Endogenous Exergy Destruction Research Articles

Sustainable production of ammonia and formic acid using three chemical looping reactors and CO2 electroreduction cell

Chemical looping reforming (CLR) is emerging as a promising technique with potential to produce useful chemicals while capturing carbon dioxide emissions. This study proposes a novel multigeneration system consisting of three-reactor CLR integrated with CO2 electroreduction cell to produce 13.3 kg/s commercial-grade of ammonia and 0.0113 kg formic acid/kg ammonia. In order to achieve the maximum use of waste heat and cold energy, the system also includes interconnected power generation cycles. The net power generation is 816.54 kJ/kg ammonia, along with 305 kg/s hot water at 80 °C. A technoeconomic analysis is conducted to economically assess the process and the results are compared with other studies. The results of technoeconomic analysis give a comparatively low levelized cost of ammonia equal to 0.48 $/kg. Furthermore, to understand the irreversibilities involved in the integrated process, a thermodynamic analysis is conducted in terms of conventional and advanced exergy analyses along with sensitivity analysis which help determine the practical limitations and opportunities for process improvement of the overall system. According to the results, the overall process exergy efficiency is 74.80% and the overall exergy destruction rate is 116338.53 kW. The results of advanced exergy analysis show strong interdependencies between the components of the system due to major endogenous exergy destruction rates. Additionally, the results are used to propose three distinct strategies for design optimization and process intensification. This work is the first to look at the potential of improving the sustainability index (SI) by using the results of an advanced exergy analysis.

Exergy analysis for enhanced performance of integrated batch reverse osmosis – Forward osmosis system for brackish water treatment

Concept of integrated brackish water batch reverse osmosis – forward osmosis (BRO – FO) is relatively new in the field of desalination. Recovery of the reverse osmosis (RO) system can be elevated using the free piston BRO concept, and the specific energy consumption (SEC) can be reduced. This study focuses on exergy, advanced exergy, and parametric analysis of the integrated BRO - FO system. The conventional exergy analysis suggests that the largest rate of exergy destruction is associated with the system's RO membrane module and pump, with values of 6.51 and 87.06 %, respectively. However, advanced exergy analysis shows that this rate can be reduced by 3.53 % and 62.89 %, respectively. Endogenous exergy destruction of pumps and membranes is higher compared to exogenous exergy destruction. These results are utilised to determine the system's optimal performance using parametric analysis. Sensitivity analysis was utilised to obtain an optimum recovery for system design, fresh water cost (FWC), and specific solution cost (SSC). The BRO can achieve >80 % recovery with 0.43 kWh/m3 SEC, and FO technology helps to attain minimal/zero waste discharge. Thus, a FWC of 0.50 $/m3 and a SWC of 0.44 $/m3 can be achieved.

Advanced Exergy and Exergoeconomic Analysis of Cascade High-Temperature Heat Pump System for Recovery of Low-Temperature Waste Heat

Cascade high-temperature heat pumps (CHTHPs) are often applied to recover low-temperature industrial waste heat owing to their large temperature lift. Through a comprehensive consideration of thermodynamic and economic performance, conventional and advanced exergy and exergoeconomic analyses are employed in this study to evaluate the potential for the improvement in CHTHP systems. The results show that the avoidable endogenous exergy destruction in a CHTHP system accounts for 62.26% of its total exergy destruction, indicating that most of the exergy destruction comes from the components. This suggests that CHTHP systems still have significant potential for improvement. The very low exergoeconomic factor of the total system (only 0.75%) implies that the exergy destruction cost has a great influence on the economic performance of a CHTHP system. The high- and low-temperature compressors are the two components with the highest exergy destruction, accounting for 34.14% and 26.79% of the total exergy destruction in the system, respectively. Moreover, their exergy destruction cost is much larger than that of the other components. Thus, the priorities for improvement should be the high- and low-temperature compressors. The decrease in exergy destruction in compressors produces a reduction in carbon emissions. This comprehensive analysis of thermodynamic and economic performance supplies guidance for the engineering application of CHTHPs in low-temperature waste heat recovery.

Conventional and advanced exergy-exergoeconomic-exergoenvironmental analyses of an organic Rankine cycle integrated with solar and biomass energy sources

Considering the huge consumption of traditional energy and the rising demand for electricity, the development of renewable energy is very necessary. In this paper, an energy system integrating biomass energy, solar and two-stage organic Rankine cycle (ORC) is proposed, which uses the stable energy output of biomass energy to compensate for the volatility of solar modules. The proposed system comprises a biomass boiler, photovoltaic thermal panels (PV/T), evaporators, condensers, working medium pumps, turbines, a preheater and an air preheater. In addition, conventional and advanced exergy, exergoeconomic and exergoenvironmental (3E) analyses are carried out. Conventional 3E analyses reveal two components that require priority improvement. They are respectively evaporator 1 with the largest exergy destruction (708.2 kW) and exergy destruction environmental impact rate (775.3 mPt/h) and evaporator 2 with the largest exergy destruction cost rate (19.15$/h). The results of advanced 3E analyses show that the largest avoidable endogenous exergy destruction is condenser 1 (136.6 kW), the largest avoidable endogenous exergy destruction cost rate is condenser 2 (3.377$/h), and the largest avoidable endogenous exergy destruction environmental impact rate is condenser 1 (196.1mPt/h). These mean that these components have great potential for improvement in reducing exergy destruction, saving cost and protecting the environment. In addition, the avoidable endogenous exergy destruction/cost/environmental impact rate of evaporator 2 are negative, so evaporator 2 is not suitable as a priority component for improvement, which is contrary to the conclusions of conventional 3E analyses. It is found that conventional 3E analyses can only point out the biggest exergy destruction point, but cannot indicate whether the components with the greatest exergy destruction have the greatest potential for improvement. However, advanced 3E analyses can show the improvement potential of each component by improving its own performance and the external conditions. Therefore, it is necessary to conduct advanced 3E analyses.

Advanced exergy analysis of a double absorption heat transformer recovering industrial waste heat for pure water production

Conventional and advanced exergy analyses are applied for a double absorption heat transformer system utilizing low-grade industrial waste heat for water purification. The conventional method provides information about the location and magnitude of inefficiencies and does not consider any interaction among components regarding the irreversibility occurring in them. The advanced exergy technique however, exposes the real origins of irreversibilities and latent potential for enhancement of each system component, along with reflecting mutual interaction between components. The simulation of the cycle is carried out with engineering equation solver software. The results show that the overall improvement potential of the cycle is only 16%. Also, the mutual interdependency between components is found to be weak as 97% of the total exergy destruction appeared as endogenous. Furthermore, considering endogenous avoidable exergy destruction, the improvement strategy of components obtained by advanced exergy method is disparate from conventional one. Results of the advanced exergy suggest this order: condenser, absorber, evaporator, absorber/evaporator assembly, generator assembly, economizer 1, and economizer 2. Additionally, a parametric investigation is performed to assess the sensitivity of splitting exergy destruction to the variation of heat source temperature for highlighting improvement opportunities. The results indicate that the condenser yields the highest amount of avoidable endogenous exergy destruction in the entire studied temperature range.

Wind energy-driven medical waste treatment with polygeneration and carbon neutrality: Process design, advanced exergy analysis and process optimization

This study presents a novel and original rich-O2 steam gasification-based process integrated with carbon capture, utilization, and storage (CCUS) for carbon-neutral valorization of medical waste (MW) in Hong Kong. The process employs a calcium loop to capture CO2 from the gasifier-produced syngas, simultaneously facilitating H2 production. The captured CO2 is optionally utilized to generate high value-added products like ethylene, ethanol, and acetic acid through electroreduction. The process incorporates multiple steam Rankine cycles for converting waste heat to power, achieving a total energy efficiency of 71.38%. Advanced exergy analysis reveals over 70% endogenous exergy destruction in reactors and 79.05% avoidable exergy destruction in heat exchangers and condensers, which may be paid more attention. Driven by wind energy, the process has the potential to achieve zero CO2 emission. Economic optimization demonstrates potential profits up to 10410 HK$/h with sufficient wind energy supply and 2548.2 HK$/h with limited supply. This study highlights the novelty of a carbon-neutral MW valorization approach, CCUS utilization, advanced exergy analysis, optimization and renewable wind energy for sustainability and economic viability.

Conventional and advanced exergy and exergoeconomic analysis of a biomass gasification based SOFC/GT cogeneration system

In this paper, a small scale biomass gasification based solid oxide fuel cell/gas turbine (SOFC/GT) combined heat and power (CHP) plant is investigated by means of both conventional and advanced exergy and exergoeconomic analysis. A one-dimensional model of an internal reforming planner SOFC is employed to account for the temperature gradient within the fuel cell solid structure, which is maintained at the maximum allowable temperature gradient (150 K) under different operating conditions. Two main parameters of the gasification process, namely, air-to-steam ratio and modified equivalence ratio, are investigated, and the key parameters of the cycle exergy and exergoeconomic study are analyzed. Moreover, a multi-objective optimization procedure is applied to determine the unavoidable gasifier conditions required for the advanced exergy analysis of the system. The results of the conventional exergy and exergoeconomic analysis reveal that the highest rate of exergy destruction occurs in the gasifier, followed by the afterburner (AB) with 41.87% and 21.98%, respectively. Also, the lowest exergoeconomic factor is related to AB by 5.34%, followed by heat recovery steam generator (HRSG), gasifier, air compressor, and SOFC, which implies that the priority is to improve these components to reduce the exergy destruction cost rate. The results obtained from the advanced exergy and exergoeconomic analysis indicate that the most of the total exergy destruction rate is unavoidably in the CHP plant. The AB shows the least improvement potential in terms of reduction of the exergy destruction by almost 2% avoidable part, followed by Heat Exchanger 3 (H.X.3), gasifier, and SOFC duo to their lowest avoidable exergy destruction parts of almost 5%, 10% and 13%f respectively. Furthermore, the unavoidable part of the investment cost rate for all the components of the cogeneration plant is larger than the avoidable part, which means that it is difficult to reduce the investment cost rate of the system components. Meanwhile, the endogenous/exogenous analysis shows that the exergy destruction is completely endogenous for all components of the integrated plant, except for HRSG, GT, and HX1. Compressors and turbines have the highest potential to reduce endogenous exergy destruction. This is due to their higher avoidable endogenous exergy destruction. Reducing the investment cost rate seems difficult, as the main investment cost rate was found to be an unavoidable endogenous part for all system components. Finally, some results obtained from the advanced analysis approach are the opposite to those of the conventional method. This fact emphasizes that the results of conventional exergy analysis alone are insufficient and unreliable. For example, based on the advanced analysis perspective, the gas turbine and H.X.2 by 8.9% and 8.46% modified exergoeconomic factor, respectively, should be considered for reducing investment cost rate, while the conventional method gives opposite results.

Advanced exergy analysis of a solar-driven cascade ejector system with heat storage

Solar-driven cascade ejector system can not only promote the utilization reliability of the solar energy, but also improve the performance of solar energy systems. This paper proposes a novel solar-driven cascade ejector system which integrates a solar ejector heat storage subsystem with R290 as refrigerant and a transcritical CO2 ejector-expansion refrigeration subsystem. The conventional and advanced exergy analysis methods are utilized to investigate the system’s exergetic performance, and explore the interactions among the system components and their improvement potential. The effects of compressor efficiency, area ratio and efficiency of ejectors on the exergetic performance of system and components are investigated. The analysis results show that the transcritical CO2 ejector-expansion refrigeration subsystem has higher exergy efficiency and its components have small exogenous exergy destruction. The endogenous exergy destruction and the avoided exergy destruction of cascade system account for 88.85% and 41.46%, respectively. Besides, 66.35% of exogenous exergy destruction of middle heat exchanger is caused by the compressor and CO2 ejector, and to improve the performance of CO2 ejector and CO2 evaporator would decrease the exergy destruction of compressor. CO2 ejector and R290 ejector have the superior improvement potential in each subsystem compared to other components. The rise of ejector area ratio would increase the endogenous exergy destruction of ejector. For the system with high-performance compressor, R290 ejector efficiency should be attracted more attention to reduce its avoided endogenous exergy destruction.

Advanced exergoeconomics of a steam power plant based on double-tier splitting of exergy destruction rates

The advanced exergy and exergoeconomic analysis methodologies excel over their conventional counterparts by quantifying endogenous/exogenous exergy destruction rates and related costs that reveal the interactions among system components. By quantifying avoidable/unavoidable losses, they also reveal the potentials for component improvements that are practically achievable. These data are imperative for improving the cost-effectiveness of thermal systems. This paper presents the performances of a complex gas-fired power plant obtained via an advanced exergoeconomic analysis. The study advances existing studies by undertaking second-tier splitting of exergy destruction cost rates and associated investment cost rates to determine avoidable endogenous, unavoidable endogenous, avoidable exogenous, and unavoidable exogenous cost rates of the plant’s components. Component exergy destruction cost rates were found to be predominantly unavoidable, while most of the component investment cost rates were avoidable. Except for the low pressure heater 3, exergy destruction cost rates are endogenous in all the plant components, contributing 84% of overall cost rates. The proportions of exergy destruction cost rates and investment cost rates of the plant that are avoidable endogenous were 21% and 28% respectively, while the respective portions that are avoidable exogenous were 4% and 27%. Furthermore, it was shown that as much as 96.3% improvements in overall plant cost-effectiveness were achievable by eliminating avoidable endogenous exergy destruction cost rates and investment cost rates for major components of the plant.

Thermodynamic performance and the exergy destruction of the transcritical CO2 two-stage compression and ejector expansion direct cooling ice making system

The transcritical CO2 direct cooling technology was used for the first time in the indoor venues of the 2022 Beijing Winter Olympic Games. The technology has a series of advantages, such as compact structure, uniform ice temperature, energy saving and environmental protection. In order to further reduce the power consumption and improve the performance, two-stage compression and low-pressure ejector direct cooling ice making system (TCLES) and two-stage compression and intermediate-pressure ejector direct cooling ice making system (TCIES) are studied. The performance of the systems is simulated by constructing a one-dimensional thermodynamic model. The impacts of the gas cooler pressure, gas cooler outlet temperature, evaporation temperature and intermediate pressure on these systems are considered. It is showed that TCLES and TCIES have their own advantages when the system is without and with heat recovery, and found that two-stage compression improves the system performance more than the ejector when only the cooling performance is considered. On this basis, conventional exergy and advanced exergy analysis of both systems are carried out. Advanced exergy analysis shows that exergy destruction can be avoided mainly in low-pressure and high-pressure compressors of both systems. Besides, the influence of gas cooler pressure on exergy destruction characteristics is discussed. When controlling the gas cooler pressure at 9.0 MPa, the avoidable endogenous exergy destruction of compressors and ejector is reduced by a maximum of 4.14% and 13.92% for TCLES and reduced by a maximum of 16.15% and 28.04% for TCIES.

Geothermal power generation improvement of organic Rankine flash cycle using exergy, advanced exergy and exergoeconomic analyses

Replacing fossil energy with geothermal energy can effectively alleviate the energy crisis and many environmental problems. However, the low heat-to-power conversion efficiency of geothermal resources hinders its development and utilization. Therefore, in-depth analysis of the causes of irreversible losses and costs of each component of the geothermal power generation system can provide theoretical support for improving its performance. In this paper, an organic Rankine flash cycle (ORFC) power generation system is analyzed and evaluated based on conventional and advanced exergy/exergoeconomic methods. The results showed that the exergy destruction of each component is mainly caused by its own performance, so the endogenous exergy destruction of all components is higher than the exogenous exergy destruction. The avoidable endogenous exergy destruction/cost of the system accounts for 54.07 % and 48.94 % of the total exergy destruction/cost, respectively. The avoidable endogenous exergy destruction/cost of two turbines accounts for 74.35 % and 86.76 % of the total endogenous exergy destruction/cost, indicating that the turbine has the greatest performance improvement potential and cost saving potential, and the exergy destruction of each component is the main factor affecting the economic benefit of the system. Hence, increasing the isentropic efficiency of the turbines and the working fluid pumps and decreasing the heat transfer temperature difference of the evaporator and the condenser can reduce their own exergy destruction, thereby improving the system performance.

Advanced exergy analysis of the modified process of natural gas cryogenic helium extraction

Conventional exergy analysis can only quantitatively analyse the exergy destruction of devices, while advanced exergy analysis can obtain the detailed distribution of the exergy destruction. For analysing the energy consumption of equipment in the process of cryogenic helium extraction, advanced exergy analysis was performed on a modified process for natural gas helium extraction. The analysis results show that the largest part of most equipment exergy destruction is endogenous exergy destruction; the equipment that has the largest exergy destruction in the process is the cold box, followed by the compressor. The exergy destruction of compression equipment is similar, but the exergy destruction of different heat exchange equipment is different. Sensitivity analysis shows that in order to minimise the unavoidable exergy destruction of the column, the recommended feed pressure of the primary concentration column is 3,200, and the recommended feed temperature of the cryogenic separator before the secondary concentration column is -150. [Received: January 18, 2022; Accepted: June 14, 2022]

Advanced Exergy Analysis of an Absorption Chiller/Kalina Cycle Integrated System for Low-Grade Waste Heat Recovery

Exergy analysis and advanced exergy analysis of an absorption chiller/Kalina cycle integrated system are conducted in this research. The exergy destruction of each component and overall exergy efficiency of the cascade process have been obtained. Advanced exergy analysis investigates the interactions among different components and the actual improvement potential. Results show that among all the equipment, the largest exergy destruction is in the generators and absorber. System exergy efficiency is obtained as 35.52%. Advanced analysis results show that the endogenous exergy destruction is dominant in each component. Interconnections among different components are not significant but very complicated. It is suggested that the improvement priority should be given to the turbine. Performance improvement of this low-grade waste heat recovery process is still necessary because around 1/4 of the total exergy destruction can be avoided. Exergy and advanced exergy analysis in this work locates the position of exergy destruction, quantifies the process irreversibility, presents the component interactions and finds out the system improvement potential. This research provides detailed and useful information about this absorption chiller/Kalina cycle integrated system.

Conventional and advanced exergy analysis of large-scale adiabatic compressed air energy storage system

Identifying the main sources of exergy destruction is a significant method for promoting high-efficiency operation of compressed air energy storage (CAES) systems. Advanced exergy analysis is free from the limitations of traditional exergy analysis and identifies the optimization order of the components and clarifies their relationships. This method is significant for developing system improvement strategies for optimization. In this study, based on the actual engineering of an adiabatic compressed air energy storage (A-CAES) power plant, real, unavoidable, and hybrid thermodynamic cycles were established, and conventional and advanced exergy analyses were conducted for it. The results showed that the endogenous exergy destruction of the main components was greater than the exogenous exergy destruction, implying the importance of optimizing the components. In addition, endogenous and exogenous exergy destructions indicate a highly complex relationship between the components. The avoidable exergy destruction indicated the highest potential for improvement of the third-stage heat exchanger (HEX3) contributing to 13.15 % of the total avoidable exergy destruction of the system; therefore, HEX3 has the highest optimization value. In addition, the second-stage heat exchanger (HEX2) and first-stage heat exchanger (HEX1) also exhibited higher potential for improvement.

Sensitivity analysis of avoidable and unavoidable exergy destructions in a parallel double‐effect LiBr–water absorption cooling system

AbstractA parametric study has been carried out based on advanced exergy assessment for a parallel double‐effect LiBr–water absorption chiller. The advanced exergy method provides the real potential of the equipment for improvement in the system by recognizing the avoidable irreversibilities. The sensitivity analysis of various parts of the exergy destruction (endogenous avoidable, endogenous unavoidable, exogenous avoidable, and exogenous unavoidable) in system components and the overall performance (coefficient of performance, exergy efficiency, and modified exergy efficiency) of the system has been done to identify the actual potential of different components of the system to improve the overall performance of the system relative to operational parameters. Operational parameters include the heat source temperature, heat sink temperature, and the difference between the absorption and condensation temperature. The results showed that by modifying and improving the performance of components, 18.5% of the total exergy destruction of the system could be reduced. The modified exergy efficiency reduces sharply (nearly 40%) with the rising heat sink temperature. The endogenous exergy destruction increases in all system components except the condenser and evaporator by the heat source temperature increase. With the increase in the heat sink temperature, the avoidable part of the total exergy destruction in the system increases due to the significant rise in the avoidable exogenous exergy destruction in the components, and the unavoidable endogenous exergy destruction decreases (more than 200%) in the absorber. Increasing the condensation temperature reduces the unavoidable endogenous exergy destruction in the components except for the low‐temperature heat exchanger.

Comprehensive performance exploration of a novel pumped-hydro based compressed air energy storage system with high energy storage density

A compressed air energy storage system is the key issue to facilitating the transformation of intermittent and fluctuant renewable energy sources into stable and high-quality power. The improvement of compression/expansion efficiency during operation processes is the first challenge faced by the compressed air energy storage system. Therefore, a novel pumped-hydro based compressed air energy storage system characterized by the advantages of high energy storage density and utilization efficiency is proposed in this study. To perform a comprehensive investigation on the system, the locations and magnitudes of irreversible sources within the system are estimated through the conventional exergy method, and the interactions among components and realistic potential for system performance improvement are identified by the advanced exergy method. The results indicate that the interactions among components are complex but not very significant since the endogenous exergy destruction is larger than the exogenous exergy destruction for all components within the system. Furthermore, the conventional exergy analysis reveals that the expander, compressor1, and pump are the most important components, accounting for 25.99%, 22.55%, and 15.34% of the total exergy destruction, respectively. Nevertheless, advanced exergy analysis recommends that the hydraulic turbine, pump, and expander have the optimization priorities since they share 28.61%, 27.72%, and 10.07% of the total endogenous avoidable exergy destruction. Finally, the overall system exergetic efficiency achieves a higher value of 18.49% under unavoidable conditions than that under real conditions.

Advanced exergy analysis of NH3/CO2 cascade refrigeration system with ejector

Aiming at the NH3/CO2 Cascade Refrigeration System with ejector (CRSE), this paper establishes the mathematical model of the refrigeration cycle and compares it with the literature based on thermodynamic analysis. Advanced exergy analysis on the CRSE is carried out and the exergy destruction of components in the system are divided into two ways: avoidable/unavoidable exergy destruction and endogenous/exogenous exergy destruction, and combine the two ways to clarify the types and values of exergy destruction in components, which provides a direction for the improvement of the system. The analysis shows that the avoidable exergy destruction of the system accounts for 38.674% of the total exergy destruction, and the proportion of endogenous exergy destruction and exogenous exergy destruction in the total exergy destruction of the system is 77.772% and 22.228% respectively. The improvement potential of the system is very high, and the low-temperature compressor, condenser, and intermediate heat exchanger should be paid special attention to in the improvement.

Conventional and advanced exergy analysis of a novel wind-to-heat system

Exergy analysis are carried out on a novel 100 kW wind-to-heat system to investigate the thermodynamic performances by employing the experimental data. Nine different combinations of the wind speed and water flow rate are selected as the typical operating conditions. Through the conventional exergy analysis, the wind turbine accounts for the most of the system's total exergy destruction at high wind speeds. In terms of the heat pump subsystem, the compressor is the most critical component, followed by the condenser. For further obtaining the optimal design directions of the key components, the advanced exergy analysis is applied to this system. By establishing the mathematical model, the exergy analysis of the ideal conditions, experimental conditions and unavoidable conditions are compared. Averagely 87.32% exergy destruction of the compressor is endogenous and avoidable, which can be decreased by optimizing the internal structure parameters. The average proportions of the avoidable exergy and endogenous exergy destruction of the condenser are 57.48% and 68.41%, respectively. Both the internal parameters of the condenser and interactions with other components must be taken into consideration. The optimizing potential of the wind-to-heat system through exergy analysis methods can be used for further research and improvement of the system.

Advanced exergoenvironmental analysis of the oil shale retorting process with SJ-type rectangular retort

A framework integrated the advanced exergy analysis with the theory of life cycle assessment is constructed to evaluate and optimize the environmental performance of energy conversion systems. The SJ-type oil shale retorting (SJ-OSR) process as one of the energy conversion systems with severe environmental pollution is employed to demonstrate the feasibility of the proposed approach. The results show that the avoidable exergy destruction of the retorting unit and recycle gas combustion unit to their total exergy destruction are 56.49% and 65.12%, respectively. The environmental impact for the total exergy destruction (BD,tot) of the SJ-OSR process is 1219.05 mPts/s. The environmental impact for total endogenous exergy destruction is 984.63 mPts/s, which accounts for 80.77% of the BD,tot. As for the drying, retorting and recycle gas combustion units, their environmental impacts related to avoidable exergy destruction rates are larger than the unavoidable parts. Most of the environmental impact with respect to the avoidable exergy destruction rates is endogenous (350.22 mPts/s). Based on the proposed two improvement strategies, ultimately, the BD,tot of the improved process decreases obviously from 1219.05 mPts/s to 1037.87 mPts/s, and the proportion of BD,tot reduced is about 14.86% of that for the existing process.

Exergy destruction characteristics of a transcritical carbon dioxide two-stage compression/ejector refrigeration system for low-temperature cold storage

Cold storage is an irreplaceable part of food cold chain. In the present study, a novel transcritical carbon dioxide two-stage compression/ejector refrigeration system for low-temperature cold storage is proposed to freeze and refrigerate food. Also, a combination of conventional and advanced exergy methods is adopted to evaluate the exergy destruction characteristics inside the system. According to the advanced exergy method, 69.88% of the exergy destruction is endogenous, which suggesting that exergy destruction of the system is primarily contributed to the irreversibility of components themselves, meanwhile, the interactions between components are not very close. Among all components, the high-pressure compressor possesses the most tremendous avoidable endogenous exergy destruction, indicating that it has the maximum potential for optimization. And the rest of the improved order is the ejector, low-pressure compressor, evaporator, gas cooler and intercooler, while the high-pressure and low-pressure expansion valves have the lowest priority for optimization. Remarkably, there is a significant discrepancy between the conclusions obtained from these two exergy methods. Furthermore, the system was also found to have a COP of 0.42 under the actual operating condition and 0.62 under the unavoidable operating condition. The impacts of the ejector and compressor efficiencies, as well as discharge and intermediate pressure upon theoretical exergetic performance of the system are considered. The system exergy efficiency is respectively increased by 5.78%, 36.2% and 50% as the ejector, low-pressure and high-pressure compressor efficiencies ranging from 0.5 to 0.9. In addition, there exists an optimal intermediate pressure of 4.25 MPa and an optimal discharge pressure of 8.75 MPa from the perspective of system exergy efficiency.