced flow. Results show that this new scaling method exhibits reasonable accuracy for both forced and mixed convection heat transfer with supercritical fluids.

Evaluation of Working Fluids for Organic Rankine Cycle Based on Exergy Analysis

One of the crucial aspects to determine the performance of Organic Rankine Cycle (ORC) is the selection of appropriate working fluids. This paper describes the simulative performance of several organic fluid and water as working fluid of an ORC based on exergy analysis with a heat source from waste heat recovery. The simulation was conducted by using Engineering Equation Solver (EES). The effect of several parameters and thermodynamic properties of working fluid was analyzed, and part of them was used as variables for the simulation in order to determine their sensitivity to the exergy efficiency changes. The results of this study showed that water is not appropriate to be used as working fluid at temperature lower than 130 °C, because the expansion process falls in saturated area. It was also found that Benzene had the highest exergy efficiency, i.e. about 10.49%, among the dry type working fluid. The increasing turbine inlet temperature did not lead to the increase of exergy efficiency when using organic working fluids with critical temperature near heat source temperature. Meanwhile, exergy efficiency decreasing linearly with the increasing condenser inlet temperature. In addition, it was found that working fluid with high latent heat of vaporization and specific heat exert in high exergy efficiency.

Thermodynamic assessment of zero-emission power, hydrogen and methanol production using captured CO2 from S-Graz oxy-fuel cycle and renewable hydrogen

Thermodynamic analysis of a novel combined system, including geothermal driven dual fluid organic Rankine cycle (ORC), S-Graz cycle, proton exchange membrane electrolyzer (PEME) and methanol synthesis unit (MSU) are carried out using energy and exergy principles. The presented system produces zero emission power and hydrogen using oxy-fuel combustion technology and splitting water into hydrogen and oxygen in the PEME, which consumes the dual fluid ORC produced power. Then, methanol production from captured CO2 and produced H2 is suggested to eliminate the captured CO2 from S-Graz cycle. The energy and exergy efficiencies and sustainability index of 14.7%, 42.43% and 1.737 are obtained for the presented cogeneration system, respectively. Also, results associated with the exergy destruction reveal that most of the destroyed exergy in the system is due to the dual fluid ORC in compare with the other units. Furthermore, it is observed that, within the dual fluid ORC, low pressure evaporator (LPE) is responsible for 33% of exergy destruction. Moreover, the results indicate that increasing higher pressure of the dual fluid ORC improves the system energetic and exergetic performance while an increment in geothermal hot water temperature leads to a reduction in system energy and exergy efficiencies.

Isobaric Heat Capacity Measurements of Supercritical R1234yf

R1234yf (2,3,3,3-tetrafluoropropene) is a low global warming potential (GWP) replacement for R134a, a fluid commonly used as a refrigerant in domestic and automotive air conditioning systems and as a working fluid in organic Rankine cycles. The use of R1234yf in energy conversion cycles requires an accurate determination of its thermophysical properties, including the isobaric heat capacity (cP). Using a custom-made flow calorimeter, experimental cP measurements of supercritical R1234yf were made at temperatures T = 373.15 to 413.5 K and absolute pressures P = 3.5 to 10 MPa. The experimental apparatus was calibrated using R134a to increase the measurement accuracy. The heat capacity measurements of R1234yf were compared to available published experimental data and to values obtained from a multiparameter equation of state (EOS). The measured cP values agreed well with the EOS, providing an average absolute deviation (AAD) of 1.6%.

A gene expression programming approach for thermodynamic properties of working fluids used on Organic Rankine Cycle

The selection of appropriate fluid in Organic Rankine Cycles (ORC) is an important issue. The thermodynamic properties of working fluids affect the efficiency, operation and environmental impact of the system. Many working fluids for ORC power plants are available. Pentane (R-601) and R245fa are the most used for ORC. In this work, a gene expression programming (GEP) model for estimating thermodynamic properties of pentane (R-601) and R245fa was used. New formulations are presented for determination of the enthalpy and entropy values of two working fluids. The actual results were compared to the results obtained with GEP. The obtained results showed that the formulations are effectively capable of evaluating the thermodynamic properties of working fluids. The GEP-based formulations are simple. These formulations will be helpful for thermodynamic analysis of ORC systems.

Novel classification of pure working fluids for Organic Rankine Cycle

Power generation from low-temperature heat sources (80–300 °C) like thermal solar, geothermal, biomass or waste heat has been becoming more and more significant in the last few decades. Organic Rankine Cycle (ORC) uses organic working fluids, obtaining higher thermal efficiency than with water used in traditional Rankine Cycles, because of the physical (thermodynamic) properties of these fluids. The traditional classification of pure (one-component) working fluids is based on the quality of the expanded vapour after an isentropic (adiabatic and reversible) expansion from saturated vapour state, and distinguishes merely three categories: wet, dry and isentropic working fluids. The purpose of this paper is to show the deficiencies of this traditional classification and to introduce novel categorisation mostly to help in finding the thermodynamically optimal working fluid for a given heat source.

Safe performance analysis of flammable mixture working fluid leakage in Organic Rankine Cycle

As working fluid in Organic Rankine Cycle (ORC) used for engine waste heat recovery, the unavoidable leakage of HCs/CO2 mixture in practical application is a threat for safety due to its flammability. In this work, safe performance comparison of propylene/carbon dioxide and pentane/carbon dioxide is carried out through concentration distribution analysis and explosion damage degree evaluation. In addition, influence factors of dispersion characteristic were analysed. According to the assessment results, the concentration of propylene is higher than pentane at the same position after leakage. However, the damage degree to buildings and individuals caused by explosion increase from propylene/carbon dioxide to pentane/carbon dioxide. Wind speed and hydrocarbon ratio have obvious effect on the dispersion characteristics. To improve the safe performance of hydrocarbon/carbon dioxide mixture in practical application, gas detectors in the middle of the ceiling can detect the occurrence of gas leakage in less than one second. The maximum ratio of propylene and pentane should be 0.4 when blending with carbon dioxide in view of explosion risk.

Numerical study of using different Organic Rankine cycle working fluids for engine coolant energy recovery

Engine waste heat recovery technology especially Organic Rankine cycle (ORC) has been widely studied in order to achieve higher overall thermal efficiency, reduce the engine emissions and improve the fuel economy. The coolant energy occupies around 30% of the fuel energy can be used as the heat source for ORC system. This paper studies thermal status of the engine heated components when using different ORC working fluids as engine coolant to avoid the heat loos using heat exchanger to transfer coolant to the ORC fluid. A Solid-Liquid Conjugated Heat Transfer (SLCHT) calculation method is developed to calculate the heat transfer inside the engine, which can solve the temperature field of both solid zone and fluid zone. The simulation results have been validated by the experimental data from a 6-cylinder medium duty diesel engine, when water is the coolant in the system. The simulation model is then used to predict the temperature profile using different ORC working fluids and investigate the influence of different ORC working fluids on the cooling effects of the engine heated parts. The maximum temperature of the heated components has been selected as the evaluation parameters. The results reveals that applying selected ORC working fluids in engine as coolant is not practical under the designed conditions, which will make the engine overheated. Further investigation showed that increasing mass flow rate of the coolant can decrease the thermal status of the heated components but still cannot meet the cooling demands even under 200% of the original mass flow rate. The variations of the coolant outlet temperature and exergy were also analysed.

An experimental investigation on a recuperative Organic Rankine Cycle (ORC) system for electric power generation with low-grade thermal energy

In this study, a small-scale recuperative Organic Rankine Cycle (ORC) system capable of generating electric power using low-grade thermal energy was setup and experimentally investigated at different operating conditions. The experimental setup consisted of typical recuperative ORC system components, such as a turboexpander with high speed electricity generator, plate recuperator, finned-tube air cooled condenser, ORC fluid pump and plate evaporator. R245fa was used as the working fluid in the recuperative ORC system due to its zero ozone depletion potential (ODP) and appropriate thermophysical properties for ORC system. The working fluid was evaporated in a plate evaporator by hot thermal oil flow which was circulated and heated by exhaust flue gases of an 80kWe microturbine CHP unit. The test rig was fully instrumented and extensive experiments were carried out to compare and examine the influences of various important parameters including heat source temperature and ORC pump speed etc. on the performances of ORC systems with and without recuperator integration. The test results are essential in understanding the system operations and can contribute significantly to optimal system design, component selections and controls.

Thermodynamic analysis and optimization of a novel zeotropic organic Rankine Cycle

The decrease of fossil energy reserves and the increase of energy prices have resulted in a strong interest in utilizing renewable heat sources or waste heat for power generation. Organic Rankine cycle (ORC) is a promising heat-to-power conversion technology. Although significant efforts have been devoted to improve the thermo-economic performance of the pure fluid ORC, the improvement is limited due to the isothermal nature of evaporation and condensation. ORC using zeotropic mixture is superior to ORC using pure fluid in thermodynamic performance due to the lower irreversibility in heat transfer process of the zeotropic mixture. In the present study, a novel zeotropic ORC with liquid-separation condensation and multi-pressure evaporation is proposed. Thermodynamic analysis and optimization model of the novel ORC is developed. The objective function of the optimization is the maximization of net power output. A case study is elaborated to test the proposed methodology. The results show that the thermodynamic performance can be significantly improved compared to the basic zeotropic ORC.

Zeotropic mixture active design method for organic Rankine cycle

Using mixture working fluids has been proven to be an effective way to improve the performance of organic Rankine cycles (ORC). Instead of recommending mixtures or compositions for a specific heat source, this work is focused on providing preliminary design guidelines of zeotropic mixture working fluids for ORC. The zeotropic mixture active design method is provided for mixture selection without massive calculation or blind trial. The designed mixtures have better performance than the optimal pure working fluid. The designed zeotropic mixtures should have the same key properties as the optimal pure working fluids and also have temperature glide matching with the cooling source. Firstly, the authors’ previous study on the optimal pure working fluid screen criteria is simply reviewed. Then, how the mixture’s temperature glide influencing on the cycle performance is analyzed. Finally, the zeotropic mixture active design method is provided and verified. Some debatable questions regarding to the mixtures are also answered.

Waste energy recovery including pressure and thermal energy from lng regasification

The world has been concentrating on waste heat recovery for several decades, with the vast majority of achievements making a significant contribution to the development of industry and economy. The attention has recently been paid to waste energy from cold streams. This work focuses on the recovery of waste cold energy released from Liquefied Natural Gas (LNG) regasification process, including pressure and thermal energy. A direct expansion configuration involving different steps of expansion and mass flow rate extraction at intermediate pressure levels is adopted in the mathematical models for pressure and cold energy recovery. An equation of state for methane (the main component of LNG) is employed to calculate LNG thermodynamic properties in a long-range phase transition of the regasification process. A direct configuration of the organic Rankine cycle (ORC) is employed to recover waste cold energy. The modified Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) equations of state (EOS) play important roles in calculation of thermodynamic properties of the organic working fluids used in the ORC. Several fluids are examined to select the most suitable one as the ORC working fluid. All the models are developed and solved by using MATLAB. By adopting propane as the working fluid in the ORC, the multi-stage expansion along with thermal energy extraction is shown to be the most thermodynamically efficient configuration with the recovery of up to 170 kJ pressure energy for each kg of the flowing LNG, and up to 74 kJ from thermal energy recovery.

Technical performance analysis and economic evaluation of a compressed air energy storage system integrated with an organic Rankine cycle

Energy storage becomes increasingly important in balancing electricity supply and demand due to the rise of intermittent power generation from renewable sources. The compressed air energy storage (CAES) system as one of the large scale (>100MW) energy storage technologies has been commercially deployed in Germany and the USA. However, the efficiency of current commercial CAES plants still needs to be improved. In this study, an integrated system consisting of a CAES system and an organic Rankine cycle (ORC) was proposed to recover the waste heat from intercoolers and aftercooler in the charging process and exhaust stream of the recuperator in discharging process of the CAES system. Steady state process models of the CAES system and ORC were developed in Aspen Plus®. These models were validated using data from the literature and the results appear in a good agreement. Process analysis was carried out using the validated models regarding the impact of different organic working fluids (R123, R134a, R152a, R245fa, R600a) of ORC and expander inlet pressures of the ORC on system performance. It was found that integrating ORC with the CAES system as well as selecting appropriate working fluid was a reasonable approach for improving performance of the CAES system. The round-trip efficiency was improved by 3.32–3.95% using five working fluids, compared to that of the CAES system without ORC. Economic evaluation on levelized cost of electricity (LCOE) was performed using Aspen Process Economic Analyser® (APEA). Different working fluids in ORC and different power sources (e.g. wind and solar) associated with the integrated system were considered to estimate the LCOEs. It was found that the LCOEs for the integrated system were competitive with fossil-fuel fired power and even lower than offshore wind power and solar power. The proposed research presented in this paper hopes to shed light on how to improve efficiency and reduce cost when implementing CAES.

Thermal stability of some hydrofluorocarbons as supercritical ORCs working fluids

Organic Rankine Cycles (ORC) are used to generate electricity in renewable energy and industrial waste heat recovery systems. Supercritical ORCs have recently attracted much interest due to their higher thermal and exergy efficiencies. Hydrofluorocarbons are used as working fluids in subcritical ORCs and are also good choices for supercritical ORCs due to their good thermal performance. However, the thermal stability of the hydrofluorocarbons must be considered at high temperatures in supercritical ORCs. This study used the fluoride ion concentration to be the indicator of hydrofluorocarbon decomposition. A test system was designed and decomposition temperatures of HFC245fa, HFC152a, HFC134a, HFC236fa and HCFC123 were measured experimentally. The relationships between thermal stability and molecular structures of hydrofluorocarbons were then analyzed. The results show that these hydrofluorocarbons were good selections for supercritical ORCs by thermal stability. The hydrofluorocarbon decomposition characteristics and the effects of the decomposition products on the test system were also analyzed in this paper.

Exploration and Analysis of CO2 + Hydrocarbons Mixtures as Working Fluids for Trans-critical ORC

ORCs (Organic Rankine Cycles) are of great significance for energy conservation and CO2 emission reductions owing to their usage in converting low grade waste heat to electrical power. The working fluid properties are the key to getting good cycle performance. The advantages of CO2 and hydrocarbon working fluids for use in ORCs are investigated here for six binary mixtures: CO2+propane/n-butane/iso-butane/n-pentane/iso-pentane/neo-pentane. A transcritical ORC simulation program was used to predict the cycle performance for various hydrocarbon mass fraction working fluids for a turbine inlet temperature of 453.15 K and inlet pressure of 1.3pc. The predicted cycle performance is used to divide the six working fluids into three categories relative to the thermal efficiency of an ORC using pure CO2. The first working fluid type (CO2+propane) has increasing efficiencies with increasing hydrocarbon mass fraction while the second type have maximum thermal efficiencies at hydrocarbon mass fractions of 0.3, and the third type has maximums at a mass fraction of 0.2. In addition, the thermodynamics relative efficiency, ξ, is highest for CO2+propane and then decreases for CO2+iso-butane, CO2+ n-butane, CO2+neo-pentane, CO2+iso-pentane, and CO2+n-pentane with the lowest. Then, the cycle efficiencies of the three working fluid types were analyzed from the perspective of the hydrocarbon molecular structure. The results show that longer main carbon chains and branched carbon chains have larger pc and Tc, with this critical property leading to higher efficiencies at low hydrocarbon mass fractions. Lower p′c and T′c lead to higher efficiencies at high hydrocarbon concentrations. Thus, there is a trade-off between the cycle efficiency and the hydrocarbon mass fraction. Finally, the three best working fluid mixtures, 0.3CO2/0.7propane, 0.7CO2/0.3neo-pentane and 0.8CO2/0.2pentane, were further analyzed according to their cycle characteristics. The 0.3CO2/0.7propane mixture working fluid had the best cycle efficiency for these conditions.

Converting a commercial scroll compressor into an expander: experimental and analytical performance evaluation

The development of low cost small scale Organic Rankine Cycle (ORC) has a very interesting potential in generating electricity using low temperature waste heat sources. Moreover, HVAC companies could significantly extend their market if a commercial scroll compressor can be converted into an expander using similar units. Therefore, this work reports experimental test funded by an Italian HVAC company on a scroll compressor modified to work as scroll expander in a non-regenerative cycle and a subcritical fluid regime, aimed at reducing system cost and complexity. The scroll expander has been tested with its fluid R410A in a ORC cycle in order to obtain the isentropic efficiency of the scroll expander (0.5) and the pump (0.4).On the basis of the experimental tests, a model accomplished by means of MATLAB/CoolProp has been set up to evaluate the performance of the ORC group to achieve 10 kWe as target power output. Four operative fluids have been simulated, i.e. R245fa, R134a, R1234yf, R1234ze, fixing 100°C as evaporating temperature and considering the condenser temperature in the range 20-50°C. The results have showed that R245fa is the most promising working fluid since there is a higher expansion ratio within lower pressure values. As a consequence, not only a lower mass flow rate is necessary, but overall a lower pump consumption is needed, reaching greater overall conversion efficiency (about 6.5% with condensing temperature of 20°C) and power. Thus, a commercial heat pump scroll compressor can be effectively converted into an expander. The fluid selection shows that the most common ORC fluid can be used with relative low performance but at low cost and easy management.

Techno-economic analysis of novel working fluid pairs for the Kalina cycle

The organic Rankine cycle (ORC) and the Kalina cycle (KC) are well established thermodynamic concepts for decentralized power generation based on waste heat at low and medium temperature level. In a previous exergetic analysis, it has been shown that second law efficiency of KC can be increased by applying alcohol/alcohol mixtures as working fluid instead of ammonia/water. The aim of this work is to provide a detailed evaluation of operational parameters of a novel ethanol/hexanol mixture as a working fluid for the KC. Therefore, process simulations in ASPEN PLUS V8.0 are conducted. As a benchmark the KC with standard working fluid ammonia/water and the ORC are examined. Next to thermodynamic aspects, a techno-economic evaluation of the KC and the ORC is conducted. For 200 °C, 300 °C and 400 °C heat source temperature the pressure, power output, heat exchange capacity and the size parameter are analyzed. Compared to ammonia/water alcohol/alcohol mixtures offer an up to 1.5 times higher power output, an up to 66.6 % lower pressure and heat exchange capacity, but lead to 5.6 times higher size parameters. Compared to the KC, the subcritical ORC leads to an up to 3.4 % lower power output. The heat exchange capacity is at least 33.3 % and the size parameter up to 6.3 times lower. For the considered concepts, ammonia/water leads to the lowest specific cost with 619.4 €/kW. However, the cost estimation for the KC is related to several uncertainties. Therefore, the pure fluid ORC should be preferred in terms of techno-economic considerations.To sum up, the results show that the pure fluid ORC should be preferred in terms of techno-economic considerations.

A multi-objective optimization approach for the selection of working fluids of geothermal facilities: Economic, environmental and social aspects

The selection of the working fluid for Organic Rankine Cycles has traditionally been addressed from systematic heuristic methods, which perform a characterization and prior selection considering mainly one objective, thus avoiding a selection considering simultaneously the objectives related to sustainability and safety. The objective of this work is to propose a methodology for the optimal selection of the working fluid for Organic Rankine Cycles. The model is presented as a multi-objective approach, which simultaneously considers the economic, environmental and safety aspects. The economic objective function considers the profit obtained by selling the energy produced. Safety was evaluated in terms of individual risk for each of the components of the Organic Rankine Cycles and it was formulated as a function of the operating conditions and hazardous properties of each working fluid. The environmental function is based on carbon dioxide emissions, considering carbon dioxide mitigation, emission due to the use of cooling water as well emissions due material release. The methodology was applied to the case of geothermal facilities to select the optimal working fluid although it can be extended to waste heat recovery. The results show that the hydrocarbons represent better solutions, thus among a list of 24 working fluids, toluene is selected as the best fluid.