Organic Rankine Cycle Evaporator Research Articles

Automated configuration of organic Rankine cycle system based on process simulations

In organic Rankine cycle (ORC) system, the selection of the structure and the optimization of operating parameters are usually studied separately, which makes the existing design principles inadequate to describe the impacts of the structure on ORC performance, and thus greatly limits its practical applications. To this end, this paper addresses an automated scheme for the optimal configuration of the ORC system, aiming to simultaneously determine the working fluid, the structure and the operating parameters corresponding to the maximal system efficiency. In this work, a comprehensive simulation and optimization of four common ORC structures is first conducted with 18 working fluid candidates and the most common heat sources with the inlet temperature ranging at 200–120 °C. The influence mechanisms of the ORC structure on the thermodynamic performance of the ORC system are then explored to generalize the qualitative and quantitative principles for ORC configuration. Results show that, for the single-pressure evaporation ORC (SPC) with a given inlet temperature of the heat source, the Pinch point type formed by the working fluid and the heat source changes from vaporization Pinch point (VPP) to VPP&PPP (preheat Pinch point) and then to PPP as the critical temperature of the working fluid decreases. The system efficiency of SPC reaches its maximum when the double Pinch point of VPP and PPP is formed and double-pressure evaporation ORC (DPC) reaches its maximum system efficiency when the double Pinch points of HVPP and LVPP are formed. Besides, it is found that when compared with the basic ORC, the regenerative ORC enables to improve the thermal efficiency and system efficiency of the system at the same time, whereas the bleeding regenerative ORC increases the thermal efficiency while reduces the system efficiency. For the DPC, the type of Pinch point formed by the heat source and the working fluid in the optimal working condition is the key to determine whether a system has multiple evaporation potential. Specially, for the heat source with variable inlet temperature, a linear relationship between the inlet temperature of the heat source and the critical temperature of the working fluid can be established to determine the multiple evaporation potential of a certain working fluid. Based on these results, an automatic scheme for ORC configuration is developed and expected to provide an effective guidance for the selection of working fluid, configuration optimization and parameter determination of the actual ORC heat recovery process.

Energy production and storage from a polygeneration system based on parabolic trough solar collector, proton exchange membrane fuel cell, organic Rankine cycle, and alkaline electrolyzer

Due to the limitations with fossil fuels consumption, it is necessary to pay more attention to clean energy sources such as solar energy. Using solar energy systems as a driven of hybrid systems and integrating with an energy storage technology can alleviate some of the problems. In the present paper, a novel solar driven-polygeneration energy system with electrical energy storage is introduced and investigated. The cycle power generation section is composed of parabolic trough collector field, proton-exchange membrane fuel cell, organic Rankine cycle, alkaline electrolyzer, and thermoelectric generator. The light energy of the sun is converted into thermal energy by the solar collector. This thermal energy prepares the necessary thermal duty of the organic Rankine cycle's evaporator. Then, the electricity generated by the organic Rankine cycle is used to electrify an electrolyzer. The oxygen and hydrogen obtained from electrolyzer are used to generate power and heat by the fuel cell. Then, the waste heat of the fuel cell goes to the hot end of the thermoelectric generator and electricity is generated. The electricity generated from the fuel cell, thermoelectric generator, and surplus of the organic Rankine cycle is stored via a hybrid storage system for times of need (at night, cloudy days and/ or peak consumption hours). Findings revealed that the proposed polygeneration cycle is capable of generating 22.5 kW of power. Furthermore, 140.8 kW of thermal power and 97.3 g/h of hydrogen fuel are generated by the solar collector field and electrolyzer, respectively. Moreover, sum exergy destruction, system efficiency, and net capacity of the storage system are 18.83 kW, 60.3%, and 89 m3, respectively. Three different scenarios are considered for the solar field design based on the determined collector numbers.

Synthesis and simultaneous optimization of multi-heat source multi-pressure evaporation organic Rankine cycle with mixed working fluid

The organic Rankine cycle (ORC)’s performance relies on heat source properties, cycle structures, and working fluids. Influences of these factors, however, are often interdependent, making it difficult to determine the optimal design of ORC systems. To address this issue, we propose a mixed-integer nonlinear programming model for the multi-pressure evaporation ORCs driven by multiple heat sources that not only optimizes operating conditions, but also identifies the best working fluid composition and cycle structure. Results demonstrate that the proposed model is accurate and effective. The dual-pressure evaporation ORC driven by multi-heat source shows a 10.87% rise in the net power output (NPO) compared to the one with single pressure evaporation. In general, 0.9/0.1 n-pentane/n-heptane is the optimal mixed working fluid generating 2.71–12.11% more power than the pure working fluids for the numerical studies. Dual-pressure evaporation ORC is found to be a balanced design for the multi-heat source driven ORC in terms of its NPO and cycle complexity.

Integrated Vapor Compression Chiller with Bottoming Organic Rankine Cycle and Onsite Low-Grade Renewable Energy

The present study offers a scheme to improve the performance of existing large-scale chillers. The system involves raising the temperature of the chiller’s cooling water stream using renewable energy sources by incorporating an organic Rankine cycle (ORC). The thermal analysis was conducted by raising the temperature of one-third of the approximately 200 ton chiller’s cooling water. The investigation was considered for ORC evaporator inlet temperature of 90~120 °C by the step of 10 °C. Various working fluids for the different ORC evaporator inlet temperatures were examined. Sensitivity analyses conducted on the degree of superheating, degree of subcooling, condenser saturation temperature, pinch point temperature differences of the ORC evaporator and condenser, and the mass flowrates of the heating and cooling streams were also reported. Genetic algorithm was employed to carry out the optimization. The best options for the ORC working fluid at the heating source ORC evaporator inlet temperatures of 90 °C was found to be DME, presenting an improvement of 48.72% in comparison with the rated coefficient of performance (COP) value of the VCC, with a renewable energy input requirement of 710 kW. At the heat source temperatures of 100 °C and 110 °C, butene, which presented an improvement in the COP equal to 48.76% and 68.85%, respectively, with the corresponding renewable energy requirements of 789.6 kW and 852 kW, was found to be the ideal candidate. Meanwhile, at the heat source inlet temperature of 120 °C, R1233zd (E), representing an improvement of 140.88% with the renewable energy input of around 1061 kW, was determined to be the most favorable ORC working fluid candidate.

Heat Exchangers in Carnot Batteries: Condensation and Evaporation in a Reversible Device

The combined heat pump–organic Rankine cycle is a thermal–electrical storage concept which allows the reversible use of components in both operation modes (loading and unloading the storage). This saves in terms of investment costs but also creates challenges during design and operation. A heat exchanger is an expensive component destined to be used for the reversible purposes of a heat pump condenser and an organic Rankine cycle evaporator. In this study, the operation of such an apparatus was evaluated based on an analytical model, experimental data and thermal imaging. This study shows that the model can predict the filling of the apparatus distinguished by liquid, vapour and the two-phase region. The thermal imaging supports the model and gives the location of the regions. Connecting both methods, a valid statement about the current condition of the heat exchanger is possible. Due to very small pinch points, the apparatus is not efficiently used in the investigated modes. Extending the pinch to 2 K can already save up to 46.1% of the heat exchange area. The quality of the heat transfer in the evaporator (q˙ORC = 10.9 kW/m2) is clearly higher than in the condenser (q˙HP = 6.1 kW/m2).

Thermodynamic performance analyses for CCHP system coupled with organic Rankine cycle and solar thermal utilization under a novel operation strategy

The combination of an organic Rankine cycle (ORC) and solar collector (ST) with combined cooling heating and power (CCHP) system has the potential to improve its thermodynamic performances. Therefore, in this paper, fully considering thermal cascade utilization, a novel CCHP system combining ORC and ST is proposed, which uses solar energy and exhaust heat as ORC driving heat sources. Compared with traditional CCHP and CCHP-ST systems, the advantages of the CCHP-ORC-ST system are demonstrated. A collaborative optimization method considering both system configuration optimization and operation strategy is proposed, which realizes the improvement of system energy efficiency and economy. Based on the collaborative optimization method, the CCHP system with different integrated configurations is applied to commercial and office buildings. Through parameter analysis, the effects of parameters (such as the rated capacity of internal combustion engine and ORC evaporation temperature, etc.) are discussed. The findings portrays that the cost per unit supply area of the CCHP-ORC-ST system in typical years of office and commercial buildings is about 19.8 $/m2 and 25.9 $/m2. Compared with the CCHP system, the cost-saving rates of the CCHP-ORC-ST system in commercial and office buildings increase by 15.0% and 27.0%, respectively. The CCHP-ORC-ST system can flexibly utilize ORC to generate more electricity, improving energy efficiency and reducing wasted heat. Consequently, the proposed CCHP-ORC-ST system based on the collaborative optimization method can determine the optimal system configuration and operation design for energy savings and consumption reduction.

Simulation research of TEG-ORC combined cycle for cascade recovery of vessel waste heat

ABSTRACT The waste heat of ships has the characteristics of large temperature gradient, huge recoverable amount and various properties. Traditional technologies such as thermoelectric power generation (TEG) and organic Rankine cycle (ORC) are difficult to take into account the different characteristics of the various types of waste heat from ships. In order to realize the efficient cascade utilization of various waste heat from ships, improve the efficiency and meet the stringent requirements of the new international carbon emission regulations, a cascade recovery system of ship waste heat based on TEG-ORC (thermoelectric power generation/organic Rankine cycle) is proposed in this paper. After designing of the TEG-ORC combined cycle experimental system, the simulation research has also been carried out. The effects of TEG bottom cycle scale, ORC working medium flow rate and evaporation pressure on system net output power, system power generation cost and bottom cycle ratio (QTEG/QORC ) are studied. The results show that the TEG-ORC combined cycle overcomes the limitation of single waste heat utilization method, realizes multi-stage utilization of ship waste heat, as well as improves the utilization of waste heat. Of course, the stability and safety of ORC bottom cycle operation are also promoted. The simulation results show that when the TEG bottom cycle scale is 22 pieces, ORC working medium flow rate is 0.44 kg/s, and ORC working medium evaporation pressure is 2.2MPa, the system economy is optimal. At this time, the system power generation cost is 0.26 $/kW·h, the system net output power is 2520.3 W, and the thermal efficiency is 23.33%.

Thermodynamic feasibility evaluation of an innovative salinity gradient solar ponds-based ORC using a zeotropic mixture as working fluid and LNG cold energy

Thermodynamic feasibility evaluation is investigated for a salinity gradient solar pond-based power generation system. A dual-pressure evaporation organic Rankine cycle using the zeotropic mixture as working fluid is employed for power generation. To increase power generation, liquefied natural gas cold energy is used as a heat sink. Furthermore, to reach reliable results, transient analysis is conducted on the overall system. Also, the solar pond walls shading effect as well as heat losses that could not be ignored (i.e., evaporation heat loss from pond surface) is considered in the simulation. The results show that for a system located in Urmia, Iran, the annual average solar pond energy efficiency for the first year is obtained 20%. Different zeotropic mixtures are examined to achieve the optimal thermal performance of the system. It is concluded that the system using R245ca/R236ea with a mass fraction of 0.6/0.4 has the optimal thermal performance among selected zeotropic mixtures. In this case, the values of the system generated power and exergy destruction are obtained 95.67 MJ year−1 m−2 and 133.7 MJ year−1 m−2, respectively, with an energy efficiency of 3.28%.

Experimental investigation of performance of plate heat exchanger as organic Rankine cycle evaporator

In this study, the experimental investigation of a plate heat exchanger was conducted, which was used as an organic Rankine cycle evaporator. The heat exchanger performance was investigated at the low mass flux and moderate evaporation temperature ranges, as it was designed for a cascade heat utilization in a fourth-generation district heating and cooling system. The experiments were conducted under various operating conditions by changing the R-245fa mass flux, evaporation pressure, R-245fa inlet temperature, heat source inlet temperature, and heat source mass flux. Moreover, the heat transfer and pressure drop mechanisms were thoroughly investigated with an internal process analysis. The two-phase heat transfer coefficient exhibited a strong dependency on the heat flux, indicating that flow boiling heat transfer was mainly dominated by the nucleate boiling mechanism. When both the evaporation pressure and R-245fa mass flux were increased, the single-phase heat transfer accounted for a significantly large portion of the total heat transfer, leading to the rapid decrease in the heat transfer rate. The port and elevation pressure drops together accounted for 27–47% of the total pressure drop due to the low R-245fa mass flux range. With the heat source side variation, the two-phase heat transfer coefficient was more affected by the heat source inlet temperature than the heat source mass flux owing to the increase in the excess temperature. Additionally, the overall heat transfer coefficient strongly depended on the R-245fa side heat transfer coefficient, because the R-245fa had lower heat transfer coefficients than the heat source, even in the two-phase region.

Thermodynamic and thermoeconomic performance analyses and optimization of a novel power and cooling cogeneration system fueled by low-grade waste heat

The ejector refrigeration cycle (ERC) is often used as the sub-cycle of power and cooling cogeneration systems due to its advantages of simple structure design, few moving parts, less system investment, good stability and reliable operation. Considering the potential of organic Rankine cycle (ORC) in the utilization of low-grade heat, and the advantages of simple structure and many choices for working fluid, a novel power and cooling cogeneration system was designed, based on dual-pressure evaporation ORC (DORC) and ERC. In this system, a common condenser was used by DORC and ERC, and a part of working fluid separated from low-pressure steam generator outlet in DORC was used as the primary flow of ejector to drive ERC. A mathematical model for calculating the thermodynamic and thermoeconomic performances of the system was established. Sensitivity analysis was performed to determine the key parameters of the system, the results showed that the low-pressure evaporation temperature (TLPSG,out), high-pressure evaporation temperature (THP,E), vapor fraction of the low-pressure steam generator outlet (xLPSG,out) and working fluid mass flow ratio of high-pressure stage to low-pressure stage (k) were the four key parameters of the system. Parametric analysis showed that higher TLPSG,out and lower xLPSG,out were beneficial to increase the cooling output and thermal efficiency, while higher THP,E and larger k were helpful to increase the net power output and exergy efficiency. The net power output and exergy efficiency can be optimized by TLPSG,out and xLPSG,out. Using genetic algorithm (GA), multi-objective function optimization was carried out with THP,E, TLPSG,out, xLPSG,out and k as the decision variables. Moreover, the system's adaptability for working fluids R245fa, R236ea, R600, R600a, R601 and R601a was investigated. According to the optimization results, R236ea was the most suitable working fluid. When THP,E was 402.43 K, TLPSG,out was 392.42 K, xLPSG,out was 0.357 and k was 0.321, the calculated net power output, cooling output, thermal efficiency, exergy efficiency and total cost of unit exergy product (SUCP) were 273.00 kW, 121.80 kW, 14.52%, 44.03% and 42.62$/MWh, respectively. Exergy analysis was conducted using R236ea as working fluid. The results showed that the two steam generators and condenser were the main components responsible for most of the exergy destruction of the system in both basic case and optimized case. Compared to the basic case, the exergy destruction of the steam generators decreased by about 32.3%, and the total exergy destruction of the system decreased by about 5.0% in optimized case.

Thermo-economic analyses and evaluations of small-scale dual-pressure evaporation organic Rankine cycle system using pure fluids

Thermo-economic performance of small-scale (<1 MW) dual-pressure evaporation organic Rankine cycle (DPORC) system using pure fluids was studied. The effects of heat source conditions (flow rate and temperature), pinch point temperature differences (PPTDs), and heat sink condition on the thermo-economic performance of DPORC system were analyzed. The distribution characteristics of component costs were given. Traditional single-pressure evaporation ORC (SPORC) system was selected as the compared object to evaluate the thermo-economic performance of DPORC system. Results show that the minimized specific investment cost (SIC) of DPORC system substantially decreases with increasing flow rate and temperature of heat source. The DPORC system can obtain a better thermo-economic performance compared with SPORC system. For R245fa, the minimized SIC of DPORC system can be 0.6% lower than that of SPORC system, meanwhile its net power output also increases by 21.9%. The substantial increase of heat absorbers cost generally results in the minimized SIC of DPORC system is higher. Furthermore, the DPORC system has a larger application potential at the working conditions of the large flow rate of heat source and PPTD during heat absorption process. The working fluid with a high critical temperature is more suitable to adopt the DPORC system.

Parametric analysis on the low temperature wet air waste heat recovery through an organic Rankine cycle

Removing moisture in the wet farm products through the open air ventilation is a traditional postharvest treatment method. However, its overall energy consumption is high and the drying time is long. Here, a combined system is proposed through integrating a bottom organic Rankine cycle (ORC) to a top closed farm products air drying cycle to save energy and decreasing the drying time. Based on a steady state thermodynamic model without any losses, a theoretical work on thermal performance the proposed system has been conducted. The key operation parameters and suitable operation conditions for both the prominent energy saving ratio and high moisture extraction characteristics have been analyzed. The calculation results showed both the prominent energy saving and high moisture extraction performances can be achieved at the low ORC evaporating pressure condition, but the optimal energy saving performance and the moisture extraction performance could not be achieved simultaneously. Increasing dew point temperature of the humid air leaving the drying chamber improves both the energy saving and moisture extraction performances of the drying system significantly. Under the working conditions in this research, with the dew point temperature value of the humid air leaving the drying chamber over 323 K, the optimal energy saving performance is achieved at a fixed low ORC evaporation pressure. But, the prominent energy saving and high moisture extraction performances could not be accomplished simultaneously, with the dew point temperature value of the humid air leaving the drying chamber lower than 323 K.

Techno-economic performance of two-stage series evaporation organic Rankine cycle with dual-level heat sources

With the increasing of fossil energy crisis and environmental pollution, the development and utilization of renewable energy has become a hot spot in recent years. Based on two-stage serial organic Rankine cycle system of single heat source, the two-stage series organic Rankine cycle system of double heat source is proposed in this paper to realize cascade utilization of heat sources with different temperatures. The composite heat source grade is correlated with the two-stage series evaporation temperature to improve the power generation power and economic benefit of the system. Taking R245fa as an example, the mass flow rate of heat source is 10 kg/s, and the effects of evaporation temperature and heat source temperature on the net output power, thermal efficiency, exergy efficiency, electricity production cost and investment payback period of organic Rankine cycle, two-stage serial organic Rankine cycle system of single heat source and two-stage series organic Rankine cycle system of double heat source are optimized and analyzed. It is found that the electricity production cost and investment payback period of two-stage series organic Rankine cycle system of double heat source are always the smallest, followed by two-stage serial organic Rankine cycle system of single heat source, and the largest is the electricity production cost and investment payback period of organic Rankine cycle system. At the same time, it is found that two-stage series organic Rankine cycle system of double heat source is more suitable for low-temperature heat source.

Thermo-economic performance evaluation of emerging liquid-separated condensation method in single-pressure and dual-pressure evaporation organic Rankine cycle systems

Liquid-separated condensation is an emerging and effective heat transfer enhancement method to reduce the condenser cost, which can help with the widespread application of organic Rankine cycle (ORC) systems. However, few related studies have focused on the shell-and-tube condenser even if it is widely used in ORC systems. The thermo-economic performance superiorities and applicable working conditions of shell-and-tube liquid-separated condenser should be investigated, especially in novel ORCs. This study introduced the liquid-separated condensation method into single-pressure and dual-pressure evaporation ORC systems with shell-and-tube condenser. R245fa, R600a, and R1234ze(E) were used as working fluids. Optimal liquid-separated thermodynamic states and decrements in condenser heat transfer area of one to four stages liquid-separated condensations were provided. Decrements in specific investment cost (SIC) of ORC system by adopting the liquid-separated condensation method were evaluated. Results show that the liquid-separated condensation method is more suitable for low tube diameter, low inlet velocity in tubes, high inlet velocity outside tubes, and low cooling water temperature rise. Optimal liquid-separated thermodynamic states remain constant with increasing the tube diameter and inlet velocities. The SIC of ORC system can be reduced by 4.6% at most by adopting the liquid-separated condensation over the conventional condensation method. Increasing the mass flow rate of heat source fluid and pinch point temperature difference during heat absorption process is beneficial to increase the superiority of liquid-separated condensation method. Compared with dual-pressure evaporation ORC, the effects of liquid-separated condensation method on reducing the SIC are more remarkable in single-pressure evaporation ORC.

Energy benefits of organic Rankine cycle in a liquid desiccant and evaporative cooling-assisted air conditioning system

This study evaluated the effect of an organic Rankine cycle (ORC), powered by various renewable heat sources, on enhancing the primary energy benefits in a liquid desiccant (LD) and indirect and direct outdoor evaporative cooling-assisted air system (LD-IDECOAS). Combining an LD-assisted air conditioning system with the ORC, which uses renewable or waste heat sources, may be beneficial in terms of energy saving and environment efficiency. However, only a few studies have addressed the benefits of integrating the ORC with this air conditioning system as the heat source for the ORC. A solar thermal system, district heat source, and conventional boiler were used respectively in a 2-kW ORC which was integrated with the LD-IDECOAS to establish three system cases, and a comparative investigation was conducted. The power and heat generated by the ORC are supplied to the LD-IDECOAS to satisfy the power and solution heating demands, respectively. By performing a detailed energy simulation, the primary energy consumption in each ORC-integrated case was calculated during the cooling season and compared with that of a conventional LD-IDECOAS driven by grid power and a gas boiler. The results revealed that three ORC-integrated system cases consumed additional electric power compared with the base case (21.6% more electric power consumed in case 1 and 20.0% more electric power consumed in cases 2 and 3). Additional heat input to the ORC evaporator was also required, however, the ORC-integrated LD-IDECOAS powered by a solar thermal system (case 1) and district heat source (case 2), which are renewable energy sources, achieved a primary energy saving of 15.7% and 35.4%, respectively, compared with the base case. In conclusion, the district heat source was more feasible than the other heat sources considered in this study, achieving the best primary energy saving (35.4%) and CO2 emission reduction (23.9%).

Thermo-economic assessment of a novel trigeneration system based on coupling of organic Rankine cycle and absorption-compression cooling and power system for waste heat recovery

A novel trigeneration system consisting of organic Rankine cycle (ORC) and absorption-compression cooling and power system (ACCPS) is proposed to recover diesel engine waste heat. A compressor is used between evaporator and absorber in ACCPS. Moreover, a screw expander is applied in ACCPS to increase power generation and domestic hot water (DHW) is prepared by DHW heat exchanger (HX) in ORC. Exergoeconomic analysis using the specific exergy costing (SPECO) method is implemented for system assessment. Base case and parametric analysis as well as the effects of using expander and compressor in ACCPS on total performance of the system are examined. The results show that the best thermo-economic performance of the system is achieved when ORC condensation temperature and generator outlet temperature have respectively the lowest (85 °C) and the highest (80 °C) values. It is difficult to select the proper ORC evaporation temperature, because the lower this temperature is, the higher net power output is obtained while higher ORC evaporation temperature leads to less total cost rate and more total energy efficiency of system. Moreover, system exergy efficiency has an optimum point versus variation of ORC evaporation temperature. By increasing expander inlet steam quality, ACCPS coefficient of performance (COP) and total energy efficiency of system decrease; however, due to increase in net power output and system exergy efficiency respectively equal to 18.23% and 3%, total system performance improves from exergy viewpoint. By utilizing a compressor in ACCPS and increasing its pressure ratio, the performance of ACCPS improves from energy viewpoint; however, system thermo-economic performance decreases.

Thermodynamic performance comparison between singlepressure and dual-pressure evaporation organic Rankine cycles for heat sources with outlet temperature limit

Organic Rankine cycle (ORC) is a widely used technology to generate power from renewable energy and waste heat. Dual-pressure evaporation cycle holds immense potential to be used in ORC systems because it can remarkably increase the heat-work conversion efficiency and improve the adaptability of ORC to heat sources with various characteristics. This study compared the thermodynamic performance of single-pressure and dual-pressure evaporation ORCs for heat sources with outlet temperature limit, based on five organic fluids. Effects of the heat source outlet temperature limit on the characteristics of dual-pressure evaporation ORC system were also discussed. Results show that the dual-pressure evaporation cycle can substantially increase the net power output by increasing system efficiency, and is beneficial to reduce the charge volume of organic fluid in the system, compared to the single-pressure evaporation cycle. The increments in net power output of dual-pressure evaporation cycle over the single-pressure evaporation cycle can be 4.9%, 8.3%, 10.8%, 12.8%, and 14.0% at most for R227ea, R1234ze(E), R600a, R245fa, and R601a, respectively. While, compared to heat sources without outlet temperature limit, the increment in net power output of dual-pressure evaporation cycle is remarkably lower for heat sources with outlet temperature limit.

Exergy analysis of novel dual-pressure evaporation organic Rankine cycle using zeotropic mixtures

Organic Rankine cycle is a power system for the widely use of medium- to low-grade (<200 °C) thermal energy. Adopting the dual-pressure evaporation cycle and zeotropic mixtures are important approaches to increase the efficiency of organic Rankine cycle system. The combination of two approaches presents considerable potential to integrate their advantages and further increase the system efficiency significantly. However, for the dual-pressure evaporation organic Rankine cycle using zeotropic mixtures, the studies on the exergy performance are blank up to now. The obtained efficiency increments by combination, reached exergy efficiency, and characteristics of irreversible loss distribution remain unclear. This study carried out a comprehensive exergy analysis on the dual-pressure evaporation organic Rankine cycle using isobutane/isopentane mixtures. The exergy efficiency and irreversible loss distribution were analyzed. The superiority of combining dual-pressure evaporation cycle and zeotropic mixtures was evaluated. Results show that the largest exergy efficiency of dual-pressure evaporation cycle using isobutane/isopentane mixtures is 43.1%–61.7% for heat sources of 100–200 °C, and the largest exergy efficiency initially increases and then decreases with increasing heat source temperature. The largest irreversible loss occurs in the heat absorption, expansion, and heat release processes for heat sources of 100–160 °C, 170–180 °C and 190–200 °C, respectively. The exergy efficiency of dual-pressure evaporation cycle using isobutane/isopentane mixtures is the largest for heat sources below approximately 180 °C, and the maximum relative increments are 11.6% and 25.6% compared with the dual-pressure evaporation cycle using isobutane and single-pressure evaporation cycle using isobutane/isopentane mixtures, respectively.

Effects of heat source temperature and mixture composition on the combined superiority of dual-pressure evaporation organic Rankine cycle and zeotropic mixtures

Abstract Dual-pressure evaporation cycle is an emerging cycle type in the organic Rankine cycle (ORC) that can remarkably reduce the irreversible loss during heat absorption process and improve the adaptability to various heat sources. Introduction of zeotropic mixtures in dual-pressure evaporation ORC presents great potential to further increase the heat-power conversion efficiency due to the variable phase-change temperature. This study investigated the dual-pressure evaporation ORC using isobutane/isopentane mixtures for heat sources of 100–200 °C. Effects of heat source temperature and mixture composition on the system thermodynamic performance were analyzed. Application potential and applicable ranges of introducing isobutane/isopentane mixtures in dual-pressure evaporation ORC were evaluated. Results showed that the introduction of zeotropic mixtures in dual-pressure evaporation ORC substantially increased the power output. The maximum power output of dual-pressure evaporation ORC using isobutane/isopentane mixtures could increase by 11.9% and 15.2% at most compared with those of using isobutane and isopentane, respectively. It could increase by 25.7% at most compared with that of single-pressure evaporation ORC using isobutane/isopentane mixtures. In terms of being introduced into dual-pressure evaporation ORC, the zeotropic mixture with a high critical temperature can generally achieve larger range in applicable heat source temperature and larger increment in the power output.

Off-design optimisation of organic Rankine cycle (ORC) engines with piston expanders for medium-scale combined heat and power applications

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising and predicting system performance, which can be used, along with the optimisation tool, by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.