Cogeneration and District Energy Systems: Modelling, Analysis and Optimization
District energy (DE) systems use central heating and/or cooling facilities to provide heating and/or cooling services for communities and can be particularly beneficial when integrated with cogeneration plants for electricity and heat. This book provides information on district energy and cogeneration technologies, and the systems that combine them, with a focus on their modelling, analysis and optimization. Topics covered include a brief introduction to district heating and cogeneration; background material on thermodynamics and exergy analyses; models for cogeneration, heating and district heating, and chilling and district cooling; descriptions and analyses of configurations for integrating cogeneration and DE technologies; economics of cogeneration and DE; environmental impact of cogeneration systems, including wastes and carbon dioxide emissions and their allocations; modelling and optimization of cogeneration-based district energy systems accounting for economics and environmental impact; developments and advances in technologies and systems for cogeneration and DE; and future directions. Examples and case studies are included throughout the book to illustrate the material covered, and to demonstrate the importance, benefits and value of cogeneration and district energy technologies in achieving sustainable and efficient energy systems.
Inspec keywords: thermodynamics; optimisation; cogeneration
Other keywords: system modeling; district energy system; economic assessment technique; thermodynamic analysis; DE systems; optimization; cogeneration; objective function
Subjects: Optimisation techniques; Thermal power stations and plants
- Book DOI: 10.1049/PBPO093E
- Chapter DOI: 10.1049/PBPO093E
- ISBN: 9781785611261
- e-ISBN: 9781785611278
- Page count: 316
- Format: PDF
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Front Matter
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1 Introduction
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An introduction is provided to the material covered in the book, starting with the groundwork related to cogeneration and district energy systems, and their modeling, analysis and optimization. The motivation for the book is also provided, in order to explain its rationale and the needs it seeks to satisfy - in terms of the technologies involved as well as the emphasis on modeling, analysis and optimization. The aims of the book are also given, complementing the material given in the motivation. The approach taken in preparing the book is described, to ensure that it provides appropriate and comprehensive coverage of cogeneration and district energy systems as well as their modeling, analysis and optimization. As part of that coverage, the approach involving an emphasis on modeling, analysis and optimization is detailed and explained. The scope of the book is explained to make it clear to the reader what is and is not covered, and a detailed outline of the book is provided.
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2 Thermodynamic analysis: fundamentals, energy and exergy
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The fundamentals of thermodynamics and thermodynamic analysis are provided, as they are central to the contents of the book. The centrality of thermodynamics can be seen by noting that it constitutes the study of the concepts and laws describing energy and its conversion in processes and systems. The means by which thermodynamics permits the behavior, performance and efficiency of energy systems to be described, particularly via energy and exergy analyses, is described at length. Details on aspects of thermodynamics most relevant to energy and exergy analyses are presented and illustrated, along with descriptions of energy and exergy analyses themselves. Thermodynamic balances and basic quantities in them are described, including the exergy of matter, heat, work and electricity. The reference environment used in exergy analysis is described and models for it (e.g., natural-environment-subsystem, reference-substance, equilibrium and constrained-equilibrium, and process-dependent models) are detailed. Various energy and exergy efficiencies are defined and properties for energy and exergy analyses are provided. The implications are explained of energy and exergy analyses, particularly on research and development. A step-by-step procedure is given for energy and exergy analyses, and a detailed example is given to illustrate how the analyses are applied. Finally, exergy values for typical commodities encountered in cogeneration and district energy (e.g., electricity, work, heated and cooled substances) are provided, and extensions are explained of exergy methods beyond thermodynamics to areas such as economics and environmental studies. The coverage in this chapter is kept general where possible, although points relevant to cogeneration and/or district energy are raised as appropriate.
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3 Cogeneration systems
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Cogeneration - and systems for it - are described, starting with fundamentals and definitions and extending to benefits, uses, operation and applications. Although the term cogeneration can in general refer to any process that generates two (and sometimes more than two) products, we typically in this book restrict our use of the term cogeneration to its most common meaning: the simultaneous production in a single process of electricity and heat (typically in the form of steam and/or hot water). In such instances, cogeneration is often referred to as CHP. For clarity, cogeneration is contrasted with thermal electrical generation, pointing out similarities and differences, and highlighting how cogeneration is a logical extension of many systems for thermal electricity generation. The trade-offbetween thermal and electrical outputs of cogeneration is discussed, demonstrating how electrical output decreases as thermal output (and the quality of the thermal output as measured by its temperature) increases. The interface between cogeneration and energy storage is also discussed, where the storage can apply to the thermal and/or electrical product of cogeneration. A general cogeneration system model, suitable for thermodynamic and engineering assessments, is provided that is sufficiently flexible and robust to model most cogeneration system options. Energy and exergy balances for the general cogeneration system model are given and illustrated. The advantages and applications of cogeneration are given, and the main categories of heat demands that can normally be satisfied with cogeneration are described: residential, commercial and institutional processes (e.g., heating of air and water) and industrial processes (e.g., drying, heating, boiling). The selection of the size and type of a cogeneration system to match as optimally as possible the thermal and electrical demands is discussed, as are matching schemes that can be used. Systems for thermal electricity generation are described, including steam-turbine, gas-turbine, combined-cycle, reciprocating engine and renewable energy-based thermal electricity-generation technologies. Cogeneration systems are also detailed, and it is shown how the types of cogeneration systems available parallel to the types of systems for thermal electricity generation.
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4 Heating and district heating systems
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Systems for heating and district heating are described. This material is of great importance to the book for at least two reasons: heating systems are the foundation of most district heating systems and heating systems constitute one of the main technologies against which cogeneration must compete. Heating plants typically convert energy in the form of a fossil fuel or electricity to direct thermal heat or a heated medium such as hot gases, steam or hot water. A general model for heating and district heating systems is provided and illustrated, along with analyses of it using energy and exergy analyses. The resultant energy and exergy balances and efficiencies are also given. The various types of heating technologies are listed along with their characteristics. The types of heating technologies include fuel-based heating (e.g., heating using furnaces), electricity-based heating, heating via waste heat recovery, groundbased heating, solar-based heating and heating using heat pumps. The coverage for these systems includes basics, applications, types, classifications and operation factors. Extending this material, the many types of technologies for district heating are listed and described. Characteristics of district heating systems are provided, including advantages, operation, applications and energy sources. The types of district heating systems covered include fossil fuel, geothermal, solar, biomass and waste-toenergy-based district heating. Typical energy conversion efficiencies are given for heating and district heating, and it is demonstrated how the thermal product can be used in the residential, commercial and institutional sectors for processes such as space and water heating, and in the industrial sector for a wide range of activities. Attention is paid to the use of a central heat supply to meet residential-commercial heat demands via district heating given its prevalence (sometimes in tandem with the use of cogeneration-derived heat to drive absorption chillers for applications such as district cooling). Many of the uses for heat from heating systems are seen to be the same as the uses for heat from cogeneration systems.
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5 Chilling and district cooling systems
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Systems for cooling and district cooling are described. This material is of great consequence to the book since cooling systems are the foundation of most district cooling systems as well as a technology against which cogeneration systems sometimes must compete. A general model for cooling and district cooling systems is provided to facilitate engineering assessments and descriptions, along with thermodynamic analyses based on energy and exergy. The resultant energy and exergy balances and efficiencies are also provided. The various types of cooling technologies are listed along with their characteristics. The types of cooling technologies include electrically driven vapor-compression chillers and heat-driven absorption chiller systems as well as systems for free cooling. The relation between chillers and heat pumps, which typically operate on the same thermodynamic cycle, is outlined. The coverage for cooling systems includes subjects such as basics, applications, types, classifications, operation, performance and efficiency. Extending the coverage of cooling or chilling, the many types of technologies for district cooling are listed and described. Characteristics of district cooling systems are provided, including advantages, operation, applications, distribution and capacity. The district cooling systems covered that operate using hydrocarbon fuels, electricity, surface waters, the ground, solar energy, biomass and industrial waste energy. It is shown how district cooling can be used to meet the space cooling requirements of buildings in the residential, commercial and institutional sectors, and how district cooling systems often allow cooling services to be provided to buildings with advantages over conventional cooling systems.
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6 Integrated systems for cogeneration and district energy
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Systems that integrate cogeneration and district energy are described. Included are descriptions of district energy (i.e., district heating and cooling), showing how it builds on and combines technologies for district heating and district cooling. Cogeneration-based district energy is defined and many facets of it are discussed, including possible variations of cogeneration-based district energy. Cogeneration-based district energy is compared with alternative and conventional systems for electrical and thermal energy, which constitute the main competing technologies. A general model for cogeneration-based district energy suitable for engineering and thermodynamic assessments is presented and illustrated and various operation modes for it identified. A detailed thermodynamic description of a combined cogeneration-based district energy system is given, along with corresponding energy and exergy balances and efficiencies. Alternative measures of system efficiency are also provided. The need for proper measures of merit for systems for cogeneration-based district energy is emphasized, especially when they include chiller systems, since useful, meaningful and logical measures are lacking. Finally, systems for integrated cogeneration-based district energy are described, by drawing on the material presented for both cogeneration and district energy. It is explained how district cooling systems using absorption chillers often complement district heating systems when both use heat supplied from a cogeneration plant because the demand for heat in a district heating system is lower in summer than in winter and heat-driven district cooling, which requires heat mainly in the summer, can help to balance the seasonal demands for cogeneration-derived heat.
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7 Comparison of systems for integrated cogeneration and district energy
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Systems for integrated cogeneration and district energy are compared, pointing out similarities and differences. The cases considered in the comparison exercise are carefully described, including the specification of efficiency measures. Performance and efficiency measures are defined and discussed for the chillers, for cogeneration and heating, and for integrated cogeneration-based district energy. The comparison of several systems for integrated cogeneration and district energy demonstrates the merits of different integrated systems for cogeneration and district energy, and also clarifies the separate technologies involved and how they operate in tandem in mutually beneficial manners. The comparison highlights the need for proper measures of merit for systems for cogeneration-based district energy, particularly when they include cooling, and the way in which district cooling systems using absorption chillers can complement district heating systems when both use heat supplied from a cogeneration plant because the demand for heat become more balanced seasonally. Comparative assessments are demonstrated to be both useful and necessary, given the breadth of technologies available for cogeneration, heating, cooling, district heating and district cooling, particularly as a way of assisting designers in selecting parameter values and system configurations, and in helping decision-makers choose among competing options. The comparisons in this chapter also serve to illustrate the technologies that are involved in cogeneration-based district energy, and to help demonstrate the merits of different integrated cogeneration and district energy systems by providing fair comparative assessments.
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8 Economics of cogeneration and district energy
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The economics of cogeneration and district energy are described and examined, starting with fundamentals and general economic considerations such as methods for estimating TCI, performing economic evaluations and calculating revenue requirements. A detailed case study on the economics of cogeneration is provided. For the scenario considered in the case study, the fixed and TCI, as well as fuel costs, start-up costs, and working capital (WC), are explained. The cogeneration project economics are provided, and the facility investment, modified accelerated cost recovery system and product costs are described. Modern and historical applications of the case study are examined. Various economic considerations involved in integrated cogeneration-based district energy systems are considered, including economic considerations for generation and distribution as well as economic considerations for consumers. An analogy is presented between allocating wastes for cogeneration among products, and allocating economic costs among products. Optimization schemes and procedures involving economics are discussed, to provide a foundation for detailed routines and computer codes for assessing, comparing and optimizing the benefits of such systems. Various investigations of economics of cogeneration and district energy are described.
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9 Environmental impact of cogeneration systems: wastes and their allocation
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The environmental impact of cogeneration systems is considered, with a focus on wastes from cogeneration and their allocations to products. The question of how to allocate wastes for an energy system with multiple products such as cogeneration has long provoked much debate and disagreement, in part because proposed methods lack consistency, simplicity, sound reasoning, ease of use, and universal - or at least widespread - acceptability. Various allocation methods for cogeneration wastes are described, including methods based on techno-economic measures such as energy content of products, exergy content of products, economic value of products, incremental fuel consumption to electrical production, incremental fuel consumption to thermal energy production and shared waste savings between electrical and thermal energy, as well as less rigorous waste-allocation methods. The rationale for the methods for allocating cogeneration wastes are detailed, with a particular focus on energy and exergy factors. The advantages of utilizing exergy methods for allocating cogeneration wastes and the balance they provide among cogenerated products are described. The authors in fact propose that exergy methods can form the basis of rational and meaningful allocation methods for wastes that are superior to other such allocation methods. Three detailed case studies are considered. Steam and hot water-based cogeneration systems are considered in the first two case studies. In the third, a comparison is presented of the waste allocations for cogeneration and equivalent independent plants. The material in this chapter is particularly important because, by permitting wastes to be allocated more appropriately among the commodities of cogeneration, its environmental benefits can be better understood and exploited. These benefits should accrue to society through the better design and utilization of cogeneration technologies based on environmental considerations, and through better decision and policy-making by companies and government.
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10 Climate change and cogeneration: addressing carbon dioxide emissions
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Climate change issues related to cogeneration are examined, with a focus on carbon dioxide emissions and how to address them. The allocation of carbon dioxide emissions for cogeneration is highly contentious, in part because of the importance of mitigating them to address and also because proposed methods are often viewed as unsound and inconsistent, difficult of use because of their complexity, and lacking in widespread acceptability. After detailing carbon dioxide emissions from cogeneration, allocation methods for these emissions are described. This includes descriptions of selected methods for allocating carbon dioxide emissions for cogeneration, basic considerations in allocating such emissions, and a comparison of exergy and energy-based methods for allocating such emissions. The authors contend that exergy methods provide the underpinnings of logical and practical allocation methods for carbon dioxide emissions, unlike other such allocation methods. A comprehensive case study is presented that compares carbon dioxide emissions allocations for cogeneration and independent plants and explains how to use this information for determining and trading carbon dioxide emissions credits. The latter includes discussions of carbon dioxide emissions credits for trading purposes from switching to cogeneration from equivalent independent plants, carbon dioxide emissions credits when an electricity user switches to cogeneration, carbon dioxide emissions credits when a heat user switches to cogeneration, and carbon dioxide emissions credits for other cases. The importance of the material in this chapter is noteworthy because it can permit the environmental benefits of cogeneration to be better understood and exploited - by allocating carbon dioxide emissions more appropriately among the electrical and thermal products of cogeneration. These benefits can improve cogeneration design and utilization, and enhance related decision-making by companies and policy-making by governments. In addition, exergy-based carbon dioxide emissions allocations provide a sensible, meaningful fair way to establish schemes for emissions trading.
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11 Modeling and optimization of cogeneration-based district energy systems accounting for economics and environmental impact
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The modeling and optimization of cogeneration-based district energy systems, especially while accounting for economics and environmental impact, are described. An energy equilibrium model is given and its mathematical formulation specified. Models and descriptions are presented for the principal relevant technologies: cogeneration production, heating and district heating, chilling and district cooling, and configurations for integrating cogeneration and district energy technologies. The economics of cogeneration and district energy technologies needed in optimization procedures are discussed. Methodologies for the analysis of economic impacts are outlined, including means of determining partial social welfare change and payback period. Various methodologies for the analysis of environmental impacts are explained, and an illustrative case study is presented. After describing the scenarios considered in the case study, parameter values for the illustrative model are presented as are the resulting partial social welfare changes and possible improvements. The modeling and optimization of cogeneration-based district energy systems help illustrate and demonstrate the comparative merits of different integrated systems for cogeneration and district energy. Often the optimization of cogeneration and district energy technologies accounts for environmental emission limits in terms of constraints, although the emissions can also be addressed as objectives.
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12 Developments and advances in technologies and systems for cogeneration and district energy
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Developments and advances in technologies and systems for cogeneration and district energy, and related technologies such as trigeneration, multigeneration and distributed energy systems, are described, drawing extensively on recent and ongoing research and development activities throughout the world. Such advances help predict how the technologies will perform and be utilized in the future. Many of the advances involve renewable energy and advanced technologies such as energy storage, fuel cells and new HVAC devices, and utilize advanced methods such as exergy analysis. The material is loosely divided and organized for convenience, with the first part focusing on technical and related factors for cogeneration and extended cogeneration, heating and cooling, district energy, and integrated systems for cogeneration and district energy. The second part is centered on advances related to the economics of systems and technologies related to cogeneration and district energy, while the third part addresses environmental impact and climate change aspects of cogeneration and district energy and how the technologies can mitigate them. The final part describes advances regarding the optimization of systems related to cogeneration and district energy.
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13 Closing and future considerations
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The material covered in this book on cogeneration and district energy systems, as well as their modeling, analysis and optimization, is summarized. Several closing remarks are provided, highlighting key points raised in the book that readers should be aware of and take into account in designs and applications of cogeneration and/or district energy systems. The closing remarks also draw on the insights of the authors and include the advantages of cogeneration and district energy systems over conventional heating and cooling systems in terms of factors such as efficiency, reliability and safety, reduced environmental impact and economics. The importance is emphasized of modeling activities, including characterizing configurations and components of cogeneration and district energy systems, analysis based on thermodynamic, economic and environmental factors, and optimization using appropriate objective functions and constraints. Finally, the authors speculate on future issues and considerations relating to the topics covered in this book, based on material included in this book and in the literature as well as their own insights and views. This includes views on the significant likelihood of increasing use in the future of systems that integrate district energy systems with cogeneration plants for electricity and heat, due to the significant technical, environmental and economic benefits.
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Back Matter
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