Exergy Destruction Distribution Research Articles

Exergy analysis of a negative CO[formula omitted] emission gas power plant based on water oxy-combustion of syngas from sewage sludge gasification and CCS

A power cycle with water-injected oxy-combustion (water cycle) is investigated by exergy analysis. It is fueled with syngas (aka. producer gas) from gasification of sewage sludge. The cycle is equipped with a spray-ejector condenser (SEC). CO2 is separated and compressed for transportation and storage. The net delivered electric power is 31% of the fuel exergy. The task efficiency is 39% when the flue gas bleed to gasification and O2 penalty are subtracted from fuel, and CO2 capture is included in the useful product. The large part of exergy destruction, 80%, pertains to the combustor. Increasing the temperature or the pressure of the combustor outlet (turbine inlet) lead, as expected, to reduced exergy destruction and more power delivery. Reducing pressure of the gas turbine outlet (SEC inlet) also increases power production. Varying pressure and temperature of the SEC outlet affects the distribution of exergy destruction among units of the condenser, however scarcely the overall efficiency. Reducing the ambient temperature, including cooling water temperature, reduces the efficiency of the plant, contrary to the effect of conventional plants. The reason is that the low pressure of SEC relies on the pressure and mass flow of injected water, rather than the temperature.

Thermo-economic analysis and multi-objective optimization of supercritical Brayton cycles with CO2-based mixtures

• A thermo-economic analysis of Brayton cycle with CO 2 -based mixtures is performed. • The Pareto fronts of 3 working fluids are obtained by multi-objective optimization. • SCO 2 -Xe cycle shows the best thermodynamic performance compared to others. • CO 2 -Kr shows the potential for development through thermo-economic analysis. The Supercritical carbon dioxide recompression Brayton cycle (SCO 2 RBC) is regarded as one of the most promising thermal power conversion systems and mixing other gases with CO 2 to change the critical point represents an effective strategy to improve the performance of the Brayton cycle. In this work, the feasibility of using xenon and krypton as additives to the SCO 2 RBC is investigated via thermodynamic analysis. The effects of the key parameters on the thermo-economic performance of recompression Brayton cycles using CO 2 , CO 2 /xenon (mass fraction 0.55/0.45) and CO 2 /krypton (mass fraction 0.755/0.245) as working fluids are discussed by means of parametric analysis. A non-dominated sorting genetic algorithm is employed to simultaneously optimize the cycle exergy efficiency and the levelized cost of energy. The total thermal conductance, the main compressor outlet pressure, pressure ratio and split ratio are selected as the decision variables. The optimum solutions with their corresponding decision variables are selected from the Pareto frontiers. The results show that the supercritical CO 2 /krypton cycle has the huge development potential. Specifically, the optimum exergy efficiencies of Brayton cycles using CO 2 , CO 2 /xenon and CO 2 /krypton as working fluids are 0.585, 0.646 and 0.664. The addition of xenon can improve cycle efficiency up to 12.08 % higher than SCO 2 RBC while the levelized cost of energy also increased by 10.04%. The cycle efficiency of the supercritical CO 2 /krypton cycle is 9.44% higher while the cost is 2.49% higher compared to the SCO 2 RBC. There are suitable decision variables to make the cost of the cycle using CO 2 / krypton lower than that using CO 2 while achieving the same exergy efficiency when the required exergy efficiency is in the range of 0.60-0.62. The exergy destruction distribution of Brayton cycle illustrates that high temperature recuperator is the highest exergy destruction component and using CO 2 -based binary mixtures can reduce the total exergy destruction effectively.

Energy, conventional and advanced exergy analysis for the organic Rankine cycle-vapor compression refrigeration combined system driven by low-grade waste heat

Global issues such as energy crisis and environmental pollution have driven the development of waste heat recovery technology. The organic Rankine cycle (ORC)-vapor compression refrigeration (VCR) combined system is a promising solution to achieve refrigeration requirements by recycling waste heat. How to determine the irreversible losses of the system and the improvement priority of the components is of great significance to enhance the system performance. Firstly, the energy and conventional exergy analysis of the ORC-VCR combined system is carried out to obtain thermodynamic performance of the system. Then, the advanced exergy analysis of the system is conducted and the exergy destruction distribution characteristics and improvement potential of the components are obtained. In addition, the interactions between the components are investigated in detail to achieve the improvement priority of the components. Finally, the improvement strategies for each component are proposed based on the influence of the components on the system. The results show that the coefficient of performance (COP) and exergy efficiency of the system are 0.44 and 14.97 %, respectively. The condenser contributes the most (44148 W) to the exergy destruction of the system. The largest endogenous (32935 W), avoidable (28530 W) and endogenous-avoidable (21577 W) exergy destruction also belong to the condenser. The condenser is determined as the most important component to promote system performance. The turbine and compressor have dominant influence on the system and the improvement depends on themselves. While the generator, condenser and evaporator only affect themselves and the improvement depends on all the components of the system. The results can provide theoretical guidance for further improving system performance.

Thermal assessment and optimization of process fluids in transcritical organic and transcritical CO2 Rankine cycle for waste energy recuperating system

• Thermodynamic modeling of basic TORC and CTRC is conducted. • Evaluation of optimum operating conditions for the working fluids. • Energy and exergy analysis performed with optimized input parameters. As natural resources are depleting gradually, scientists are inclining toward the quest for an alternate source of energy and searching for efficient ways to mitigate wastage of energy. The potential of the organic Rankine cycle (ORC) in the application of recovering energy from medium and low-temperature heat sources has been explored by researchers for a long time. Over the years numerous optimizations and modifications have appeared to extract usable power from sources that are fundamentally discharging waste energy to the environment. In this study, a thermodynamic model of basic transcritical Organic Rankine Cycle (TORC) and carbon dioxide-based transcritical Rankine cycle are modeled using a validated MATLAB code, scrutinizing the optimum condition of eleven working fluids. This study prioritizes environment-friendly working fluids and the fluids with lower ODP and GWP indices have been selected for experimentation. These working fluids have been tested setting optimized input parameters and a comparative study of working fluids has been done using exergy destruction distribution. According to the second law analysis, at low turbine inlet temperature, R32 works better than other process fluids. Whereas, at higher temperatures, carbon dioxide delivers better performance compared to other working fluids.

Exergetic Effects of Cooled Cooling Air Technology on the Turbofan Engine during a Typical Mission

The cooled cooling air technology (CCA technology) shows expected performance in solving the growing thermal challenge for advanced aero engines by reducing the temperature of cooling air. The effect of CCA technology on the overall propelling performance with or without adjusting cycle parameters is controversial. Based on this, both the energy and exergy methods have been adopted to elaborate the specific mechanisms of the above energy utilization discrepancy. As a result, the scheme of CCA technology without optimizing cycle parameters has lower propelling work and efficiency with the total exergy destruction increasing 0.5~2%. Oppositely, as for the scheme of CCA with meliorated cycle parameters, the propelling efficiency improved by around 2~4% with total exergy destruction reduced by 1~3.5%. By analyzing the distribution of exergy destruction, the avoidable and unavoidable exergy destruction caused by the combustion chamber, compressors, and turbines accounts for the largest proportion, which indicates that more attention needs to be paid in the future. During the whole flight mission, the percentage of exergy destruction is much higher in supersonic, subsonic cruise, combat, and escape conditions. In conclusion, the improvement of cycle parameters to reduce the exergy destruction should be considered when introducing CCA technology.

Completely Recuperative Supercritical CO2 Recompression Brayton/Absorption Combined Power/Cooling Cycle: Performance Assessment and Optimization

Excessive heat losses and water consumption in cooling units are significant constraints restricting the application circumstances and performances for the SCO2 Brayton cycle, and the heat exchange capacity in the precooler (PRC) is typically 1.5 times that of power generation. Therefore, this research offers a high-integrated combined power/cooling system in which two waste heat exchangers (WHEs) and a rectifier (RET) are used instead of the PRC to achieve 100% exhaust heat recovery. Each component’s energy and exergy models are developed, and the operational characteristics, coupling relationships, and exergy destruction distribution are examined. Results indicate that, when compared to the Brayton cycle, the thermal and exergy efficiency is considerably increased, and the concentration difference and WHE1 pitch point difference have significant influences on system performance. Further exergoeconomic and optimization analysis reveals that the superior exergy case is mostly recommended for relevant thermal and exergy efficiency increasing rates of 13.7% and 9.17%, respectively, and the unit cost is 81.33% that of the base case. Turbine 1 (TUR1) and main compressor (MCP) are the first and second highest cost rates, respectively, and RET and generator (GEN) account for roughly 34% exergy destruction rate and 20% exergy destruction cost rate, respectively. In addition, reducing heat transfer differences in relevant equipment can further promote system performance.

Conventional and enhanced exergy analyses of a parallel integrated thermal management system for pure electric vehicles

To improve the efficiency of the integrated thermal management system (ITMS) of pure electric vehicles, an enhanced exergetic analysis method was proposed. According to enhanced exergetic analysis, the exergy destruction distribution of main components of ITMS was detected in-depth. The results indicate that the exergy destruction within the evaporator entirely belonged to the endogenous part, and the exergy destruction of the condenser is mainly caused by the condenser itself; its endogenous exergy destruction (78.8%) is greater than the exogenous exergy destruction (21.2%). Meanwhile, the enhanced exergetic analysis suggested that the avoidable exergy destruction of the system accounts for about 68.7% of the total exergy destruction of the system; the potential for improvement of the system is huge. The interaction among the components is not strong, due to the 72.38% of exergy destruction in the system is endogenous. The highlights of the paper are: 1) thermodynamic modelling of the parallel ITMS is carried out; 2) enhanced exergetic analysis is applied to ITMS of pure electric vehicles; 3) avoidable exergy destruction is obtained, which represents the potential for system improvement; 4) the improvement priority for system components is acquired.

Thermodynamic Limitations and Exergy Analysis of Brackish Water Reverse Osmosis Desalination Process.

The reverse osmosis (RO) process is one of the most popular membrane technologies for the generation of freshwater from seawater and brackish water resources. An industrial scale RO desalination consumes a considerable amount of energy due to the exergy destruction in several units of the process. To mitigate these limitations, several colleagues focused on delivering feasible options to resolve these issues. Most importantly, the intention was to specify the most units responsible for dissipating energy. However, in the literature, no research has been done on the analysis of exergy losses and thermodynamic limitations of the RO system of the Arab Potash Company (APC). Specifically, the RO system of the APC is designed as a medium-sized, multistage, multi pass spiral wound brackish water RO desalination plant with a capacity of 1200 m3/day. Therefore, this paper intends to fill this gap and critically investigate the distribution of exergy destruction by incorporating both physical and chemical exergies of several units and compartments of the RO system. To carry out this study, a sub-model of exergy analysis was collected from the open literature and embedded into the original RO model developed by the authors of this study. The simulation results explored the most sections that cause the highest energy destruction. Specifically, it is confirmed that the major exergy destruction happens in the product stream with 95.8% of the total exergy input. However, the lowest exergy destruction happens in the mixing location of permeate of the first pass of RO desalination system with 62.28% of the total exergy input.

Synergistic effect of geometric parameters on CO2 ejector based on local exergy destruction analysis

Ejector is valuable to improve the transcritical CO2 refrigeration cycle. The improvement on geometric parameters is significant for the performance enhancement of CO2 ejector. In this paper, numerical simulation based on a homogeneous equilibrium model is used to implement the sensitivity analysis on the key geometric parameters. With the simulation, the influence mechanisms of the key geometric parameters on the macro-performance under overall operation conditions of the ejector are discovered based on the analysis of flow field structure and local exergy destruction distribution. The results show that the outlet area of primary nozzle, convergent angle of pre-mixer and nozzle exit position are strongly coupled. A sensitivity matrix of these three key geometric parameters to local exergy destruction distribution is obtained to evaluate their synergism quantitatively. This work would provide a new way to guide the optimization on the pre-mixer of the ejector considering the coupling effect of geometric parameters.

Optimum Expanded Fraction for an Industrial, Collins-Based Nitrogen Liquefaction Cycle.

Industrial nitrogen liquefaction cycles are based on the Collins topology but integrate variations. Several pressure levels with liquefaction to medium pressure and compressor–expander sets are common. The cycle must be designed aiming to minimise specific power consumption rather than to maximise liquid yield. For these reasons, conclusions of general studies cannot be extrapolated directly. This article calculates the optimal share of total compressed flow to be expanded in an industrial Collins-based cycle for nitrogen liquefaction. Simulations in Unisim Design R451 using Peng Robinson EOS for nitrogen resulted in expanded flow, which is greater than the 75–80% for conventional Collins cycles with helium or other substances. Optimum specific compression work resulted kWh/ton of liquid nitrogen. For some operating conditions, the relation between liquid yield and specific power consumption was counterintuitive: larger yield entailed larger consumption. Exergy analysis showed exergy efficiency of the optimised process. The exergy destruction distribution and exergy flow across the cycle is provided. Approximately 40% of the exergy destruction takes place in the cooling after compression. This exergy could be used for secondary applications such as industrial heating, energy storage or for lower temperature applications as heat conditioning.

Economic, Exergy, Environmental (3E) Analysis of Methanol Production from Shale Gas

Compared with direct utilisation as fuel, the downstream processing of shale gas shows great potential in economic competitiveness, in which methanol is a particularly important product. This study simulates a technical route for producing methanol from shale gas, which is evaluated from the points of economic, exergy and environment. The economic target is estimated by purchased cost curves, while exergy analysis demonstrates the distribution of exergy destruction. A hybrid LCI (Life cycle inventory) model is established to assess pollutant emissions by sources. The results show that methane reforming features the highest capital cost, while methanol synthesis requires the highest operating cost. The reaction systems account for 73.3 % of total exergy destruction. Electricity consumption dominates the environmental impacts of the whole process.

Optimal Design Method for Absorption Heat Pump Cycles Based on Energy-Utilization Diagram

Optimization for energy systems is considered at three levels: synthesis (configuration), design (component characteristics), and operation. This paper aims to evaluate the system performance and margins for improvement of two absorption heat pump systems, including an absorber heat exchanger (AHX) and a solution heat exchanger (SHX), and perform their design/operation optimization efficiently based on an energy-utilization diagram (EUD) for performance improvement. Before optimization, exergy efficiency is higher in the SHX cycle, while the margin for improvement is larger in the AHX cycle. The optimization attempts to reduce exergy destruction in the components where dominant exergy destruction caused by heat transfer occurs. In the absorber, the operating points are adjusted to make the temperature slopes at the hot and cold sides coincide. The design parameters in other components are adjusted to improve the heat transfer performances. The distribution of exergy destruction of each component leads to improve exergy efficiency. After these improvements, exergy efficiency is higher in the AHX cycle. It is concluded that we could efficiently realize the design/operation optimization of thermodynamic systems using an EUD, because the diagram presents both exergy destruction and margin for improvement at the components comprehensively, as well as the operating properties of working fluids.

Thermodynamic analysis of a micro turbine CHP system integrated with absorption-compression heat pump

Micro gas turbine is a high-efficiency power generation device, which is easy to realize miniaturization. Included compressor, turbine and generator share only one shaft, thus reducing the required moving parts and the unit size. It is widely used in civilian use, industrial and agricultural sectors due to its low fuel consumption, noise and emissions. Meanwhile, steam is widely used in industrial processes such as iron and steel. Steam is generally generated by coal-fired boiler or heat recover steam generator. It is estimated that industrial boilers take up to 30% of the manufacturing industry energy consumption. Micro gas turbine combined with waste heat boiler is a common way to produce heat and electric power simultaneously. However, large amount of waste flue gas with a high temperature above 170°C is rejected to ambient, causing large energy waste and heat island effect. To solve this problem, a micro turbine CHP system integrated with absorption-compression heat pump (ACHP) is proposed. The waste flue gas of the Micro turbine CHP system is adopted as the heat resource, which could produce 0.5 MPa saturated steam in the proposed system. As a result, the energy and environmental performance of the proposed system can be improved significantly. The thermodynamic performances of the proposed system are mathematically investigated. The results show that the primary energy ratio and energy saving ratio of the new system reach 67.3% and 23.56%, respectively, which are 11.2% and 14.76% higher than those of a traditional micro turbine CHP system. An exergy analysis is carried out to determine the distribution of exergy destruction for further optimizing the system performance. The exergy analysis shows that the irreversible loss in the waste flue gas could be lowered by 78% in the proposed system, and the exergy efficiency is 1.79% higher than that of the reference system. Moreover, parametric analysis is conducted to study the effect of the reboiler outlet temperature, partial condenser outlet temperature, the rectifier inlet ammonia-water concentration and the generation pressure on the system performance. Meanwhile, the coefficient of the performance (COP) is selected to evaluate the performance of the system. The outlet temperature of reboiler and the rectifier inlet ammonia-water concentration have a great influence on the COP, while the effect of the generation pressure and the outlet temperature of the partial condenser is relatively small. The COP increase as the outlet temperature of reboiler rises, while there is a peak value (0.29) with the increase of the inlet ammonia-water concentration. The COP changes slowly as the outlet temperature of the partial condenser increases from 45°C to 75°C. The COP almost keeps constant before generation pressure reaches 0.6 MPa, and then decreases slightly afterwards.

Thermodynamic performance of cycle combined large temperature drop heat exchange process:Theoretical models and advanced process

High-temperature-drop heat exchange technology, which can be realized by the proper configuration of heat-driven heat pump cycles, is an important solution for the Large-scale District Heating project. Cycle combined heat exchange process model is a theoretical standard for the analysis of High-temperature-drop heat exchange process, the thermodynamic performance and the heat-work conversion relationship of the this kind of process need to be researched to approach the theoretical optimal heat exchange effect. In this paper, with the analytical method, the thermodynamic performance including the medium temperature variation, useful work transmission, operation efficiency and heat exchange effect. Various values of heat engine high-grade temperature, heat exchange efficiency and endo-irreversibility factor are considered to analyze their effects on the outlet temperature lift/drop. The quantity and location of the exergy destruction distribution is also researched. Moreover, the relation between the heat exchange effect and the distribution of the exergy loss is researched, by which the advanced process model is proposed. With re-distribution of the exergy destruction, the advanced process has a promoted heat exchange effect compared with the original one. The result and conclusion of the this paper will offer a guidance principle for the component configuration of the practical application.

Thermodynamic analysis of a new combined cooling and power system using ammonia–water mixture

In order to achieve both power and cooling supply for users, a new combined cooling and power system using ammonia–water mixture is proposed to utilizing low grade heat sources, such as industrial waste heat, solar energy and geothermal energy. The proposed system combines a Kalina cycle and an ammonia–water absorption refrigeration cycle, in which the ammonia–water turbine exhaust is delivered to a separator to extract purer ammonia vapor. The purer ammonia vapor enters an evaporator to generate refrigeration output after being condensed and throttled. Mathematical models are established to simulate the combined system under steady-state conditions. Exergy destruction analysis is conducted to display the exergy destruction distribution in the system qualitatively and the results show that the major exergy destruction occurs in the heat exchangers. Finally a thermodynamic sensitivity analysis is performed and reveals that with an increase in the pressure of separator I or the ammonia mass fraction of basic solution, thermal efficiency and exergy efficiency of the system increase, whereas with an increase in the temperature of separator I, the ammonia–water turbine back pressure or the condenser II pressure, thermal efficiency and exergy efficiency of the system drop.

Renewable and non-renewable exergy costs and CO2 emissions in the production of fuels for Brazilian transportation sector

An exergy and environmental comparison between the fuel production routes for Brazilian transportation sector, including fossil fuels (natural gas, oil-derived products and hydrogen), biofuels (ethanol and biodiesel) and electricity is performed, and the percentage distribution of exergy destruction in the different units of the processing plants is characterized. An exergoeconomy methodology is developed and applied to properly allocate the renewable and non-renewable exergy costs and CO2 emission cost among the different products of multiproduct plants. Since Brazilian electricity is consumed in the upstream processing stages of the fuels used in the generation thereof, an iterative calculation is used. The electricity mix comprises thermal (coal, natural gas and oil-fired), nuclear, wind and hydroelectric power plants, as well as bagasse-fired mills, which, besides exporting surplus electricity, also produce sugar and bioethanol. Oil and natural gas-derived fuels production and biodiesel fatty acid methyl-esters (FAME) derived from palm oil are also analyzed. It was found that in spite of the highest total unit exergy costs correspond to the production of biofuels and electricity, the ratio between the renewable to non-renewable invested exergy (cR/cNR) for those fuels is 2.69 for biodiesel, 4.39 for electricity, and 15.96 for ethanol, whereas for fossil fuels is almost negligible.

Exergy-based optimal control of a vapor compression system

Exergy-based analysis and optimization has been successfully used to design a variety of thermal systems to achieve greater efficiency. However, the advantages afforded by exergy destruction minimization (EDM) at the design stage have not been translated to closed-loop operation of thermal systems such as vapor compression systems (VCSs). Through online optimization and control, VCSs can effectively respond to disturbances, such as weather or varying loads that cannot be accounted for at the design stage, while simultaneously maximizing system efficiency. Furthermore, in applications where VCSs encounter high frequency disturbances, such as in refrigerated transport applications or passenger vehicles, optimizing efficiency at steady-state conditions alone may not lead to significant reductions in energy consumption. In this paper we design the first exergetic, or second law, optimal controller for a canonical four-component vapor compression system (VCS). A lumped parameter moving boundary modeling framework is used to model the two heat exchangers in the VCS. A model predictive controller is then designed and implemented in simulation using a dynamic exergy-based objective function to determine the optimal control actions for the VCS to maximize exergetic efficiency while achieving a desired cooling capacity. Simulation results show that an exergy-based model predictive controller minimizing exergy destruction achieves over 40% greater exergetic efficiency during operation than a comparable first law MPC. Moreover, the distribution of exergy destruction across individual components offers new insight into the effect of variable-speed actuators on system efficiency in VCSs.

Organic Rankine cycle for power recovery of exhaust flue gas

To study the effects of different working fluids on the performance of organic Rankine cycle (ORC), three working fluids, a mixture that matches with heat source, a mixture that matches with heat sink and a pure working fluid, are selected in this paper. Thermodynamic models were built in Matlab together with REFPROP, with which, the physical properties of the selected working fluids can be acquired. Heat source of the ORC system is the exhaust flue gas of boiler in a 240 MW pulverized coal-fired power plant. Some indicators such as thermal efficiency, inlet temperature of expander, superheat degree, mass flow, volumetric flow, and exergy destruction distribution, as well as the influence of recuperator are studied. The analytical results show that the mixture that matches with heat sink has the greatest efficiency and the mixture that matches with heat source has the lowest superheat degree. The rate of heat exchanged in recuperator to that in evaporator has a maximum value with evaporating pressure. There exists no optimal working fluid for all indicators (thermal efficiency, heat exchanger area, mass flow and volumetric flow etc.). An appropriate working fluid should be chosen by taking both investment cost and power generating benefits into account. The cost-benefit ratio of the proposed ORC plant was evaluated either.

Exergy Analysis of Coal-Fired Power Plants in Two Cooling Condition

The distribution of exergy destruction is different in power plant systems with water cooled condenser and air cooled condenser. A detailed comparison based on a conventional Rankine cycle with two different cooling configurations is carried out in this paper in an exergy perspective. The inefficiency of the overall systems is analyzed in the same amount of heat transfer capacity condition, it shows that the distribution of exergy destruction among components is similar in both conditions. And this study of energy consumption is benefit for realization of auto-efficient-working power plants in future.

Comprehensive exergy-based evaluation and parametric study of a coal-fired ultra-supercritical power plant

In this paper, both conventional and advanced exergy analyses were conducted to a large-scale ultra-supercritical coal-fired power plant. The objectives of the conventional one are to compare the exergetic performances of different components, to identify and quantify the sites with the largest exergy destruction and losses, and to find the fuel-savings potential by improving each component in isolation. The advanced exergetic analysis focuses on the thermodynamic interactions among components and the sources for energy-saving potential of each component. Moreover, comparisons with several subcritical units are conducted and a sensitivity analysis shows the dependencies of the overall exergetic efficiency on a number of key design parameters. The results display the spatial distribution of exergy destruction and losses in detail and three performance ranges for different types of heat exchangers involved in the system. The energy-saving potentials at both the system and the component levels by improving an individual component are not in accordance with the amount of its exergy destruction. Improvement strategies for different components differ significantly due to the varied contributions of endogenous/exogenous parts to their avoidable exergy destructions. With an increase in the steam conditions, the exergy destruction ratio of the boiler is significantly reduced, contributing mainly to the system improvement. The most effective and achievable measure for reducing the fuel consumption is still the reasonable utilization of available low-grade heat. This framework provides a basis for the quantifying proposals of exergy-driven strategies for improving the system.