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[en] Highlights: • A detailed model is developed without using CFD. • Twelve types of inefficiencies of a gas turbine system are identified and rated. • Largest inefficiencies natural gas: stoich. combustion, addition of excess air. • In case of using syngas, the effect of adding excess air and mixing increases. • Negligible inefficiencies: convective cooling, heat loss, shaft and generator loss. - Abstract: Gas turbine systems are widely used for the production of electricity in a simple or combined-cycle mode today. Based on their ability to allow a fast load change, gas turbine systems will become even more important in the future since the volatile production of renewable energies will increase. In this study, a state-of-the-art gas turbine running on natural gas, having an overall net efficiency of approximately 40%, is modeled using Aspen Plus® and characteristic parameters are identified. Based on these parameters, a gas turbine running on syngas was simulated. The emphasis here is on a very detailed evaluation of the inefficiencies. The models consider cooling and sealing flows. The syngas considered in this study is typically used in IGCC processes with carbon capture resulting in a high concentration of hydrogen. For both systems, twelve types of inefficiencies were identified and rated. A comparison of the inefficiencies within each system and between both systems represented by their exergy destruction ratios is presented. In case of the gas turbine running on natural gas, the most important results show that the stoichiometric combustion, followed by the addition of excess air represent the largest inefficiencies. When just applying an isentropic efficiency, the exergy destructions associated with expansion and mixing at different temperatures and pressures of a gas turbine stage cannot be further sub-divided. Hence, this grouping of inefficiencies results to the third position. The effect of mixing at different compositions and the compression follows. In the second case considered here (use of syngas instead of natural gas), the effects of mixing and adding excess air become more significant due to a higher specific heat capacity of the combustion gas. In both cases, the exergy destruction associated with mixing at different compositions can be neglected except the one at the inlet of the pre-mixed combustor, which strongly depends on the particular conditions of the fuel gas. Inefficiencies such as convective cooling of the vanes and blades, heat loss, losses associated with the shaft and generator were found to represent a very small part of the overall exergy destruction. The resulting exergy destructions and losses are shown in an exergy flow diagram.
[en] Highlights: • A new concept for power generation including carbon capture was found. • The air reactor temperature significantly influences the net efficiency. • The use of a CO2 turbine decreases the net efficiency. • Compared to a conventional IGCC with 90% CO2 capture the net efficiency increases. - Abstract: Chemical-looping combustion (CLC) is a novel and promising combustion technology with inherent separation of the greenhouse gas CO2. This paper focuses on the design and thermodynamic evaluation of an integrated gasification combined-cycle (IGCC) process using syngas chemical looping (SCL) combustion for generating electricity. The syngas is provided by coal gasification; the gas from the gasifier is cleaned using high-temperature gas desulfurization (HGD). In this study, the oxygen carrier iron oxide (Fe2O3) is selected to oxidize the syngas in a multistage moving-bed reactor. The resulting reduced iron particles then consist of FeO and Fe3O4. To create a closed-cycle operation, these particles are partially re-oxidized with steam in a fluidized-bed regenerator to pure Fe3O4 and then fully re-oxidized in a fluidized-bed air combustor to Fe2O3. One advantage of this process is the co-production of hydrogen diluted with water vapor within the steam regenerator. Both the HGD and CLC systems are not under commercial operation so far. This mixture is fed to a gas turbine for the purpose of generating electricity. The gas turbine is expected to exhibit low NOx emissions due to the high ratio of water in the combustion chamber. Cooling the flue gas in the HRSG condenses the water vapor to yield high-purity CO2 for subsequent compression and disposal. To evaluate the net efficiency, two conventional syngas gasifiers are considered, namely the BGL slagging gasifier and the Shell entrained-flow gasifier. The option of using a CO2 turbine after the SCL-fuel reactor is also investigated. A sensitivity analysis is performed on the SCL-air reactor outlet temperature, this being a key design parameter. It was found that the best net efficiency of 43% (based on HHV) can be obtained using a BGL gasifier without a CO2 turbine at an air reactor temperature of 1000 °C including CO2 compression for transport and storage. Simulation data are based on software Aspen Plus® and EES (Engineering Equation Solver)
[en] An industrial ammonia synthesis loop is a complex interconnected system. With the synthesis reactor operated at high-pressure levels and with synthesis gas made of hydrogen and nitrogen, a highly efficient process design is necessary in order to meet the requirements in terms of cost-efficiency and environmental impact. The evaluation and optimization of different designs in the process synthesis phase are generally done by considering mass and energy balances. However, the conclusions drawn from such an analysis can be misleading and provide, if any, little useful information with respect to system improvement. In order to address these issues, an exergy analysis is used to identify the real thermodynamic inefficiencies of a system and its components. Furthermore, a subsequently conducted advanced exergy analysis provides the means to determine the structural interactions within a system and the thermodynamic improvement potential of its components. In this context, two different ammonia synthesis loop configurations are analyzed. The first concept consists of a three-staged adiabatic reactor with intermediate quench cooling, whereas the second design features a cooled reactor. - Highlights: • The information provided by thermodynamic, exergetic and advanced exergetic analyses is evaluated and compared. • Advanced exergetic analysis identifies the main design aspects and their impact for the ammonia synthesis loop. • Reactor design is the key decision variable for the thermodynamic efficiency of the synthesis loop. • Means for further system improvement are identified.
[en] A new approach for the calculation of the interactions among the components of a thermal system for application to the concept of advanced exergy-based analysis is presented. The approach can be used to determine the thermodynamic interactions of system components, and to evaluate alternative designs. The new approach puts the calculation of endogenous and exogenous exergy destruction on a proper thermodynamic basis and introduces a straightforward and time-saving calculation procedure in contrast to various approaches used in the past. When employed to the analysis of the CGAM-problem, the new approach complies with qualitative reasoning, resolves the shortcomings and shows comparable results with previous approaches. The top-down hierarchical approach assists in achieving the best system design possible by identifying the effects of design decisions and by stimulating the engineer's creativity in terms of design alternatives and optimization options. Furthermore, the generalization of the approach allows for any level of aggregation, thus, making the determination of improvement potentials easier. By providing profound thermodynamic understanding for processes, the advanced exergy-based analysis is a promising tool for designing, analyzing and optimizing processes for higher efficiencies and lower costs. - Highlights: • A new concept for the application in advanced exergy-based analysis is introduced and verified. • Problems with previous approaches for advanced exergy analysis are solved. • The concept easily integrates into existing process design methodologies. • Thermodynamic interactions among components are straightforwardly determined.
[en] As part of the German transition towards a low-carbon economy, renewable energies are set to account for more than half of the gross electricity consumption by 2035, resulting in a rising flexibility demand. Flexibility is required to balance fluctuations in the residual load. In addition, uncertainties in the wind and solar power generation cause an increased demand for control reserve. A unit commitment model of the German power system is used to analyze the value of power plant flexibility in systems with high shares of variable renewables. To investigate the value of power plant flexibility, the additional revenue that can be generated by flexibility improvements is calculated. The results indicate, that power plant flexibility has a small positive impact on the power plant's contribution margin in 2014. A future power system configuration according to the German Grid Development Plan would provide sufficient flexibility to integrate high shares of renewables without power plant flexibility being very valuable. However, integrating variable renewables into a system relying on coal-fired and nuclear power stations results in power plants being able to significantly increase their revenue with improved flexibility. Under these circumstances, power plant flexibility has a considerable value. - Highlights: • A unit commitment model is applied to study future value of power plant flexibility. • Detailed analysis of the German day-ahead spot market and control reserve market. • Promising approach to determine prices in markets with non-convexities is proposed. • Assessment of power plant fleet dominated by nuclear and coal vs. gas-fired units. • Low carbon power system provides low cost flexibility at high share of renewables.
[en] The thermodynamic inefficiencies associated with any energy conversion process are expressed by the exergy destruction and the exergy losses associated with the process. Combustion processes exhibit very high thermodynamic inefficiencies caused by chemical reaction, heat transfer, friction, and mixing. In this paper, we discuss how to estimate the thermodynamic inefficiencies resulting from each one of these sources. The thermodynamic evaluation can be conducted with the aid of either a conventional exergetic analysis or an advanced one. The latter allows estimation of the potential for improvement of the process being considered and demonstrates the interactions among the components of the system in which combustion takes place. The paper discusses how advanced exergy-based evaluations can be used to reduce the thermodynamic inefficiencies, costs, and environmental impacts associated with energy conversion systems including combustion processes
[en] Highlights: ► Demonstrates that previous work on the Voorhees compression process was incorrect. ► Provides a thermodynamically consistent way for simulating this process. ► Shows the formation of irreversibilities in the process. ► Demonstrates an interesting application of advanced exergy analysis. - Abstract: The Voorhees’ compression process is used in refrigeration as an alternative process to the two-stage vapor-compression refrigeration machines that are commercially available. This process is a combination of a compression process initially at constant total volume and then at near isentropic conditions. The compression process at constant total volume is realized with the help of injection of working fluid in the beginning of the compression process. Several factors (related to the simulation of the process and to stable operation) did not allow the wide use of this process (compressor) in the past. With today’s state of the art of compressors, automation systems and control systems, the idea of Voorhees became reality with screw and scroll compressors. However, all developments are based on data obtained from experimental research. All publications, where the concept of the Voorhees’ compression process is discussed, deal only with energetic analyses. For the first time, in addition to the detailed energetic analysis, and in order to show the limitations of the energetic analysis for such a complex process, a conventional and an advanced exergetic analysis are presented.
[en] Highlights: • An exergetic evaluation of the Siemens H-Class and F-Class CCPP is conducted. • The H-Class CCPP shows a better thermodynamic performance and higher profitability. • A comparative exergoeconomic evaluation is conducted on both processes. • The gas turbine system has the highest cost contribution among all units. • Levelized cost of electricity is reduced within an exergoeconomic optimization. - Abstract: Combined-cycle power plants are one of the main pillars of the global power sector. Worldwide, different stakeholders are on the race of developing highly efficient power plants through investing in metallurgical, thermodynamic, and technological developments. The purpose of this study is a comprehensive exergoeconomic evaluation and comparison of two cases of the latest combined-cycle power plant generation - the triple pressure F-Class and H-Class technologies of the Siemens AG. Taking into consideration the specific design differences, rigorous simulations are set up by implementing real plant data prior to an application of exergetic, economic and exergoeconomic analyses to evaluate the processes. The exergy analysis shows a higher exergetic efficiency of 58.3% for the H-Class, while this value is calculated to be 56% for the F-Class. The NPV of the H-Class exceeds that of the F-Class by 69% after 20 years of operation. Accordingly, the total capital investment of the H-Class is recovered one and a half years earlier. The levelized costs of electricity generated by the H-Class and F-Class are 31.7 $/MWh and 32.5 $/MWh, respectively. The exergoeconomic evaluation demonstrates that on the component basis the gas turbine system has the highest contribution to the overall cost caused by investment and irreversibilities within the processes. Design improvements obtained from an iterative exergoeconomic optimization of some important design parameters (decision variables) result in further reduction of the levelized cost of electricity for the H-Class design.
[en] Splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts represents a new development in the exergy analysis of energy conversion systems. This splitting improves the accuracy of exergy analysis, improves our understanding of the thermodynamic inefficiencies and facilitates the improvement of a system. An absorption refrigeration machine is used here as an application example. This refrigeration machine represents the most complex type of a refrigeration machine, in which the sum of physical and chemical exergy is used for each material stream
[en] Exergy-based (exergetic, exergoeconomic and exergoenvironmental) analyses, are used for designing, assessing and improving energy conversion systems. In an exergoeconomic analysis, thermodynamic inefficiencies – represented by exergy destruction – are used in combination with investment costs to calculate the “cost-optimal” layout of a plant. Analogously, in an exergoenvironmental analysis, the aim is to minimize the total environmental impact of a plant. Until today exergoeconomic and exergoenvironmental analyses have been used as separate and distinct tools and the improvement of a plant has been considered in terms of the reduction of either costs or environmental impact. To simultaneously decrease the investment costs and the component-related (manufacturing or construction-related) environmental impacts, their relationship with exergy destruction must be studied in parallel. This paper examines the relationship between exergoeconomic and exergoenvironmental data under various plant operating conditions. A combined-cycle power plant is analyzed and options for a simultaneous improvement from the thermodynamic, economic and environmental viewpoints are discussed. - Highlights: • Novel analysis combining exergoeconomic and exergoenvironmental methods. • Simultaneous thermodynamic, economic and environmental improvements of a power plant. • In most cases, efficiency improvements decrease costs and environmental impacts. • The trends of investment cost and environmental impact functions do not always agree.