Dual-loop Organic Rankine Cycle System Research Articles

Performance analysis of a dual-loop organic Rankine cycle system for waste heat recovery from engine coolant and exhaust of a heavy-duty truck

• Heat exchanger employed influences the system efficiency. • Significant heat loss observed from engine block at part-load. • Variable expander-to-engine speed gear ratio gives a small gain in power. • Performance comparison for unique Scandinavian motorway data. Strict legislations over emissions from heavy-duty trucks are pushing the manufacturers towards improving their brake thermal efficiency, thereby mitigating fuel consumption and reducing CO 2 emissions. Organic Rankine cycle-based waste heat recovery system has proven to be an inevitable technology in boosting the engine performance besides the other methods and approaches. In addition to recovering the exhaust heat from the engine, recovering heat from the engine coolant is also beneficial, provided its nominal temperature is raised. Thus, this work investigates the scope of improvement in performance of a heavy-duty truck engine when integrated with a dual-loop organic Rankine cycle system for recovering heat from its exhaust and coolant, simultaneously. The analysis has been performed as a simulation study on GT-SUITE using the one-dimensional model of the engine with its organic Rankine cycle heat recovery setup. The model was developed based on the components of a real commercial truck engine connected with a simple exhaust waste heat recovery system. For the work described in this paper, model of the single-loop exhaust waste heat recovery system was modified to a dual-loop circuit comprising of two scroll expanders and R1233zd(E) as the working fluid. Notably, performance investigation was carried out using a unique Scandinavian motorway road data retrieved from the truck. This paper addresses the challenges associated with simultaneous heat recovery from engine exhaust and coolant. Performance comparison at four steady-state engine operating points reveal that the low temperature radiator installed for the indirect condensation of the working fluid is the major influencing component on system efficiency. At higher engine loads, the overall system efficiency considerably decreased due to limited heat rejection capacity of this radiator. Moreover, on assessing the scope of improving the system’s performance by having variable gear ratio between the engine and the expanders, 1.6% points (0.12 kW) gain in power was observed by having the high-pressure expander’s speed fixed and the low-pressure expander’s speed varied. Furthermore, elevating the engine coolant temperature from 120 °C to 140 °C, significant heat loss from the coolant to the engine surroundings resulted in a substantial drop in net power at lower part loads although it had improved at higher engine operating points.

Investigation of a Dual-Loop ORC for the Waste Heat Recovery of a Marine Main Engine

As carbon dioxide emissions arising from fossil energy consumption and fossil fuels are gradually increased, it is important for the low-carbon operation of ships to recover diesel engine waste heat. A newly developed dual-loop organic Rankine cycle (ORC) system to recover waste heat from a marine main engine (M/E) was designed in this paper. The exhaust gas (EG) heat was recovered by the high-temperature (HT) loop. The jacket cooling water (JCW) heat and the condensation heat of the HT loop were recovered by the low-temperature (LT) loop. Toluene, cyclohexane, benzene, R1233zd (E), R245fa, and R227ea were selected as the working fluids. The influence of the condenser thermal parameters on the LT loop was analyzed using the pinch point method. The performance of the dual-loop ORC was investigated under various working fluid combinations. The maximum net power of the HT loop can reach 253.4 kW when using cyclohexane as the working fluid, and the maximum thermal efficiency of the HT loop can reach 18.5% with benzene as the working fluid. Meanwhile, higher condensation temperatures and levels of condensation heat of the HT loop have a positive effect on the performance of the LT loop. However, in most conditions, the HT loop condensation heat could not provide enough heat for the LT loop’s working fluid to start the boiling process. The total net power of the dual-loop ORC system was 410.6 kW with Cyclohexane in the HT loop and R1233zd (E) in the LT loop, resulting in a 10.9% improvement in the marine main engine thermal efficiency.

Rankine cycle waste heat recovery for a heavy-duty natural gas engine meeting China VI emission standards

The China VI natural gas engine is based on the theory of the air–fuel ratio method, leading to a high exhaust temperature, large heat load and difficult cooling. The paper carried out a thermal balance experiment of the China VI natural gas engine under different working conditions. A dual-loop organic Rankine cycle system is designed to reduce the difficulty of cooling system and improve the thermal efficiency. The high-temperature loop uses water as working fluid to recover the heat of engine exhaust gas and recirculation exhaust gas. The low-temperature loop recovers the heat from cooling water and waste heat of the high-temperature loop. The high-temperature and low-temperature loops are coupled through a shared heat exchanger. The results show that the China VI natural gas engine has a higher and more stable waste heat temperature than traditional engines and is more adaptable to waste heat recovery under variable operating conditions. For fixed evaporation temperature, with the increase of inlet temperature of the expander, the net output power of high-temperature loop increases first and then decreases, reaching the maximum value at about 800 K. In the low-temperature loop working fluids R601a, R601b, R600, R600a and R12333zd, The maximum net output power of the waste heat recovery system based on the combination of water and R12333zd is approximately 75.4 kW under the rated condition, which is 22.24% of the rated power of the engine.

Analyzing the dual-loop organic rankine cycle for waste heat recovery of container vessel

Increasing the efficiency of ships via waste heat recovery systems is one of the most effective methods to minimize emissions from ships, especially carbon dioxide emissions. The organic rankine cycle is a promising system. The purpose of this study is to analyze the dual-loop organic rankine cycle system using jacket cooling water, exhaust gas and scavenge air. Two model is designed to analyze the waste heat recovery system for the different main engine load. In the first model, while the exhaust gas is used as the heat source in the high temperature cycle, jacket cooling water and high temperature loop waste heat are used as heat sources in low temperature cycle. Scavenge air and exhaust gas and jacket cooling water are used as heat sources in the second model. Different working fluid combinations that have been familiar from literature and their alternatives are analyzed in two models and the net power output by using thermodynamics, reduction in carbon dioxide and fuel consumption are calculated. In low engine load, since the temperature range of scavenge air is low the second model is suitable for high main engine load. Results show that with the second model when benzene-R245fa(1,1,1,3,3-pentafluoropropane) combination is used as working fluid, 3373 kW net power can be produced, 13588,3 tonnes/year Carbon dioxide emissions can be reduced. However, since benzene is a flammable fluid, it is not recommended for use in the ship. Instead of benzene, R1233zd(E) (trans-1-chloro-3,3,3-trifluoro-1-propene) is more suitable for high temperature source.

Dynamic Response Characteristics Analysis and Energy, Exergy, and Economic (3E) Evaluation of Dual Loop Organic Rankine Cycle (DORC) for CNG Engine Waste Heat Recovery

Frequent fluctuations of CNG engine operating conditions make the waste heat source have uncertain, nonlinear, and strong coupling characteristics. These characteristics are not conducive to the efficient recovery of the DORC system. The systematic evaluation of the CNG engine waste heat source and the comprehensive performance of the DORC system is conducive to the efficient use of waste heat. Based on the theory of internal combustion (IC) engine thermal balance, this paper analyzes the dynamic characteristics of compressed natural gas (CNG) engine waste heat energy under full operating conditions. Then, based on the operating characteristics of the dual loop organic Rankine cycle (DORC) system, thermodynamic models, heat transfer models, and economic models are constructed. The dynamic response characteristics analysis and energy, exergy, and economic (3E) evaluation of the DORC system under full operating conditions are carried out. The results show that the maximum values of net power output, heat exchange area, and the minimum values of EPC (electricity production cost) and PBT (payback time) are all obtained under rated condition, which are 174.03 kW, 25.86 kW, 37.54 kW, 24.76 m2, 0.15 $/kW·h and 3.46 years. Therefore, the rated condition is a relatively ideal design operating point for the DORC system. The research in this paper not only provides a reliable reference for the comprehensive analysis and evaluation of the performance of the DORC system, but also provides useful guidance for the selection of appropriate DORC system design operating points.

Thermodynamic analysis and high-dimensional evolutionary many-objective optimization of dual loop organic Rankine cycle (DORC) for CNG engine waste heat recovery

The complex and changeable working conditions of compressed natural gas (CNG) engines have brought great challenges to the efficient recovery of waste heat energy. The dual loop organic Rankine cycle (DORC) system can effectively recover and utilize CNG engine waste heat due to its structural advantages. Determining the nonlinear variations between operating parameters and thermodynamic performance serves as the basis for obtaining the thermodynamic performance limits of the DORC system. However, traditional analysis methods have obvious limitations in this area. Based on the bilinear interpolation algorithm, this paper comprehensively analyzes and evaluates the nonlinear and strong coupling characteristics between operating parameters and net power output, thermal efficiency, and exergy destruction. In addition, the comprehensive thermodynamic performance limits of the DORC system have a critical effect on its engineering application yet have not been thoroughly and comprehensively optimized. Therefore, this paper proposes an equilibrium-based thermodynamic high-dimensional evolutionary many-objective optimization (EMO) method for the DORC system. The optimal thermal efficiency, net power output, and total exergy destruction of the DORC system reached 37.11 kW, 14.27%, and 104.58 kW, respectively. Based on the optimization results under equilibrium and unequilibrium weight, the dominant relationship between the different thermodynamic performances of the DORC system is then analyzed. This research can provide a direct reference for analyzing and optimizing the comprehensive thermodynamic performance of the DORC system.

Thermodynamic, economic, and environmental analysis and multi-objective optimization of a dual loop organic Rankine cycle for CNG engine waste heat recovery

• Thermodynamic, economic, and environmental analysis are presented. • Proposing a multi-model driven NSGA-III method for DORC performance optimization. • A tradeoff appears among thermal efficiency, payback period and CO 2 emission. Waste heat energy exists in the exhaust, intercooler, and coolant during the operation of the compressed natural gas (CNG) engine. The cascade utilization of the waste heat energy of the CNG engine can be realized considering the structural advantages of the dual loop organic Rankine cycle (DORC) system. Operating parameters have an important influence on DORC system performance. Traditional analysis methods have limitations in analyzing the relationship between operating parameters and system performance, and little attention is paid to the environmental impact of the system. The environmental performance of the system plays an important role in saving energy and protecting the environment. Therefore, the bilinear interpolation algorithm is introduced to analyze the waste heat sources of the CNG engine and the thermodynamic, economic, and environmental performances of the DORC system. In addition, the comprehensive operating performance of the DORC system has not been thoroughly and comprehensively optimized. Therefore, this study proposes a multi-model driven NSGA-III method for DORC comprehensive performance optimization. Results show that the optimal thermal efficiency, payback period (PB), and emissions of CO 2 equivalent (ECE) are 11.98%, 4.02 years, and 43.74 ton CO 2,eq , respectively. Moreover, decreasing the thermal efficiency of the system will significantly decrease ECE and increase PB at the same time. This research provides a reliable reference for the analysis, design, evaluation, and optimization of the comprehensive performance of the DORC system.

Performance analysis of a dual-loop bottoming organic Rankine cycle (ORC) for waste heat recovery of a heavy-duty diesel engine, Part I: Thermodynamic analysis

In this paper, which is the part 1 of this study, the energy and exergy characteristics of a dual-loop organic Rankine cycle (ORC) combining with a heavy-duty diesel engine were investigated. The proposed cycle is driven by the waste heat of the exhaust gases, intake air, and the coolant. Central composite design (CCD), which is a standard response surface methodology (RSM) technique, was applied to design of experiments (DoE) and to investigate the influence of both the engine and cyclic parameters such as the engine speed, the start of injection (SOI), the higher pressure of the high-temperature loop, and the higher pressure of the low-temperature loop. Adequacy of the developed RSM models has been evaluated by the ANOVA tables. The results revealed an increase in whether the engine or cycle parameters led to an enhance in the produced power of the system. As a comparison, the variation in engine variables has more impact on the produced power. On the opposite side, a change in the engine speed and SOI have a minor effect on the thermal efficiency of the system. The maximum produced power of the dual-loop ORC system was identified as 310 kW, which was resulted at the engine speed of 1800 RPM, and 0.0 degrees of CA bTDC for SOI; while the higher pressures of the HT and LT loops were in 2400 and 2100 kPa, respectively. This obtained power is 31% of the engine brake power. The maximum values for the thermal and exergy efficiencies were observed as 9.5% and 43%, respectively. Also, the exergy analysis demonstrated that the highest exergy destruction rate of the system is determined to be 500 kW.

Fluid selection and advanced exergy analysis of dual-loop ORC using zeotropic mixture

The comprehensive performance of a dual-loop organic Rankine cycle (DORC) is mainly influenced by working fluid pairs and key components. Based on multi-objective optimization, we conducted the working fluid selection and advanced exergy analysis for the DORC system driven by flue gas at 300℃. The exergy efficiency, payback period (PBP), and annual CO2 emission reduction (AER) were selected as the objective functions. Operating parameters and mass fraction of mixtures were optimized by non-dominated sorting genetic algorithm-II (NSGA-II). A suitable fluid pair for the DORC system was selected from 13 candidate fluid pairs. Then, the improvement potential of each component was estimated based on advanced exergy analysis. The results show that there is an optimal mass fraction for different zeotropic mixtures. When cyclohexane/cyclopentane (0.2/0.8) and butane/pentane (0.65/0.35) are used for the high-temperature (HT) loop and low-temperature (LT) loop respectively, the DORC system can achieve the best performance. Its exergy efficiency and AER increase by 14.7% and 19.1% respectively, while PBP only increases by 2.6%, compared with the fluid pair of cyclohexane-butane. There is 261.9 kW endogenous avoidable exergy destruction in the DORC system, accounting for 53.5% of the total exergy destruction. The HT turbine (HTT) is the first component that needs technical modifications, followed by the HT evaporator (HTE), intermediate heat exchanger (IHE), and LT turbine (LTT).

Thermo-Economic Study of a Regenerative Dual-Loop ORC System Coupled to the Main Diesel Engines of a General Support Vessel

A thermo-economic analysis of a regenerative dual-loop organic Rankine cycle (ORC) is conducted, which will be coupled with the main diesel engines of a general support vessel. An energy and exergy analysis of the regenerative dual-loop ORC is conducted. The energy and exergy analysis results of the regenerative dual-loop ORC are compared with pertinent results for a simple dual-loop ORC without regeneration. A mission analysis that was based on a vessel speed profile with the proposed ORC was conducted. A heat transfer analysis was performed for dimensioning the heat exchangers of both ORC loops. Finally, an economic analysis is conducted to calculate the total capital cost and the payback period of the proposed ORC. The results showed that the proposed ORC is thermodynamically superior in both energetic and exergetic terms compared to the simple dual-loop ORC. The total fuel cost saving is 337,493 Euros, the total CO2 emission saving is 1,153,416.4 kg, and the SO2 emission saving is 36,044.3 kg. The total capital cost of the proposed ORC is 2,546,000 Euros. Finally, the installation of the proposed ORC in the examined vessel is economically feasible because it results in a reasonable payback period, which is less than nine years.

Thermo-economic selection criteria of working fluid used in dual-loop ORC for engine waste heat recovery by multi-objective optimization

The dual-loop organic Rankine cycle(DORC) is considered as an important way for engine waste heat recovery (WHR). Currently, researchers mainly focus on thermodynamic analysis and the performance comparison of multiple existing working fluid pairs without optimizing the working fluid pairs. Therefore, a novel analysis method of selection criteria based on thermodynamic and economic is provided to select a suitable working fluid using the multi-objective optimization. Firstly, a multi-objective optimization was performed using the Non-dominated sorting genetic algorithm-Ⅱ(NSGA-Ⅱ) to maximize exergy efficiency and minimize payback period under 24 candidate working fluid pairs from the references. And the Technical for Order Preference by Similarity to an Ideal Solution (TOPSIS) method was applied to select the Pareto optimal solutions using different kinds of working fluid pairs. Finally, the optimization results were analyzed by the grey relational analysis(GRA). The results show that the critical temperature can be used as the thermo-economic critical for working fluids selection in DORC system, such as the ideal working fluid pairs with the critical temperature in the range of 580–600 K of the HT cycle and the critical temperature in the range of 380–415 K of the LT cycle. And the optimal candidate is toluene/R124.

Working-fluid selection and thermoeconomic optimisation of a combined cycle cogeneration dual-loop organic Rankine cycle (ORC) system for solid oxide fuel cell (SOFC) waste-heat recovery

A novel combined-cycle system is proposed for the cogeneration of electricity and cooling, in which a dual-loop organic Rankine cycle (ORC) engine is used for waste-heat recovery from a solid oxide fuel cell system equipped with a gas turbine (SOFC-GT). Electricity is generated by the SOFC, its associated gas turbine, the two ORC turbines and a liquefied natural gas (LNG) turbine; the LNG supply to the fuel cell is also used as the heat sink to the ORC engines and as a cooling medium for domestic applications. The performance of the system with 20 different combinations of ORC working fluids is investigated by multi-objective optimisation of its capital cost rate and exergy efficiency, using an integration of a genetic algorithm and a neural network. The combination of R601 (top cycle) and Ethane (bottom cycle) is proposed for the dual-loop ORC system, due to the satisfaction of the optimisation goals, i.e., an optimal trade-off between efficiency and cost. With these working fluids, the overall system achieves an exergy efficiency of 51.6%, a total electrical power generation of 1040 kW, with the ORC waste-heat recovery system supplying 20.7% of this power, and a cooling capacity of 567 kW. In addition, an economic analysis of the proposed SOFC-GT-ORC system shows that the cost of production of an electrical unit amounts to $33.2perMWh, which is 12.9% and 73.9% lower than the levelized cost of electricity of separate SOFC-GT and SOFC systems, respectively. Exergy flow diagrams are used to determine the flow rate of the exergy and the value of exergy destruction in each component. In the waste-heat recovery system, exergy destruction mainly occurs within theheat exchangers, the highest of which is in the LNG cooling unit followed by the LNG vaporiser and the evaporator of the bottom-cycle ORC system, highlighting the importance of these components’ design in maximising the performance of the overall system.

Working fluid selection of dual-loop organic Rankine cycle using multi-objective optimization and improved grey relational analysis

Working fluid selection is still a great challenge for a dual-loop organic Rankine cycle (DORC) to achieve better comprehensive performance. In this paper, a method combing multi-objective optimization with improved grey relational analysis (GRA) was proposed to select a suitable fluid pair for the DORC system, which was driven by the flue gas from a rotary kiln with a temperature of 573.15 K. A multi-objective optimization was conducted for the thermodynamic performance (exergy efficiency, EXE), economic performance (payback period, PBP), and environmental performance (annual CO2 emission reduction, AER) by using non-dominated sorting genetic algorithm-II (NSGA-II). Then, priorities of 27 candidate pairs were sequenced based on their grey relational degree. The results show that cyclohexane/butane has the best comprehensive performance among 27 alternatives, and the EXE, AER, and PBP are 0.53, 3.43 years, and 14,100 tons, respectively. Boiling point temperature is a criterion of fluid selection for the DORC system. The most suitable boiling point temperature of working fluid in high-temperature (HT) loop and low-temperature (LT) loop is 330–363 K and 255–305 K, respectively. This research provides a general methodology to evaluate working fluids of different ORC systems to recover low-medium temperature waste heat.

Parametric analysis and optimization of transcritical-subcritical dual-loop organic Rankine cycle using zeotropic mixtures for engine waste heat recovery

Dual-loop organic Rankine cycle is a great potential technology to recover engine waste heat for energy saving. Transcritical cycle can improve exergy efficiency of heat transfer process in the evaporator, and zeotropic mixtures furtherly can improve thermal match with heat source and sink. Thus, a transcritical-subcritical dual-loop organic Rankine cycle system using zeotropic mixtures is adopted for engine waste heat recovery, and system without and with regenerator is considered. The whole system is assumed in the steady state, R600a/R601a and R134a/R245fa mixtures are used as working fluid of transcritical and subcritical cycle, respectively. Parametric analysis and optimization about turbine inlet temperature and pressure of high-temperature loop, evaporation temperature of low-temperature loop are carried out. According to the results, the designed parameters have great effect on performance of system, and for the system without and with regenerator, the optimal turbine inlet temperature, pressure and evaporation temperature are 260 °C, 11 MPa, 85 °C and 250 °C, 10 MPa, 85 °C respectively. Moreover, the effects of components of mixtures on the performance of system are analyzed. The results demonstrate that system with zeotropic mixtures can significantly improve the performance of system. The system without regenerator using R600a/R601a (0.2/0.8) and R134a/R245fa (0.4/0.6) shows the maximum power output of 97.49 kW which is higher 5.48–20.00% than pure fluids, and system with regenerator using R600a/R601a (0.3/0.7) and R134a/R245fa (0.4/0.6) achieves the maximum power output of 97.95 kW, increment of 6.52–19.78% compared with using pure fluids. And the power output of engine can be improved by 9.79% and 9.83% for the system without and with regenerator, respectively. Furthermore, the exergy destruction analysis demonstrates that zeotropic mixtures can reduce heat transfer exergy destruction and total exergy destruction of system.

Thermoeconomic multi-objective optimization of a dual loop organic Rankine cycle (ORC) for CNG engine waste heat recovery

Asbtract In this paper, a thermoeconomic model of a dual loop organic Rankine cycle (ORC) system has been developed to analyze both the thermodynamic and economic performance of several working fluid groups for the purpose of compressed natural gas (CNG) engine waste heat recovery. The effects of six key parameters on the thermoeconomic indicators of the dual loop ORC system are investigated. Furthermore, a multi-objective genetic algorithm (GA) is employed to solve the Pareto optimal solutions from the viewpoints of maximizing net power output and minimizing total investment cost over the whole operating range of the CNG engine. The most suitable working fluid group is screened out, then the optimal parameter regions are determined. The results show that a higher evaporation pressure and a lower condensation temperature exhibit a positive effect on the thermoeconomic performances of the dual loop ORC system while the effects of variation in superheat degree and exhaust outlet temperature on the thermoeconomic performances are not obvious. The optimal evaporation pressure of the high temperature loop ORC (HT cycle) is always above 2.5 MPa. The optimal condensation temperature of the HT cycle, optimal evaporation temperature and condensation temperature of the low temperature loop ORC (LT cycle) are all kept almost constants. In addition, the optimal exhaust outlet temperature is mainly influenced by the engine speed. At the rated condition, the dual loop ORC system has the maximum net power output of 23.62 kW and the minimum electricity production cost (EPC) of 0.41 $/kW h. The thermal efficiency of the dual loop ORC system is in the range of 8.97–10.19% over the whole operating range.

A novel cascade organic Rankine cycle (ORC) system for waste heat recovery of truck diesel engines

Waste heat recovery (WHR) of engines has attracted increasingly more concerns recently, as it can improve engine thermal efficiency and help truck manufacturers meet the restrictions of CO2 emission. The organic Rankine cycle (ORC) has been considered as the most potential technology of WHR. To take full advantage of waste heat energy, the waste heat in both exhaust gases and the coolant need to be recovered; however, conventional multi-source ORC systems are too complex for vehicle applications. This paper proposed a confluent cascade expansion ORC (CCE-ORC) system for engine waste heat recovery, which has simpler architecture, a smaller volume and higher efficiency compared with conventional dual-loop ORC systems. Cyclopentane is analyzed to be regarded as the most suitable working fluid for this novel system. A thermodynamic simulation method is established for this system, and off-design performance of main components and the working fluid side pressure drop in the condenser have been taken into consideration. System performance simulations under full engine operating conditions are conducted for the application of this system on a heavy-duty truck diesel engine. Results show that the engine peak thermal efficiency can be improved from 45.3% to 49.5% where the brake specific fuel consumption (BSFC) decreases from 185.6g/(kWh) to 169.9g/(kWh). The average BSFC in the frequently operating region can decrease by 9.2% from 187.9g/(kWh) to 172.2g/(kWh). Compared with the conventional dual-loop ORC system, the CCE-ORC system can generate 8% more net power, while the total volume of heat exchangers is 18% less.

Thermo-economic analysis of zeotropic mixtures based on siloxanes for engine waste heat recovery using a dual-loop organic Rankine cycle (DORC)

Siloxanes are usually used in the high temperature organic Rankine cycle (ORC) for engine waste heat recovery, but their flammability limits the practical application. Besides, blending siloxanes with retardants often brings a great temperature glide, causing the large condensation heat and the reduction in net output power. In view of this, the zeotropic mixtures based on siloxanes used in a dual-loop organic Rankine cycle (DORC) system are proposed in this paper. Three kinds of binary zeotropic mixtures consisting of R123 and various siloxanes (octamethylcyclotetrasiloxane ‘D4’, octamethyltrisiloxane ‘MDM’, decamethyltetrasiloxane ‘MD2M’), represented by D4/R123, MDM/R123 and MD2M/R123, are selected as the working fluid of the high temperature (HT) cycle. Meanwhile, R123 is always used in the low temperature (LT) cycle. The net output power and utilization of heat source are considered as the evaluation indexes to select the optimal mixture ratios for further analysis. Based on the thermodynamic and economic model, net output power, thermal efficiency, exergy efficiency, exergy destruction and electricity production cost (EPC) of the DORC system using the selected mixtures have been investigated under different operating parameters. According to the results, the DORC based on D4/R123 (0.3/0.7) shows the best thermodynamic performance with the largest net power of 21.66kW and the highest thermal efficiency of 22.84%. It also has the largest exergy efficiency of 48.6% and the smallest total exergy destruction of 19.64kW. The DORC using MD2M/R123 (0.35/0.65) represents the most economic system with the smallest EPC of 0.603 $/kWh. Besides, the irreversibility in the internal heat exchanger, turbine and evaporator of HT cycle contributes most to the total exergy destruction which can serve as the parameter to be optimized in the further study.

A regenerative supercritical-subcritical dual-loop organic Rankine cycle system for energy recovery from the waste heat of internal combustion engines

Organic Rankine cycle (ORC) system is considered as a promising technology for energy recovery from the waste heat rejected by internal combustion (IC) engines. However, such waste heat is normally contained in both coolant and exhaust gases at quite different temperatures. A single ORC system is usually unable to efficiently recover energy from both of these waste heat sources. A dual loop ORC system which essentially has two cascaded ORCs to recover energy from the engine’s exhaust gases and coolant separately has been proposed to address this challenge. In this way, the overall efficiency of energy recovery can be substantially improved. This paper examines a regenerative dual loop ORC system using a pair of environmentally friendly refrigerants, R1233zd and R1234yf, as working fluids, to recover energy from the waste heat of a compressed natural gas (CNG) engine. Unlike most previous studies focusing on the ORC system only, the present research analyses the ORC system and CNG engine together as an integrated system. As such, the ORC system is analysed on the basis of real data of waste heat sources of the CNG engine under various operational conditions. A numerical model is employed to analyse the performances of the proposed dual loop cycle with four pairs of working fluids. The effects of a regenerative heat exchanger and several other key operating parameters are also analysed and discussed in detail. The performance of the integrated engine-ORC system is then analysed under actual engine operating conditions which were measured beforehand. The performance of the proposed system under off-design conditions has also been analysed. The obtained results show that the proposed dual loop ORC system could achieve better performance than other ORC systems for similar applications.

Parametric optimization and heat transfer analysis of a dual loop ORC (organic Rankine cycle) system for CNG engine waste heat recovery

In this study, a dual loop ORC (organic Rankine cycle) system is adopted to recover exhaust energy, waste heat from the coolant system, and intercooler heat rejection of a six-cylinder CNG (compressed natural gas) engine. The thermodynamic, heat transfer, and optimization models for the dual loop ORC system are established. On the basis of the waste heat characteristics of the CNG engine over the whole operating range, a GA (genetic algorithm) is used to solve the Pareto solution for the thermodynamic and heat transfer performances to maximize net power output and minimize heat transfer area. Combined with optimization results, the optimal parameter regions of the dual loop ORC system are determined under various operating conditions. Then, the variation in the heat transfer area with the operating conditions of the CNG engine is analyzed. The results show that the optimal evaporation pressure and superheat degree of the HT (high temperature) cycle are mainly influenced by the operating conditions of the CNG engine. The optimal evaporation pressure and superheat degree of the HT cycle over the whole operating range are within 2.5–2.9 MPa and 0.43–12.35 K, respectively. The optimal condensation temperature of the HT cycle, evaporation and condensation temperatures of the LT (low temperature) cycle, and exhaust temperature at the outlet of evaporator 1 are kept nearly constant under various operating conditions of the CNG engine. The thermal efficiency of the dual loop ORC system is within the range of 8.79%–10.17%. The dual loop ORC system achieves the maximum net power output of 23.62 kW under the engine rated condition. In addition, the operating conditions of the CNG engine and the operating parameters of the dual loop ORC system significantly influence the heat transfer areas for each heat exchanger.

Thermodynamic analysis of a novel dual-loop organic Rankine cycle for engine waste heat and LNG cold

The marine sector produces a large portion of total air pollution, so the emissions of the engines used must be improved. This can be achieved using a new eco-friendly engine and waste-heat recovery system. A dual-fuel (DF) engine has been introduced for LNG carriers that is eco-friendly and has high thermal efficiency since it uses natural gas as fuel. The thermal efficiency could be further improved with the organic Rankine cycle (ORC). A novel dual-loop ORC system was designed for DF engines. The upper ORC loop recovers waste heat from the exhaust gas, and the bottom ORC loop recovers waste heat from the jacket cooling water and LNG cold. Both ORC loops were optimized to produce the maximum net work output. The optimum simple dual-loop ORC with n-pentane and R125 as working fluids produces an additional power output of 729.1 kW, which is 4.15% of the original engine output. Further system improvement studies were conducted using a recuperator and preheater, and the feasibility of using boil-off gas as a heat sink was analyzed. Optimization of the system configuration revealed that the preheater and recuperator with n-pentane and R125 as working fluids increase the maximum net work output by 906.4 kW, which is 5.17% of the original engine output.