Organic Rankine Cycle Model Research Articles

Preliminary design and performance analysis of a centrifugal turbine for Organic Rankine Cycle (ORC) applications

A thermodynamic model of Organic Rankine Cycle (ORC) system and an aerodynamic design model of a centrifugal turbine were conducted in this paper. Thermodynamic performance comparison of ORC using different working fluids (R123, R245fa, R600, R601a, R245ca, R141b) was achieved for a given amount of heat absorbed by the system. For a known heat source term whose thermophysical properties were specified, the aerodynamic and mechanical design of three centrifugal turbines having different stages have been undertaken, in which organic fluid like R123 was used as the working fluid. Three-dimensional modeling and optimization were subsequently applied to improve the design of the centrifugal turbine whose preliminary thermodynamic design was conducted based on one-dimensional theoretical model. The power and efficiency characteristics of the designed centrifugal turbines were then assessed by means of numerical simulations. Numerical results suggested that isentropic efficiency and output work of all the proposed centrifugal turbines operating at design conditions can exceed 82.7% and 330 kW, which satisfy the requirements of ORC system applications.

A Comparative Study of the Effect of Turbocompounding and ORC Waste Heat Recovery Systems on the Performance of a Turbocharged Heavy-Duty Diesel Engine

In this study the influence of utilization of two Waste Heat Recovery (WHR) strategies, namely organic Rankine cycle (ORC) and turbocompounding, have been investigated based on the performance of a heavy-duty diesel engine using 1-D simulation engine code (GT-Power) in terms of Brake Specific Fuel Consumptions (BSFC) at various engine speeds and Brake Mean Effective Pressures (BMEP). The model of a 6-cylinder turbocharged engine (Holset HDX55V) was calibrated using an experimental BSFC map to predict engine exhaust thermodynamic conditions such as exhaust mass flow rate and exhaust temperature under various operating conditions. These engine exhaust conditions were then utilized to feed the inlet conditions for both the ORC and turbocompounding models, evaluating the available exhaust energy to be recovered by each technology. Firstly the ORC system model was simulated to obtain the power that can be generated from the system. Having this additional power converted to useful work, the BSFC was observed to reduce around 2–5% depending upon engine’s speed and BMEP. The initial model of the engine was then modified by considering a second turbine representing turbocompounding heat recovery system. The BSFC was increased due to the back-pressure from the second turbine, but the energy generated from the turbine was sufficient to reduce the BSFC further. However, by application of turbocompounding no improvement in BSFC was achieved at low engine’s speeds. It is concluded that ORC heat recovery system produces a satisfactory results at low engine speeds with both low and high loads whereas at medium and high engine speeds turbocompounding heat recovery system causes higher BSFC reduction.

Working fluid charge oriented off-design modeling of a small scale Organic Rankine Cycle system

Organic Rankine Cycle system is one of the most widely used technique for low-grade waste heat recovery. Developing of dynamic Organic Rankine Cycle models played an increasingly important part in system performance prediction. The present paper developed a working fluid charge oriented model for an small scale Organic Rankine Cycle to calculate the theoretical value of working fluid charge level for the system under rated condition. The two heat exchangers are divided into three different zones and related heat transfer correlations are employed to estimate the length variation of each zones. Steady state models have been applied to describe the performance of pump and expander. Afterwards, an overall solution algorithm based on the established model has been proposed in order to exact simulate the system’s off-design performance. Additionally, the impact of different working fluid charge volumes has also been discussed. Simulation results clearly shows the variation trend of different zones in both heat exchangers, as well as the variation trend of system operating parameters under various expander output work. Furthermore, the highest thermal efficiency can be reached 6.37% under rated conditions with a working fluid charge volume of 34.6kg.

Simultaneous working fluids design and cycle optimization for Organic Rankine cycle using group contribution model

The performance of Organic Rankine Cycle (ORC) is significantly influenced by the used working fluid and the operating condition. Consequently, this paper presents a systematic model for the efficient design of working fluids and the optimization of cycle parameters at the molecular scale, so that optimal working fluids can be identified by simultaneously considering cycle parameters, environmental and safety properties. In the proposed model, working fluids are generated via the combination of groups. The required properties, which consist of thermodynamic, environmental and safety properties, are estimated by the published group contribution methods. Based on these estimated properties, cycle optimizations are performed to obtain the optimal performance of working fluids using an ORC model. Thereafter, optimal working fluids are identified, according to the cycle parameters, environmental and safety properties. Furthermore, working fluids design and cycle optimization for an example are conducted to demonstrate the proposed model. The optimal candidates, namely R254eb, R254cb, are found for the considered example through proposed methodology. The novel working fluids, which are firstly reported in ORC applications, are worth being studied in-depth through time-consuming and expensive experiments.

Experimental and modeling investigation of an organic Rankine cycle system based on the scroll expander

A comprehensive model for organic Rankine cycle (ORC) was presented in this work. Several sub-models were included. Fewer empirical coefficients were needed in the improved expander model than the previous one as several parameters were obtained directly from the actual structure parameters of scrolls. The Dalton's law was used to simulate the pressure in the liquid tank, consisting of partial pressures of non-condensable gas and saturated vapor. A spray cooling tower model was built to take account of the influence of environment temperature and humidity on the ORC operation and performance. The simulations agree well with the tested data. Then, the model was used to analyze the mismatch among components of the cycle and evaluated the potential optimizations to the ORC prototype. For the present ORC system, the model predicts that the output power can be doubled after the optimization measures are taken. The environment temperature and humidity apparently influence the system performance. The effect of humidity becomes significant at high environment temperature. The expander power can be decreased by one third when the humidity is increased from 50% to 90% in summer season. The present work provides the guidance for design and operation of the ORC system.

Dynamic modelling of ORC-based solar thermal power plant integrated with multitube shell and tube latent heat thermal storage system

In this paper, dynamic model for organic Rankine cycle (ORC) based solar thermal power plant integrated with latent heat thermal energy storage (LHTES) system is developed to understand its interaction with other components in the power plant for extending electricity generation after sunset. A multitube shell and tube latent heat thermal storage system is considered, in which phase change material (PCM) is stored in the shell side and heat transfer fluid (HTF) flows through the tubes. The multitube shell and tube latent heat storage is modelled considering an elemental unit obtained by encircling each HTF tube to the extent within which the surrounding PCM and HTF exchanges heat. A reduced order model coupled with the enthalpy technique is developed for analysing heat transfer in the LHTES system during charging and discharging periods. The model is validated with the experimental results reported in literature. Simplified transient energy equations are used to model collector and heat exchanger. Further, a latent heat thermal storage system is designed to generate 200kWt after sunset and the overall performance of the solar thermal power plant is evaluated for ten days of operation under Indian conditions.

Simulation Study of an ORC System Driven by the Waste Heat Recovered from a Trigeneration System

Trigeneration produces power, heat and cooling simultaneously. It utilises the waste heat from generator for heating and generating cooling. From previous studies, it is found that some part of the waste heat recovered by the trigeneration is not being used when it is in operation. It is therefore necessary to find a way to utilise the heat. Integrating an ORC (organic Rankine cycle) system into a micro trigeneration system is studied. The waste heat recovered by the trigeneration system is utilised by the ORC system to generate electrical power. An ORC model is set up in Matlab and a micro trigeneration system based a diesel engine generator in the laboratory at Newcastle University is used as the case study. R245fa is used as the working fluid. The result showed that the ORC system may generate 0.737kW electrical power with 9.25% efficiency at the generator full-load operation point.

An efficient integration strategy for a SOFC-GT-SORC combined system with performance simulation and parametric optimization

The present study aims to establish an integrated modelling and optimization framework for a combined power system comprising fuel cell, gas turbine and Organic Rankine Cycle (ORC). The method is based upon the coupling of a syngas fueled solid oxide fuel cell-gas turbine (SOFC-GT) hybrid model and a bottoming supercritical ORC model developed using gPROMS to efficiently determine the range of appropriate operating conditions. A comprehensive and detailed analysis is carried out to simulate system behavior and to investigate the influence of key design and operating parameters on the system performance. The fuel flowrate and operating temperature of fuel cell, the pressure ratios of turbines, together with the flow split ratio of the ORC cycle are all set to be controllable variables. The results indicate that an electrical efficiency of 66.27% and a CHP efficiency of 88.43% are achievable for the integrated system. The analysis enables us to identify the system design tradeoffs, and therefore to determine optimal integration strategies for complex polygeneration systems.

Uncertainty assessment of equations of state with application to an organic Rankine cycle

ABSTRACTEvaluations of equations of state (EoS) should include uncertainty. This study presents a generic method to analyse EoS from a detailed uncertainty analysis of the mathematical form and the data used to obtain EoS parameter values. The method is illustrated by comparison of Soave–Redlich–Kwong (SRK) cubic EoS with perturbed-chain statistical associating fluid theory (PC-SAFT) EoS for an organic Rankine cycle (ORC) for heat recovery to power from the exhaust gas of a marine diesel engine using cyclopentane as working fluid. Uncertainties of the EoS input parameters including their corresponding correlation structure, are quantified from experimental measurements using a bootstrap method. Variance-based sensitivity analysis is used to compare the uncertainties from the departure function and the ideal-gas contribution. A Monte Carlo procedure propagates fluid parameter input uncertainty onto the model outputs. Uncertainties in the departure function (SRK or PC-SAFT EoS) dominate the total uncertainties of the ORC model output. For this application and working fluid, SRK EoS has less predictive uncertainty in the process model output than does PC-SAFT EoS, though it cannot be determined if this is due to differences in the data for parameter estimation or in the mathematical form of the EoS or both.

Developing a performance evaluation model of Organic Rankine Cycle for working fluids based on the group contribution method

An Organic Rankine Cycle (ORC) model is presented in this paper to easily, quickly, and inexpensively evaluate the performance potentials of various working fluids. When given molecular structure of working fluid, the normal boiling temperature, critical properties, liquid density and ideal gas heat capacity can be obtained via the existing group contribution methods (GCMs). Other properties required in the ORC model are calculated out by the thermodynamic relationships with the estimated properties of GCMs. Based on the calculated properties, four basic processes of the ORC including compression, evaporation, expansion and condensation are modeled. Meanwhile, the cycle parameters of 21 potential working fluids for typical ORC operating conditions are obtained from the molecular structures by the developed model. Compared with the REFPROP, the model shows sufficient accuracy for engineering purposes. The relative errors of thermodynamic properties and cycle parameters are less than 10% for most of working fluids. It is concluded that the proposed model can estimate the ORC characteristics of any pure working fluid only based on its molecular structure. Thus, a large amount of working fluids formed by the combination of groups can be directly screened by this model, and the optimal working fluids can be identified for a quick assessment in engineering field.

Economical evaluation and optimization of organic Rankine cycle with mixture working fluids using R245fa as flame retardant

A detailed economic model of organic Rankine cycle (ORC) system using mixtures as working fluid has been built. By using R245fa as the flame retardant, R245fa/Isopentane, R245fa/Pentane, R245fa/Cisbutene and R245fa/Butene were considered as the potential working fluids. The heat source temperatures investigated were 373.15–453.15K. The optimization of 4 mixture and 5 pure working fluids were carried out with the objective function of minimizing the Electricity Production Cost (EPC). Genetic Algorithm (GA) was employed as the optimized method. Different from the most of the optimization researches about ORC with mixture working fluid, the mixture composition was set as one of the variables in the present work. The results show that the mixture working fluid is more economic-efficient than pure working fluid. The lower EPC value obtained by mixture working fluid was mainly due to the decrease of capital cost, which is essentially caused by the decrease of capital cost of the evaporator. R245fa/Isopentane and R245fa/Pentane were recommended as the optimal working fluids for their minimum EPC values. In addition, the correlations between the optimized mixture composition and heat source temperature were obtained.

Modelling of organic Rankine cycle efficiency with respect to the equivalent hot side temperature

An indicator, namely equivalent hot side temperature (TEHST) is proposed for the organic Rankine cycle (ORC). TEHST is derived from ideal thermodynamic process, but can denote the efficiency of irreversible ORC. Study on 27 fluids shows that given the operating conditions, fluid of higher TEHST generally offers higher ORC efficiency. This relationship is stronger and more universal than those established with respect to the critical temperature, boiling point temperature, Jacobs number and Figure of Merit. An ORC model by the method of error transfer and compensation is further built, in which the efficiency is quantitatively correlated with TEHST. Unlike the conventional ORC efficiency model, this one consists of thermodynamic parameters on the liquid/vapor curve and is independent on fluid properties at superheated state, and hence is more convenient. It has high accuracy especially for basic ORC and the relative deviation of the estimated efficiency from that calculated by the conventional model is from −0.7% to 3.4%. The novel model is applied for the thermodynamic performance prediction of a recently developed fluid of HFO1336mzzZ based on the phase equilibrium data. The results indicate HFO1336mzzZ is more efficient than R245fa on the conditions of high evaporation temperature and low pump efficiency.

A novel solar-geothermal trigeneration system integrating water desalination: Design, dynamic simulation and economic assessment

In this paper, an innovative solar-geothermal polygeneration system is investigated. The system supplies a small community with electricity, desalinated water and space heating and cooling through a district network. The hybrid multi-purpose plant, based on an Organic Rankine Cycle (ORC) supplied by medium-enthalpy geothermal energy and by solar energy; this latter is provided by Parabolic Trough Collectors (PTC). The geothermal brine is first used to drive the ORC loop, then to provide space heating at around 85÷90 °C (in the winter), or cooling (in the summer, by means of a single-effect absorption chiller). Finally, the geothermal brine drives a Multi-Effect Distillation (MED) system, where seawater is converted into freshwater. For such a system, a dynamic simulation model was developed in TRNSYS environment. In particular, the ORC model, developed in Engineering Equation Solver (EES), was based on zero-dimensional energy and mass balances and includes specific algorithms to evaluate the off-design performance. Similarly, a novel model of the MED unit was developed in EES. Suitable control strategies were implemented for the optimal management of system. The energy and economic performance of the system under analysis was investigated, using different time bases (day, week, month, year). Finally, a sensitivity analysis was performed to determine the set of system, design/control parameters able to minimize the simple payback period. The results showed that the novel system is highly flexible and efficient. On the other hand, a significant capital cost must be taken into account, so that the system is economically profitable only when the majority of the energy available for heating and cooling purposes is actually used.

Part-Load Performance Prediction and Operation Strategy Design of Organic Rankine Cycles with a Medium Cycle Used for Recovering Waste Heat from Gaseous Fuel Engines

The Organic Rankine Cycle (ORC) is regarded as a suitable way to recover waste heat from gaseous fuel internal combustion engines. As waste heat recovery systems (WHRS) have always been designed based on rated working conditions, while engines often work under part-load conditions, it is quite significant to analyze the part-load performance and corresponding operation strategy of ORC systems. This paper presents a dynamic model of ORC with a medium cycle used for a large gaseous fuel engine and analyzes the effect of adjustable parameters on the system performance, giving effective control directions under various conditions. The results indicate that the intermediary fluid mass flow rate has nearly no effect on the output power and thermal efficiency of the ORC, while the mass flow rate of working fluid has a great effect on them. In order to get a better system performance under different working conditions, the system should be operated with the working fluid mass flow rate as large as possible, but with a slight degree of superheating. Then, with the control of constant superheat degree at the end of the heating process, the performance of the combined system that consists of ORC and the engine at steady state under seven typical working conditions is also analyzed. The results indicate that the energy-saving effect of WHRS becomes worse and worse as the working condition decreases. Especially at 40% working condition the WHRS nearly has no energy-saving effect anymore.

Development and a Validation of a Charge Sensitive Organic Rankine Cycle (ORC) Simulation Tool

Despite the increasing interest in organic Rankine cycle (ORC) systems and the large number of cycle models proposed in the literature, charge-based ORC models are still almost absent. In this paper, a detailed overall ORC simulation model is presented based on two solution strategies: condenser subcooling and total working fluid charge of the system. The latter allows the subcooling level to be predicted rather than specified as an input. The overall cycle model is composed of independent models for pump, expander, line sets, liquid receiver and heat exchangers. Empirical and semi-empirical models are adopted for the pump and expander, respectively. A generalized steady-state moving boundary method is used to model the heat exchangers. The line sets and liquid receiver are used to better estimate the total charge of the system and pressure drops. Finally, the individual components are connected to form a cycle model in an object-oriented fashion. The solution algorithm includes a preconditioner to guess reasonable values for the evaporating and condensing temperatures and a main cycle solver loop which drives to zero a set of residuals to ensure the convergence of the solution. The model has been developed in the Python programming language. A thorough validation is then carried out against experimental data obtained from two test setups having different nominal size, working fluids and individual components: (i) a regenerative ORC with a 5 kW scroll expander and an oil flooding loop; (ii) a regenerative ORC with a 11 kW single-screw expander. The computer code is made available through open-source dissemination.

Maximizing ORC performance with optimal match of working fluid with system design

This article compares constrained system design to non-constrained system design for the basic Organic Rankine Cycle (ORC). The 12 working fluids studied include eight dry-type, three isentropic, and one wet-type fluids. The ORC model was developed using Aspen HYSYS® and validated with data obtained from the literature. The constrained design compared the performance of working fluids for a fixed heat exchanger and turbine configuration. A non-constrained design was studied by altering the design specifications for the heat exchangers and turbine to match the working fluid. An energy and exergy analysis was performed using first and second law efficiency. The exergy analysis was also used to study exergy destruction across the ORC components. Cost analysis was performed by comparing the levelized cost of electricity (LCOE) for each working fluid in both designs.It was observed that non-constrained design favored working fluids with higher critical temperatures. Switching from constrained to non-constrained design lowered the LCOE for higher critical temperature working fluids such as R601, R601a, R123, R245ca, R245fa, R600, and R236ea. R245ca, R601, and R236ea show 11%, 10%, and 9% decrease in LCOE, respectively. No significant change in efficiency is observed for lower critical temperature working fluids such as R236fa and R134a. Also, no increase in net power was observed for lower critical temperature working fluids, suggesting that modifying design does not affect the performance of ORC. LCOE increased for R600a, R152a, and R227ea and remained unchanged for R236fa and R134a.

Dynamic modelling and validation of a commercial scale geothermal organic rankine cycle power plant

The organic rankine cycle (ORC) is a heat recovery technology used for renewable energy generation such as the large-scale production of electricity from geothermal resources. In this paper a dynamic model of a commercial-scale geothermal ORC is developed in process simulation software and validated against plant data. The difference in power output between the model and plant was found to be 0.24% with a standard deviation of 1.40% of the average power output. The results from model validation are used to suggest improvements to the model, which is intended to be used for further investigation of optimisation and process control.

Dynamic Model of a Multi-Evaporator Organic Rankine Cycle for Exhaust Heat Recovery in Automotive Applications

Abstract: This paper presents a dynamic model of an organic Rankine cycle (ORC) for exhaust heat recovery (EHR) in automotive applications. EHR converts the thermal energy of the exhaust gas into mechanical power. Dynamic components like evaporator, condenser, etc. are modeled based on physical conservation laws given as partial differential equations (PDE). The system involves two parallel evaporators coupled via an open t-piece which is handled numerically efficiently by introduction of algebraic constraints. An integral method improves the evaluation of the fluid properties which partly contain discontinuities. They can have negative effects on accuracy and simulation performance if evaluated inappropriately. The model is implemented by signal-based programming suitable for simulation in Matlab/Simulink. This enables convenient software-in-the-loop (SIL) engineering since many control unit producers provide rapid prototype programming via Matlab/Simulink. A simplified model is proposed for the increase of robustness and simulation performance or for application of controller design methods. Simulation shows both the entire ORC system compared measurements from a Daimler prototype truck, and comparison between complex and simplified model.

SIMULATION OF ORGANIC RANKINE CYCLE THROUGH FLUEGAS TO LARGE SCALE ELECTRICITY GENERATION PURPOSE

A simulation model of Organic Rankine Cycle (ORC) was developed with HYSYS software driven by R245fa, with NOVEC7000 and R141b as working fluids and Fluegas of boilers as a heat source of shell and tube Heat Exchanger to generate large scale electricity. The initial working condition was in subcooled liquid and steady state condition. R141b was found to generate the highest electricity power increment in specific mass flow rates and inlet pressures of Expander because of approaching its critical temperature to inlet Fluegas temperature. Howeever, in terms of economic considerations and cost of shell and tube Heat Exchanger that related to total heat transfer capacity, NOVEC7000 is the optimum selection. Furthermore, R245fa has the highest total effiiciency of ORC compared with other working fluids in this study.

Mass-conserving dynamic organic Rankine cycle model to investigate the link between mass distribution and system state

This study presents a mass-conserving dynamic numerical model to capture dynamic response of ORCs (organic Rankine cycles) when the system experiences a change in expander's rotational speed, pump's capacity factor, and conditions of hot and cold heat transfer fluids. ORC's dynamic response is tracked specifically considering evaporator pressure, condenser pressure, degree of superheating at the evaporator exit, degree of sub-cooling at the condenser exit, and the mass distribution along evaporator, condenser and liquid receiver tank. The developed model is novel due to the way subcomponent models are integrated together. Specifically, this integration includes fully coupled tank and condenser models. The model is validated against an experimental benchmark study for various steady state conditions and further verified considering mass and energy conservation principles. A parametric study is carried out to identify parameters which can be used for devising a new autonomous control strategy for organic Rankine cycles. It is illustrated through simulations that the mass distribution over ORC sub-components are linked to the system's overall state; and both the liquid level in the tank and the degree of sub-cooling at the tank exit can be used to devise a control system by changing pump's capacity factor and expander's rotational speed.