Small-scale Organic Rankine Cycle Research Articles

Numerical analysis-based performance assessment of the small-scale organic Rankine cycle turbine design for residential applications

The organic Rankine cycle (ORC) is one of the most common ways to use energy resources to utilise waste heat. Integrating ORC with other systems, such as cooling or heating applications, is also helpful for residential uses. In the ORC system, the turbines are the most critical component determining the system's efficiency. This study conducted a computational fluid dynamics study on a regenerative flow turbine that can be used in small-scale organic Rankine cycle systems. For the higher temperature applications, the analyses were performed using n-pentane working fluid for 1500 and 3000 rpm turbine speeds, inlet temperature between 475–550 K, outlet pressure between 0.9–1.2 MPa, and mass flow rates between 0.5–1.0 kg/s. As a result of the analysis, the isentropic efficiency varies between 4.97 % and 12.52 %. It was concluded that the output power obtained from the turbine varies between 0.53 kW and 2.96 kW. The analyses were also performed for lower temperature (363 and 393 K), pressure (0.05 MPa and 0.3 MPa) and mass flows (0.1 to 0.25 kg/s) applications. In this case, the isentropic efficiency of the turbine varies between 6.23 % and 14.52 %, whereas the power generation varies between 0.068 and 0.521 kW.

Organic Rankine Cycle (ORC) Systems: A fundamental Overview of Small-scale Applications Fuelled by Low-grade Heat Sources

Environmental issues shift energy production from conventional methods to new and more efficient alternatives. One of these alternatives is the use of organic Rankine cycles (ORC) in low-grade heat sources to generate both heat and power at small scales. Among different technologies available for this purpose, ORC-based systems seem to be the most suitable and promising option due to their simplicity and versatility. Thus, such systems have been investigated intensively. However, current studies often focus on only one aspect of these systems due to the massive research scale in this field. Therefore, this study aims to provide a fundamental and holistic overview to evaluate ORC-based low-heat sourced and small-scale applications from multiple perspectives. As a result, the basic operating principles and application areas of ORCs, selection and design criteria of their working fluids and all other system components, methods of improving their performance, and other thermodynamic cycles that can be ORC alternatives are examined in detail. The results of this study show that ORC applications can enable small-scale combined heat and power generation, while geothermal and solar energy sources have the potential to scale the size of such applications up to kW capacities. The results also showed that dry & isentropic fluids and vane & scroll expanders are the most suitable refrigerant and expander types, respectively, for small-scale ORC applications. Furthermore, the implications of all findings are critically discussed.

Multi-condition operating characteristics and optimization of a small-scale ORC system

The multi-condition operating characteristics of an organic Rankine cycle (ORC) are crucial to the development of the practical unit. This work developed a steady-state model of a small-scale ORC prototype built and tested in the lab. The influence of heat source/sink parameters, component design, and operation strategies are analyzed. It is found that the optimal output power and thermal efficiency correspond to the similar expander rotating speed but diverse working fluid mass flow rate. The higher mass flow rate is beneficial to the output power whereas deteriorates the thermal efficiency. Heat source temperature and flow rates mainly affect the unit output power while the ambient temperature and humidity influence both the output power and thermal efficiency. The condenser fouling resistance leads to a significant reduction of expander output power and system thermal efficiency. However, the effect of the evaporator fouling resistance could be neglected due to its sufficient redundant area. The long-term climate conditions and operating strategies profoundly impact the unit's off-design performance. The annual constant output power operation can be achieved by adjusting the working fluid mass flow rate. The thermal efficiency of the unit is damaged while pursuing the output power stability in winter and spring.

Numerical simulation and low speed experiment of a low partially admitted rate axial turbine for small scale organic Rankine cycle

High rotating speed is a prominent feature of turbine used for small-scale organic Rankine cycle (ORC) system, which can bring the problems of safety, reliability, sealing, vibration, economics, etc. Partial admission method has potential to reduce the rotating speed of small turbine. In this study, an axial turbine with low partially admitted rate of 0.033 was tested in ORC loop using R1233zde as working fluid, and numerically researched using Ansys CFX software with k-omega turbulence model. Results showed that the specific powers of turbine resulting from CFD should be corrected with losses to match well with the experimental results. Losses could reduce the optimal rotating speed of turbine. The optimal rotating speed, mass flow rate, power and efficiency of turbine under rated condition were 8000 RPM, 115.8 g/s, 611.8 W and 35.8 %, respectively. The corresponding power and thermal efficiency of ORC were 551.0 W and 2.15 %. The influences of inlet and back pressures on the performance of turbine were different. With constant inlet pressure and temperature, the mass flow rate kept almost unchanged after chocking, but the power continued to increase. Under allowable conditions, reducing back pressure of turbine played an important role in improving turbine performance.

Experimental validation of a 0.3 kW ORC for the future purposes in the study of low-grade thermal to power conversion

Organic Rankine cycle (ORC) is a promising development direction for energy conservation and environmental protection, as it can effectively convert low-temperature waste heat into mechanical work. This article focuses on the experimental performance study of what is likely the world's smallest scroll expander used in a 0.3 kW micro-ORC system. The system uses R1233zd-E as the working fluid, and the expander is repurposed from a scroll compressor of an air conditioner, with a displacement of 11.1 mL/rev. In the first part of the experiments, the influence of the system pressure difference and the expander rotational speed on the system performance is investigated while keeping the expander inlet superheating fixed. The maximum system efficiency, expander efficiency, and power output were 5.46 %, 72.4 %, and 0.266 kW, respectively. In the second part of experiments, the effect of changing the heating source temperature on the system is explored while keeping the expander rotational speed and inlet pressure fixed. As the superheat increased, both the expander and cycle efficiency increased. In addition, this study combines the experimental results with previous data on small-scale ORC systems and discusses the similar trends, indirectly demonstrating that the 0.3 kW micro-ORC can achieve similar behavior as predicted in a large-scale ORC.

Small-Scale Organic Rankine Cycle Performance Using an Additional Heat Exchanger

Small-Scale Organic Rankine Cycle Performance Using an Additional Heat Exchanger

Control strategies of pumps in organic Rankine cycle under variable condensing conditions

The working medium pump and cooling water pump are important components in organic Rankine cycle system. However, they have not received as much attention as the expander and working medium. Simultaneously, the dramatic changes in condensing environment deviate the organic Rankine cycle working point from the designed operating conditions. Hence, this paper focused on the performance improvements and control strategies of working medium pump and cooling water pump in small-scale organic Rankine cycle under variable condensing conditions. A model was established to predict the performances of an organic Rankine cycle system with R245fa, especially the pumps in the system, under variable condensing conditions. The results showed that the overall efficiency of multi-stage centrifugal pump was between 14% and 39% at an estimated speed of approximately 1600 RPM to 2328 RPM. In winter conditions, the back work ratio increased from 0.05 to 0.19 with increasing evaporation pressure and decreasing mass flow rate. In other words, it is unnecessary to pursue excessively high evaporation temperature at low rotational speed of multi-stage centrifugal pump. As for the cooling water pump, it is feasible to reduce the power consumption of the cooling system through changing the rotational speed of water pump. The water pump can be operated with high efficiency (between approximately 58.4% and 65.3%, corresponding to an estimated speed range of 1374 RPM to 1813 RPM) when the system run in optimum operating conditions. It is also found that the performance of organic Rankine cycle was strongly influenced by fluctuating ambient temperature, with the net efficiency and net power increase of 3.36 % and 5.11 kW in winter compared to summer.

Novel multi-helical coil heat exchanger-based small-scale organic Rankine cycle: A detailed heat transfer analysis

This paper discusses a prototype of the ORC, which is developed utilizing a novel multi-coil evaporator and condenser for the waste heat source with a temperature below 100 °C. R123 acts as a working fluid for the ORC. The thermal performance of a multi-helical coil is evaluated experimentally and compared to the single-helical coil heat exchanger available in the literature.The performance of a novel multi-coil heat exchanger depicts that when the Reynolds Number varies from 60000 to 120000, the heat transfer coefficient varies from 1560 W/(m2K) to 2900 W/(m2K) for a single coil heat exchanger and 3014 W/(m2K) to 4531 W/(m2K) for multi helical coil evaporator. The Nusselt Number varies from 300 to 570 for single-coil heat exchangers and 715 to 1076 for multi-helical coil heat exchangers. The results show that the expander's power output is around 2.14 kW and proportional to the evaporator temperature and pressure increase. The high condensing temperature decreases the expander power output. The main contribution of the present work is developing a small ORC system with novel multi-helical coil heat exchangers and testing them on a lab scale for the most probable application to fulfill the electricity requirement in remote areas in the future.

A Review on the Preliminary Design of Axial and Radial Turbines for Small-Scale Organic Rankine Cycle

Organic Rankine cycle (ORC) is an effective technology to harness low-grade energy. Turbine, as a key component of ORC, takes advantages of its high efficiency and compact size compared with other expanders. Currently, developing suitable turbines with a high performance and a low cost is one of the bottlenecks for wide applications of various ORCs. In this context, technical progress on radial inflow turbines (RITs), axial turbines (ATs), and radial outflow turbines (ROTs) is introduced, and loss models used in the preliminary design are compared, especially for small-scale ORCs. RIT is recommended for medium and small ORCs with an expansion pressure ratio of <10. The power outs and rotational speeds of the designed RITs spanned the ranges of 9.3–684 kW and 3000–114,000 r/min with an efficiency of 56.1–91.75%. In comparison, the power outputs and speeds of ATs were 3–2446 kW and 3000–91,800 r/min with an efficiency of 63–89.1%. AT is suitable for large-scale ORCs with a power output of greater than hundreds of kW. However, AT with impulse stages is feasible for small-scale ORCs when the pressure ratio is high, and the mass flow rate is small. The power outputs of the designed ROTs were relatively small, at 10–400 kW with a speed of 7200–42,700 r/min and an efficiency of 68.7–85%. For organic working fluids with a large expansion pressure ratio, ROT might be employed. Conventional mean-line models may neglect the effects of supersonic flow, which will be encountered in many ORC turbines. Therefore, adequate models for supersonic expansion loss and shock loss need to be added. Meanwhile, a proper multivariable optimization algorithm such as a gradient-based or stochastic search method should be selected. Finally, the challenges and potential research directions are discussed. The outcomes can provide some insights for the development of ORC turbines and the optimization of ORC systems.

Experimental investigation and machine learning optimization of a small-scale organic Rankine cycle

The system behaviors of a small-scale organic Rankine cycle (ORC) system using a scroll expander under various operating conditions, as well as the machine learning optimization based on back propagation artificial neural network (BPANN), are investigated in this study. The effects of pump outlet pressure, expander inlet temperature and mass flow rate on net power output and thermal efficiency are examined experimentally. A BP-ORC neural network model is constructed, and the prediction accuracy is analyzed by the errors between the experiment and predicted data. The effects of eight operating parameters on system performance are discussed. The bi-objective optimization for maximum net output power and maximum thermal efficiency is addressed. Results indicate that as the mass flow rate increases, the net power output presents an increase trend, whereas the thermal efficiency yields a smooth tendency. The absolute error in the net power output from the BP-ORC model is between −0.05 kW and 0.05 kW, while that in the thermal efficiency is between −8 × 10-5 and 8 × 10-5. The Pareto-optimal solution of thermal efficiency is 9.32 % and the net output power is 2.42 kW.

Definition of a general performance map for single stage radial inflow turbines and analysis of the impact of expander performance on the optimal ORC design in on-board waste heat recovery applications

Small-scale Organic Rankine Cycles (ORCs) represent a promising technology for on-board waste heat recovery (WHR) from internal combustion engines (ICEs) especially in innovative long-haul trucks transport applications. Despite market leader companies have already proved its effectiveness, detailed system design procedures are scarcely available in the open literature and proposed solutions are often more simplified with respect to current industrial state-of-art. The present work describes a methodology to include within the ORC design and optimization procedure an efficiency map of the turbo-expander, retrieved exploiting a reduced-order method developed in-house. The proposed approach allows therefore to design the thermodynamic cycle considering a realistic performance of the expander and to retrieve the best cycle architectures and turbine geometry depending on heat source characteristics and active constraints. First the turbine performance is defined in general terms and investigation of its applicability to different fluids and to expansions characterized by various degrees of non-ideality is provided. Then, the methodology is applied to different WHR applications highlighting for several potential working fluids the impact of adopting turbine performance maps within the cycle optimization procedure rather than assuming constant efficiency values. Results show that the cycle configuration and the main design parameters are strongly affected by the plant size and the working fluid selection, and that the adoption of a turbine performance map is particularly valuable when dealing with small size power plants adopting high density fluids. In particular, the assumption of constant turbine efficiency may lead to the definition of thermodynamic cycles extremely far from the optimal ones (e.g. adoption of supercritical cycles rather than subcritical ones) that in turn can result in an unfeasible design of the turboexpander. Moreover, this assumption may also lead to an incorrect comparison between different candidate working fluids lacking in identify those more suitable for a specific plant size. Both those limits can be solved with the adoption of the expander performance map proposed in this work. Final results show that on board WHR ORC can reach an efficiency of at least 20% in a relatively large range of internal combustion engine size. On the contrary, the choice of the best working fluid is affected by engine size and chosen hot sources: for small available thermal power the use of complex and flammable hydrocarbons (cyclopentane, cyclohexane, toluene) is the most efficient choice while the use of low GWP non-flammable halogenated hydrocarbons (R1233zd and R245fa) becomes promising only for relatively large available thermal power.

Performance analysis of a bio-diesel fired engine bottoming with micro-ORC

The purpose of this work is to investigate the performance in terms of electric power and electric efficiency of a 11kW bio-diesel internal combustion engine (ICE) coupled with a micro Organic Rankine Cycle (ORC) both in design and off-design conditions. A zero dimensional (0D) thermodynamical engine model has been developed to predict the electric and thermal power with 100% biodiesel fuel (B100). B100 has been selected for the performance analysis of the integrated system due its lower environmental impact. For the ORC system, a subcritical thermodynamic model has been used with various working fluid (R245fa, R1233zd(E), R134a, R1234ze(E) and R1234ze(Z)). A plate heat exchanger (PHEX) has been adopted as evaporator of the organic cycle to directly recover the thermal power of the exhaust gas. Both models have been validated using experimental data from literature. Two different expander configurations, dynamic and volumetric, has been investigated. Model validations show good agreement with the experimental and literature data, respectively. Moreover, the results highlight that the micro-ORC could achieve a maximum electric efficiency of about 7% at full load with R1234ze(Z). Although at part load the dynamic expander show better performance, the volumetric expander has been selected for the system analysis due to its real suitability for small and micro scale ORC. Combined system results show a maximum enhancement of engine efficiency of about 5%, and better results, in terms of good compromise between electric efficiency and operating range width, has been shown for R1233zd(E). Furthermore, the direct coupling of the PHEX with the exhaust gases allows to have heat exchange evaporator areas below 2 m2 for all analysed fluids.

Experimental evaluation of organic Rankine cycle using zeotropic mixture under different operation conditions

The zeotropic mixture is a feasible way to improve the thermodynamics performance of organic Rankine cycle (ORC). A small-scale ORC experimental apparatus with a scroll expander was established. Then, the system performance using different zeotropic mixtures (including 0.75R245fa/0.25R141b, 0.5R245fa/0.5R141b and 0.25R245fa/0.75R141b) was tested and compared with the pure fluid of R245fa and R141b to determine whether the mixture fluid can improve the system performance. The experimental results show that the thermal efficiency and exergy efficiency of ORC system gradually increase with the increase of cooling water flow rate and the decrease of expander rotational speed. Under different working conditions, mixture fluids can produce more shaft power and have lower condenser exergy destruction than pure fluids. The zeotropic mixture is not always better than the pure fluid, while it can improve the thermal efficiency and exergy efficiency of the ORC system with an appropriate mass fraction. In the experiment, the best mass fraction of R245fa/R141b is 0.25/0.75, and its thermodynamic performance is significantly higher than that of R141b and R245fa. The maximum thermal efficiency and exergy efficiency of 0.25R245fa/0.75R141b are 5.58% and 23.2% respectively.

Thermodynamic performance comparison of organic Rankine flash cycle with and without ejector for geothermal power output

To improve the matching between heat source and working fluid and alleviate the problem of high pump power and low system efficiency in small-scale organic Rankine cycle (ORC), an organic Rankine flash cycle with ejector (EORFC) was proposed. In this paper, the theoretical analysis of the EORFC is conducted based on one-dimensional global model of the ejector and one-dimensional analytical thermodynamic model of the system. A parametrical analysis on the EORFC with R245fa is conducted, the influences of the entrainment ratio and area ratio on the ejector mechanism and system performance are investigated in turn. Then, a comparative analysis is further carried out, which mainly focus on the comparison between the thermodynamic performance of the EORFC and organic Rankine flash cycle (ORFC) by analyzing the influences of the system operating parameters, including overall efficiency of the pump, flashing temperature, condensing temperature and dryness degree at the evaporator outlet. The results show that with the decrease of the entrainment ratio and the area ratio, the pressure lift and ejector efficiency increase, thereby improving the performance of the EORFC. The temperature at the ejector outlet is also inversely proportional to the entrainment ratio, while it is almost unaffected by the area ratio. Meanwhile, the four system operating parameters have positive influence on the net power output of the EORFC and ORFC. When the overall efficiency of the pump is lower than 30% and the condensing temperature is higher than 30 °C, the EORFC can put more net power. Compared with the ORFC, the EORFC always has higher thermal efficiency and exergetic efficiency due to the heating effect of the ejector, besides the EORFC is superior at low cycle efficiencies.

Numerical simulation and optimal design of scroll expander applied in a small-scale organic rankine cycle

As an important component in the organic Rankine cycle (ORC), the operation characteristics of the scroll expander directly determine the ORC efficiency. In this study, numerical simulation and optimal design of scroll expander applied in a small-scale organic Rankine cycle are conducted. The expander unsteady flow characteristics using R245fa are examined. The dynamic mesh technology is applied to accurately achieve the eccentric motion of the orbiting scroll. The effects of four key parameters (inlet temperature, inlet pressure, outlet pressure and rotation speed) on the expander output power and isentropic efficiency are discussed. Considering maximum isentropic efficiency and maximum expander output power simultaneously, the optimal solution using response surface methodology (RSM) is yielded. Results indicate that the distribution of velocity and pressure in different working chambers are not completely symmetric, and the expander isentropic efficiency can be significantly improved by changing the boundary conditions. The maximum output power and isentropic efficiency are 2.458 kW and 72.98%, which deviates from the simulated value by 6.51% and 5.12%, respectively.

Application of novel framework based on ensemble boosted regression trees and Gaussian process regression in modelling thermal performance of small-scale Organic Rankine Cycle (ORC) using hybrid nanofluid

This work examined the thermal performance of a small-scale solar organic Rankine cycle system, in which a flat plate solar collector was employed to supply heat to the organic Rankine cycle system. In the ORC system, WO 3 +MWCNT/water nanofluid was employed for the solar collector, while MWCNT/R141b nano-refrigerant was used in the ORC system. In the first stage, the thermal and exergy efficiency of the ORC system was experimentally examined at different flow rates and concentrations of nanofluid. As a result, the peak thermal efficiency of 73.21% was attained at 3 L per minute (lpm) of nanofluid flow rate and 1.5 vol% of nanofluid concentration. Moreover, a significant increase in energy and exergy efficiency by 8.52% and 6.30%, respectively, was observed for 0.5 vol% of nanofluid concentration and 3 lpm of nanofluid flow rate in the collector. In the second stage, the experimental data was utilized to develop contemporary ensemble machine learning algorithms such as boosted regression trees (BRT) and Gaussian process regression (GPR) aiming to predict the solar collector's energy efficiency and the ORC system's energy-exergy efficiency. The GPR-based models performed exceptionally well as Pearson's coefficient (R), coefficient of determination (R 2 ), mean squared error (MSE), and root mean squared error (RMSE) were 0.9999, 0.9998–0.9999, 0.0015–0.0524, and 0.0124–0.0723, respectively. For BRT-based model the R, R 2 , MSE and RMSE were 0.9604–0.9949, 0.97–0.99, 0.0969–0.8783, and 0.3113–0.9372. Theil's U2 was employed to calculate the uncertainty in the prediction framework, which was determined to be in the range of 0.002–0.00369 for GPR and 0.00228–0.1414 for the BRT-based model. Overall, both machine learning algorithms performed well, although GPR did better than BRT across the board. • Thermal performance of a solar-aided ORC system was modeled with GPR and BRT. • Peak thermal efficiency of ORC system reached 73.21% at 3 lpm of nanofluid flow rate. • Energy and exergy efficiency were increased by 8.52% and 6.30% as using nanofluid. • GPR-based models worked with exceptionally good indicators of R, R 2 , MSE, and RMSE. • Nanofluid flow rates were the most influential factor on ORC system performance.

Effect of resistive load characteristics on the performance of Organic Rankine cycle (ORC)

The effects of various organic Rankine cycle (ORC) components, such as evaporator, expander and condenser on the performance of the system have been extensively studied. However, only a handful of studies have focused on studying the influence of external load characteristics. A small-scale ORC test rig of 3 kWe was constructed to experimentally investigate the effect of resistive load on the performance of the system. In order to achieve this, the study combined the thermodynamic method with electrical analysis. The results showed that the increase in output power caused a simultaneous increase in rotational speed, torque and inlet pressure of the expander under constant load resistance conditions. However, the fluid mass flow rate only affected the expander's inlet pressure. The decrease in load resistance increased the expander's inlet pressure and torque, and reduced the superheating and rotational speed. The influences of load resistance on the isentropic efficiency and net efficiency were mainly thorough the expander's inlet pressure and superheating. The net efficiency also benefited from the ideal cycle efficiency due to that decreasing load resistance enlarged the enthalpy drop of expander. The effects of load resistance on performance parameters were restricted by the mass flow rate of the fluid.

Experimental research and artificial neural network prediction of free piston expander-linear generator

For the purpose of better matching the performance of the organic Rankine cycle (ORC) system concerning the vehicle engine waste heat recovery, this paper studies the output performance of free piston expander-linear generator (FPE-LG). A test bench of FPE-LG is established for small scale ORC system, and timing and displacement control strategy is proposed. Furthermore, the impact of the intake pressure and the torque on motion characteristics and output performance of FPE-LG are analyzed. According to evaluating different learning rates, number of hidden artificial neural networks and training functions, a prediction model of FPE-LG based on artificial neural network is established. Genetic algorithm is used to optimize the key operating parameters, to maximize the power output of FPE-LG. In consideration of the mean square error and determination coefficient, the artificial neural network model is verified and tested by experimental data. Finally, combining genetic algorithm with artificial neural network model, the maximum power output of FPE-LG is optimized and its performance is predicted. The results show that the maximum value of electric current, voltage and power output are 2.8 A, 14.75 V and 28.5 W, respectively. Based on artificial neural network, this method can provide useful guidance for performance prediction and coordinated optimization, with advantages of minimum deviation and high precision.

Thermodynamic Analysis of Small Scale Organic Rankine Cycle (ORC) System Based on Dry, Wet and Dry/Wet (Hybrid) Cooling Towers Under Six Climatic Zones of India

Thermodynamic Analysis of Small Scale Organic Rankine Cycle (ORC) System Based on Dry, Wet and Dry/Wet (Hybrid) Cooling Towers Under Six Climatic Zones of India

Experimental Investigation of a 560 Watt Organic Rankine Cycle System using R134a as Working Fluid and Plat Solar Collector as Heat Source

New and renewable energy sources such as solar, geothermal, and waste heat are energy sources that can be used as a source of energy for Organic Rankine cycle system because the organic Rankine cycle (ORC) requires heat at low temperatures to be used as energy source. The experimental of Organic Rankine Cycle (ORC) systems with solar energy as a heat source was conduct to investigate a small-scale ORC system with R134a as a working fluid by varying the heat source at temperature 75⁰C-95⁰C. The experiment resulted a maximum efficiency, power of system is 4.30%, and 185.9 Watt, where the temperature of heat source is 95⁰C, the pressure and temperature of steam inlet turbine is 1.38 MPa and 67.9oC respectively. Solar energy as the main energy source in the ORC system can reduce energy use up to 49.9% or 4080.8 kJ where the temperature of the water as the heat source in the evaporator is 51°C.