Performance Of Organic Rankine Cycle System Research Articles

Performance Analysis and Rapid Optimization of Vehicle ORC Systems Based on Numerical Simulation and Machine Learning

The organic Rankine cycle (ORC) system is an important technology for recovering energy from the waste heat of internal combustion engines, which is of significant importance for the improvement of fuel utilization. This study analyses the performance of vehicle ORC systems and proposes a rapid optimization method for enhancing vehicle ORC performance. This study constructed a numerical simulation model of an internal combustion engine-ORC waste heat recovery system based on GT-Suite software v2016. The impact of key operating parameters on the performance of two organic Rankine cycles: the simple organic Rankine cycle (SORC) and the recuperative organic Rankine cycle (RORC) was investigated. In order to facilitate real-time prediction and optimization of system performance, a data-driven rapid prediction model of the performance of the waste heat recovery system was constructed based on an artificial neural network. Meanwhile, the NSGA-II multi-objective algorithm was used to investigate the competitive relationship between different performance objective functions. Furthermore, the optimal operating parameters of the system were determined by utilizing the TOPSIS method. The results demonstrate that the highest thermal efficiencies of the SORC and RORC are 6.21% and 8.61%, respectively, the highest power outputs per unit heat transfer area (POPAs) are 6.98 kW/m2 and 8.99 kW/m2, respectively, the lowest unit electricity production costs (EPC) are 7.22 × 10−2 USD/kWh and 3.15 × 10−2 USD/kWh, respectively, and the lowest CO2 emissions are 2.85 ton CO2,eq and 3.11 ton CO2,eq, respectively. The optimization results show that the RORC exhibits superior thermodynamic and economic performance in comparison to the SORC, yet inferior environmental performance.

Performance Improvement of a Limaçon Gas Expander Using an Inlet Control Valve: Two Case Studies

Renewable energy-based compact energy-generation systems based on the organic Rankine cycle (ORC) can be employed to meet the ever-growing thirst for affordable and clean energy. The overall performance and effectiveness of ORC systems are constrained by the low efficiency of the gas expander, specifically the positive displacement expander, which is responsible for energy conversion from the working fluid. This low-efficiency scenario can be significantly improved by employing a control valve to regulate and restrict the flow of the working fluid into the expander. A control valve can effectively curve the loss of costly compressed and energized working fluids by allowing them to expand in the expander chamber before discharging through the outlet port. They can thus be used to regulate the amount of energy yield and output power. In this work, two direct drive rotary valves (DDRVs) operated by a stepper motor (SM-DDRV) and rotary solenoid (RS-DDRV) are suggested, and the behavior of the valves is examined. The effect of friction and temperature on the valve response is also studied. Additionally, the effect of inlet control valves on the overall system performance of the limaçon expander is assessed. Thermodynamic properties such as the isentropic efficiency and filling factor are also computed. The effect of leakage due to valve response delay is analyzed at different inlet pressures. The performance indices are compared to the expander performance without any inlet valve. The SM-DDRV setup results in a 14.86% increase in isentropic efficiency and a 220% increase in the filling factor, whereas the RS-DDRV performs moderately with a 2.58% increase in isentropic efficiency and an 80% increase in the filling factor compared to a ported expander. The SM-DDRV provides better performance indices compared to the RS-DDRV and without valve setups. However, the performance of the limaçon expander with the SM-DDRV is sensitive to the inlet pressure and degrades at higher pressure. Overall, the valves proposed in this work present key insights into improving the performance characteristics of gas expanders of ORC systems.

Comparative study on the organic rankine cycle off-design performance under different zeotropic mixture flow boiling correlations

Zeotropic mixture utilization enhances the performance of the Organic Rankine cycle (ORC) significantly. For geothermal ORC power plants, the zeotropic mixture flow boiling correlation plays a vital role in both evaporator design and system performance evaluation. Based on the screened zeotropic mixture R1224yd(Z)/R1234ze(E) (0.452/0.548), a comparative analysis of 4 flow boiling correlations is performed to assess their impact on both evaporator design and system performance under off-design conditions. The results underscore a substantial impact of these correlations on determining the plate length, with the largest relative difference being 53.8%, consequently resulting in uncertainties of 31.7% and 23.4% for the evaporator exergy loss and cost, respectively. Under off-design working conditions, the relative difference in net power output varies from 16.43% to 7.67%. The exergy efficiency variation is up to 11.82% and the electricity production cost (EPC) uncertainty is up to 11.2% at the working condition with the maximum difference in net output work. This study provides valuable insights for selecting the appropriate zeotropic mixture flow boiling correlation in practical evaporator design and evaluating the off-design performance of ORC systems.

A multi-objective optimization energy management strategy for marine hybrid propulsion with waste heat recovery system

To improve the efficiency of marine hybrid propulsion systems (HPS) and make full of the potential of the waste heat recovery system, this paper presents a powerplant scheme which includes marine HPS with thermoelectric generator (TEG) and organic rankine cycle (ORC) system. The original energy management strategy (EMS) based on the multi-objective optimization algorithm is proposed, which considers the objectives of minimum total energy consumption, maximum output energy of TEG and ORC, and the optimal engine efficiency. The EMS is validated with different initial states of charge of the battery, and the influence of weight factors in the EMS on the system performance is also investigated. Additionally, the interactions between various ship driving cycles and performance of TEG and ORC system are analyzed. Compared with the traditional engine propulsion, the maximum energy saving of the HPS is 13.74%, while the efficiency of the engine and ORC system has been improved by 1.53% and 1.17%, respectively. This paper provides a new algorithm of waste heat recovery system regulation and gives a valuable reference for EMS optimization in marine HPS.

A comparative study on design and performance evaluation of Organic Rankine Cycle (ORC) under different two-phase heat transfer correlations

Organic Rankine cycle (ORC) is considered to be a good solution to the high-efficient recovery of low-grade heat for saving energy and reducing carbon emissions. As the core component, the heat exchanger plays a significant role in the thermo-economic performance evaluation of an ORC plant and its heat transfer correlations greatly influence the cycle performance. In this paper, 8 two-phase heat transfer correlations (4 flow boiling ones and 4 flow condensation ones) are utilized to design the plate-type heat exchangers, and a comparative study among different correlations is carried out. The result shows the designed evaporators are finitely affected by heat transfer correlations whereas the situation with the designed condensers is the opposite. The net output power is slightly influenced with a relative difference within 1.1% but the system's economic performance is significantly affected with the maximum relative difference of electricity production cost up to 11.2%. The environmental performance is hardly affected by correlations with the maximum relative difference of total greenhouse gas emission within 0.96%. This paper analyzed the performance variation of the ORC under different correlations and provided a guideline for designing heat exchangers and evaluating the off-design performance of ORC systems.

Research on the Operational Performance of Organic Rankine Cycle System for Waste Heat Recovery from Large Ship Main Engine

Based on the analysis of the waste heat distribution characteristics of a typical ship two-stroke low-speed main engine (model: MAN 8S65ME-C8.6HL, the specified maximum continuous rating SMCR: 21,840 kW) under different loads, two different types of organic Rankine cycle (ORC) systems, namely the basic system (BORC) and the preheated system(PORC), were constructed to recover the ship main engine’s exhaust gas waste heat and jacket cooling water waste heat. Using the thermodynamic simulation model of the system, the main performance indexes, including net output power of the two ORC systems were studied with the variation of seawater temperature and main engine load, and the annual ship fuel saving and annual carbon emission reduction generated by the two systems were compared and analyzed. It was found that the maximum net output power of the BORC system and PORC system were 445.3 kW and 491.3 kW, respectively, when the ship’s main engine load was 100%, and the outboard seawater temperature was 20 °C; the maximum thermal efficiency was 12.84% and 12.71%, respectively; under the annual operation, the fuel saving of BORC system and PORC system can be 456 tons and 510 tons, respectively, and the carbon emission reduction was 1416 tons and 1581 tons, respectively. The analysis found that the net output power of the PORC system is always greater than that of the BORC system. When the outboard seawater is lower, and the main engine load is more than 80%, the net output power difference between the PORC system and BORC system gradually expands, and the improvement of ORC system performance is more evident by adding a preheater. It can be concluded that when the ship was mainly operated in the sea area with low seawater temperature and the main engine was running under high load most of the time, selecting the PORC system to recover the waste heat of the main engine was more advantageous.

Machine learning-based multi-objective optimization and thermodynamic evaluation of organic Rankine cycle (ORC) system for vehicle engine under road condition

Organic Rankine cycle (ORC) cooperative multi-objective optimization under dynamic driving cycle is the key to obtain the potential of waste heat recovery of internal combustion (IC) engine. The exhaust waste heat energy of IC engine has the characteristics of complex change and strong fluctuation under actual road conditions. The analysis and optimization of the performance of ORC system under single engine operation has some limitations in evaluating the actual waste heat recovery characteristics. Based on Japan10-15 road condition, this paper builds ORC system for vehicle engine model oriented to driving environment. The interaction between operating parameters of ORC system and thermodynamic and thermal economic performance is analyzed. Based on the operating characteristics of the ORC system, the thermal efficiency and power output per unit heat transfer area (POPA) data driven models are established, and the structural parameters of the network are optimized. Based on the multi-objective optimization algorithm, the thermodynamic performance of ORC system under road condition is optimized. The results show that the ORC system has a maximum thermal efficiency of 2.23% and a maximum POPA of 0.55 kW/m2 under Japan10-15 road condition. In this paper, the performance evaluation and multi-objective optimization results of ORC system under road condition can provide a certain reference for the practical application potential of ORC system.

Organic rankine cycle systems with mixture of pure fluids: On infeasible fluid’s fractions due to the interaction between the mixture glide and the hexs pinchs

The Organic Rankine Cycle (ORC) is a promising technology for power generation from low-grade heat. The selection of working fluids is one of the important key points to improve the performance of an ORC system. Zeotropic mixtures show promising performances as working fluids. In fact, their temperature glide during phase change enables better match between the working fluid and the heat source/sink temperatures. In order to reveal the performance of mixture in ORC system, this paper deals with the thermodynamic model of the subcritical Organic Rankine Cycle (ORC) systems driven by low grade heat source while using zeotropic mixture working fluids with a special consideration to the interaction between phase change glides and the pinch value and their location in both the evaporator and the condenser (HEXs). Zeotropic mixtures of seven pure fluids are evaluated as working fluids for a subcritical ORC system. The mass fraction effects of mixtures on the thermal efficiency are analyzed. For given working conditions (working fluid mass flow, pressure and bubble temperature) the results show that for each considered zeotropic mixture there exist mass fraction ranges that are not consistent with the pinch values constraint in the HEXs and leads to so-called ‘infeasible zones’ with unreal HEXs dimensions. Results shows also that, out of these “infeasible fractions” zone, keeping unchanged the working conditions, the thermal performances of ORC system using zeotropic mixture are always better than the thermal performances of the same systems using the correspondent pure fluids. In addition, out of these highlighted “unfeasible zones” it was found that mixture with high temperature glide improve the thermal efficiency of ORC system.

Simulations on organic Rankine cycle with quasi two-stage expander under cross-seasonal ambient conditions

Organic Rankine cycle (ORC) plays an important role in addressing the issue of energy shortage and climate change. The condenser should release heat to the environment. Due to the seasonal and daily variations of ambient conditions, the condensation temperature, power consumption of the cooling system, the operation conditions of the working medium pump and expander would suffer drastic changes in different condensing ambient conditions. Therefore, the main aim of this study is to investigate the cross-seasonal performance of the organic Rankine cycle under variable condensation conditions. Simultaneously, the single screw expander with quasi two-stage expansion was used to improve the performance of ORC in different condensing ambient conditions. A model was developed to depict the variations of operation parameters of ORC system. The results show that the performance of ORC was adversely correlated with the wet-bulb temperature throughout the year. The net power and net efficiency can be reached at 8.04 kW and 11.31 % when the wet-bulb temperature is 1.1 ℃, respectively. Additionally, the variation in cooling water flow rate has little effect on the volumetric expansion process of the new single screw expander, but has a significant influence on the velocity expansion process, which increases by nearly 56.4 %. Meanwhile, decreasing the pipeline impedance coefficients of cooling system can optimize ORC system performance effectively. It was also discovered that the isentropic efficiency of single screw expander decreases by only 8.68 percentage points during the expansion ratio increase from 3.31 to 11.54.

Nonlinear modeling and multi-scale influence characteristics analysis of organic Rankine cycle (ORC) system considering variable driving cycles

The reasonable construction of the organic Rankine cycle (ORC) system model under road conditions is the key to analyze, evaluate, and optimize the performance of the ORC system. However, due to the variability of high-temperature waste heat source and the strong coupling correlation of operating parameters, the operation characteristics of the ORC system show evident time-varying characteristics. Based on the coupling correlation and redundancy characteristics of the ORC system in complex environment, this paper presents a nonlinear modeling framework for the multi-scale influence analysis of the ORC system under road conditions. The nonlinear modeling framework can improve the prediction accuracy of ORC model by at least 68.36% and reduce the time cost by 53.37%. Based on the nonlinear model, the synergistic effects of multiple variables on ORC system performance at different scales are studied. The frequent fluctuation of vehicle speed enhances the coupling correlation of operating parameters, resulting in nonlinear, hysteretic dynamic characteristics of ORC thermal efficiency. The maximum thermal efficiency of ORC is only 3.28%. The nonlinear modeling framework proposed in this paper can provide a practical solution for constructing the intelligent analysis, design, and optimization models of ORC systems under complex road conditions.

Thermodynamic analysis of Tesla turbine in Organic Rankine Cycle under two-phase flow conditions

In the field of clean energy, Organic Rankine Cycle (ORC) is considered a vital method to utilize low-grade waste heat effectively. Due to certain application advantages in low power level ORC systems, the Tesla turbine is considered one of the essential choices for the core components of ORC systems. Given the inadequacy of the current research on the performance of the Tesla turbine under two-phase flow conditions, a one-dimensional model of the Tesla turbine was constructed in this paper to make it equally applicable to the two-phase flow conditions of wet working fluids. A model of the Tesla turbine and ORC system was constructed from the thermodynamic point of view, and five typical wet working fluids (R22, R417a, R134a, R152a, and R290) were selected to study the effects of the five wet working fluids on the thermal performance of Tesla turbine and ORC system. The effects of the changes in key structural parameters of the Tesla turbine on the performance of the Tesla turbine and the ORC system were further analyzed when different wet working fluids were used in the ORC system. The results showed that the highest turbine efficiency of 42.0 % was achieved when the Tesla turbine used R22 as the working fluid under the design conditions, while the highest turbine output power and the highest system thermal efficiency of 1.19 kW and 3.96 % were achieved when R417a was used as the working fluid. The analysis of the key structural parameters of the system showed that for the five different wet working fluids, the turbine efficiency, turbine output power, and system thermal efficiency increased monotonically with increasing rotor inlet radius. For each of the five wet working fluids, an optimum rotor speed, rotor outlet radius, and rotor gap distance exist, resulting in optimal turbine efficiency and system performance. The related work provides an important reference for the optimal design of Tesla turbines with wet working fluids.

Numerical investigation on power generation performance of enhanced geothermal system with horizontal well

• Combined with thermodynamic cycle, EGS-ORC coupling model is proposed. • The optimal operation parameters of EGS-ORC are obtained. • The effect of various parameters on the performance of ORC system is probed. Enhanced geothermal system (EGS) can efficiently extract heat stored underground, while Organic Rankine Cycle (ORC) system can generate electricity with heat extracted by EGS. A combination of underground heat extraction and ground power generation systems is proposed in present work. The underground heat extraction system that consists of an EGS coupled with hydraulic-thermal horizontal wells and five disc-shaped fractures. The ground power generation system is comprised of a basic ORC system, and R245fa is used as the working fluid. The performance of combined system is numerically investigated in present work. The results indicate that with the increase of geothermal fluid mass flow rate, the geothermal fluid outlet temperature decreases rapidly, and the thermal reservoir life exhausts more earlier. In addition, the optimization of EGS is conducted as well, and the optimal case with the maximum production temperature has been obtained. It’s found that when the mass flow rate and injection temperature of geothermal fluid are 30 kg/s and 50 °C respectively, the production temperature of considered EGS drops to 441.3 K after thirty years of operation, the increase in pump power consumed by transporting geothermal fluid rises to 0.53 MW, and the net output power of ORC system drops to 895 kW. Furthermore, it’s found that the geothermal fluid mass flow rate has an obvious influence on the power generation performance. In addition, a sensitivity analysis of the three considered parameters (production pressure, fracture distance, well spacing) has been performed, which reveals that well spacing has the greatest effect on EGS power generation performance.

Dynamic response assessment and multi-objective optimization of organic Rankine cycle (ORC) under vehicle driving cycle conditions

Organic Rankine cycle (ORC) can effectively utilize the waste heat energy of internal combustion (IC) engine. The ORC system has obvious large time-varying characteristic, strong fluctuation, and hysteresis under the synergistic influence of frequent fluctuation of vehicle speed and operating parameters in the system. The power system, waste heat recovery system, transmission system, and other subsystem models are constructed in this study. The key components are verified. An integrated system model of IC engine–ORC for driving cycle is constructed on the basis of the subsystem model. The dynamic response of ORC system is evaluated under different road conditions. The synergistic influence of multiple variables on system performance is analyzed. Obvious strong coupling and nonlinear characteristics are observed among different performances of ORC system under road conditions. Therefore, a multi-objective cooperative optimization framework of ORC system considering actual road conditions is proposed on the basis of multidimensional data. In the process of collaborative optimization, thermal efficiency and emissions of CO2 equivalent show a clear trade-off under different road conditions. The limit value of thermal efficiency under new European driving cycle is also 6.48% higher than that under worldwide harmonized light vehicles test cycle. Frequent fluctuation of vehicle speed is not conducive to obtaining the limit value of thermal efficiency. The evaluation and optimization of the comprehensive performance of ORC system under complex road conditions can provide a reference for the practical application of vehicle ORC system.

Investigation of an artificial cold source with variable temperature for ORC test rig

Organic Rankine cycle (ORC) power generation system is applied to the cascade heat utilization system. The temperature of cold and heat sources has the significant effect on the performance of ORC system. To study this effect efficiently and easily operated, an artificial cold source with variable temperature in the range of -25–35°C was designed for ORC test rig, and an experimental study was conducted. The experiment investigated the effect of outdoor temperature and compressor frequency on the cold source temperature for a heat source output temperature in the range of 50–120°C. An experimental empirical formula for adjusting the cold source temperature independently was established, modified, and experimentally verified. The results demonstrated that the modified formula with two variables exhibited a quadratic polynomial relationship with the correlation coefficient R2 values in the range of 0.9992–0.9995. The verification experiments demonstrated that the calculated value was within ±0.5°C error range from the experimental value. The empirical formula can realize the accurate cold source temperature for ORC test rig. Eventually, the novel artificial cold source with variable temperature test rig can be used to conduct experimental studies on the operational performance of ORC systems under different temperature conditions from subtropical to temperate regions.

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 Climatic Conditions On Performance Of ORC Using Environment-Friendly Working Fluids – Special Reference To India

<p>Efficient heat recovery from low-grade heat sources is an achievable scientific frontier in the coming decade. One of the promising technologies in waste heat recovery is the organic Rankine cycle (ORC) system. Major obstacles to low-grade heat recovery include economic viability, scale, system efficiency, and exergy efficiency. In low temperature ORC systems, the cost of condenser becomes significant and hence air-cooled condenser is a feasible option. However, the performance of the air-cooled condenser is sensitive to the variation in ambient air conditions. India, being a tropical country, have villages and cities whose average temperature varies from -2oC in winter to 50oC in summer. The present work involves the study of heat transfer effects on the condenser using common organic working fluids and quantifying the consequences of environmental temperature variations. The constant thermal evaporator load is supplied with a medium-enthalpy heat source at 100 and 150oC. The atmospheric temperature associated with heat rejection from the condenser is varied from 273K to 313K for 5 working fluids. A MATLAB model of the ORC system is developed to study the effect of condenser performance on the first and second law efficiency of the ORC system. The MATLAB model is used to investigate the effect of varying inlet temperatures of the working fluid on the performance of ORC system to choose an organic working fluid suitable for Indian climatic conditions. R365 mfc, and R1233zd exhibit higher expander work and show higher heat rejection. At higher heat source temperatures of 150oC, the simulation shows a higher second law efficiency for the working fluid R1224yd(Z), which has critical temperature closed to the heat source temperature.</p>

Non-steady thermodynamic characteristics of a pilot-scale organic Rankine cycle system with a thermally-driven pump

A pilot-scale organic Rankine cycle (ORC) power generation system with a thermally-driven pump is constructed, and the working fluid of R245fa and a screw expander are adopted. Different heat source temperatures in the range of 90.00 °C–130.00 °C and the ambient temperature around 22 °C are employed to investigate the performance of the ORC system in the thermally-driven mode. The experimental results show that the generated electrical power indicates a stable fluctuation with the maximum time-averaged power of 11.61 kW at the heat source temperature of 130.00 °C, and the peak and valley values of 13.9 kW and 7.2 kW, respectively. The maximum thermal efficiency of 4.3% and the exergetic efficiency of 20.9% are obtained at the heat source temperature of 130.00 °C and 100.00 °C, respectively. Meanwhile, the proportion of full width at half maxima (PFWHM) is defined to illustrate the fluctuation characteristics of the output power, and the value is always between 0.70 and 0.72 at different heat source temperatures. Finally, the performance of the thermally-driven ORC system is compared with that of the conventional ORC system, and the results show that the ORC system in the thermally-driven mode adapts better to the heat source temperature below 110.00 °C.

Selection of working fluids for solar organic Rankine cycle—A review

The performance of renewable energy systems can be significantly improved if power is produced from low-temperature heat sources. The organic Rankine cycle (ORC) is suitable for producing power from low-temperature heat sources. ORC finds applications alongside solar power, geothermal power, waste heat and biomass. ORC-enabled systems can contribute significantly to raising the effectiveness of renewable power sources. Solar power, for example, can deliver affordable electricity to remote locations and small industries. By employing ORC, power generation is possible at lower temperatures on cloudy days. Its main difference from the steam Rankine cycle is its working fluid. The ORC makes use of an organic, high molecular mass fluid as the working fluid. Compared to steam Rankine cycles, ORC works at a lower temperature (<250°C) and pressure. The only working fluid in the steam Rankine cycle is water, but the ORC can operate on hundreds of different working fluids. The design and performance of ORC systems are entirely dependent upon the suitable working fluid and, hence identification of the working fluid for ORCs is of utmost importance for diverse applications. Properties of working fluid have a considerable impact on the efficiency of the system. The main objective of this study is to identify the most suitable organic fluids having characteristics necessary for use in solar ORCs. In addition, a detailed review of the various types of heat transfer fluids now available in the market that are often used in ORC systems was conducted. This research assists in selecting the most appropriate organic fluids for various solar ORC applications in terms of operating circumstances.

Energy, economic and environmental dynamic response characteristics of organic Rankine cycle (ORC) system under different driving cycles

Internal combustion (IC) engine as the main power source of vehicle, its energy saving and emission reduction is of great significance to alleviate the energy crisis. Organic Rankine cycle (ORC) system has a good prospect in the field of waste heat recovery of IC engine. However, in the actual operation of the vehicle, the hysteresis time, time-varying and uncertainty of ORC system are more obvious. Moreover, under the influence of the fluctuation of IC engine and the hysteresis time of ORC system, the performance of the system is heavily dependent on the change of operating parameters. This paper firstly constructs four key models of power system, waste heat recovery system, chassis and driving cycle. Then, the driving cycle-based vehicle-ORC combined system model is proposed. From the perspective of thermodynamic performance, economic performance and environmental impact, the dynamic response characteristics of ORC system performance and operating parameters under real driving cycles are systematically evaluated. The performance of ORC system can be improved by 9.25–17.72% with the increase of the large speed gradient. The hysteresis time of ORC system is 16.6–144.3 s. Hysteresis time and performance degradation will become more serious with the superposition of decrease of the large speed gradient, low speed and idle speed. The hysteresis time under the influence of superposition is up to 150.4–295.3 s. There is also a 39.59–62.45% decline in performance. The analysis of dynamic characteristics of ORC system under complex real driving cycles can provide useful guidance for practical engineering application of vehicle ORC system.

Thermodynamic analysis and optimization of a vapor injection organic Rankine cycle system for low-grade heat recovery

The organic Rankine cycle (ORC) system has been widely used in waste heat recovery and geothermal utilization, but the system efficiency is limited due to the large exergy loss in the evaporation process. To improve the performance of ORC systems, in this paper, a vapor injection organic Rankine cycle (VIORC) system using positive-displacement expanders was proposed. The thermodynamic model of the proposed system was developed in views of energy and exergy. The effects of system operation parameters on system performance were investigated, and the system was optimized under different waste heat temperatures. Furthermore, the optimized systems were compared with basic ORC (BORC) systems. Results showed that compared with the BORC system, the proposed system increased the net output power by 3-8% and the exergy efficiency by 0.8-3.2% under the heat source temperature of 90-150 °C. The exergy loss analysis further showed that the reduction of the exergy loss in the evaporation process was the main contribution to the efficiency improvement of the proposed system.