Avoidable Endogenous Exergy Destruction Research Articles

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.

Advanced Exergy Analysis of Cascade Rankine Cycle‐Driven Reverse Osmosis System

A novel solar‐assisted cascade Rankine cycle‐driven reverse osmosis (RO) system is proposed and evaluated using conventional and advanced exergy analysis. The system comprises a solar collector as a heat input, steam Rankine cycle (SRC), and organic Rankine cycle (ORC) to generate turbine work and RO units for brackish water treatment. Water and R1233zd(E) are used as the working fluid in the SRC and ORC, respectively. The RO system is proposed to produce 1 m3 h−1 of permeate water from each RO unit. Conventional exergy analysis shows that the solar collector exhibits the highest exergy destruction of 0.798 kW, followed by the RO unit. However, advanced exergy analysis reveals that only 16.16% of the avoidable exergy in solar collectors can be reduced due to technical constraints. Advanced exergy analysis identified that endogenous avoidable exergy destruction contributes 24% of overall exergy destruction, which can be reduced by optimizing operating parameters and making technical improvements. The RO unit has a higher exogenous avoidable exergy, potentially suggesting improvements in other system component parameters. The advanced exergy analysis also prioritized the components for improving system performance in a sequence of RO high‐pressure pumps, RO modules, solar collector, ORC condenser, evaporative condenser, ORC expander, and SRC turbine.

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.

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 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.

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.

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.

Advanced exergy analysis and optimization of a CO2 to methanol process based on rigorous modeling and simulation

The efficient conversion and utilization of CO2 is of great strategic significance to achieve the goal of “carbon neutrality”. The production of green methanol from the captured CO2 of industrial tail gas and green hydrogen of renewable energy can not only effectively reduce carbon emissions, but also solve the problem of green hydrogen storage and transportation. Facing with the unsatisfactory thermodynamic performance of CO2 to methanol (CTM) process, however, little literature has been reported on the systematic investigation of its thermodynamic performance to reduce its avoidable exergy destruction instead of spending wasted effort to reduce unavoidable one. This study conducted an advanced exergy analysis of the CTM process to ascertain its real improvement potential and interactions among the components. Results show that the real improvement potential of the CTM process is 46.55%. Most of the exergy destruction of the CTM process is endogenous, which accounts for 94.47% of the total exergy destruction. After combination of splitting the exergy destruction, it founds that the unavoidable endogenous exergy destruction of the CTM process has the largest proportion, 50.93%, followed by avoidable endogenous exergy destruction, 43.55%. Besides, several improvement strategies are proposed to reduce the avoidable exergy destruction, which indicate that the total exergy destruction of the improved CTM process is reduced by 14.78% and exergy efficiency is increased by 4.91%.

Advanced exergy investigation of a combined cooling and power system with low-temperature geothermal heat as a prime mover for district cooling applications

Present work deals with the conventional and advanced exergy analyses for an ammonia-water combined power and cooling cycle integrated with a low-temperature geothermal heat resource for district cooling applications. The energy and exergy calculation of the mentioned cycle was carried out by the Engineering Equation Solver (EES) software. The thermodynamic cycle method is used for advanced exergy analysis. The advanced exergy analysis by dividing the exergy destruction into the endogenous/exogenous and avoidable/unavoidable portions reveals the reducible exergy destruction part in each cycle element. Consequently, it determines the real improvement potential in the system. Results represented that the overall exergy efficiency with real and unavoidable conditions is 41.03% and 47.29%, respectively. It was found that from the 3681.2 kW of total exergy destruction for this cycle, about 941.5 kW (26% of total exergy destruction) can be mitigated by technology modification of components, where 75% is the endogenous avoidable, and 25% is related to the exogenous avoidable part. It also showed that the prioritizing of the components improving in terms of conventional and advanced exergy is different. The conventional exergy destruction presents the priority as the boiler, the absorber, the rectifier, the HER, the turbine, the RHE, and the pump. In contrast, the advanced exergy analysis’s suggested order is the turbine, the boiler, the rectifier, the HER, the absorber, the RHE, and the pump. They suggested that improving the Turbine and the boiler, and the rectifier is more efficient in increasing the system efficiency due to high endogenous avoidable exergy destruction.

Conventional and advanced exergy analyses of an organic Rankine cycle by using the thermodynamic cycle approach

AbstractIn this study, a basic organic Rankine cycle (ORC) is introduced in an air separation process for waste heat recovery. Conventional and advanced exergy analyses are adopted to investigate the thermodynamic properties of components in the ORC. A comprehensive thermodynamic model is constructed to improve the advanced exergy analysis in the ORC, thereby encompassing real, theoretical, unavoidable, and hybrid cycles. Nine organic working fluids are introduced to investigate the influence on the ORC performance. (1) The conventional exergy analysis reveals the following: (a) The expander constantly demonstrates the maximum exergy efficiency except when R227ea is used. (b) The evaporator constantly exhibits the maximum exergy destruction regardless of the working fluid used. (c) The maximum product exergy is obtained when R114 is used. (d) Key components must focus on the condenser and evaporator to improve the ORC performance. (2) The advanced exergy analysis reveals that the expander demonstrates maximum potential for improvement because its endogenous avoidable exergy destruction accounts for approximately 90% of its real exergy destruction for all working fluids. The expander must be improved to achieve the optimal ORC performance. The advanced exergy analysis can distinguish the source of exergy destruction and the magnitude for possible improvement via the proposed thermodynamic model in this study. The comprehensive thermodynamic model can promote the investigation of the advanced exergy analysis in the ORC. Applying conventional and advanced exergy analyses to investigate the thermodynamic performance of a system or its components is highly recommended.

Advanced exergy and exergoeconomic analyses and a case study of a novel trans-critical CO2 cycle with pressurization process for hot dry rock

Using hot dry rocks instead of fossil fuels can greatly alleviate the problem of environmental pollution. However, the low thermoelectric conversion efficiency of dry hot rock resources may hinder its commercial development. Therefore, an in-depth analysis of the operating efficiency and the relationship between the components in the hot dry rock power generation system can provide a basis for improving the system performance. In this study, the influence of the initial operating parameters on the thermodynamic performance of a trans-critical CO2 cycle with pressurization process applied to the utilization of hot dry rock resources is analyzed, so as to make the system more universal. On the other hand, the thermo-economic performance of the system is evaluated by traditional and advanced exergy/ exergoeconomic analysis respectively. The results show that the increase in production well-head temperature and CO2 capture pressure, the decrease in the CO2 capture temperature will improve the thermodynamic performance of the system to a certain extent. In general, the ratio of endogenous avoidable exergy destruction in trans-critical CO2 generation cycle and CO2 pressurization process is 46.48% and 34.41%, respectively. The exergy efficiency of condensers and intercoolers in the system is low, which have high performance improvement potential. The exergoeconomic analysis shows that the investment cost of each component is the main factor affecting the economic performance. Meanwhile, advanced exergy/ exergoeconomic analysis provides the valuable information for a clearer understanding of the interactions between different components of the system and the potential for improvement.

Conventional and advanced exergy analyses of transcritical CO2 ejector refrigeration system equipped with thermoelectric subcooler

This paper assesses the exergetic performance inside transcritical CO2 ejector refrigeration system integrated with thermoelectric subcooler (EJE + TES) through conventional and advanced exergy analyses. The results point out that 89.44% of the total exergy destruction is endogenous, which clearly demonstrates that it is not close in the interrelations among the system components. In addition, increasing the efficiency of the different components can decrease the exergy destruction by about 53.36%. The avoidable exergy destructions induced by the compressor is the highest, which indicates that priority should be given to the improvement of the compressor, followed by the thermoelectric subcooler, evaporator, ejector and gas cooler, while the expansion valve has almost no potential for improvement. This conclusion differs from that obtained by the conventional exergy method. The effects of the pinch temperature difference in each heat exchanger, as well as the compressor and ejector efficiencies on cycle theoretically exergetic performance are considered. When the compressor and ejector efficiencies increase from 0.5 to 0.9, the avoidable endogenous exergy destruction taking place in the corresponding system component are reduced by 93.61% and 82.33%, respectively. The performance of the EJE + TES system can be enhanced through reducing the pinch temperature difference.

Enhanced exergy analysis of a full-scale brackish water reverse osmosis desalination plant

Brackish water desalination by reverse osmosis membranes is energy-driven process. With the purpose of improving the energy performance of such unit a full-scale BWRO desalination plant located in Canary Island (Spain) has been working under intermittent operating conditions for 14 years was assessed with conventional and enhanced (exogenous and endogenous/avoidable and unavoidable exergy destructions) exergy analysis. The exergies across the major components of the plant are calculated. The conventional exergy analysis reveals that, largest irreversibilities were identified in the RO system (membrane modules), the high pressure pump and the feed pump which amounts to about 64.28%, 40.84% and 38.48% respectively. Results of advanced exergy analyses shows that 70.61%, 92.94% and 7.83% of the total exergy destruction in the high pressure pump, feed pump and the RO system respectively are avoidable. Moreover, the highest endogenous avoidable exergy destruction rate was determined owing to the feed and high pressure pumps. Using these findings, instructions were proposed to ensure optimal effectiveness of the RO system and to improve the exergy destruction in the pumps currently in use.

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

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

Exergy destruction analysis of a low-temperature Compressed Carbon dioxide Energy Storage system based on conventional and advanced exergy methods

Compressed Carbon dioxide Energy Storage (CCES) system is a novel energy storage technology, which provides a new method to solve the unstable problem of renewable energy. Since the CCES system using low-temperature thermal energy storage can avoid the technical difficulties from high-temperature thermal energy storage, the low-temperature Compressed Carbon dioxide Energy Storage (LT-CCES) system has more obvious application feasibility compared with the high-temperature energy storage system. As the research on energy conversion, transfer, and loss in CCES system under low-temperature heat storage is still missing, while it is important to understand the energy losses for the further optimization of this kind of system, in this paper, the conventional exergy analysis and advanced exergy analysis were utilized to analyze the thermodynamic characteristics of a LT-CCES system consisting of CO2 Brayton cycle, low-temperature thermal energy storage and cold energy storage, whose thermal energy storage temperature is below 200 °C. As the advanced exergy analysis takes into account the correlation between system components as well as the technical limitations of each component, exergy destruction can be split into more detailed parts (i.e., avoidable exergy destruction, unavoidable exergy destruction, endogenous exergy destruction, and exogenous exergy destruction), which makes the results of the advanced exergy analysis different from those of conventional exergy analysis. The results showed that, advanced exergy analysis pointed out that HEX3 should be given priority as endogenous avoidable exergy destruction in HEX3 was the largest, while the results from the conventional exergy analysis showed that HEX1 should be given priority. For all the components, the two methods indicated that the compressor should be given the highest priority for improvement, followed by the turbine. Besides, as the total exergy efficiency of the system was 55.3% under the real condition and the total exergy efficiency was 76.7% under the unavoidable condition, there was still room for improvement of the system. In brief, based on the advanced exergy analysis method, the real component that should be paid attention can be determined, which gives the designers a more profound understanding of exergy destruction in the LT-CCES system.

Comparison of Conventional and Advanced Exergy Analysis for Dual-Loop Organic Rankine Cycle used in Engine Waste Heat Recovery

At present, the dual-loop organic Rankine cycle (DORC) is regarded as an important solution to engine waste heat recovery (WHR). Compared with the conventional exergy analysis, the advanced exergy analysis can better describe the interactions between system components and the irreversibility caused by economic or technical limitations. In order to systematically study the thermodynamic performance of DORC, the conventional and advanced exergy analyses are compared using an inline 6-cylinder 4-stroke turbocharged diesel engine. Meanwhile, the sensitivity analysis is implemented to further investigate the influence of operating parameters on avoidable-endogenous exergy destruction. The analysis result of conventional exergy analysis demonstrates that the priorities for the components that should be improved are in order of the high-temperature evaporator, the low-temperature turbine, the first low-temperature evaporator and the high-temperature condenser. The advanced exergy analysis result suggests that the avoidable exergy destruction values are the highest in the low-temperature turbine, the high-temperature evaporator and the high-temperature turbine because they have considerable endogenous-avoidable exergy destruction. The sensitivity analysis indicates that reducing the evaporation pinch point and raising the turbine efficiency can decrease the avoidable exergy destruction.

Advanced exergy analysis of a combined gas power cycle with humidification

ABSTRACT A combined gas power cycle with humidification has been investigated using the advanced exergy analysis. The combustion chamber and humidifier were found to be the components with the largest exergy destruction. For a prescribed overall power generation, the exergy destruction at the combustion chamber increased, while that at the humidifier decreased when the humidification ratio at the bottoming cycle was increased. However, most of the irreversibility generation at these components was shown to be unavoidable. The topping cycle turbine and compressor were revealed to be the components with the largest endogenous and avoidable irreversibility generation. This means that there is a real potential of improving the overall performance of the cycle by improving the operation of these two components. The endogenous avoidable exergy destruction at the topping cycle turbine and compressor decreased monotonically when the humidification ratio, the combustion chamber exit temperature or the mass flowrate ratio between the bottoming and topping cycles were increased, and increased when the topping cycle pressure ratio was increased. On the other hand, varying the humidification ratio at the bottoming cycle from zero to one resulted in as much as 26% reduction of the irreversibility generation at the topping cycle turbine and compressor.

Conventional and advanced exergy analysis of an air-cooled type of absorption-ejection refrigeration cycle with R290-mineral oil as the working pair

An absorption-ejection refrigeration cycle used R290-mineral oil mixture is investigated depending on both conventional and advanced exergy analysis. In this cycle, an ejector and an air-cooled type absorber are utilized to improve the exergetic efficiency of the system. The effects of generator temperature, condensing temperature, evaporating temperature, absorber outlet temperature, absorber efficiency, and pressure ratio on system exergetic efficiency have been discussed and compared with a single-effect absorption cycle in this study. Furthermore, the exergy destruction of each component has been calculated and analyzed. Besides, the exergy destruction of the major components are further researched through the advanced exergy analysis. Analytical results show that the absorption-ejection refrigeration cycle has a higher exergetic efficiency and a more extensive working condition range than the single effect absorption cycle and its exergetic efficiency can reach 0.3554. It is proved the R290/mineral oil as absorption refrigeration has great research value. In addition, the vast majority of exergy destruction appears in the generator and absorber, totally above 60%. The optimization of the generator has the highest priority due to its maximum endogenous avoidable exergy destruction. The absorber is in second place and its exergy destruction can be effectively reduced by optimizing other components.