Smart switchable glazing for solar energy and daylight control

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Abstract

The exciting field of chromogenic materials for smart windows and other large-area applications is discussed. A selection of switchable glazing devices that change color electrically are detailed. The types of devices covered are the electrochromic which change color electrically, covering electrochromic, dispersed liquid crystal and dispersed particle glazing that switch under an applied electric field. Device structures and switching characteristics are compared. The status of prototype and commercial devices from commercial and university labs through out the world are covered. A discussion of the future of this technology is made including areas of necessary development for the realization of large area glazing in excess of 1m2.

Introduction

Chromogenic materials have a significant place in future “smart windows” for architectural, vehicle, aircraft, spacecraft and marine glazing. Smart windows are one of the most exciting topics in advanced glazing. Smart windows have the potential to change the function of a glazing from a fixed element to a dynamic one. Over the last decade there has been growing interest in this technology and is expected to continue well into the next century for a wide variety of products. There are various physical processes that can be used for the control and modification of incident daylight, solar energy, and glare. The processes covered in this study are the electrically activated kind, covering electrochromic, dispersed particle and dispersed liquid crystal glazing. There are other types of chromogenic materials, such as thermotropic, thermochromic and photochromic. These are covered in other studies 1, 2, 3. The function of a smart window is to control the flow of light and hear into and out of a glazing, according to occupant comfort. Also, smart windows can regulate lighting and heating levels for energy load management. Computer modeling of energy efficiency of electrochromic windows in buildings has shown that electrochromic windows can provide significant energy performance improvement compared to conventional double glazed windows 4, 5, 6. Energy savings for specific conditions can result in above 30% energy savings over conventional glazing [7]. The major issues challenging the development of smart windows have to do with cost and quality. One major goal is to develop good stable devices with a cyclic lifetime that match the application. Another goal is to fabricate complicated devices of large-area and low cost with high optical quality. A third goal is to fully integrate the smart window with a control system and include as part of a building energy management system.

The use of flat glass is very wide spread, the world production of flat glass is about 2 billion m2 per year, with the largest portion going to building and automotive glazing, Electrochromic products currently in the marketplace are automobile and truck mirrors, and sunglasses. The mirrors are designed to automatically regulate glare in response to incident light levels. The mirrors are produced by Donnelly (Holland, MI) and Gentex (Zeeland, MI). Electrochromic sunglasses were introduced as a product by Nikon (Tokyo, Japan) in 1994. Near-future applications include glazing panels for automobile sun-roofs and visors, automobile side and rear windows, small area architectural glazing, aircraft windows and prescription eyeglasses. Switchable glazing can also be used for large-area information displays in applications where high switching speed is not required, such as for airport display boards.

One of the most significant issues of electrically switchable glazing is the cost of the devices and the trade-offs between cost, benefit, and lifetime. The cost of an electrochromic window has been estimated to range from 100–1000 US $/m2. Some companies have set cost goals of 100–250 US $/m2. Both dispersed particle and liquid crystal windows fall within this range too. Current electrochromic development is aimed at long-life devices with durability similar to regular coated windows. Another approach is the development of low-cost limited-life switching devices. There is growing investment by larger companies and national governments in promoting this technology. Major project developments are centered in Japan, Europe, USA and Australia.

The fundamental property of an electrically activated chromogenic material is that it exhibits a large change in optical properties upon a change in either electrical field or injected or ejected charge. The change in optical properties can be in the form of absorptance, reflectance or scattering. This optical change results in a transformation from a highly transmitting state to a partly reflecting or absorbing state. This change can be either totally or partly over the visible and solar spectrum. Typically it is over some portion of the spectra. The electrically activated devices has the advantage of user or automatic control.

Thin and thick film fabrication is relevant to all of the switchable glazing technologies. Several large-area deposition technologies are necessary, such as planar reactive DC magnetron sputtering, different types of chemical vapor deposition, and sol–gel coating. Since electrochromic and conductor layers are fairly thick, of the order of 200–400 nm thick for each active layer, high rate is important. These coatings are about 10 times thicker than the typical interference coating. High quality is important since electrically activated coatings don't tolerate pin holes. Fortunately, high-rate sputtering is advancing rapidly. The advent of rotating cathodes, with superior materials utilization, leads to a drop in manufacturing cost. Chemical vapor deposition (spray pyrolysis) makes it possible to coat directly on the glass ribbon of the float line in the manufacturing plant. This process can give an inexpensive coating. Currently, it is used to deposit transparent conductors (low-e coatings). Also, plasma enhanced chemical vapor deposition ends itself to very high deposition rates for certain compounds. Sol–gel deposition processes for electrochromic layers are being developed by several groups including Donnelly, SAGE (Piscataway, NJ) and LBNL (Berkeley, CA) [8]. Advances in fabrication technology will help reduce the cost of switchable windows. With these technologies there are still significant challenges for the process designer to adapt large-scale deposition technologies to the fabrication of square meter size multilayer switchable glazing with high optical quality.

A major issue for all electrically activated devices is the quality ad cost of transparent conductors. Examples are In2O3 : Sn (also known as ITO) and SnO2 : F. The transparent conductors are a significant cost of the switchable glazing, and necessary for all device types 9, 10. There is considerable development effort on low-resistivity ITO or ZnO transparent conductors deposited onto plastic substrates at low temperatures [11]. Fairly new coated glass products for Low-c surfaces, e.g., Tech Glass (LOF Glass, Toledo, OH), K-Glass (Pilkington Glass, UK), can be used a substrates for electrochromic windows. The cost in quantity is about 15 US $/m2 which is considerably less than the electronic grade ITO/glass. It consists of a low resistivity coating of doped tin oxide produced by thermal pyrolysis directly on the glass float line. Pilkington uses this material for their prototype electrochromic windows. Asahi glass (Yokohama, Japan) has shown a very large area ITO glass, over 2 m2 with 1 Ω/square and low haze. Unfortunately it is expensive. For the field effect devices, liquid crystals and dispersed particles, the need for the lowest resistivity for large areas is less than for electrochromics.

Section snippets

Electrochromics

Electrochromic windows are the most popular area of all switching technology. Over the last ten years about 200 US and international patents have been granted per year on electrochromics. The major advantages of electrochromic materials are: (1) they only require power during switching; (2) require a small voltage to switch (1–5 V); (3) are specular under all conditions; (4) have continuous dimming; (5) many designs have a long-term memory (12–48 h). Typical electrochromics have upper visible

Dispersed liquid crystals

Liquid crystal based systems offer another approach to electrically switchable glazing. The basic classes of liquid crystals are the twisted nematic, guest–host, surface stabilized ferroelectric, and dispersed liquid crystals. The mechanism of optical switching in liquid crystals is to change the orientation of liquid crystal molecules interspersed between two conductive electrodes with an applied electric field. The orientation of the liquid crystals change with the field strength that alters

Dispersed particles

The development of suspended particle of electrophoretic devices and glazing has spanned many years. Some of the earliest work was done by Edwin Land of Polaroid in 1934. A suspended particle device consists of 3–5 layers. The active layer has needle shaped particles of (dihydrocinchonidine bisulfite polyiodide) or heraphathite (<1 μm long) suspended in an organic fluid or gel. This layer is laminated or filled between two electrical conductors. In the off condition the particles are random and

Conclusions

In this study, electrochromic devices were compared to dispersed liquid crystals, and suspended particle devices. Of the three major types of electronically activated glazing all three have their particular applications. Electrochromic smart windows will ultimately lead to better building and vehicle glazing, and display technology. In a number of uses, it will add to the comfort and well-being of its users. Smart windows will help lower of the demands on precious non-renewable fuels for

Acknowledgements

The author wishes to thank Prof. Jaime G. Cervantes, IIM, UNAM, Temixco, Mexico for inviting this paper to the International Symposium on Solar Energy Materials. I wish to thank the cited authors and glass and window company representatives who provided me with advice and technical information for this study.

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