Performance Of Organic Rankine Cycle System Research Articles

Influences of fluid corrosivity and heat exchanger materials on design and thermo-economic performance of organic Rankine cycle systems

Various heat sources and cooling fluids bring different requirements on the anti-corrosion performance of heat exchangers in organic Rankine cycle (ORC) systems. Using different materials may change optimal parameters and thermo-economic performance of ORC systems due to substantially different thermal conductivities and cost coefficients. This paper studied the influences of heat exchanger materials on optimal design and thermo-economic performance of ORC systems for various heat source conditions. The widely-used shell-and-tube heat exchanger was selected, and carbon steel, copper, and stainless steel were focused. The utilization costs of different types of heat sources were compared. Results indicate that the corrosivity of heat source and cooling fluid is a key factor deciding the utilization cost. The difference of specific investment cost (SIC) caused by different heat exchanger materials is up to 25.5%, which substantially exceeds that caused by different working fluids (2.3%). Even the corrosivity restriction is not considered, the maximum difference in SIC is still 11.0%, and the low-cost material is preferred. The lower the temperature and flow rate of heat source, the more significant the influences of heat exchanger materials. The improper selection of heat exchanger materials will underestimate the economic benefits of ORC technology.

Grundfos Holdings A/S, Denmark

Various heat sources and cooling fluids bring different requirements on the anti-corrosion performance of heat exchangers in organic Rankine cycle (ORC) systems. Using different materials may change optimal parameters and thermo-economic performance of ORC systems due to substantially different thermal conductivities and cost coefficients. This paper studied the influences of heat exchanger materials on optimal design and thermo-economic performance of ORC systems for various heat source conditions. The widely-used shell-and-tube heat exchanger was selected, and carbon steel, copper, and stainless steel were focused. The utilization costs of different types of heat sources were compared. Results indicate that the corrosivity of heat source and cooling fluid is a key factor deciding the utilization cost. The difference of specific investment cost (SIC) caused by different heat exchanger materials is up to 25.5%, which substantially exceeds that caused by different working fluids (2.3%). Even the corrosivity restriction is not considered, the maximum difference in SIC is still 11.0%, and the low-cost material is preferred. The lower the temperature and flow rate of heat source, the more significant the influences of heat exchanger materials. The improper selection of heat exchanger materials will underestimate the economic benefits of ORC technology.

Characterization analysis of dynamic behavior of basic ORC under fluctuating heat source

Organic Rankine Cycle (ORC) is widely investigated for low-grade waste heat recovery, but its performance deteriorates under heat source with thermal power fluctuations. As the effects of fluctuating heat source on ORC system performance is less investigated, the operating rules of ORC systems under fluctuating heat source are not clear. In this study, the effects of fluctuating amplitude and period for fluctuating heat sources on the performance of a basic ORC system are studied in detail, considering the proposed indicators including safety and efficiency. Afterwards, the allowable fluctuation ranges of heat source are obtained based on requirements of safety and efficiency. Finally, the impact of the average temperature of fluctuating heat sources on the allowable fluctuation range is systematically investigated. The results indicate that the safety limitations and efficiency of the basic ORC system are generally negatively related to fluctuating amplitude and period, and the allowable fluctuation range is generally positively related to average temperature of heat source. To ensure the efficient operation of basic ORC, the allowable relative amplitude is 25–40 K under large fluctuating period while it is higher than 35 K with the fluctuating period smaller than 200 s. This study contributes to provide valuable reference for evaluating the performance of an existing ORC system under different fluctuating heat source conditions.

Performance Investigation of Solar Organic Rankine Cycle System With Zeotropic Working Fluid Mixtures for Use in Micro-Cogeneration

Abstract Overall, there are numerous sustainable sources of renewable, low-temperature heat, principally solar energy, geothermal energy, and energy produced from industrial wastes. Extended utilization of these low-temperature alternatives has a certain capacity of decreasing fossil fuel use with its associated very hazardous greenhouse gas emissions. Researchers have commonly recognized the organic Rankine cycle (ORC) as a feasible and suitable system to produce electrical power from renewable sources based on its advantageous use of volatile organic fluids as working fluids (WFs). Researchers have similarly shown an affinity to the exploitation of zeotropic mixtures as ORC WFs due to their capability to enhance the thermodynamic performance of ORC systems, an achievement supported by improved fits of the temperature profiles of the WF and the heat source/sink. This paper determines both the technical feasibility and the benefits of using zeotropic mixtures as WFs by means of a simulation study of an ORC system. This study analyzes the thermodynamic performance of ORC systems using zeotropic WF mixtures to produce electricity driven by low-temperature solar heat sources for use in buildings. A thermodynamic model is created with an ORC system with and without a regenerator. Five zeotropic mixtures with diverse compositions between 0 and 1 in 0.2 increments of R245fa/propane, R245fa/hexane, R245fa/heptane, pentane/hexane, and isopentane/hexane are assessed and compared with identify the best blends of mixtures that are able to produce superior efficiency in their system cycles. Results disclosed that R245fa/propane (0.4/0.6) with regenerator produces the highest net power output of 7.9 kW and cycle efficiency of 9.4% at the operating condition with a hot source temperature of 85 °C. The study also investigates the effects of the volume flow ratio, and evaporation and condensation temperature glide on the ORC’s thermodynamic performance. Following a thorough analysis of each mixture, R245fa/propane is chosen for a parametric study to examine the effects of operating factors on the system’s efficiency and sustainability index. It was found that the highest cycle efficiency and highest second law cycle efficiency of around 10.5% and 84.0%, respectively, were attained with a mass composition of 0.6/0.4 at the hot source temperature of 95 °C and cold source temperature of 20 °C with a net power output of 9.6 kW. Moreover, results revealed that for zeotropic mixtures, there is an optimal composition range within which binary mixtures are tending to work more efficiently than the component pure fluids. In addition, a significant increase in cycle efficiency can be achieved with a regenerative ORC, with cycle efficiency in the range 3.1–9.8% versus 8.6–17.4% for ORC both without and with regeneration, respectively. In conclusion, utilizing zeotropic mixtures may well expand the restriction faced in choosing WFs for solar-powered ORC-based micro-combined heat and power (CHP) systems.

Thermodynamic, Economic and Sustainability Analysis of Solar Organic Rankine Cycle System with Zeotropic Working Fluid Mixtures for Micro-Cogeneration in Buildings

Globally there are several viable sources of renewable, low-temperature heat (below 130 °C), particularly solar energy, geothermal energy, and energy generated from industrial wastes. Increased exploitation of these low-temperature options has the definite potential of reducing fossil fuel consumption with its attendant very harmful greenhouse gas emissions. Researchers have universally identified the organic Rankine cycle (ORC) as a practicable and suitable system to generate electrical power from renewable sources based on its beneficial usage of volatile organic fluids as working fluids (WFs). In recent times, researchers have also shown a preference towards deployment of zeotropic mixtures as ORC WFs because of their capacity to improve thermodynamic performance of ORC systems, a feat enabled through the greater matching of the temperature profiles of the WF and the heat source/sink. This paper demonstrates the thermodynamic, economic and sustainability feasibility, and the notable advantages of using zeotropic mixtures as WFs through a simulation study of an ORC system. The study examines first the thermodynamic performance of ORC systems using zeotropic mixtures to generate electricity powered by a low-temperature solar heat source for building applications. A thermodynamic model is developed with a solar-driven ORC system both with and excluding a regenerator. Twelve zeotropic mixtures with varying compositions are evaluated and compared to identify the best combinations of mixtures that can yield high performance and high efficiency in their system cycles. The study also examines the effects of the volume flow ratio, and evaporation and condensation temperature glides on the ORC’s thermodynamic performance. Following a detailed analysis of each mixture, R245fa/propane and butane/propane are selected for parametric study to investigate the influence of operating parameters on the system’s efficiency and sustainability index. For zeotropic mixtures, results disclosed that there is an optimal composition range within which binary mixtures are inclined to perform more efficiently than the component pure fluids. In addition, a substantial enhancement in cycle efficiency can be obtained by a regenerative ORC, with cycle efficiency ranging between 3.1–9.8% and 8.6–17.4% for ORC both without and with regeneration, respectively. Results also revealed that exploiting zeotropic mixtures could enlarge the limitation experienced in selecting WFs for low-temperature solar ORCs. Moreover, a detailed economic with a sensitivity analysis of the solar ORC system was performed to evaluate the cost of the electricity and other economic criteria. The outcome of this investigation should be useful in the thermo-economic feasibility assessments of solar-driven ORC systems using working fluid mixtures to find the optimum operating range for maximum performance and minimum cost.

Optimal design of an organic Rankine cycle system considering the expected variations on heat sources

This paper addresses the optimal design of an organic Rankine cycle (ORC) system with consideration of the expected variations on heat sources in multi-product plants and multi-period operation processes. For a series of the given heat sources with variable numbers and temperatures, a two-step method was proposed to reach the maximum thermal efficiency and to reduce the investment cost of the system as much as possible. In the first step, a multi-period optimization model that contains a rigorous thermodynamic sub-model is developed to determine the optimal configuration of ORC system, including the optimal working fluid and the corresponding operating parameters of an ORC system. In the second step, the heat recovery network connecting the heat sources and the ORC system is optimized to minimize the investment cost of the integrated system through a multi-period time-sharing mechanism. The effects of the heat source variations on the selection of working fluid, the performance and economy of ORC system are thus analyzed. The effectiveness and application of the proposed method are finally verified via a case study of a keto-benzene dewaxing process.

New classification of dry and isentropic working fluids and a method used to determine their optimal or worst condensation temperature used in Organic Rankine Cycle

The organic Rankine cycle (ORC) is a popular technology for utilization of medium-low grade thermal energy. The thermophysical properties of working fluids can greatly affect the economic performance of ORC system that is an important factor affecting its application and development. Heat exchangers account for the largest proportion of investment in the whole ORC system. In this paper, a curved triangle is defined to describe the condensation characteristics of dry and isentropic working fluids. Using this model, a new classification of dry and isentropic working fluids and their application characteristics in ORC are proposed. Moreover, a method used to determine the theoretical optimal or the theoretical worst condensation temperature of dry and isentropic working fluids is proposed. The work in this paper may offer a good help for the determination of practical condensation temperature of ORC system and its design and establishment.

Integration of solar latent heat storage towards optimal small-scale combined heat and power generation by Organic Rankine Cycle

Abstract Thermal energy and distributed electricity demand are continuously increased in areas poorly served by a centralized power grid. In many cases, the deployment of the electricity grid is not economically feasible. Small-scale Organic Rankine Cycle (ORC) appears as a promising technology that can be operated by solar energy, providing combined heat and power (CHP) generation. Additionally, thermal energy storage can ensure stable and continuous operation in case of scarce thermal energy availability. This paper evaluates the potential application of latent heat storage to enhance solar ORC performance at operating temperatures between 80 °C and 140 °C, aiming at improving the efficiency and capacity of ORC for low-cost non-concentrating solar-thermal collectors. Three thermal energy storage scenarios are considered. Scenario 1 and 2 consist of reference cases based on a solar ORC system integrated with a conventional hot water tank and a pressurised water tank. Scenario 3 implements a storage unit based on a phase change material. The simulation was carried out through models developed in TRNSYS for solar energy balance and ASPEN for ORC system performance. The results show that solar latent heat storage tank can provide 54% of useful collector gains with a higher and narrower temperature range in the evaporator, increasing the annual thermal energy capacity by 19%, reducing annual heat losses by 66% and decreasing the investment cost by 50% in comparison with a pressurised water tank. It also allows increasing the efficiency of ORC cycle by approximately 18% (from 8.9% to 10.5%) with a higher net generated power than a conventional water tank integration, scaled up from 498 W to 1628 W. These results highlight the potential benefits that latent heat integration provides to improve the low-cost solar ORC performance for powering electricity and thermal energy supply.

Experimental investigation of operation behavior of plate heat exchangers and their influences on organic Rankine cycle performance

As an efficient and reliable heat-to-power conversion technology, the Organic Rankine Cycle (ORC) has been recognized and accepted throughout academia and industry. Much effort has been devoted to the theoretical analysis, numerical simulation, and optimization in past decades. Experimental investigation has recently drawn considerable attention to practical applications. Most of experimental studies focused on the performance test of working fluid, expander, pump, and system. Although heat exchangers are the major contributors of exergy destruction and investment cost, few experimental studies have paid attention to heat exchangers and their effects on the performance of the ORC system. In this study, an ORC test rig with switchable evaporators and condensers is established to investigate the operation behavior of these heat exchangers and the influences of their heat transfer area on the performance of ORC system. An equivalent overall heat transfer coefficient, together with the degree of superheat, degree of subcooling, temperature difference and location at pinch point are used to evaluate operation behavior of the evaporators and condensers. A heat exchanger area utilization indicator is proposed, and applied along with the net power output and thermal efficiency to access the performance of the ORC with different heat exchanger combinations. The operation characteristics of these heat exchangers and the ORC system are experimentally investigated. Experimental results demonstrate that the operation behavior of these heat exchangers and the ORC systems are remarkably influenced by their heat transfer areas and combinations.

Analysis of heat transfer and irreversibility of organic rankine cycle evaporator for selecting working fluid and operating conditions

Organic Rankine cycle (ORC) is suitable to converting the normally hard to utilize low temperature thermal energies, such as geothermal energy, solar energy, and industrial waste heat, to electricity through utilizing low boiling organic working fluids. The performance of ORC system is dramatically affected by the selections of working fluid and working conditions. As a key component of waste heat recovery, the irreversible loss of evaporator also has great influence on the performance of ORC system. In this paper, we study the heat transfer performance in evaporator under the condition that the heat source parameters and pinch point temperature difference are identified. It is found that the heat transfer performance is affected by Cr, the ratio of heat capacity flow rates between the working fluid and the heat source fluid. The equivalent thermal resistance, deducing from the concept of entransy, to measure the irreversability during the heat transfer process is used. Then, the parameter ?r, the ratio between latent heat and sensible heat of working fluid is defined. With the parameters Cr and ?r, we investigate the relationship between the heat transfer and irreversible loss, and deduce the condition that maximum heat transfer and minimum equivalent thermal resistance occurs. Finally, a calculation method is established to choose the optimum working fluid and the evaporation condition.

Performance analysis of an Organic Rankine Cycle system using evaporative condenser for sewage heat recovery in the petrochemical industry

An Organic Rankine Cycle (ORC) system using evaporative condenser was developed and built for sewage heat recovery in the petrochemical industry. The ORC system using evaporative condenser enhances the heat rejection process by utilizing the evaporation cooling effect and therefore can improve the efficiency of energy conversion. A 4E (Energy, exergy, economy, environment) analysis method is used to investigate and assess the performance of the presented ORC system based on the actual operating data. The performance of ORC system using evaporative condenser is greatly affected by the ambient temperature. The result shows that when the ambient temperature rose from 0 °C to 30 °C, the energy efficiency of the ORC system decreased by 2.11% and the average cost per unit exergy increased by 1.87 $/GJ. The investment payback period of the integrated ORC system is estimated to be 2.3 years. Finally, the emission reduction of CO2, SO2 and NOx is 1513.02 t, 46.89 t and 23.45 t per year. Therefore, the results show the presented system has good performance and can produced great economic and environmental benefits.

Effects of shell-and-tube heat exchanger arranged forms on the thermo-economic performance of organic Rankine cycle systems using hydrocarbons

Shell-and-tube heat exchangers are crucial and widely used components in organic Rankine cycle (ORC) systems. Heat exchanger arranged forms, namely, the organic fluid flows either inside or outside the tubes, substantially affect the purchased cost and exergy loss of heat exchangers; and thereby considerably influencing the system thermo-economic performance. Based on various heat exchanger arranged forms, the thermo-economic performance of ORC systems may differ remarkably and even result in opposite conclusions, such as in selecting optimal working fluids and ratios of component purchased cost. However, no published studies have focused on this specific issue. Influence degree exerted by the heat exchanger arranged form on the thermo-economic performance of ORC system remains unclear. Selection guidelines for the optimal arranged forms also need to be developed. This study analyzed the effects of heat exchanger arranged forms on the thermo-economic performance of subcritical ORC systems with shell-and-tube heat exchangers. Three common heat exchanger arranged forms were studied. Heat source temperatures are 100–200 °C, and five hydrocarbons are working fluids. Results show that the heat exchanger arranged forms substantially affect the specific investment cost (SIC) of ORC system, and the largest difference in SIC reaches 14.7% among various arranged forms. The optimal working fluid and system parameters, and ratios of component purchased costs also differ remarkably. Optimal heat exchanger arranged form depends on the working fluid and heat source temperature. This study provides the optimal heat exchanger arranged forms for five hydrocarbons at heat source temperatures of 100–200 °C.

Experimental study of operating load variation for organic Rankine cycle system based on radial inflow turbine

Based on the self-developed high-speed radial inflow turbine and R245fa as the working fluid, organic Rankine cycle (ORC) testing rig of low-temperature waste heat power generation was built. The effects of different operating load power on the ORC system performance were studied through the experiment. The results show that the isentropic efficiency of the turbine, output power, electric power, thermal efficiency, and exergy efficiency of system, all reach the maximum values at the optimal load power of 1.7 kW, and at this condition, electric power, thermal efficiency and exergy efficiency of system are 1.424 kW, 2.52% and 8.09%, respectively. Therefore, when the ORC system is designed, the turbine, generator and load power should be matched to the optimal performance of the system. By grey relational analysis (GRA), it is concluded that electric power has the closest correlation with load power, with its grey relational grade being 0.632. Compared with the exergy efficiency, the thermal efficiency maintains a higher relational grade. In addition, according to exergy analysis, the evaporator is the component with the highest exergy loss, followed by the condenser.

Thermoeconomic optimisation of small-scale organic Rankine cycle systems based on screw vs. piston expander maps in waste heat recovery applications

The high cost of organic Rankine cycle (ORC) systems is a key barrier to their implementation in waste-heat recovery (WHR) applications. In particular, the choice of the expansion device has a significant influence on this cost, strongly affecting the economic viability of an installation. In this work, numerical simulations and optimisation strategies are used to compare the performance and profitability of small-scale ORC systems using reciprocating-piston or single/two-stage screw expanders when recovering heat from the exhaust gases of a 185-kW internal combustion engine operating in baseload mode. The study goes beyond previous work by directly comparing these small-scale expanders for a broad range of working fluids, and by exploring the sensitivity of project viability to key parameters such as electricity price and onsite heat demand. For the piston expander, a lumped-mass model and optimisation based on artificial neural networks are used to generate performance maps, while performance and cost correlations from the literature are used for the screw expanders. The thermodynamic analysis shows that two-stage screw expanders typically deliver more power than either single-stage screw or piston expanders due to their higher conversion efficiency at the required pressure ratios. The best fluids for the proposed application are acetone and ethanol, as these provide a compromise between the exergy losses in the condenser and in the evaporator. The maximum net power output is found to be 17.7 kW, from an ORC engine operating with acetone and a two-stage screw expander. On the other hand, the thermoeconomic optimisation shows that reciprocating-piston expanders show a potential for lower specific costs, and since piston-expander technology is not mature, especially at these scales, this finding motivates further consideration of this component. A minimum specific investment cost of 1630 €/kW is observed for an ORC engine with a piston expander, again with acetone as the working fluid. This system, optimised for minimum cost, gives the shortest payback time of 4 years at an avoided electricity cost of 0.13 €/kWh. Finally, financial appraisals show a high sensitivity of the investment profitability to the value of produced electricity and to the heat-demand intensity.

Experimental study of micro-scale organic Rankine cycle system based on scroll expander

The organic Rankine cycle (ORC) is a promising technology to recover low grade waste heat. In this study, the experimental research is conducted to investigate the dynamic response performance of a micro-scale ORC system driven by low-grade heat sources at different loads. A scroll compressor is modified as the expansion machine in the ORC module and its isentropic efficiency under variable loads conditions is tested in the experiments. This work is investigated to assess the steady-state and dynamic performance of micro-scale ORC system at different loads conditions. The maximum electricity production and isentropic efficiency of the expander are 120 W and 43%, respectively, under the steady state experiments. On the other hand, in the dynamic experiment, the micro-scale ORC system has balance ability for disturbance caused by the load changes. If the disturbance is too large, the ORC system is unstable and could not be restored to a stable state by simple load reduction operation. The outcomes of experiment can establish foundation to the optimal system controls and give more information of optimal system design for the micro-scale ORC system.

Organic Rankine cycle systems for engine waste-heat recovery: Heat exchanger design in space-constrained applications

Organic Rankine cycle (ORC) systems are a promising solution for improving internal combustion engine efficiencies, however, conflicts between the pressure drops in the heat exchangers, overall thermodynamic performance and economic viability are acute in this space-constrained application. This paper focuses on the interaction of the heat exchanger pressure drop (HEPD) and the thermo-economic performance of ORC systems in engine waste-heat recovery applications. An iterative procedure is included in the thermo-economic analysis of such systems that quantifies the HEPD in each case, and uses this information to revise the cycle and to resize the components until convergence. The newly proposed approach is compared with conventional methods in which the heat exchangers are sized after thermodynamic cycle modelling and the pressure drops through them are ignored, in order to understand and quantify the effects of the HEPD on ORC system design and working fluid selection. Results demonstrate that neglecting the HEPD leads to significant overestimations of both the thermodynamic and the economic performance of ORC systems, which for some indicators can be as high as >80% in some cases, and that this can be effectively avoided with the improved approach that accounts for the HEPD. In such space-limited applications, the heat exchangers can be designed with a smaller cross-section in order to achieve a better compromise between packaging volume, heat transfer and ORC net power output. Furthermore, we identify differences in working fluid selection that arise from the fact that different working fluids give rise to different levels of HEPD. The optimized thermo-economic approach proposed here improves the accuracy and reliability of conventional early-stage engineering design and assessments, which can be extended to other similar thermal systems (i.e., CO2 cycle, Brayton cycle, etc.) that involve heat exchangers integration in similar applications.

Evaluation of organic Rankine cycle by using hydrocarbons as working fluids: Advanced exergy and advanced exergoeconomic analyses

Hydrocarbons (HCs) are characterized by extremely low global warming potential (GWP) and are promising working fluid for organic Rankine cycle (ORC). A comprehensive advanced evaluation model is developed to study the energetic, advanced exergy and economic performance of ORC system by using twelve HCs, considering the heat recovery from geothermal, low-temperature solar, engine waste gas heat, and high-temperature solar applications. The results show that cyclohexane obtains the highest value of thermal efficiency for high-temperature solar energy. The exergy efficiency of ORC is improved by about 20% after the system optimization through the advanced exergy analysis. In addition, the recoverable effect for the four major components can be ranked as expander, evaporator, condenser, and pump. Exergetic improvement potential ratio of expander by employing propyne and isopentane obtains the highest value of 12.41% and 12.60% at the heat source temperature of 115 and 140 °C, respectively. The lowest value of levelized energy cost are propyne, pentane, cyclohexane, and cyclohexane, which are 1.46, 1.28, 1.05, and 0.95 USD/kWh. Isobutene, isopentane, cyclohexane, and cyclohexane obtain the highest endogenous avoidable cost corresponding to the four heat sources. The endogenous avoidable cost is relatively sensitive to the heat source temperature, and it is reduced by 28% with the heat source temperature increasing from 100 to 150 °C, while it is insensitive to the expander efficiency.

Experimental investigation of a small-scale Organic Rankine Cycle under off-design conditions: From the perspective of data fluctuation

The Organic Rankine Cycle (ORC) technology is applicable for conversion of low-grade waste heat to electric power. Off-design performance of ORC system is one of the research hotspot. In the present paper, a small-scale ORC experimental bench was constructed with a scroll expander whose nominal output power was 3 kW. Experimental data including thermodynamic parameters at inlet and outlet of the main components from 15 steady-state operation conditions were collected. Special attention has been paid to analyzing the amplitude of data fluctuation. Standard deviation has been introduced to quantify the fluctuation amplitude. The comparison between the accuracy of measuring sensors and the amplitude of data fluctuation was conducted. Moreover, critical parameters affecting the expander output power calculation were investigated. Results indicated that the largest amplitude of the measured pressure and temperature fluctuation were at the pump outlet and evaporator outlet respectively. Besides, the fluctuation amplitude of measured volume flow rate in gas phase was greater than the liquid phase due to strong turbulence of gas phase. Furthermore, the measured volume flow rate of working fluid had a vital effect on the calculated expander output power compared to the measured temperatures and pressures.

Design space exploration for waste heat recovery system in automotive application under driving cycle

Organic Rankine Cycle (ORC) systems have been recently considered for the promising application to Waste Heat Recovery (WHR) in automotive application. However, the design of ORC systems for highly transient operations typical of heavy-duty vehicles faces several challenges. This work extends the method of Design Space Exploration (DSE) to optimize the design of an ORC system for heavy-duty vehicle applications. The goal is to match the size of ORC system with the feature areas of driving cycle. Starting from drive cycle data, the feature areas are identified to represent the driving cycle. Then, a Weighted Least Square method is adopted to determine the initial design condition from feature areas, and based on the design condition, the initial ORC model is built for the further optimization. Particle Swarm Optimization (PSO) with parallel computation is adopted to optimize the performance of nonlinear ORC system with coupled constraints. The result shows that the optimal system can output 6.87% more power than the initial system and 20.08% more power than the inferior system over the feature areas of WHTC test. The analysis shown in this work also includes general design guidelines for optimizing the system architecture and optimally sizing the heat exchangers to improve the ORC thermal efficiency.

Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications

Abstract Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow-rate variations). Maximising ORC system performance while accounting for off-design operation in response to such variations is of crucial importance for the financial viability and wider adoption of this technology. In this paper, the off-design performance of an ORC engine is investigated in a heat-recovery application from a stationary internal combustion engine (ICE), employing a screw expander and two heat exchanger (HEX) architectures. Unlike previous studies where the ORC expander and HEX performance are assumed fixed during off-design operation, here we consider explicitly the time-varying characteristics of these system components. Nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, and that the heat transfer area requirements of plate HEXs (PHEXs) are 50% lower than those of double-pipe HEXs (DPHEX). Following nominal system design, the ORC engine operation is optimised at conditions relating to part-load (PL) operation of the ICE. Although, the heat transfer coefficients in the evaporator decrease by 30% at PL, the HEX effectiveness increases by up to 20% due to higher temperature differences between the working fluid and the heat source, and the performance of the PHEX appears less sensitive to off-design operation conditions. The optimum PL screw expander efficiency maps reveal only a minor reduction of the expander efficiency (by 2%) relative to the design conditions. Optimum off-design maps indicate that the power output of the ORC engine reduces to 72% of full-load for an ICE at 60% of full-load, and that ORC engines with PHEXs generate slightly more power for the same heat-source conditions. Overall, the tool developed predicts ORC performance over an operating envelope and allows the selection of optimal HEX and expander designs. The findings can be used by ORC plant operators to optimise the ORC engine power output given varying on-site heat-source conditions, and by ORC vendors to inform HEX and expander design decisions.