Performance Of Organic Rankine Cycle Research Articles

Modelling and parametric analysis of small-scale axial and radial-outflow turbines for Organic Rankine Cycle applications

The existing literature pays limited attention to the design and 3D analysis of small-scale axial and radial–outflow turbines that can be utilised in Organic Rankine Cycles (ORC) for power generation with a low–temperature (<100°C) heat source and low mass flow rate. Turbine efficiency significantly affects an ORC’s efficiency because the turbine is considered a key component of the ORC. Therefore, obtaining high cycle thermal efficiency requires high turbine efficiency and power output. This work presents an integrated mathematical model for developing efficient axial and radial-outflow (centrifugal) turbines using a range of organic working fluids (R141b, R245fa, R365mfc, isobutane and n–pentane). This mathematical approach integrates mean-line design and 3D CFD analysis with ORC modelling. The ANSYSR17CFX is used to predict 3D viscous flow and turbine performance. To achieve accurate prediction, the ORC/turbines model uses real gas formulations based on the REFPROP database. The results showed that the axial turbine performed better, with efficiency of 82.5% and power output of 15.15kW, compared with 79.05% and 13.625kW from the radial–outflow turbine, with n-pentane as the working fluid in both cases. The maximum cycle thermal efficiency was 11.74% and 10.25% for axial and radial-outflow turbines respectively with n-pentane as the working fluid and a heat source temperature of 87°C. The large tip diameter of the axial turbine was 73.82mm compared with 108.72mm for the radial-outflow turbine. The predicted results are better than others in the literature and highlight the advantages of the integrated approach for accurate prediction of ORC performance based on small-scale axial and radial-outflow turbines.

Integrated thermoeconomic optimization of standard and regenerative ORC for different heat source types and capacities

An integrated thermoeconomic optimization approach of standard and regenerative Organic Rankine Cycles (ORC) is presented. The impact of different heat source temperatures and capacities is investigated. Different expander types and configurations are selected based on technical operation limits. Four working fluids are examined, in accordance with the latest European Union F-gas regulations. The optimization variables are the evaporating pressure, condensation temperature, pinch point in the heater and cooler and recuperator effectiveness. Cyclopentane was found to be the optimal fluid for small scale applications, while for larger ones propane and R1234ze exhibit the best performance. The expander selection, mainly affected by the operating pressure and condensation temperature, plays a predominant role on the economic performance of ORCs. The pinch point in the heater is also an important variable, while the pinch point in the cooler is comparatively insignificant. The recuperative ORC was found to be appealing for high temperature, closed loop (CL) systems and unfavorable for open loop (OL) ones. An economy of scale effect was observed for small system capacities, becoming decreasingly significant for larger scales. The specific investment cost is highly variable, ranging from 15,067 €/kWe (R1234ze, Ths = 100 °C, 1.41 kWe) to 770 €/kWe (propane, Ths = 300 °C, 110.58 kWe).

Impact of molecular structure of working fluids on performance of organic Rankine cycles (ORCs)

Performance of organic Rankine cycle (ORC) power plants can be predicted based on the molecular structures of working fluids.

Energetic Investigation of Organic Rankine Cycles (ORCs) for the Exploitation of Low-Temperature Geothermal Sources – A possible application in Slovakia

Abstract: The paper aims at analysing the energetic performances of Organic Rankine Cycles (ORCs) for the exploitation of low temperature heat sources. Specifically, the attention has been focused on low-enthalpy geothermal energy for small-scale applications. To this purpose a thermodynamic model has been developed and a parametric investigation has been performed considering different organic fluids (isobutane, isopentane, and R245ca). Saturated conditions at the expander inlet have been adopted and the effect of the internal regenerator on the system performances has been evaluated. The investigation highlights the large impact of the evaporation temperature on the ORC behaviour and the positive influence of the internal regenerator at the highest temperatures. Conversely, the effect of the internal heat exchanger reduces when the evaporation temperature decreases. Furthermore, a possible application in the Kosice Basin (Slovakia) has been analysed. The investigation demonstrates that the organic Rankine cycle represents an interesting option for efficient valorisation of low-enthalpy geothermal resources and electricity production in small-scale applications.

Performance investigation of reciprocating pump running with organic fluid for organic Rankine cycle

Organic Rankine cycles (ORC) are used to convert lowgrade heat sources into power. Current research and development investigate small scale and variable heat sources application such as waste heat recovery. Many experimental data on ORC are available. Feed-pump performances achieved are lower than expected and some authors reported cavitation issue. Pump performance has a non-negligible impact over the ORC performance, especially for transcritical cycles. Operations of diaphragm pumps in three different test benches with different fluid and pump size are analyzed. A semi-empirical model of the pump power chain is proposed and validated. Energetic analysis show highlevel of losses in the variable speed drive and electric motor, mainly due to design oversizing. Then a model and analysis of reciprocating pump volumetric efficiency is proposed, taking into account fluid properties. Finally, cavitation limits in different conditions are calculated. Required Net Positive Suction Head (NPSHr) calculated for R134a are found to be in accordance with manufacturer limits for water. Pump vibration sensor could be used for cavitation monitoring. This work gives information for ORC feed-pump simulation, design and operation.

Three dimensional optimization of small-scale axial turbine for low temperature heat source driven organic Rankine cycle

Advances in optimization techniques can be used to enhance the performance of turbines in various applications. However, limited work has been reported on using such optimization techniques to develop small-scale turbines for organic Rankine cycles. This paper investigates the use of multi-objective genetic algorithm to optimize the stage geometry of a small-axial subsonic turbine. This optimization is integrated with organic Rankine cycle analysis using wide range of high density organic working fluids like R123, R134a, R141b, R152a, R245fa and isobutane suitable for low temperature heat sources <100°C such as solar energy to achieve the best turbine design and highest organic Rankine cycle efficiency. The isentropic efficiency of the turbine in most of the reported organic Rankine cycle studies was assumed constant, while the current work allows the turbine isentropic efficiency to change (dynamic value) with both operating conditions and working fluids. Three-dimensional computational fluid dynamics analysis and multi-objective genetic algorithm optimization were performed using three-dimensional Reynolds-averaged Navier-Stokes equations with k-omega shear stress transport turbulence model in ANSYSR17-CFX and design exploration for various working fluids. The optimization was carried out using eight design parameters for the turbine stage geometry optimization including stator and rotor number of blades, rotor leading edge beta angle, trailing edge beta angle, stagger angle, throat width, trailing half wedge angle and shroud tip clearance. Results showed that using working fluid R123 for a turbine with mean diameter of 70mm, the maximum isentropic efficiency was about 88% and power output of 6.3kW leading to cycle thermal efficiency of 10.5% showing an enhancement of 14.08% compared to the baseline design. Such results highlight the potential of the 3D optimization technique to improve the organic Rankine cycle performance.

Development of micro-scale axial and radial turbines for low-temperature heat source driven organic Rankine cycle

Most studies on the organic Rankine cycle (ORC) focused on parametric studies and selection working fluids to maximize the performance of organic Rankine cycle but without attention for turbine design features which are crucial to achieving them. The rotational speed, expansion ratio, mass flow rate and turbine size have markedly effect on turbine performance. For this purpose organic Rankine cycle modeling, mean-line design and three-dimensional computational fluid dynamics analysis were integrated for both micro axial and radial-inflow turbines with five organic fluids (R141b, R1234yf, R245fa, n-butane and n-pentane) for realistic low-temperature heat source <100°C like solar and geothermal energy. Three-dimensional simulation is performed using ANSYSR17-CFX where three-dimensional Reynolds-averaged Navier-Stokes equations are solved with k-omega shear stress transport turbulence model. Both configurations of turbines are designed at wide range of mass flow rate (0.1–0.5)kg/s for each working fluid. The results showed that n-pentane has the highest performance at all design conditions where the maximum total-to-total efficiency and power output of radial-inflow turbine are 83.85% and 8.893kW respectively. The performance of the axial turbine was 83.48% total-to-total efficiency and 8.507kW power output. The maximum overall size of axial turbine was 64.685mm compared with 70.97mm for radial-inflow turbine. R245fa has the lowest overall size for all cases. The organic Rankine cycle thermal efficiency was about 10.60% with radial-inflow turbine and 10.14% with axial turbine. Such results are better than other studies in the literature and highlight the potential of the integrated approach for accurate prediction of the organic Rankine cycle performance based on micro-scale axial and radial-inflow turbines.

Experimental investigation on the performance of ORC power system using zeotropic mixture R601a/R600a

An experimental study on the practical performance of organic Rankine cycle (ORC) system using zeotropic mixture is performed by using a small scale ORC power generation experimental setup. R601a/R600a is selected as the working fluid. The effects of mixture composition, heat source temperature, and working fluid flow rate on the performance of ORC system are investigated. The experimental results indicate that the net power output first increases and then decreases as the R600a concentration increases. The optimal mixture composition with the maximum net power output is 0.6/0.4 (mass fraction) at the heat source temperature of 115°C. The net power output of R601a/R600a (0.6/0.4) is higher than that of R601a by 25%, indicating that the performance of ORC system can be clearly improved by using the zeotropic mixture. For a fixed working fluid flow rate, both net power output and thermal efficiency first decrease slowly and then drop sharply with the decrease of the heat source temperature. The appropriate superheat degree of R601a/R600a is in the range of 15 to 20°C when the heat source temperature has a small variation. In addition, the optimal working fluid volume flow rates yielding the maximum net power output are obtained for different compositions of R601a/R600a. The experimental results in the study can be of great significance for the design and operation of ORC power system using zeotropic mixture. Copyright © 2016 John Wiley & Sons, Ltd.

Multi-objective optimization and grey relational analysis on configurations of organic Rankine cycle

Concerning the comprehensive performance of organic Rankine cycle (ORC), comparisons and optimizations on 3 different configurations of ORC (basic, regenerative and extractive ORCs) are investigated in this paper. Medium-temperature geothermal water is used for comparing the influence of configurations, working fluids and operating parameters on different evaluation criteria. Different evaluation and optimization methods are adopted in evaluation of ORCs to obtain the one with the best comprehensive performance, such as exergoeconomic analysis, bi-objective optimization and grey relational analysis. The results reveal that the basic ORC performs the best among these 3 ORCs in terms of comprehensive thermodynamic and economic performances when using R245fa and driven by geothermal water at 150°C. Furthermore, R141b shows the best comprehensive performance among 14 working fluids based on the Pareto frontier solutions without considering safe factors. Meanwhile, R141b is the best among all 14 working fluids with the optimal comprehensive performance when regarding all the evaluation criteria as equal by using grey relational analysis.

Comparison of a Basic Organic Rankine Cycle and a Parallel Double-Evaporator Organic Rankine Cycle

ABSTRACTThe applications of zeotropic mixtures and multi-evaporator systems are two viable options to improve the performance of the organic Rankine cycle (ORC). This paper conducts the thermo-economic comparison of a basic ORC with R245fa/R600a and a parallel double-evaporator organic Rankine cycle (PDORC) with R245fa. Four indicators are used to evaluate the system performance: net power, cycle efficiency, area of heat exchangers, and area of heat exchangers per net power output. Submodels of condensers and evaporators are established specially for pure organic fluids and zeotropic mixtures. The performance optimization using genetic algorithm is conducted to compare the two systems quantitatively. The optimization indicates a zeotropic mixture is more profitable than a pure work fluid in a basic ORC with a worthy additional investment of heat exchanger. Though PDORC can increase net power obviously, it would decrease the thermo-economic performance of ORC.

Study of efficiency of a multistage centrifugal pump used in engine waste heat recovery application

A test bench of a multistage centrifugal pump was constructed using R123 as working fluid in simulative organic Rankine cycle (ORC) conditions. Experimental results of the pump under various operating conditions were obtained based on controlled working frequency and mass flow rate. The effect of the key pump parameters on the ORC performance was analyzed in this study. In addition, the control strategy of the pump was presented. Results show that the overall pump efficiency was between 15% and 65.7%. The outlet pressure, pump efficiency, and ORC thermal efficiency increased with the working frequency of the pump. The mass flow rate needed to be regulated as the frequency became increasingly high. The maximum thermal efficiencies of the ORC system corresponding to various working frequencies of the pump were observed. Furthermore, back work ratio (BWR) can reach up to 0.45 with the increase of the evaporating temperature of the ORC system. Pumping power should not be neglected for small-scale ORC applications, and pump efficiency assumptions should be dependent on experiments. Low pump efficiency affected the increase of the thermal efficiency and net power of the ORC system. The superheat degree was also discussed.

Comparison of Organic Rankine Cycle Systems under Varying Conditions Using Turbine and Twin-Screw Expanders

A multi-variable optimization program has been developed to investigate the performance of Organic Rankine Cycles (ORCs) for low temperature heat recovery applications using both turbine and twin-screw expanders when account is taken of performance variation due to changes in ambient conditions. The cycle simulation contains thermodynamic models of both types of expander. In the case of the twin-screw machine, the methods used to match the operation of the expander to the requirements of the cycle are described. The performance of turbine expanders in a superheated ORC has been modelled using correlations derived from operational data for single stage reaction turbines to predict the turbine efficiency at “off-design” conditions. Several turbine configurations have been considered including variable nozzle area and variable speed. The capability of the cycle model has been demonstrated for the case of heat recovery from a steady source of pressurized hot water at 120 °C. The system parameters are optimised for a typical operating condition, which determines the required size of heat exchangers and the expander characteristics. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on the system performance, and the seasonal variation in net power output for the turbine and twin-screw ORC systems. A case study is presented for a low temperature heat recovery application with system electrical power output of around 100 kWe at design conditions. The results indicate that similar overall performance can be achieved for ORC systems using either type of expander.

Unsteady analysis of a bottoming Organic Rankine Cycle for exhaust heat recovery from an Internal Combustion Engine using Monte Carlo simulation

An optimization model is developed to maximize the net power output of a bottoming Organic Rankine Cycle (ORC) with ten working fluids for exhaust heat recovery from an Internal Combustion Engine (ICE) theoretically. The ICE-ORC system is influenced by several unsteady parameters which make it difficult to determine the optimal design parameters. Therefore, we introduce probability density functions in order to investigate the impacts of the ICE power output, the sink temperature and the pinch point temperature difference on the ORC performances. Each unsteady parameter is illustrated to analyze the performances of the ICE-ORC system. Furthermore, Monte Carlo simulation is introduced to investigate the role played by the unsteady parameters, each of which obeys different probability distributions. By these methods, we obtained the convergence values, the frequency distributions and the cumulative probability distributions of various performance parameters. These results can provide valuable suggestions for the design of ICE-ORC system.

Theoretical analysis and comparison of rankine cycle and different organic rankine cycles as waste heat recovery system for a large gaseous fuel internal combustion engine

As oil crisis and environment pollution problem become increasingly prominent, gaseous fuel internal combustion engines (gas engines) have been valued more and more because of the great resources potential and the feature of low pollution of gaseous fuel. However, their average efficiency is just about 30–40% wasting large amount of energy by exhaust, cooling water and so on. Therefore, it is an effective way of improving the efficiency to recover waste heat. Both the RC (steam Rankine Cycle) and the ORC (Organic Rankine Cycle) are considered as quite suitable methods of WHR (waste heat recovery), especially for large engines. As the WHR system for a specific large gas engine, RC and ORC both have advantages and disadvantages. A gas engine of rated power 1000kW is the studied object in this paper. According to the characters of its waste heat, the WHR systems of RC and ORC with different working fluids, including a two-stage Organic Rankine Cycle (DORC) are built in math model and they are compared from the aspects of recovered power, thermal efficiency, system feasibility and so on, which provides a reference for the engineering application. Finally, the economic performance of ORC, RC and DORC are evaluated and compared.

An innovative organic Rankine cycle approach for high temperature applications

Organic Rankine Cycles (ORC) using toluene and hexamethyldisiloxane (MM) are put forward as a means of improving the efficiency of automotive heavy duty engines, and provide a reference for comparison in this study. Despite an efficiency improvement potential of 4–4.7%, the current ORC approach is not reaching the required fuel savings within the expected costs. As such, innovative pathways to improve the ORC performance and cost-effectiveness are of great importance to the research community. This paper presents a partial solution by means of a conceptual overview and simulation results for ORCs especially tailored for high-temperature applications. A fundamental revision of the heat transfer and expansion characteristics is presented, without increasing the system integration complexity. These characteristics are attributed to the use of formulated organic blends with toluene and MM as a significant blend component. The developed 22 criteria blend screening methodology is presented. Simulation results show that for an equivalent expansion volume flow ratio, and product of heat transfer coefficient and area, the blends offer a 22–24% improvement in the net power. This resulted in a 15–18% cost savings compared to the reference ORC. The simulations were conducted in Aspen HYSYS V8 using the Peng-Robinson and Wilson fluid property packages.

Experimental Study of the Influence of Light Intensity on Solar Organic Rankine Cycle Power Generation System

A low-temperature (&lt;120 °C) solar organic Rankine cycle (ORC) power generation experimental facility is designed and built. The influence of light intensity on the system performance is investigated using the experimental facility. The results indicate that the system efficiency can reach 2.2%. The temperature of heat transfer fluid (HTF) decreases linearly with light intensity (I). However, both system efficiency and thermoelectric efficiency first decrease linearly and then drop sharply as I decreases at working fluid flow rates (Vwf) of 200 and 160 L/hr, while they only decrease slightly with I at Vwf of 120 L/hr. The light intensity of the turning point is 824 W/m2 at Vwf of 200 L/hr, which corresponds to an HTF temperature of 75 °C. In addition, it is found that the influence of light intensity on the performance of ORC becomes stronger for higher working fluid flow rate. Moreover, the light intensity and HTF temperature at the turning point increase with working fluid flow rate. The experimental results are of great significance for the design and operation of low-temperature solar ORC power generation system.

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.

Investigation on working fluids selection for organic rankine cycles with low-temperature heat sources

ABSTRACTThe Influence of mixed and pure working fluids on the performance of organic Rankine cycles (ORCs) is discussed. Specifically, the performance of mixed and pure working fluids is analyzed based on certain characteristics of low-temperature heat source and heat sink. A method of selecting binary zeotropic mixed working fluids that match with different heat sinks is introduced. Thermodynamic processes of ORCs for various heat sources are simulated in Matlab. The performance characteristics of pure and mixed working fluids are compared under different inlet temperatures and temperature gradients of sensible heat sources. The results demonstrate that when the initial temperature of a heat source is lower and its temperature gradient is higher, and the temperature gradient of the heat sink is higher, mixed working fluids have better performance than pure working fluids. However, for the opposite heat source and heat sink situations, pure working fluids perform better. Mixtures with low critical temperature components exhibit the best performance among all working fluids when the temperature gradient of the heat source is large. The analysis also shows that introduction of a recuperator may reduce the cycle efficiency when the heat source temperature is low and the temperature gradient of the heat source is large.

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.

Performance analysis of sequential Carnot cycles with finite heat sources and heat sinks and its application in organic Rankine cycles

The sequential Carnot cycle, where a number of single Carnot cycles are connected in parallel, has been researched for renewable thermal energy utilization because the characteristics of its low grade heat source, are different from those of a conventional one. In this paper, the thermodynamic analysis is conducted on the sequential Carnot cycle which uses not only finite heat sources but also finite heat sinks. Equations for efficiency and power from the whole sequential system are derived from the general theory of thermodynamics and heat transfer. Based on the equation from theory, the performance of the sequential system is calculated in various system conditions, including the ideal situation where the system has an infinite number of Carnot cycles and infinite heat exchanger inventory. In addition, the sequential concept is applied to organic Rankine cycles, which is one of the most used thermodynamic systems that generate work from low grade heat sources. From a simple simulation with properties of real working fluids, the performance of a sequential ORC (organic Rankine cycle) is investigated and its result is compared with that of theoretical sequential Carnot cycles.