A Review of Evaluation, Optimization and Synthesis of Energy Systems: Methodology and Application to Thermal Power Plant...Published: 27 December 2018 by MDPI in Energies
To reach optimal/better conceptual designs of energy systems, key design variables should be optimized/adapted with system layouts, which may contribute significantly to system improvement. Layout improvement can be proposed by combining system analysis with engineers’ judgments; however, optimal flowsheet synthesis is not trivial and can be best addressed by mathematical programming. In addition, multiple objectives are always involved for decision makers. Therefore, this paper reviews progressively the methodologies of system evaluation, optimization, and synthesis for the conceptual design of energy systems, and highlights the applications to thermal power plants, which are still supposed to play a significant role in the near future. For system evaluation, both conventional and advanced exergy-based analysis methods, including (advanced) exergoeconomics are deeply discussed and compared methodologically with recent developments. The advanced analysis is highlighted for further revealing the source, avoidability, and interactions among exergy destruction or cost of different components. For optimization and layout synthesis, after a general description of typical optimization problems and the solving methods, the superstructure-based and -free concepts are introduced and intensively compared by emphasizing the automatic generation and identification of structural alternatives. The theoretical basis of the most commonly-used multi-objective techniques and recent developments are given to offer high-quality Pareto front for decision makers, with an emphasis on evolutionary algorithms. Finally, the selected analysis and synthesis methods for layout improvement are compared and future perspectives are concluded with the emphasis on considering additional constraints for real-world designs and retrofits, possible methodology development for evaluation and synthesis, and the importance of good modeling practice.
Simulation and Exergy Analysis of Energy Conversion Processes Using a Free and Open-Source Framework—Python-Based Object...Published: 30 September 2018 by MDPI in Energies
State-of-the-art thermodynamic simulation of energy conversion processes requires proprietary software. This article is an attempt to refute this statement. Based on object-oriented programming a simulation and exergy analysis of a combined cycle gas turbine is carried out in a free and open-source framework. Relevant basics of a thermodynamic analysis with exergy-based methods and necessary fluid property models are explained. Thermodynamic models describe the component groups of a combined heat and power system. The procedure to transform a physical model into a Python-based simulation program is shown. The article contains a solving algorithm for a precise gas turbine model with sophisticated equations of state. As an example, a system analysis of a combined cycle gas turbine with district heating is presented. Herein, the gas turbine model is validated based on literature data. The exergy analysis identifies the thermodynamic inefficiencies. The results are graphically presented in a Grassmann chart. With a sensitivity analysis a thermodynamic optimization of the district heating system is discussed. Using the exergy destruction rate in heating condensers or the overall efficiency as the objective function yields to different results.
Active Phase Change Material Cold Storage in Off-Grid Telecommunication Base Stations: Potential Assessment of Primary E...Published: 23 July 2018 by ASME International in Journal of Energy Resources Technology
The global demand for wireless, mobile communication and data services has grown significantly in the recent years. Consequently, electrical energy consumption to provide these services has increased. The principal contributors to this electricity demand are approximately 7 million telecommunication base stations (TBS) worldwide. They act as access points for mobile networks and have typical electrical loads of 2 3 kW. Whereas for most of the TBS the electricity is supplied by the grid, approximately 15 % are located in remote areas or regions with poor grid accessibility, where diesel generators (DG) supply the required electricity. Based on a dynamic simulation model built in Matlab/Simulink, the application of a latent heat storage (LHS) using phase change material (PCM) in existing off grid TBS has been analysed. The LHS unit has been modelled as an air based storage with phase change temperatures between 20 30 °C with the PCM being macro encapsulated in slabs. This paper demonstrates the potential to reduce the primary energy consumption in off grid TBS through the following methods: optimization of the operating point of the diesel generators and of the air conditioning unit operation schedule as well as utilization of photovoltaic energy.
Increasing the Flexibility of Combined Heat and Power Plants With Heat Pumps and Thermal Energy StoragePublished: 30 November 2017 by ASME International in Journal of Energy Resources Technology
Combined heat and power (CHP) plants are efficient regarding fuel, costs, and emissions compared to the separate generation of heat and electricity. Sinking revenues from sales of electricity due to sinking market prices endanger the economically viable operation of the plants. The integration of heat pumps (HP) and thermal energy storages (TESs) represents an option to increase the flexibility of CHP plants so that electricity can be produced only when the market conditions are favorable. The investigated district heating system is located in Germany, where the electricity market is influenced by a high share of renewable energies. The price-based unit-commitment and dispatch problem is modeled as a mixed integer linear program (MILP) with a temporal resolution of 1 h and a planning horizon of 1 yr. This paper presents the optimal operation of a TES unit and a HP in combination with CHP plants as well as synergies or competitions between them. Coal and gas-fired CHP plants with back pressure or extraction condensing steam turbines (STs) are considered, and their results are compared to each other.
Highlights•A possible SOFC system configuration for sewage treatment plant cogeneration application is investigated.•Economic analyses comparing the SOFC system to conventional CHP systems are conducted.•SOFC systems become economically viable at about 3000 EUR/kWel.•Smaller SOFC systems are currently economically favorable. AbstractFuel cells are likely to make their market introduction through high-efficiency applications in niche markets. A possible market for SOFC systems is therefore the utilization of biogas from sewage treatment plants. However, the feasibility of fuel cell applications crucially depends on the gas cleaning system. A system layout for SOFC integration is derived from an existing fuel cell system for biogas utilization, providing a feasible design for gas cleaning for contaminant removal and data on different operation regimes. The resulting SOFC plant provides electricity and heat for on-site usage at the sewage plant by cogeneration. The SOFC system is then analyzed at system level regarding its economic viability compared to a conventional CHP system. Costing for the different system components is made using cost data and literature-based estimates. Different design studies concerning system size and a subsequent sensitivity study concerning decisive economic parameters are used to provide robust decision measures. The study shows that economic feasibility of an SOFC system for biogas utilization can be achieved without subsidies in the near future if SOFC system prices are reduced from 7000 to about 3000 EUR/kWel. It is further shown that smaller SOFC systems are preferable due to their current economies of scale.
Advanced exergy analysis applied to the process of regasification of LNG (liquefied natural gas) integrated into an air ...Published: 01 December 2016 by Elsevier BV in Energy
Highlights•The integration of LNG regasification into an air separation process was studied.•Two integrated designs with different exergetic efficiencies are discussed.•Power consumption by the air separation unit decreases notably through integration.•The advanced exergy analyses highlight components that are of particular interest.•The effect of the pressure of LNG supply was investigated. AbstractNatural gas is one of the most important sources of energy, the demand for which increases continuously. The LNG (liquefied natural gas) market rises currently exponentially; many countries entered this market recently. Applying an efficient regasification process for LNG is now more important than in the past. At present, mainly regasification of LNG via direct or indirect heating is used for industrial applications. Regasification of LNG can also be combined with generation of electricity. Another possibility is the integration of the regasification into a processes requiring low temperatures. A new concept dealing with the integration of regasification of LNG into a cryogenic process of air separation has recently been developed at Technische Universität Berlin. This paper evaluates two options of integrating the regasification of LNG into an air separation system. Conventional and advanced exergy analyses are used in the evaluation.
Exergy Analysis of a Novel Cryogenic Concept for the Liquefaction of Natural Gas Integrated Into an Air Separation Proce...Published: 11 November 2016 by ASME International in Volume 8: Heat Transfer and Thermal Engineering
The increasing demand for primary energy leads to a growing market of natural gas and the associated market for liquefied natural gas (LNG) increases, too. The liquefaction of natural gas is an energy- and cost-intensive process. After exploration, natural gas, is pretreated and cooled to the liquefaction temperature of around −160°C. In this paper, a novel concept for the integration of the liquefaction of natural gas into an air separation process is introduced. The system is evaluated from the energetic and exergetic points of view. Additionally, an advanced exergy analysis is conducted. The analysis of the concepts shows the effect of important parameters regarding the maximum amount of liquefiable of natural gas and the total power consumption. Comparing the different cases, the amount of LNG production could be increased by two thirds, while the power consumption is doubled. The results of the exergy analysis show, that the introduction of the liquefaction of natural gas has a positive effect on the exergetic efficiency of a convetional air separation unit, which increases from 38% to 49%.
The aim of this work is to study a binary Rankine process with a significantly higher efficiency compared to a conventional coal-fired power plant. This paper focuses on the design of the process and especially on an efficient combination of flue gas, potassium, and water streams in the components of the steam generator, such as economizers, evaporators, and superheaters, to decrease the overall exergy destruction. Based on a literature review, a base case for a coal-fired binary Rankine cycle with potassium and water as working fluids was developed and, in order to evaluate the thermodynamic quality of several variants, comparative exergy analyses were conducted. A simulation of the process and calculation of the values for the streams were carried out by using the flow-sheeting program CycleTempo, which simultaneously solves the mass and energy balances and contains property functions for the specific enthalpy and entropy of all the substances used. Necessary assumptions are predominantly based on literature data or they are discussed in the paper. We present the exergy analysis of the overall process that includes the flue gas streams as well as the potassium and water cycles. A design analysis and sensitivity studies show the effects of stream combinations and key parameters on the net efficiency, which is higher than 50%.
Highlights•The implementation of the environmental issue to the exergy-based methods is discussed.•Exergoenvironmental analysis for the compression refrigeration machines is applied.•The effect of the selected eco-indicators to the results is evaluated.•The component-related environmental impact is negligible compared with the environmental impact of fuel. AbstractAn exergoenvironmental analysis is conducted at the component level of a system and identifies (a) the relative contribution of each component to the environmental impact associated with the entire system, and (b) options for reducing the environmental impact associated with the overall system. In an exergoenvironmental analysis a one-dimensional characterization indicator is obtained using a Life Cycle Assessment (LCA). An index (a single number) describes the overall environmental impact associated with system components and exergy carriers. It should be mentioned that the evaluation of environmental impacts would always be subjective to some degree. The paper discusses the effect of the indicator used in an exergoenvironmental analysis on the conclusions obtained from the analysis using a compression refrigeration machine as an example. The results demonstrate that the contribution of the component-related environmental impact can be neglected in the exergoenvironmental evaluation, and that only the environmental impact associated with the exergy destruction should be considered in the analysis. For the case study reported here, the conclusions extracted from the exergoenvironmental evaluation are independent of the employed environmental indicator.
Design and Assessment of an IGCC Concept with CO2 Capture for the Co-Generation of Electricity and Substitute Natural Ga...Published: 04 December 2015 by MDPI in Sustainability
The focus of this work is on the modeling and the thermodynamic evaluation of an integrated gasification combined cycle (IGCC) for the co-production of electricity and substitute natural gas (SNG). At first, an IGCC with CO₂ capture for electricity generation is analyzed. Coal-derived syngas is conditioned in a water gas shift unit (WGS), and cleaned in an acid gas removal system including carbon capture. Eventually, the conditioned syngas is fed to a combined cycle. A second case refers to a complete conversion of syngas to SNG in an integrated commercial methanation unit (TREMP™ process, Haldor Topsøe, Kgs. Lyngby, Denmark). Due to the exothermic reaction, a gas recycling and intercooling stages are necessary to avoid catalyst damage. Based on a state-of-the-art IGCC plant, an optimal integration of the synthetic process considering off-design behavior was determined. The raw syngas production remains constant in both cases, while one shift reactor in combination with a bypass is used to provide an adequate H₂/CO-ratio for the methanation unit. Electricity has to be purchased from the grid in order to cover the internal consumption when producing SNG. The resulting heat and power distributions of both cases are discussed.
Advanced Exergy Analysis Based on the Parametric Study of the Regasification of LNG Integrated Into an Air Separation Pr...Published: 13 November 2015 by ASME International in Volume 13: Vibration, Acoustics and Wave Propagation
The growing demand for natural gas leads to an increasing LNG market. The amount of traded LNG has more than doubled during the last decade. This trend is intensified by the rising number of liquefaction plants (export terminals) and regasification plants (import terminals). At the end of the year 2013 there were 86 liquefaction plants in 17 exporting countries and 104 import terminals in 29 importing countries. Also the number of floating regasification plants is growing. It is expected that the LNG market will grow with 7 % per year until 2020. In comparison, the market for gaseous natural gas only will increase with approxematly 1.8 % per year. The difference could be led back to the several advantages, when using LNG. Thus LNG enables the extraction of natural gas in offsite areas and leads to a flexible gas market. Especially with improving the efficiency of each part of the LNG chain — liquefaction, transportation, storage and regasification — and its fallen prices the LNG market will continue to grow. For the regasification of LNG different processes have been used, while mainly the vaporization via direct or indirect heating is applied. Due to their location at the coast of the importing country, seawater, air or the combustion gases coming from natural gas are used as thermal energy. A further possibility is the combination of regasification of LNG with generating electricity. Additionally, the regasification of LNG could be integrated into chemical processes (oil refinery and petrochemical plants), where low temperature refrigeration is required. The authors have already reported a concept for the integration of the regasification of LNG into an air separation and liquefactions process, i.e. into a cryogenic processes. In previous publications, an evaluation of the conventional air separation unit in combination with the LNG regasification has been reported. It was emphasized that the integration of LNG leads to a lower power consumption for the entire system. This paper deals with an improved concept for integrating the regasification of LNG into an air separation process. Due to structural changes, comparing the first design and the new design, the system can be further improved from the thermodynamic point of view. The aim of this paper is to discover the potential for improvement by the parametric study. The results obtained from the sensitivity analysis (energetic and exergetic) are reported as well as the results obtained from the advanced exergetic analysis. Some options for new designs of this system are be developed.
The idea of developing supercritical CO2 power cycles and applying them to industrial processes became increasingly popular in the last decade. Significant research has been done in this field, including the investigation of characteristics of equipment, and parametric optimization of power systems. There are only few publications on refrigeration using CO2, under hot climatic conditions. This paper deals with an application of an integrated conventional and advanced exergetic analysis to a supercritical CO2 power cycle operating in hot climatic conditions. The objective is to obtain detailed useful information about the optimization of the structure and the parameters of the system being considered. Conventional exergetic analyses have some limitations, which are significantly reduced by the so-called advanced analyses. In addition to conventional analyses, the latter evaluate, (a) the interactions among components of the overall system, and (b) the real potential for improving a system component. A conventional exergetic analysis emphasizes more the relative importance of the regenerative heat exchanger compared to the remaining four components (compressor, cooler, expander, and heat exchanger) than the advanced analysis does. The results obtained from the advanced exergetic analysis show that the system being analyzed can be improved by improving the components in isolation from the overall system, because the avoidable inefficiencies caused by the components interconnections are relatively low.
We examine the integrated advanced exergetic, exergoeconomic, and exergoenvironmental analyses, which identify the magnitude, location and causes of thermodynamic inefficiencies, costs, and environmental impacts. The analyses evaluate the interactions among the components of the overall system and the real potential for improving a system component. The results from the application of these methods are useful in understanding the operation of energy conversion systems and in developing strategies to improve them. This chapter demonstrates how exergoeconomic and exergoenvironmental analyses provide the user with information related to (a) the formation processes of costs and environmental impacts, and (b) the interactions among thermodynamics, economics, and ecology.
New technologies that lead to an increased efficiency and lower product cost in each step of the LNG chain are of particular interest to scientists and engineers. The studies and commercial applications of LNG regasification processes can be divided into three large groups: (a) direct and indirect heat transfer processes between LNG and other substances in the so-called “traditional systems”, (b) LNG-based electricity generation systems, and (c) LNG regasification within an industrial complex consisting of an LNG import terminal and at least one different refrigeration or cryogenic plant (process). In this paper the regasification of LNG is accomplished within an air separation process, to improve the overall system efficiency. The paper discusses the simulation, and the energetic, exergetic, and economic analyses of this novel cryogenic-based concept, which is characterized by a lower specific power requirement and improved cost effectiveness.
Thermodynamic and Economic Evaluation of a Novel Mixed-Refrigerant Process for the Liquefaction of Natural GasPublished: 14 November 2014 by ASME International in Volume 5: Education and Globalization
LNG technology has been in use since the 1960s and is constantly evolving. During the last 20 years the total cost of LNG technology per unit of product has decreased by 30% due to improvements of the liquefaction processes and shipping. A novel process for the liquefaction of natural gas (LNG production) combines the advantages of the three large-scale LNG processes that use a mixed refrigerant: C3MR, DMR and AP-X. The novel process is a very promising one as it consists of the three stages: pre-cooling of natural gas, liquefaction, and sub-cooling. This paper discusses the development of the novel LNG process, its simulation, as well as the results from the energy, exergy and economic analyses.Copyright © 2014 by ASME
The liquefied natural gas (LNG) market has grown significantly and the growth is expected to continue in the future. New technologies that lead to an increased efficiency in each step of the LNG chain are currently under consideration. The studies and the commercial applications associated with the LNG regasification processes can be divided into three large groups: (a) Direct and indirect heat transfer processes between water and LNG; (b) LNG-based co-generation systems, and (c) industrial complexes consisting of an LNG import terminal and an energy-conversion plant, or an energy-intensive chemical plant. In this paper a novel concept for integrating the LNG vaporization into an air separation process is presented. The simulation, energy and exergy analyses are discussed.
Systems Generating Electricity Through Expansion of Natural Gas: Effect of the Ambient Temperature to the PerformancePublished: 15 November 2013 by ASME International in Volume 3: Biomedical and Biotechnology Engineering
This paper discusses the thermodynamic performance of three different system configurations used to expand natural gas from a pressure of above 40 bar to a final pressure of approximately 12 bar: (1) System in which all natural gas passes through a throttling valve, (2) system that uses only an expander as the expansion device, and (3) system using in parallel both an expander and a throttling valve as expansion devices. The sensitivity analysis demonstrates how the mass flow rate of natural gas and the seasonal fluctuation of the ambient temperature affect the thermodynamic efficiency of these systems.
The exergetic analysis is a powerful tool for developing, evaluating and improving an energy conversion system. The strengths and limitations of the so-called conventional exergetic analysis have already been discussed. An advanced exergetic analysis can significantly reduce some of the limitations of a conventional analysis by evaluating (a) the detailed interactions between components of the overall system, and (b) the real potential for improving a system component. The main objective of advanced exergy-based analyses is to provide engineers with additional useful information for better understanding and improving the design and operation of energy-conversion systems. This information cannot be supplied by any other approach. The weaknesses of an advanced exergetic analysis are associated with (a) the subjectivity that is associated with the calculation of avoidable exergy destruction and with the definition of both the ideal and the so-called hybrid processes, and (b) the large number of calculations that need to be conducted to obtain the avoidable endogenous and the avoidable exogenous values.
Exergoeconomic Evaluation of a Solid-Oxide Fuel-Cell-Based Combined Heat and Power Generation SystemPublished: 15 November 2013 by ASME International in Volume 3: Biomedical and Biotechnology Engineering
An exergoeconomic evaluation has been conducted for a 100kW-class SOFC power generation system, in order to evaluate the cost effectiveness of the system. The exergoeconomic analysis is an appropriate combination of an exergy analysis and an economic analysis. Through an exergoeconomic analysis, we obtain the real cost associated with each stream and component in a system. We also can calculate the portion of the cost that is associated with the exergy destruction within each component. The analyzed system, a 100kW SOFC power generation system, consists of SOFC stack, reformer, catalytic combustor, heat exchangers, pumps, blowers, inverter, and HRSG for heat recovery. As a first step, mass, energy, and exergy balances were formulated. Then a conventional exergetic analysis (based on the concept of exergy of fuel/exergy of product) was performed. Next, a levelized cost for each component was calculated based on the purchased equipment costs using the Total Revenue Requirement (TRR) method with appropriate economic assumptions. Finally, the cost structure of the SOFC was figured out through an exergoeconomic evaluation. Finally suggestions have been made for reducing the cost associated with the product of the system.
Lowering the exergy content of heat required for heating purposes decreases the primary energy consumption. District heating systems are often an important link between facilities that generate heat with low exergy content and consumers. Exergetic efficiency of heat distribution is an important performance criterion in heat supply to consumers. It can serve as a criterion for optimization, towards a more sustainable distribution-network design and operation. This paper presents a methodology for an exergy-based distribution-network analysis in a district heating system. Criteria for performance evaluations are defined. They can be used to evaluate heat supply to different points in the network, or individual system components. A case study is performed on an existing district heating system. Energetic and exergetic efficiencies of supply lines are analyzed. Exergy destructions and exergy losses are studied. Large differences in efficiency of heat supply to different points in the network are discovered. Over-dimensioned parameters of the distribution network are investigated.
The paper briefly discusses the theory and applications of the conventional and advanced exergetic analyses. A conventional exergetic analysis identifies the magnitude, location, and causes of thermodynamic inefficiencies. An advanced exergetic analysis evaluates the interactions among components of the overall system and the real potential for improving a system component and the overall system. The information supplied by an advanced exergetic analysis is very useful in understanding the operation of energy conversion systems and in developing strategies for improving them.
Optimization of Thermal Power Plants Operation in the German De-Regulated Electricity Market Using Dynamic ProgrammingPublished: 09 November 2012 by ASME International in Volume 9: Micro- and Nano-Systems Engineering and Packaging, Parts A and B
The prospect of clean electrical energy generation has recently driven to massive investments on renewable energies, which in turn has affected operation and profits of existing traditional thermal power plants. In this work several coal-fired and combined cycle power units are simulated under design and off-design conditions to adequately represent the behavior of all modern thermal units included in the German power system. A dynamic optimization problem is then solved to estimate the short-run profits obtained by these units using the spot prices of the German electricity market (EEX) in years 2007–2010. The optimization model is developed using a Mixed Integer Linear Programming approach to take the on-off status into account and reduce computational effort. New market scenarios with increasing renewable shares (and consequently different spot prices) are finally simulated to analyze the consequences of a larger capacity of renewable energies on the optimal operation of traditional thermal power plants.
LNG technology has been in use since the 1960s and is constantly evolving. During the last 20 years the total cost of LNG technology has decreased by approximately 30% due to improvements of the liquefaction process and shipping. One of the last developed processes for the liquefaction of the natural gas is the so-called AP-X process that is patented by Air Products and Chemicals, Inc. (US Patent No. 6308521), and is now used for industrial applications in the Middle East. The thermodynamic processes within the components of the AP-X process are complex because a mixture is used as a working fluid. This paper discusses the exergetic analysis of the AP-X process.
The Maisotsenko-process (M-process or M-cycle) is a complex process associated with humid air. Heat transfer and evaporative cooling occur in a unique indirect evaporative cooler resulting in product temperatures that approach the dew point temperature. This process utilizes the enthalpy difference between air at its dew point temperature, and air saturated at a higher temperature. This enthalpy difference is used to reject heat from the air stream with the high temperature. The different applications of the M-process contribute to effective energy savings. The M-process technology was realized initially in the year 1984. By enhancing cooling towers with the M-process it is possible to (a) cool water to dew point temperature; (b) reduce pressure drop and required fan power, and (c) modify existing cooling towers to substantially decrease cooled water temperature. An exergetic analysis identifies the real thermodynamic inefficiencies and the potential of improvement for the M-process. This paper demonstrates the detailed exergetic analysis of the M-process with separate consideration of the thermal and mechanical exergies (as two parts of the physical exergy) and the chemical exergy.
A conventional exergy analysis can highlight the main components having high thermodynamic inefficiencies, but cannot consider the interactions among components or the true potential for the improvement of each component. By splitting the exergy destruction into endogenous/exogenous and avoidable/unavoidable parts, the advanced exergy analysis is capable of providing additional information to conventional exergy analysis for improving the design and operation of energy conversion systems. This paper presents the application of both a conventional and an advanced exergy analysis to a supercritical coal-fired power plant. The results show that the ratio of exogenous exergy destruction differs quite a lot from component to component. In general, almost 90% of the total exergy destruction within turbines comes from their endogenous parts, while that of feedwater preheaters contributes more or less 70% to their total exergy destruction. Moreover, the boiler subsystem is proven to have a large amount of exergy destruction caused by the irreversibilities within the remaining components of the overall system. It is also found that the boiler subsystem still has the largest avoidable exergy destruction; however, the enhancement efforts should focus not only on its inherent irreversibilities but also on the inefficiencies within the remaining components. A large part of the avoidable exergy destruction within feedwater preheaters is exogenous; while that of the remaining components is mostly endogenous indicating that the improvements mainly depend on advances in design and operation of the component itself.
The contribution of heat storage to the profitable operation of combined heat and power plants in liberalized electricit...Published: 01 May 2012 by Elsevier BV in Energy
Combined heat and power (CHP) plants are characterized by high fuel efficiency and are therefore usually the thermal power producing units of choice within a district heating network. The operation of CHP units is typically controlled by the current heat demand and thus delimits the range of electricity production. Heat storage devices are a promising alternative to uncouple the heat load of the district heating network from the commitment of the units and to allow for price-oriented electricity production. In this paper we present numerical results for the combined optimization of the operation of nineteen existing power plant units and the design of six proposed heat accumulators which supply the district heating network of Berlin. A mixed-integer programming problem (MIP) is formulated in GAMS and solved with CPLEX. This paper focuses on the potential for increasing profitability through the addition of heat accumulators in the energy system described above, on the optimal storage capacities for different price scenarios (variation of fuel costs, prices for carbon dioxide emission certificates, and electricity price time series) as well as on the adjustment of the operation of the power plants due to heat storage.
Conventional and advanced exergoenvironmental analysis of a steam methane reforming reactor for hydrogen productionPublished: 01 January 2012 by Elsevier BV in Journal of Cleaner Production
CO2 capture from power plants, combined with CO2 storage, is a potential means for limiting the impact of fossil fuel use on the climate. In this paper, three oxy-fuel plants with incorporated CO2 capture are evaluated from an economic and environmental perspective. The oxy-fuel plants, a plant with chemical looping combustion with near 100% CO2 capture and two advanced zero emission plants with 100% and 85% CO2 capture are evaluated and compared to a similarly structured reference plant without CO2 capture. To complete the comparison, the reference plant is also considered with CO2 capture incorporating chemical absorption with monoethanolamine. Two exergy-based methods, the exergoeconomic and the exergoenvironmental analyses, are used to determine the cost-related and the environmental impacts of the plants, respectively, and to reveal options for improving their overall effectiveness. For the considered oxy-fuel plants, the investment cost is estimated to be almost double that of the reference plant, mainly due to the equipment used for oxygen production and CO2 compression. Furthermore, the exergoeconomic analysis reveals an increase in the cost of electricity with respect to the reference plant by more than 20%, with the advanced zero emission plant with 85% CO2 capture being the most economical choice. On the other hand, a life cycle assessment reveals a decrease in the environmental impact of the plants with CO2 capture, due to the CO2 and NOx emission control. This leads to a reduction in the overall environmental impact of the plants by more than 20% with respect to the reference plant. The most environmentally friendly concept is the plant with chemical looping combustion.
During the last two decades the total cost of LNG technology has decreased significantly due to improvements of the liquefaction process. However, the regasification system has not been considerably improved. It is known that for the regasification process about 1.5% of LNG is used. Two novel, gas-turbine-based concepts for combining LNG regasification with the generation of electricity are discussed in this paper. These concepts have relatively low investment costs and high efficiencies. An advanced exergetic analysis is applied to one of these attractive LNG-based cogeneration systems to identify the potential for improvement and the interactions among components. In an advanced exergetic analysis, the exergy destruction within each component is split into unavoidable/avoidable and endogenous/exogenous parts. The advantages of this analysis over a conventional one are demonstrated. Some new developments in the advanced exergetic analysis and options for improving the concepts are also presented.
Steam methane reforming (SMR) is one of the most promising processes for hydrogen production. Several studies have demonstrated its advantages from the economic viewpoint. Nowadays process development is based on technical and economical aspects; however, in the near future, the environmental impact will play a significant role in the design of such processes. In this paper, an SMR process is studied from the viewpoint of overall environmental impact, using an exergoenvironmental analysis. This analysis presents the combination of exergy analysis and life cycle assessment. Components where chemical reactions occur are the most important plant components from the exergoenvironmental point of view, because, in general, there is a high environmental impact associated with these components. This is mainly caused by the exergy destruction within the components, and this in turn is mainly due to the chemical reactions. The obtained results show that the largest potential for reducing the overall environmental impact is associated with the combustion reactor, the steam reformer, the hydrogen separation unit and the major heat exchangers. The environmental impact in these components can mainly be reduced by improving their exergetic efficiency. A sensitivity analysis for some important exergoenvironmental variables is also presented in the paper.
Exergy Linked to Environmental Impacts: Advanced Exergoenvironmental Analysis of an Advanced Zero Emission PlantPublished: 01 January 2011 by ASME International in Volume 11: Nano and Micro Materials, Devices and Systems; Microsystems Integration
Exergy-based analyses are useful means for the evaluation and improvement of energy conversion systems. A life cycle assessment (LCA) is coupled with an exergetic analysis in an exergo-environmental analysis. An advanced exergo-environmental analysis quantifies the environmental impacts estimated in the LCA into avoidable/unavoidable parts and into endogenous/exogenous parts, depending on their source. This analysis reveals the potential for improvement of plant components/processes and the component interactions within a system. In this paper, the environmental performance of an advanced zero emission plant (AZEP) with CO2 capture is evaluated based on an advanced exergoenvironmental analysis. The plant uses oxy-fuel technology and incorporates an oxygen-separating mixed conducting membrane (MCM). To evaluate the operation of the system, a similar plant (reference plant) without CO2 capture is used. It has been found that the improvement potential of the AZEP concept is restricted by the relatively low avoidable environmental impact of exergy destruction of several plant components. Moreover, the endogenous environmental impacts are for the majority of the components significant, while the exogenous values are, generally, kept at low levels. Nevertheless, a closer inspection reveals that there are strong interactions among the components of the MCM reactor and the components constituting the CO2 compression unit. Such results are valuable, when the improvement of the environmental performance of the plant is targeted and they can only be obtained through advanced exergy-based methods.
In the first part of the paper, the advanced exergy-based analyses are applied to an air refrigeration machine. In this part, we demonstrate that the information obtained in the first part can be used to modify the values of the decision variables to reduce the cost of the final product (cold) of the overall system.
Although conventional exergy-based analyses uncover a path towards plant improvement, they suffer from some limitations, which are addressed by advanced exergy-based analyses. Advanced exergy-based methods identify interdependencies among plant components, and reveal the potential of improvement both at the component and plant level. Thus, data obtained from these methods pinpoint strengths and weaknesses of energy conversion systems and are of great importance when complex plants with a large number of interconnected components are considered. In this paper an advanced exergoeconomic analysis is applied to an advanced zero emission plant (AZEP). The most important components, in terms of the total avoidable costs, are the components constituting the main gas turbine system, while of particular importance are also the components of the mixed conducting membrane reactor incorporated in the plant. It has been found that for the most influential components of the plant, the largest part of investment cost rates and costs of exergy destruction are unavoidable. Additionally, for both the investment cost and the cost of exergy destruction, the interactions among the components are in most cases of lower importance, since for the majority of the components, the endogenous parts of the costs (related to the operation of the components themselves) are significantly larger than the corresponding exogenous parts (related to the operation of the remaining components). Nevertheless, strong interactions have been found among the components of the mixed conducting membrane reactor.
Advanced Exergoeconomic Analysis of a Refrigeration Machine: Part 1—Methodology and First EvaluationPublished: 01 January 2011 by ASME International in Volume 11: Nano and Micro Materials, Devices and Systems; Microsystems Integration
An exergoeconomic analysis identifies the location, magnitude and sources of thermodynamic inefficiencies and costs in an energy conversion system. This information is used for improving the thermodynamic and the economic performance and for comparing various systems. A conventional exergy-based analysis does not consider the interactions among the components of a system nor the real potential for improving the system. These shortcomings can be addressed and the quality of the conclusions obtained from an exergoeconomic evaluation is improved, when for each important system component the values of exergy destruction and costs are split into endogenous/exogenous and avoidable/unavoidable parts. We call the analyses resulting from such splittings advanced exergy-based analyses. The paper demonstrates how an advanced exergoeconomic analysis provides the user with information on the formation processes of thermodynamic inefficiencies and costs and with suggestions for their minimization. In the first part of the paper, the advanced exergy-based analyses are applied to an air refrigeration machine. In the second part of the paper, we demonstrate that the information obtained in the first part can be used to modify the values of the decision variables to reduce the cost of the final product (cold) of the overall system.
CO2 capture and storage from energy conversion systems is widely known as a potential method to reduce CO2 emissions to the atmosphere and to limit the impact of energy use on the climate. This study uses the exergoeconomic and exergoenvironmental analyses to provide an evaluation from an economic and environmental perspective, respectively, of an advanced zero emission plant (AZEP) and to reveal possible ways to improve the overall effectiveness of the plant. The AZEP, which is an oxy-fuel power plant, is evaluated and compared with a reference plant without CO2 capture. The exergoeconomic analysis shows a high increase in cost for the AZEP, due to the introduction of the membrane technology, while on the other hand, its environmental impact is significantly reduced. When compared with competitive alternatives, like chemical absorption—a post-combustion technology for CO2 capture—the oxy-fuel plant achieves lower relative cost and exergy expenditures.
Advanced exergetic analysis of a novel system for generating electricity and vaporizing liquefied natural gasPublished: 01 February 2010 by Elsevier BV in Energy
LNG technology has been in use since the 1960s. During the last 20 years the total cost of LNG technology has decreased by 30% due mainly to improvements of the liquefaction process and shipping. However, the regasification system has not been significantly improved. The paper presents a detailed advanced exergetic analysis of a novel co-generation concept that combines LNG regasification with the generation of electricity. The analysis includes splitting the exergy destruction within each component into its unavoidable, avoidable, endogenous and exogenous parts as well as a detailed splitting of the avoidable exogenous exergy destruction. The results of the advanced exergetic analysis are confirmed through a sensitivity analysis. Finally, some suggestions for improving the overall system efficiency are developed.
This paper discusses the thermodynamic performance of three different system configurations used to expand natural gas: (1) A system in which all natural gas passes through a throttling valve, (2) a system that uses only an expander as the expansion device, and (3) a system using in parallel both an expander and a throttling valve as expansion devices. The overall energetic efficiencies for the second system are between 89.2 and 89.4% while for the third system they are between 89.4 and 89.9% depending on mass throughput. The corresponding exergetic efficiencies are between 25% and 32% for the second system and 10% and 25% for the third one. The sensitivity analysis demonstrated that the mass flow rate of natural gas affects the thermodynamic efficiency of the systems generating electricity through expansion of natural gas. The differences in efficiencies between the second and third system, particularly at higher mass flow rates could justify the higher investment costs required for the second system and make it the most attractive system also from the cost viewpoint among the three systems studied here.
Exergy-based analyses are important tools for studying and evaluating energy conversion systems. While conventional exergy-based analyses provide us with important information, further insight on the potential for improving plant components and the overall plant as well as on the interactions among components of energy conversion systems are significant when optimizing a system. This necessity led to the development of advanced exergy-based analyses, in which the exergy destruction, as well as the associated costs and environmental impact are split into avoidable/unavoidable and endogenous/exogenous parts. Based on the avoidable parts of the exergy destruction, costs and environmental impact, the potential for improvement and related strategies are revealed. This paper presents the application of an advanced exergoeconomic analysis to a combined cycle power plant. The largest parts of the unavoidable cost rates are calculated for the components constituting the gas turbine system and the low-pressure steam turbine. The combustion chamber has the second highest avoidable investment cost, while it has the highest avoidable cost of exergy destruction. In general, most of the investment costs are unavoidable, with the exception of some heat exchangers of the plant. Similarly, most of the cost of exergy destruction is unavoidable with the exception of the expander in the gas turbine system and the high-pressure and intermediate-pressure steam turbines. In general, the advanced exergoeconomic analysis reveals high endogenous values, which suggest improvement of the total plant by improving the design of the components primarily in isolation, and lower exogenous values, which suggest that the component interactions are of lower significance for this plant.
In this paper, an advanced exergoenvironmental analysis is conducted for a steam methane reforming process for the production of hydrogen. The approach for calculating pollutant formation is generalized and the assumptions required for applying the analysis are discussed in detail. These are the main contributions of this work to the development of exergy-based methods for the analysis of energy-intensive chemical processes. In an advanced exergoenvironmental analysis, the environmental impact associated with the exergy destruction within a component as well as the component-related environmental impact and a component-related pollutant formation are split into unavoidable/avoidable and endogenous/exogenous parts. This splitting improves our understanding of the sources of thermodynamic inefficiencies and their effect to the formation of environmental impacts and pollutants, and facilitates a subsequent improvement of the overall process. Finally, some improvement options developed on the basis of the results of the advanced exergoenvironmental analysis are discussed.
Splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts has many advantages for the detailed analysis of energy conversion systems. Endogenous is the exergy destruction obtained when all other system components are ideal and the component being considered operates with its real efficiency. The difference between total and endogenous exergy destruction is the exogenous exergy destruction caused within the component being considered by the irreversibilities in the remaining components and the structure of the overall system. Unavoidable is the part of exergy destruction within one system component that cannot be eliminated even if the best available technology in the near future would be applied. The avoidable exergy destruction is the difference between total and unavoidable exergy destruction. These concepts enhance an exergy analysis and assist in improving the quality of the conclusions obtained from this analysis. The paper presents the combined application of both concepts to vapor-compression refrigeration machines using different one-component working fluids (R125, R134a, R22 and R717) as well as azeotropic (R500) and zeotropic (R407C) mixtures. The purpose of the paper is not to evaluate these working fluids, some of which cannot be used in future, but to demonstrate the effect of different material properties on the results of advanced exergy analysis.
Advanced exergetic analysis: Approaches for splitting the exergy destruction into endogenous and exogenous partsPublished: 01 March 2009 by Elsevier BV in Energy
The irreversibilities (exergy destruction) within a component of an energy conversion system can be represented by two parts. The first part depends on the inefficiencies of the considered component while the second part depends on the system structure and the inefficiencies of the other components of the overall system. Thus, the exergy destruction occurring within a component can be split into two parts: (a) endogenous exergy destruction due exclusively to the performance of the component being considered and (b) exogenous exergy destruction caused also by the inefficiencies within the remaining components of the overall system. The paper discusses four different approaches developed by the authors for calculating the endogenous part of exergy destruction as well as the approach based on the structural theory. The advantages, disadvantages and restrictions for applications associated with each approach are presented. It is concluded that all approaches developed by the authors lead to comparable and acceptable results, whereas the structural theory approach should not be used for calculating the endogenous part of exergy destruction because it delivers unacceptable results. Splitting the exergy destruction into endogenous and exogenous parts improves our understanding of the interactions among system components and provides very useful information for improving an exergy conversion system, particularly when this concept is combined with the concept of avoidable and unavoidable exergy destruction.
Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systemsPublished: 01 January 2009 by Elsevier BV in Energy
To improve the ecological performance of energy conversion systems, it is essential to understand the formation of environmental impact at component level. A method has been developed that (a) reveals the extent to which each component of an energy conversion system is responsible for the overall environmental impact and (b) identifies the sources of the impact. The approach of exergoeconomic analysis is modified to deal with an evaluation of the ecological impact instead of an economic problem. The basic idea is that exergy represents a proper basis for allocating both costs and environmental impact to components of energy conversion processes.The proposed exergoenvironmental analysis consists of three steps. In the first step, a detailed exergy analysis of the system under consideration is conducted. In the second step, the required values of the environmental impact are determined by applying an appropriate method involving an appropriate quantifier of the environmental impact. Here the Eco-indicator 99 impact assessment method is applied in conjunction with the method of life cycle assessment. In the last step, the environmental impact associated with each component is assigned to the product exergy streams of the component; subsequently exergoenvironmental variables are calculated and an exergoenvironmental evaluation is conducted.As a case study, an energy conversion system consisting of a high-temperature solid oxide fuel cell integrated with an allothermal biomass gasification process has been analyzed. Exergoenvironmental analysis allows us to identify the environmentally most relevant system components and provides information about possibilities for design improvements.
Optimization of the Design and Partial-Load Operation of Power Plants Using Mixed-Integer Nonlinear ProgrammingPublished: 01 January 2009 by Springer Nature in Optimization in the Energy Industry
This paper focuses on the optimization of the design and operation of combined heat and power plants (cogeneration plants). Due to the complexity of such an optimization task, conventional optimization methods consider only one operation point that is usually the full-load case. However, the frequent changes in demand lead to operation in several partial-load conditions. To guarantee a technically feasible and economically sound operation, we present a mathematical programming formulation of a model that considers the partial-load operation already in the design phase of the plant. This leads to a nonconvex mixed-integer nonlinear program (MINLP) due to discrete decisions in the design phase and discrete variables and nonlinear equations describing the thermodynamic status and behavior of the plant. The model is solved using an extended Branch and Cut algorithm that is implemented in the solver LaGO. We describe conventional optimization approaches and show that without consideration of different operation points, a flexible operation of the plant may be impossible. Further, we address the problem associated with the uncertain cost functions for plant components.
In this paper, a steam methane reforming (SMR) process for the production of hydrogen is studied. The process is based on two chemical reactions (reforming and water-gas-shift reaction). For each component but especially focusing on the chemical reactors, the avoidable part of the exergy destruction is estimated. The assumptions required for these calculations are discussed in detail and represent the main contribution of this work to the development of exergy-based methods for the analysis of chemical processes. In an advanced exergy analysis, the exergy destruction within a component is split into avoidable/unavoidable parts. This splitting improves understanding of the sources of thermodynamic inefficiencies and facilitates a subsequent optimization of the overall process. The overall SMR process is characterized by high energetic and exergetic efficiencies. However, the majority of the exergy destruction is caused by the irreversibility of chemical reactions and heat transfer. Results of this paper suggest options for improving the efficiency of the overall process.
Liquefied Natural Gas (LNG) is expected to contribute in future more than in the past to the overall energy supply in the world. The paper is the first part of a two-part presentation. In the first one, some novel concepts for combining LNG regasification with the generation of electricity are discussed, whereas in the second part advanced exergy-based analyses are applied to the most favorable LNG–based cogeneration system. All concepts to be evaluated in the first part use gas turbines, to keep the overall investment cost relatively low. The comparisons are conducted based on thermodynamic efficiency and on the potential for improving this efficiency.
Liquefied Natural Gas (LNG) is expected to contribute in future more than in the past to the overall energy supply in the world. The paper is the second part of a two-part presentation. In the first one, some novel concepts for combining LNG regasification with the generation of electricity are discussed, whereas in the second part advanced exergy-based analyses are applied to the most favorable LNG–based cogeneration system. These analyses include an advanced exergy analysis and an advanced exergoeconomic analysis. With the aid of these analyses the interactions among system components as well as the potential for improving the thermodynamic efficiency and the potential for reducing the overall cost will be revealed. The objective of this paper is to demonstrate (a) the potential for generating electricity while regasifying LNG, and (b) the capabilities associated with advanced exergy-based methods.
Exergy-based methods are reliable means for the comparison and the evaluation of the operation of energy conversion systems. In this paper, the Advanced Zero Emission Plant, a plant that performs combustion in a nitrogen-free environment (oxy-fuel combustion) is presented, compared to a reference plant (without CO2 capture) and evaluated based on an exergoeconomic analysis. A variation of the oxy-fuel plant with a lower CO2 capture percentage (85%) is also presented in order to (1) evaluate the influence of CO2 capture on a plant’s overall performance and cost, and (2) enable the comparison with other conventional methods, such as post-combustion with chemical absorption that also performs CO2 capture with lower effectiveness. When compared to the reference case, the oxy-fuel plants achieve a minimal decrease in exergetic efficiency, essentially due to their more efficient combustion processes. Cost calculations reveal that the membrane used for the oxygen production in the oxy-fuel plants is their main expenditure. Nonetheless, the cost of electricity and the cost of CO2 avoided for these plants are calculated to be competitive with chemical absorption.
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.