Vanadium Dioxide-Based Thermochromic Smart Windows

Yi Long, Yanfeng Gao, Yi Long, Yanfeng Gao

• Publisher: Jenny Stanford Publishing
• Year: 2021
• eBook ISBN: 9781000393620
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Biology

The usage of building energy accounts for 30–40% of total energy consumption in developed countries, exceeding the amount for industry or transportation. Around 50% energy for building services is contributed by heating, ventilation, and air-conditioning (HVAC) systems. More importantly, both building and HVAC energy consumptions are predicted to increase in the next two decades. Windows are considered as the least energy-efficient components of buildings. Therefore, smart windows are becoming increasingly important as they are capable of reducing HVAC energy usage by tuning the transmitted sunlight in a smart and favoured way: blocking solar irradiation on hot days, while letting it pass through on cold days. Compared with other type of smart windows, thermochromic windows have the unique advantages of cost-effectiveness, rational stimulus, and passive response. This book covers fabrication of vanadium dioxide–based smart windows, discusses various strategies to enhance their performance, and shares perspectives from the top scientists in this particular field.

Chapter 1

Thermochromic VO₂ for Energy-Efficient Glazing: An Introduction

Claes G. Granqvist

Department of Materials Science and Engineering, Ångström Laboratory, Uppsala University, SE-75121 Uppsala, Sweden
[email protected]

Vanadium dioxide, VO₂, has numerous potential applications in forthcoming technologies. The largest one, at least in terms of surface area, is in glazing (i.e., windows and glass facades) for energy-efficient buildings. These applications are based on the fact that VO₂ has thermochromic properties, and thin films and nanoparticle composites can transmit less solar energy above a certain “critical” temperature in the vicinity of room temperature than below this temperature, which implies that the demand on energy-guzzling air cooling can be lowered. The transmittance of visible light, on the other hand, is not influenced by temperature to a corresponding degree. This chapter outlines the basic properties of VO₂ and discusses how this material can be modified in order to be of practical interest for glazing. The roles of multilayering, doping, nanostructuring, etc., are introduced. In-depth discussions of these and other aspects are given in subsequent chapters.

1.1 The Big Challenge

Environmental challenges serve as prime drivers for new technology [1–3], and the business case for clean energy is growing [4]. The quest for new energy technology is fueled by the increasing quantity of greenhouse gases in the atmosphere. Due to the greenhouse gases, the amount of CO₂ has risen dramatically from ∼315 parts per million (ppm) at the end of the 1950s to about 410 ppm in 2018, while the growth rate has almost tripled [5]. The CO₂ injection originates largely from energy conversion, specifically the burning of coal, oil, and gas. Another greenhouse gas, which is presently emerging as an environmental threat, is hydrofluorocarbon emitted from air-conditioning units [6, 7], whose numbers are growing rapidly, particularly in developing countries. The increasing amount of CO₂ is strongly believed to be of significance for life on Earth and leads to global warming and rising sea levels [8], and there are many secondary and perilous influences of climate change connected with socioeconomic effects and increased risk for violent conflict [9–11], health issues [12, 13], and modifications of the geosphere [14] and biosphere [15–17]. Self-reinforcing feedback loops represent special threats and might lead to a transition to a “hothouse Earth” with temperatures much higher than during any interglacial period during the last millennium [18]. Furthermore, it should be noted that the global population is growing and is expected to be some 50% larger in the year 2100 than it is today [19]; this population is increasingly living in megacities [20], which act as “urban heat islands” and tend to have average temperatures several degrees Celsius higher than the temperatures of the surrounding countryside [21]. The human influence is strong enough to distinguish the present geological era (the Anthropocene) from the one prior to the Industrial Revolution (the Holocene) [22].

The gloomy prospects outlined above underscore that radical steps must be taken to decarbonize the energy sector, and this realization leads directly to a focus on buildings which account for 30% to 40% of the global use of primary energy [23]. Energy efficiency in the built environment is sometimes overlooked as an opportunity for CO₂ abatement [24], but, in fact, there are many emerging “green” technologies—often based on nanomaterials—that can be put to work [1, 25–27]. Particular attention should be given to glazing (i.e., windows and glass facades), which very often allow undesired energy transfer with excessive inflow or outflow of heat and associated need for energy-demanding cooling and heating of buildings. It is then patently clear that glazing with adjustable throughput of solar energy and visible light can lower the energy demand. Such glazing is often called “smart,” “intelligent,” or “adaptive” and uses a class of materials known as “chromogenic” [28, 29]. These materials can be of several different types and can alter their properties upon temperature change (thermochromic materials, as discussed in the present book), exposure to electrical voltage or current (electrochromic materials), irradiation by ultraviolet light (photochromic materials), and exposure to oxidizing/reducing conditions (gasochromic materials).

The present chapter introduces thermochromic vanadium dioxide (VO₂)-based materials for applications in energy-efficient glazing. This work began more than thirty years ago with research at the Lawrence Berkeley National Laboratory in the USA [30, 31] and through concurrent efforts by the author’s team in Sweden [32, 33]. Research on thermochromic VO₂-based materials has increased sharply during recent years, as can be seen from a database such as Web of Science; looking for citations to articles with “thermochromic” or “thermochromism” together with “VO₂” or “vanadium dioxide” in their titles yielded about 100 entries/year in 2005, more than 1000 entries/year in 2013, and more than 2000 entries/year in 2018. Obviously, this introductory chapter covers only a small fraction of the relevant scientific literature.

Glazing based on VO₂, or any other inorganic thermochromic material, does not yet exist as a commercial product, but the field of thermochromics has advanced greatly during recent years, and it is the contention of the author and many others that practical implementation of thermochromic glazing may be in the offing [34–43]. It should also be observed that thermochromics is of interest not only for glazing but also, for example, for roof-type ceramic tiles [44] and for a vast number of other applications.

1.2 Thermal, Solar, and Luminous Radiation

Thermochromics is about the control of luminous (visible) and solar light. Figure 1.1 reports key features of the radiation around us. Thermal radiation, introduced in Fig. 1.1a, is governed by blackbody curves (shown for ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication Page
• Contents
• Preface
• 1 Thermochromic VO2 for Energy-Efficient Glazing: An Introduction
• 2 Effect of Doping on the Thermochromic Performance of VO2
• 3 VO2 Nanocomposite Coatings for Smart Windows
• 4 Antireflection for the Performance of VO2 Thermochromic Thin Films
• 5 Controllable Synthesis of Porous Vanadium Dioxide Nanostructures
• 6 Biomimetic, Gridded Structure, and Hybridation
• 7 Hydrothermal Synthesis of Thermochromic VO2 for Energy-Efficient Windows
• 8 Chemical Vapor Deposition and Its Application in VO2 Synthesis
• 9 Physical Vapor Deposition and Its Application in Vanadium Dioxide Synthesis
• 10 Sol-Gel Synthesis of Thermochromic VO2 Coatings
• 11 VO2-Based Smart Coatings with Long-Term Durability: Review and Perspective
• 12 Conclusions and Perspectives
• Index