Author: Jeffrey R. S. Brownson
Publisher: Academic Press
ISBN: 0123973155
Category : Technology & Engineering
Languages : en
Pages : 471
Book Description
Solar energy conversion requires a different mind-set from traditional energy engineering in order to assess distribution, scales of use, systems design, predictive economic models for fluctuating solar resources, and planning to address transient cycles and social adoption. Solar Energy Conversion Systems examines solar energy conversion as an integrative design process, applying systems thinking methods to a solid knowledge base for creators of solar energy systems. This approach permits different levels of access for the emerging broad audience of scientists, engineers, architects, planners, and economists. Traditional texts in solar energy engineering have often emerged from mechanical or chemical engineering fields. Instead, Solar Energy Conversion Systems approaches solar energy conversion from the perspectives of integrative design, environmental technology, sustainability science, and materials science in the wake of amazing new thin films, polymers, and glasses developed by the optoelectronics and semiconductor industries. This is a new solar text for the new generation of green job designers and developers. It's highlighted with vignettes that break down solar conversion into useful stories and provides common points of reference, as well as techniques, for effective estimation of evolving technologies. - Contextualizes solar conversion for systems design and implementation in practical applications - Provides a complete understanding of solar power, from underlying science to essential economic outcomes - Analytical approach emphasizes systems simulations from measured irradiance and weather data rather than estimations from "rules of thumb" - Emphasizes integrative design and solar utility, where trans-disciplinary teams can develop sustainable solar solutions that increase client well-being and ecosystems services for a given locale
Solar Energy Conversion Systems
Author: Jeffrey R. S. Brownson
Publisher: Academic Press
ISBN: 0123973155
Category : Technology & Engineering
Languages : en
Pages : 471
Book Description
Solar energy conversion requires a different mind-set from traditional energy engineering in order to assess distribution, scales of use, systems design, predictive economic models for fluctuating solar resources, and planning to address transient cycles and social adoption. Solar Energy Conversion Systems examines solar energy conversion as an integrative design process, applying systems thinking methods to a solid knowledge base for creators of solar energy systems. This approach permits different levels of access for the emerging broad audience of scientists, engineers, architects, planners, and economists. Traditional texts in solar energy engineering have often emerged from mechanical or chemical engineering fields. Instead, Solar Energy Conversion Systems approaches solar energy conversion from the perspectives of integrative design, environmental technology, sustainability science, and materials science in the wake of amazing new thin films, polymers, and glasses developed by the optoelectronics and semiconductor industries. This is a new solar text for the new generation of green job designers and developers. It's highlighted with vignettes that break down solar conversion into useful stories and provides common points of reference, as well as techniques, for effective estimation of evolving technologies. - Contextualizes solar conversion for systems design and implementation in practical applications - Provides a complete understanding of solar power, from underlying science to essential economic outcomes - Analytical approach emphasizes systems simulations from measured irradiance and weather data rather than estimations from "rules of thumb" - Emphasizes integrative design and solar utility, where trans-disciplinary teams can develop sustainable solar solutions that increase client well-being and ecosystems services for a given locale
Publisher: Academic Press
ISBN: 0123973155
Category : Technology & Engineering
Languages : en
Pages : 471
Book Description
Solar energy conversion requires a different mind-set from traditional energy engineering in order to assess distribution, scales of use, systems design, predictive economic models for fluctuating solar resources, and planning to address transient cycles and social adoption. Solar Energy Conversion Systems examines solar energy conversion as an integrative design process, applying systems thinking methods to a solid knowledge base for creators of solar energy systems. This approach permits different levels of access for the emerging broad audience of scientists, engineers, architects, planners, and economists. Traditional texts in solar energy engineering have often emerged from mechanical or chemical engineering fields. Instead, Solar Energy Conversion Systems approaches solar energy conversion from the perspectives of integrative design, environmental technology, sustainability science, and materials science in the wake of amazing new thin films, polymers, and glasses developed by the optoelectronics and semiconductor industries. This is a new solar text for the new generation of green job designers and developers. It's highlighted with vignettes that break down solar conversion into useful stories and provides common points of reference, as well as techniques, for effective estimation of evolving technologies. - Contextualizes solar conversion for systems design and implementation in practical applications - Provides a complete understanding of solar power, from underlying science to essential economic outcomes - Analytical approach emphasizes systems simulations from measured irradiance and weather data rather than estimations from "rules of thumb" - Emphasizes integrative design and solar utility, where trans-disciplinary teams can develop sustainable solar solutions that increase client well-being and ecosystems services for a given locale
Renewable energy conversion systems
Author: Muhammad Kamran
Publisher: Academic Press
ISBN: 0128235985
Category : Science
Languages : en
Pages : 302
Book Description
Fundamentals of Renewable Energy Systems goes beyond theoretical aspects of advances in renewable energy and addresses future trends. By focusing on the design of developing technologies, relevant operation and detailed background and an understanding of the application of power electronics and thermodynamics processes in renewable energy, this book provides an analysis of advancing energy systems. The book will be of interest to engineering graduates, researchers, professors and industry professionals involved in the renewable energy sector and is ideal for advanced engineering courses dealing with renewable energy, sources, thermal and electrical energy production and sustainability. With increasing focus on developing low carbon energy production, audiences need to have the engineering knowledge and practical skills to develop and implement creative solutions to engineering problems encountered with renewable energy technologies. By looking at renewable energy capture and conversion, system design and analysis, project development and implementation, each modular chapter examines recent advances in specific renewable energy systems with detailed methods, calculations and worked examples. - Includes recent techniques used to design and model different renewable energy sources (RES) - Demonstrates how to use power electronics in renewable systems - Discusses how to identify, design, integrate and operate the most suitable technologies through key problems
Publisher: Academic Press
ISBN: 0128235985
Category : Science
Languages : en
Pages : 302
Book Description
Fundamentals of Renewable Energy Systems goes beyond theoretical aspects of advances in renewable energy and addresses future trends. By focusing on the design of developing technologies, relevant operation and detailed background and an understanding of the application of power electronics and thermodynamics processes in renewable energy, this book provides an analysis of advancing energy systems. The book will be of interest to engineering graduates, researchers, professors and industry professionals involved in the renewable energy sector and is ideal for advanced engineering courses dealing with renewable energy, sources, thermal and electrical energy production and sustainability. With increasing focus on developing low carbon energy production, audiences need to have the engineering knowledge and practical skills to develop and implement creative solutions to engineering problems encountered with renewable energy technologies. By looking at renewable energy capture and conversion, system design and analysis, project development and implementation, each modular chapter examines recent advances in specific renewable energy systems with detailed methods, calculations and worked examples. - Includes recent techniques used to design and model different renewable energy sources (RES) - Demonstrates how to use power electronics in renewable systems - Discusses how to identify, design, integrate and operate the most suitable technologies through key problems
Energy Harvesting
Author: Alireza Khaligh
Publisher: CRC Press
ISBN: 1351834029
Category : Science
Languages : en
Pages : 529
Book Description
Also called energy scavenging, energy harvesting captures, stores, and uses "clean" energy sources by employing interfaces, storage devices, and other units. Unlike conventional electric power generation systems, renewable energy harvesting does not use fossil fuels and the generation units can be decentralized, thereby significantly reducing transmission and distribution losses. But advanced technical methods must be developed to increase the efficiency of devices in harvesting energy from environmentally friendly, "green" resources and converting them into electrical energy. Recognizing this need, Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems describes various energy harvesting technologies, different topologies, and many types of power electronic interfaces for stand-alone utilization or grid connection of energy harvesting applications. Along with providing all the necessary concepts and theoretical background, the authors develop simulation models throughout the text to build a practical understanding of system analysis and modeling. With a focus on solar energy, the first chapter discusses the I−V characteristics of photovoltaic (PV) systems, PV models and equivalent circuits, sun tracking systems, maximum power point tracking systems, shading effects, and power electronic interfaces for grid-connected and stand-alone PV systems. It also presents sizing criteria for applications and modern solar energy applications, including residential, vehicular, naval, and space applications. The next chapter reviews different types of wind turbines and electrical machines as well as various power electronic interfaces. After explaining the energy generation technologies, optimal operation principles, and possible utilization techniques of ocean tidal energy harvesting, the book explores near- and offshore approaches for harvesting the kinetic and potential energy of ocean waves. It also describes the required absorber, turbine, and generator types, along with the power electronic interfaces for grid connection and commercialized ocean wave energy conversion applications. The final chapter deals with closed, open, and hybrid-cycle ocean thermal energy conversion systems.
Publisher: CRC Press
ISBN: 1351834029
Category : Science
Languages : en
Pages : 529
Book Description
Also called energy scavenging, energy harvesting captures, stores, and uses "clean" energy sources by employing interfaces, storage devices, and other units. Unlike conventional electric power generation systems, renewable energy harvesting does not use fossil fuels and the generation units can be decentralized, thereby significantly reducing transmission and distribution losses. But advanced technical methods must be developed to increase the efficiency of devices in harvesting energy from environmentally friendly, "green" resources and converting them into electrical energy. Recognizing this need, Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems describes various energy harvesting technologies, different topologies, and many types of power electronic interfaces for stand-alone utilization or grid connection of energy harvesting applications. Along with providing all the necessary concepts and theoretical background, the authors develop simulation models throughout the text to build a practical understanding of system analysis and modeling. With a focus on solar energy, the first chapter discusses the I−V characteristics of photovoltaic (PV) systems, PV models and equivalent circuits, sun tracking systems, maximum power point tracking systems, shading effects, and power electronic interfaces for grid-connected and stand-alone PV systems. It also presents sizing criteria for applications and modern solar energy applications, including residential, vehicular, naval, and space applications. The next chapter reviews different types of wind turbines and electrical machines as well as various power electronic interfaces. After explaining the energy generation technologies, optimal operation principles, and possible utilization techniques of ocean tidal energy harvesting, the book explores near- and offshore approaches for harvesting the kinetic and potential energy of ocean waves. It also describes the required absorber, turbine, and generator types, along with the power electronic interfaces for grid connection and commercialized ocean wave energy conversion applications. The final chapter deals with closed, open, and hybrid-cycle ocean thermal energy conversion systems.
Materials Science for Solar Energy Conversion Systems
Author: C.G. Granqvist
Publisher: Elsevier
ISBN: 1483287572
Category : Technology & Engineering
Languages : en
Pages : 212
Book Description
Rapid advances in materials technology are creating many novel forms of coatings for energy efficient applications in solar energy. Insulating heat mirrors, selective absorbers, transparent insulation and fluorescent concentrators are already available commercially. Radiative cooling, electrochromic windows and polymeric light pipes hold promise for future development, while chemical and photochemical processes are being considered for energy storage. This book investigates new material advances as well as applications, costs, reliability and industrial production of existing materials. Each contribution represents a landmark in the field of materials science.
Publisher: Elsevier
ISBN: 1483287572
Category : Technology & Engineering
Languages : en
Pages : 212
Book Description
Rapid advances in materials technology are creating many novel forms of coatings for energy efficient applications in solar energy. Insulating heat mirrors, selective absorbers, transparent insulation and fluorescent concentrators are already available commercially. Radiative cooling, electrochromic windows and polymeric light pipes hold promise for future development, while chemical and photochemical processes are being considered for energy storage. This book investigates new material advances as well as applications, costs, reliability and industrial production of existing materials. Each contribution represents a landmark in the field of materials science.
Solar Energy Conversion
Author: R.C. Neville
Publisher: Elsevier
ISBN: 0080541429
Category : Technology & Engineering
Languages : en
Pages : 441
Book Description
A large number of solar cell and solar cell systems are described in this volume. The theory of their operation, their design and the levels of their performance is discussed. Originally the book appeared in 1978 but extensive change over the intervening years in the fields of energy generation and consumption, solar energy and solar cells, has necessitated the publication of an updated version.The text initially surveys the requirements of humanity, the subsequent need for solar cells, the nature of sunlight and the properties of semiconductors. Concrete examples, extensive references and theoretical arguments are then used to present a comparison of options available in the design and operation of solar cells and solar cell systems. The cells - constructed from single, crystal, polycrystalline and amorphous semiconductors - and the systems - have varying designs and differing levels of solar energy for input and produce electricity or electrical and thermal energies. Solar cell production, economics and environmental effects are considered throughout the publication.
Publisher: Elsevier
ISBN: 0080541429
Category : Technology & Engineering
Languages : en
Pages : 441
Book Description
A large number of solar cell and solar cell systems are described in this volume. The theory of their operation, their design and the levels of their performance is discussed. Originally the book appeared in 1978 but extensive change over the intervening years in the fields of energy generation and consumption, solar energy and solar cells, has necessitated the publication of an updated version.The text initially surveys the requirements of humanity, the subsequent need for solar cells, the nature of sunlight and the properties of semiconductors. Concrete examples, extensive references and theoretical arguments are then used to present a comparison of options available in the design and operation of solar cells and solar cell systems. The cells - constructed from single, crystal, polycrystalline and amorphous semiconductors - and the systems - have varying designs and differing levels of solar energy for input and produce electricity or electrical and thermal energies. Solar cell production, economics and environmental effects are considered throughout the publication.
Photovoltaic Solar Energy Conversion
Author: Shiva Gorjian
Publisher: Academic Press
ISBN: 0128226412
Category : Science
Languages : en
Pages : 464
Book Description
Photovoltaic Solar Energy Conversion - Technologies, Applications and Environmental Impacts features comprehensive and up-to-date knowledge on the photovoltaic solar energy conversion technology and describes its different aspects in the context of most recent scientific and technological advances. It also provides an insight into future developments in this field by covering four distinct topics include "PV Cells and Modules", "Applications of PV Systems", "Life Cycle and Environmental Impacts" and "PV Market and Policies". - An up-to-date reference book on the advances of photovoltaic solar energy conversion technology - Describes different aspects of PV and PVT technologies in a comprehensive way - Provides information on design, development, and monitoring of PV systems - Covers applications of PV and PVT systems in the urban, industry, and agriculture sectors - Features new concepts, environmental impacts, market and policies of the PV technology
Publisher: Academic Press
ISBN: 0128226412
Category : Science
Languages : en
Pages : 464
Book Description
Photovoltaic Solar Energy Conversion - Technologies, Applications and Environmental Impacts features comprehensive and up-to-date knowledge on the photovoltaic solar energy conversion technology and describes its different aspects in the context of most recent scientific and technological advances. It also provides an insight into future developments in this field by covering four distinct topics include "PV Cells and Modules", "Applications of PV Systems", "Life Cycle and Environmental Impacts" and "PV Market and Policies". - An up-to-date reference book on the advances of photovoltaic solar energy conversion technology - Describes different aspects of PV and PVT technologies in a comprehensive way - Provides information on design, development, and monitoring of PV systems - Covers applications of PV and PVT systems in the urban, industry, and agriculture sectors - Features new concepts, environmental impacts, market and policies of the PV technology
Solar PV and Wind Energy Conversion Systems
Author: S. Sumathi
Publisher: Springer
ISBN: 9783319366937
Category : Technology & Engineering
Languages : en
Pages : 0
Book Description
This textbook starts with a review of the principles of operation, modeling and control of common solar energy and wind-power generation systems before moving on to discuss grid compatibility, power quality issues and hybrid models of Solar PV and Wind Energy Conversion Systems (WECS). MATLAB/SIMULINK models of fuel cell technology and associated converters are discussed in detail. The impact of soft computing techniques such as neural networks, fuzzy logic and genetic algorithms in the context of solar and wind energy is explained with practical implementation using MATLAB/SIMULINK models. This book is intended for final year undergraduate, post-graduate and research students interested in understanding the modeling and control of Solar PV and Wind Energy Conversion Systems based on MATLAB/SIMULINK. - Each chapter includes “Learning Objectives” at the start, a “Summary” at the end and helpful Review Questions - Includes MATLAB/SIMULINK models of different control strategies for power conditioning units in the context of Solar PV - Presents soft computing techniques for Solar PV and WECS, as well as MATLAB/SIMULINK models, e.g. for wind turbine topologies and grid integration - Covers hybrid solar PV and Wind Energy Conversion Systems with converters and MATLAB/SIMULINK models - Reviews harmonic reduction in Solar PV and Wind Energy Conversion Systems in connection with power quality issues - Covers fuel cells and converters with implementation using MATLAB/SIMULINK
Publisher: Springer
ISBN: 9783319366937
Category : Technology & Engineering
Languages : en
Pages : 0
Book Description
This textbook starts with a review of the principles of operation, modeling and control of common solar energy and wind-power generation systems before moving on to discuss grid compatibility, power quality issues and hybrid models of Solar PV and Wind Energy Conversion Systems (WECS). MATLAB/SIMULINK models of fuel cell technology and associated converters are discussed in detail. The impact of soft computing techniques such as neural networks, fuzzy logic and genetic algorithms in the context of solar and wind energy is explained with practical implementation using MATLAB/SIMULINK models. This book is intended for final year undergraduate, post-graduate and research students interested in understanding the modeling and control of Solar PV and Wind Energy Conversion Systems based on MATLAB/SIMULINK. - Each chapter includes “Learning Objectives” at the start, a “Summary” at the end and helpful Review Questions - Includes MATLAB/SIMULINK models of different control strategies for power conditioning units in the context of Solar PV - Presents soft computing techniques for Solar PV and WECS, as well as MATLAB/SIMULINK models, e.g. for wind turbine topologies and grid integration - Covers hybrid solar PV and Wind Energy Conversion Systems with converters and MATLAB/SIMULINK models - Reviews harmonic reduction in Solar PV and Wind Energy Conversion Systems in connection with power quality issues - Covers fuel cells and converters with implementation using MATLAB/SIMULINK
Solar Energy
Author: Arno Smets
Publisher: Bloomsbury Publishing
ISBN: 1906860750
Category : Technology & Engineering
Languages : en
Pages : 488
Book Description
This book provides a broad overview on the different aspects of solar energy, with a focus on photovoltaics, which is the technology that allows light energy to be converted into electric energy. Renewable energy sources have become increasingly popular in recent years, and solar is one of the most adaptable and attractive types – from solar farms to support the National Grid to roof panels/tiles used for solar thermal heating systems, and small solar garden lights. Written by Delft University researchers, Solar Energy uniquely covers both the physics of photovoltaic (PV) cells and the design of PV systems for real-life applications, from a concise history of solar cells components and location issues of current systems. The book is designed to make this complicated subject accessible to all, and is packed with fascinating graphs and charts, as well as useful exercises to cement the topics covered in each chapter. Solar Energy outlines the fundamental principles of semiconductor solar cells, as well as PV technology: crystalline silicon solar cells, thin-film cells, PV modules, and third-generation concepts. There is also background on PV systems, from simple stand-alone to complex systems connected to the grid. This is an invaluable reference for physics students, researchers, industrial engineers and designers working in solar energy generation, as well those with a general interest in renewable energy.
Publisher: Bloomsbury Publishing
ISBN: 1906860750
Category : Technology & Engineering
Languages : en
Pages : 488
Book Description
This book provides a broad overview on the different aspects of solar energy, with a focus on photovoltaics, which is the technology that allows light energy to be converted into electric energy. Renewable energy sources have become increasingly popular in recent years, and solar is one of the most adaptable and attractive types – from solar farms to support the National Grid to roof panels/tiles used for solar thermal heating systems, and small solar garden lights. Written by Delft University researchers, Solar Energy uniquely covers both the physics of photovoltaic (PV) cells and the design of PV systems for real-life applications, from a concise history of solar cells components and location issues of current systems. The book is designed to make this complicated subject accessible to all, and is packed with fascinating graphs and charts, as well as useful exercises to cement the topics covered in each chapter. Solar Energy outlines the fundamental principles of semiconductor solar cells, as well as PV technology: crystalline silicon solar cells, thin-film cells, PV modules, and third-generation concepts. There is also background on PV systems, from simple stand-alone to complex systems connected to the grid. This is an invaluable reference for physics students, researchers, industrial engineers and designers working in solar energy generation, as well those with a general interest in renewable energy.
Solar Energy Conversion
Author: Gertz I. Likhtenshtein
Publisher: John Wiley & Sons
ISBN: 3527647686
Category : Science
Languages : en
Pages : 425
Book Description
Finally filling a gap in the literature for a text that also adopts the chemist's view of this hot topic, Professor Likhtenshtein, an experienced author and internationally renowned scientist, considers different physical and engineering aspects in solar energy conversion. From theory to real-life systems, he shows exactly which chemical reactions take place when converting light energy, providing an overview of the chemical perspective from fundamentals to molecular harvesting systems and solar cells. This essential guide will thus help researchers in academia and industry better understand solar energy conversion, and so ultimately help this promising, multibillion dollar fi eld to expand. From the contents: * Electron Transfer Theories * Principle Stages of Photosynthetic Light Energy Conversion * Photochemical Systems of Light Energy Conversion * Redox Processes on Surface of Semiconductors and Metals * Dye-Sensitized Solar Cells * Photocatalytic Reduction and Oxidation of Water
Publisher: John Wiley & Sons
ISBN: 3527647686
Category : Science
Languages : en
Pages : 425
Book Description
Finally filling a gap in the literature for a text that also adopts the chemist's view of this hot topic, Professor Likhtenshtein, an experienced author and internationally renowned scientist, considers different physical and engineering aspects in solar energy conversion. From theory to real-life systems, he shows exactly which chemical reactions take place when converting light energy, providing an overview of the chemical perspective from fundamentals to molecular harvesting systems and solar cells. This essential guide will thus help researchers in academia and industry better understand solar energy conversion, and so ultimately help this promising, multibillion dollar fi eld to expand. From the contents: * Electron Transfer Theories * Principle Stages of Photosynthetic Light Energy Conversion * Photochemical Systems of Light Energy Conversion * Redox Processes on Surface of Semiconductors and Metals * Dye-Sensitized Solar Cells * Photocatalytic Reduction and Oxidation of Water
Renewable Energy Systems
Author: Fang Lin Luo
Publisher: CRC Press
ISBN: 1351832603
Category : Science
Languages : en
Pages : 883
Book Description
Energy conversion techniques are key in power electronics and even more so in renewable energy source systems, which require a large number of converters. Renewable Energy Systems: Advanced Conversion Technologies and Applications describes advanced conversion technologies and provides design examples of converters and inverters for renewable energy systems—including wind turbine and solar panel energy systems. Learn Cutting-Edge Techniques for Converters and Inverters Setting the scene, the book begins with a review of the basics of astronomy and Earth physics. It then systematically introduces more than 200 topologies of advanced converters originally developed by the authors, including 150 updated circuits on modern conversion technologies. It also discusses recently published topologies and thoroughly analyzes new converter circuits. Novel approaches include split-capacitor and split-inductor techniques that can be applied in super-lift and other converters. Resolve Historic Problems in Conversion Technologies Along with offering many cutting-edge techniques, the authors resolve some historic problems, such as the accurate determination of the conduction angle of single-phase rectifiers and power factor correction. They also describe a new series—laddered multilevel inverters—that uses few devices to produce more levels, overcoming the drawbacks of the pulse-width-modulation (PWM) inverter and providing great scope for industrial applications. Tap the Knowledge of Pioneers in the Field This book is written by pioneers in advanced conversion technology who have created a large number of converters, including the world-renowned DC/DC Luo-converters and super-lift Luo-converters. Featuring numerous examples and diagrams, it guides readers in designing advanced converters for use in renewable energy systems.
Publisher: CRC Press
ISBN: 1351832603
Category : Science
Languages : en
Pages : 883
Book Description
Energy conversion techniques are key in power electronics and even more so in renewable energy source systems, which require a large number of converters. Renewable Energy Systems: Advanced Conversion Technologies and Applications describes advanced conversion technologies and provides design examples of converters and inverters for renewable energy systems—including wind turbine and solar panel energy systems. Learn Cutting-Edge Techniques for Converters and Inverters Setting the scene, the book begins with a review of the basics of astronomy and Earth physics. It then systematically introduces more than 200 topologies of advanced converters originally developed by the authors, including 150 updated circuits on modern conversion technologies. It also discusses recently published topologies and thoroughly analyzes new converter circuits. Novel approaches include split-capacitor and split-inductor techniques that can be applied in super-lift and other converters. Resolve Historic Problems in Conversion Technologies Along with offering many cutting-edge techniques, the authors resolve some historic problems, such as the accurate determination of the conduction angle of single-phase rectifiers and power factor correction. They also describe a new series—laddered multilevel inverters—that uses few devices to produce more levels, overcoming the drawbacks of the pulse-width-modulation (PWM) inverter and providing great scope for industrial applications. Tap the Knowledge of Pioneers in the Field This book is written by pioneers in advanced conversion technology who have created a large number of converters, including the world-renowned DC/DC Luo-converters and super-lift Luo-converters. Featuring numerous examples and diagrams, it guides readers in designing advanced converters for use in renewable energy systems.