- Email: [email protected]
Smart switchable glazing for solar energy and daylight control
Solar Energy Materials and Solar Cells 52 (1998) 207—221
Smart switchable glazing for solar energy and daylight control Carl M. Lampert* Star Science, 2384 Stanford Place, Santa Clara, CA 95051-1530, USA Received 11 November 1996; received in revised form 6 June 1997
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. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Smart windows; Solar energy control
1. 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
* E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 2 7 9 - 1
SOLMAT 1809 BRR SAVITHA RAVI
208
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
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—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—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.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
209
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 In O : Sn (also known as ITO) and SnO : F. The 2 3 2 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 )/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.
2. 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 transmission of ¹ "0.65—0.50 and fully colored transmittance of ¹ "0.25—0.10. 7 7
SOLMAT 1809 BRR SAVITHA RAVI
210
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
Lower levels of transmittance have been noted below 1% but only in specialized devices. The shading coefficient (SC) is about SC"0.67—0.60 for the bleached condition, and SC"0.30—0.18 for the fully colored condition. Electrochromic materials change their optical properties due to the action of an electric field and can be changed back to the original state by a field reversal. There are two major categories of electrochromic materials: transition metal oxides including intercalated compounds, and organic compounds (including polymers). The electrochromic effect occurs in inorganic compounds by dual injection (cathodic) or ejection (anodic) of ions (M`) and electrons (e~). A typical reaction for a cathodic coloring material using lithium as a coloration ion is: WO (colorless)#yLi`#ye~ % Li WO (blue) 3 y 3 A typical complimentary anodic reaction is: Li V O (light yellow)!yLi`!ye~ % V O (blue) y 2 5 2 5 These reactions show, that by using two different materials, one layer that colors upon intercalation and one that colors on deintercalation, both sides of the device can color at the same time giving greater optical density. Depending on the electrochromic other coloration ions can be used, such as H`, Na` and Ag`. The inorganic materials that have gained the most research interest are various forms of WO , NiO, MoO , 3 3 and IrO . These compounds, among other transition metal oxides, are the subject of x several reviews [12—18]. An electrochromic device must use an ion-containing material (electrolyte) in close proximity to the electrochromic layer as well as transparent layers for setting up a distributed electric field. Devices are designed in such a way that they shuttle ions back and forth into the electrochromic layer with applied potential. An electrochromic window can be fabricated from five (or less) layers consisting of two transparent conductors, electrolyte or ion conductor, counter electrode, and electrochromic layer. A schematic of the device structure is shown in Fig. 1.
Fig. 1. Schematic of an electrochromic glazing. Film thickness is not to scale.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
211
Depending on the components used in devices, some of the layers can be combined serving dual functions. Some devices use even more layers depending on design. Ion conductors and solid electrolytes also require further development for this technology. The most promising ion conductors are certain immobile solvent polymer systems, ionic glasses and open channel metal oxide structures such as perovskites [19]. Typical electrochromic device structures developed by industry and university groups are given in Table 1. Certain forms of crystalline WO can have substantial near-infrared modulation. 3 This material has the potential to control part of the infrared portion of the solar spectrum. The more common amorphous form, used by most developers, has only a little effect in the near-infrared. Crystalline WO is being studied by several 3 investigators [20—22]. Peak near-infrared switching levels from 20% to greater than 75% has been achieved with crystalline tungsten oxide. The reflectance properties of tungsten oxide films produced so far seem to lie far from the theoretically limiting behavior. Organic electrochromics are based on the viologens, anthraquinones, diphthalocyanines, and tetrathiafulvalenes. With organic compounds, coloration is Table 1 Electrochromic device structures Electrochromic IC/Electrolyte
Ion storage
Maker
Application
Prussian blue NiO y CeO x MeO x
Central glass Asahi St. Gobain Pilkington Flachglas EIC Uppsala U. St. Gobain Tufts LBNL! LBNL! NREL
Glazing Building glazing Auto glazing Building/auto Glazing Building glazing Building glazing Auto glazing Building glazing Building glazing Building glazing Building glazing
Gentex Donnelly Nikon Schott Tovota Dornier PPG EIC LBNL!
Auto mirrors Auto/truck mirrors Auto mirrors Auto mirrors Auto glazing Aircraft glazing Aircraft glazing Spacecraft glazing Building glazing
Lithium systems a-Li WO x 3 a-Li WO x 3 a-Li WO x 3 a-Li WO x 3
LiClO #PC 4 Metal oxide LI-PEO Li-polymer
a-Li WO x 3 a-Li WO x 3 a-Li WO x 3 c-Li WO x 3 a-Li WO x 3 a-Li WO x 3 a-Li WO x 3
Li-B-SiO glass Li CrO x 4 PPG-LiClO -MMA Li V O 4 y 2 5 Li-PEO NiO y LiNbO Li CoO 3 x 2 Modified a-PEO Ion storage polymer Li-a-PEO Li Ni O x y Li-polymer LiV O 2 5
Proton systems Viologen a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3
PMMA#Organic Ta O 2 5 Ta O 2 5 SiO /metal 2 Polymer Polymer Poly-AMPS Ta O 2 5 a-PEO copolymer
None NiO Ir Sn O : F x y 2 WO 3 Polyaniline Polyaniline Cu grid a-IrO x Redox polymer
!Information about the LBNL devices can be obtained from the author.
SOLMAT 1809 BRR SAVITHA RAVI
212
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
achieved by an oxidation—reduction reaction, which may be coupled to a chemical reaction. The viologens are the most studied and used commercially of the organic electrochromics. Originally, organic electrochromics tended to suffer from problems with secondary reactions during switching, but recently more stable organic systems have been developed. Both the Donnelly and Gentex have commercially developed organic electrochromic materials for automotive mirrors. There are efforts at Sony and Sharp in Japan to develop a multicolor flat panel display using organic electrochromics. New techniques for the electrodeposition of polymer electrochromics from solution have been reported by Toyota (Japan) [23]. Uniform films of polythiophene, polyaniline and polypyrrole have been deposited by a oxidation polymerization technique. Polyaniline is one of the favored organic polymer electrochromics [24,25]. Electrochromic products currently in the marketplace are automotive and truck mirrors, and sunglasses. The mirrors are designed to automatically regulate glare in response to incident light levels. Both the Gentex Co. and their licensees and Donnelly Co. have commercially developed organic electrochromic materials for automotive mirrors. The truck mirror commercialized by Donnelly is based on inorganic H WO /Ta O /NiO . Mirrors are the most commercially developed electrochromic y x 2 5 x product to date. Gentex has produced over 1.5 million mirrors over the last few years. Prototype electrochromic glazing panels are being tested for automobile sun-roofs and visors. St. Gobain Glass is developing electrochromic sunroofs for French automakers. Donnelly has developed a sunroof for the Ford Motor Co. This is an ideal large-area entry market because of the smaller size and shorter lifetime required of the window compared to building glazings, although the upper temperature limits are higher (90—120°C). Several international research groups are investigating electrochromic materials and devices for building windows [26]. They are listed in Tables 2 and 3. Many of these are organized under government funded programs. Also, many of the participants have other parallel chromogenic projects. One of the bigger multinational projects have is the Joule 11 Commission of the European Communities (CEC) project. Pilkington PLC (UK) has a multilayer project electrochromic glazing under the JOULE 11 program. This project involves several organizations, including Flachglas (Germany) and Davionics AS (Denmark), Oxford Brookes University and the University of Southampton. The Oxford group is known for their work on electrochromic nickel oxide. Flachglas has recently shown a prototype switchable glazing of 0.80 in]1.2 in. For their window they obtained a range of ¹ "65—20% and 7 ¹ "45—10% [27]. The glazing takes a few minutes to change color and to bleach. 4 Flachglas has also made prototype sunroofs for cars (46 cm]78 cm) with a ¹ of 4 14%. Under this project, laminated glazing have been developed based on an Li WO /Li-polymer/metal oxide structure. Other participants in CEC projects are x 3 the Granqvist group at the University of Uppsala (Sweden) working with Coat AB (Sweden) and Leybold AG (Germany). In France, St. Gobain, Corning Europe, and C.S.T.B. and the University of Domaine are working with electrochromic devices testing and components. In Italy, there are several groups involved in electrochromic devices testing and development. In Japan, Asahi Glass has been steadily developing prototype electrochromic windows (0.4 in]0.6 in and 0.6 in]0.8 in) based on
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
213
Table 2 World research in electrochromic devices North America: Donnelly Corp (Holland, MI, Tucson, AZ) EIC Labs (Norwood, MA)/OCLI/Andersen Windows Tufts Univ. (Medford, MA) Gentex (Zeeland, MI) LOF Glass (Toledo, OH) 3M (St. Paul, MN) University of Moncton (Canada) Institute Nat. de la Rech. Sci. (Varennes, Canada) Lawrence Berkeley Nat. Lab (LBNL) (Berkeley, CA)
(Santa Rosa, CA) NREL (Golden, CO) SAGE (Piscataway, NJ) Southwall Tech. (Palo Alto, CA) Enermodel Engin. Ltd. (Waterloo, Canada) Ashwin-Ushas (Freehold, NJ) Gumbs Assoc. (E. Brunswick, NJ)
South America: Univ. of Campinas (Campinas, Brazil) Univ. of Sao Palo (San Carlos, Brazil)
Univ. Nacional de Eng. (Limam Peru)
Europe: Univ. of Uppsala (Uppsala, Sweden) Schott Glass (Mainz, Germany) Pilkington (Lathom, UK)/Flachglas AG (Gelsenkirchen, Germany) St. Gobain Recherche (Aubervilliers France) Univ. of Lanquedoc (Montpellier, France) P. and M. Curie Univ. (Paris, France) Fraunhofer Inst. (Freiburg, Germany) Imperial College (London, England) Leybold AG (Aahen, Germany) Latvian State Univ. (Latvia) Dornier (Friedrichshafen, Germany) University of Rome (Rome, Italy) Univ. of Catania (Catania, Italy) Oxford Brookes Univ. (Oxford, UK) Univ. of Bordeaux (Bordeaux. France) Elf Aqutaine (Lacq, France) SIV (San Salvo, Italy) Corning Europe (Avon, France) Univ. of Southamption (Southampton, UK) CRF Fiat (Orbassano, Italy) Davionics AS (Odense, Denmark) Coat AB (Gothenburg, Sweden) Norwegian Institute of Tech. (Trondheim) Conphoebus (Catania, Italy) ENEA (Rome, Italy) Isoclima (Este, Italy) Nat. Inst. of Chem. (Ljubljana, Slovenia) C.S.T.B. (Grenoble, France) Domaine Univ. (Grenoble, France) Japan and Asia: Asahi Glass (Yokohama) Central Glass (Tokyo) Toyota-Cars (Nagakute) Shizuoka Univ. Tokyo Metro. Univ. (Tokyo) Takai-Rika-Cars Carmate-Cars
NIRIN-(Nagoya) Tokyo Univ. (Tokyo) Ichikoh-Cars (Tokyo) Nikon-Cars/Eyeglasses (Tokyo) Nagoya Univ. (Nagoya) Murakami Kaimeido-Cars Chiba Univ. (Chiba)
Li WO /metal oxide/NiO for testing and evaluation. Part of Asahi’s work has been x 3 x funded by the Japanese Government, Sunshine project, which is part of MITI. The project goals are to develop a glazing by Spring 1996, with 50% visible change, 10 year projected lifetime and cycle lifetime [28]. For this glazing the Asahi group has obtained optical properties of ¹ "73—18% and ¹ "55—11% withstanding 25—60°C 7 4
SOLMAT 1809 BRR SAVITHA RAVI
214
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
Table 3 World research in field-effect chromogenic devices North America: G.M. Research Labs (Warren, MI)-PDLC 3M (Minneapolis, MN)-PDLC Raychem/Taliq (Sunnyvale, CA)-PDLC Kent State Univ. (Kent, OH )-PDLC Research Frontiers (Woodbury, NY)-DPS Litton Systems (Canada)-DPS Meyer and Lonberg Assoc. (Brookline, MA)-Liquid crystals Europe: Gloverbel (Brussels, Belgium)-DPS Saint Gobain Rech. (Paris, France)-PDLC Pilkington (Lathom, UK)-PDLC Asulab-Liquid Crystals-(Neuchatel, Switzerland) Isoclima (Este, Italy)-PDLC Japan and Asia: Asahi Glass (Yokohama, Japan)-PDLC Nippon Sheet Glass (Itami City, Japan)-PDLC, DPS Hankuk Glass (Korea)-DPS Japan Steel Co. (Fuchan, Japan)-DPS Toyota Research (PDLC, DPS)
long-term cycling. The transmission properties are shown in Fig. 2. About two hundred (0.4 in]0.4 in size) of an early prototype electrochromic window have been installed in the Seto Bridge Museum (Kojima, Okayama-Pref., Japan). These prototypes were evaluated in the IEA SH&C Task 10 activities [29]. Another Japanese Government funded project at NIRIN is on basic electrochromic materials for devices including characterization. Under the 1995-96 U.S. DOE (Department of Energy) Electrochromics Initiative, both Donnelly Corp., and OCLI (Santa Rosa, CA) in partnership with EIC Labs (Norwood, MA) and Anderson Windows (St. Paul, MN) have contracts to develop large-area electrochromic glazing. Also, under the U.S. Dept. of Commerce, NIST grant, the partnership of 3M (St. Paul, MN) and SAGE Corp. (Piscataway, NJ) was awarded a sizable grant to develop electrochromics on plastics. We have at LBNL developed devices and components for laminated devices based on WO , NiO, NbO 3 x and Li NiO [30—32]. Another development at LBNL has been on polymer ion x storage layers for lithium and proton storage [33]. The development of testing and evaluation procedures have been done through IEA (The International Energy Agency) [34] and at NREL (Golden, CO). Another approach is the development of low-cost switching devices based on plastic substrates. Southwall Technologies (Palo Alto, CA) has produced electrochromic electrodes of tungsten oxide/ITO on polyester. Over the next few years we expect to see further development in this field.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
215
Fig. 2. Spectral transmittance for an Asahi electrochromic window. Both the bleached (upper curve) and colored (lower curve) conditions are shown. ¹ "72—20%, ¹ "53—10%, switching at !0.7 to 2 V. 7 4
3. 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 the overall optical properties of the window. Open circuit memory is not possible with liquid crystals. Dispersed liquid crystals are the only liquid crystal type used for commercial glazing. The largest size of any chromogenic glazing in the market place is the dispersed liquid crystal glazing. This glazing is currently produced by 3M. Also, Raychem (Sunnyvale, CA) (formerly under Taliq), along with other licensees including Isoclima (Italy), St. Gobain and Nippon Sheet Glass (Japan), produce a glazing called NCAP for products known as “Varilite”, “Umu” (Japan) and “Vision Panel” for specialty automotive and building applications. The guest—host liquid crystals are other choices for large-area glazing. Of the dispersed liquid crystals, there are two types the polymer dispersed liquid crystal (PDCL) and encapsulated liquid crystal (NCAP-nematic curvilinear aligned phase) which consists of nematic liquid crystals distributed in microcavities [35,36]. PDCL and NCAP materials have very similar characteristics but are defined in the patent literature differently according
SOLMAT 1809 BRR SAVITHA RAVI
216
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
to preparation procedure. NCAP films are formed from an emulsion and PDCL films are formed from an isotropic solution which phase separates during curing. 3M with General Motors Research Laboratory and Kent State University have developed PDLC technology for automotive glazing [37]. The 3M product is used in specialty architectural windows by Viracon and Marvin Windows. Large-area NCAP glazing have been fabricated in 1 in]2.5 in sheets. With the liquid crystals are encapsulated within an index matched polymer matrix. The composite polymer is fabricated between two sheets of ITO coated polyester (PET) that serves as electrodes. A schematic of the construction and operation is shown in Fig. 3. The device can also be fabricated between one sheet of glass and one sheet of plastic or two sheets of conductive glass. The switching effect of this device spans the entire solar spectrum, up to the absorption edge of glass. In the off-state, the device appears translucent white. Since the off-state in these devices is diffusely transmitting, the device has application for privacy and security. When an electric field is applied, the liquid crystal droplets align with the field and the device becomes transparent. Typically these devices operate between 60—120 V AC, (potentially lower in the future). Their power consumption is less than 20 W/m2 but require continuous power to be clear [38]. In general, compared to electrochromics, the power consumption is higher for liquid crystals because of the need for continuous power in the activated state. The typical integrated hemispherical visible transmission values for a 3MPDLC device are ¹ (off-on)"0.49—0.80. The shading coefficient changes by 7 SC"0.63—0.79 [43]. Pleochroic dyes can be added to darken the device in the off-state film. The spectral properties of a Raychem dyed film are shown in Fig. 4. These films were evaluated in IEA SH&C Task 10 activities [39]. Dispersed liquid crystal devices have a very good future but are restricted by three characteristics: the unpowered state is diffuse, haze remains in the activated (transparent) state and UV
Fig. 3. Schematic of the construction of a dispersed liquid crystal glazing. Film thickness is not to scale. To the left are shown are the of (diffusing) and on (specular) states.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
217
Fig. 4. Spectral transmittance of a dyed dispersed liquid crystal glazing made by Raychem. The two transmission states are shown.
stability is poor. Also, for widespread use of NCAP and PDLC devices the cost will have to decline. Another possibility for a glazing is the guest—host liquid crystal. The guest—host liquid crystal uses dichroic dye molecules mixed with liquid crystals. The guest—host type is being developed mainly for display applications, but has been considered for optical shutter applications. The cholesteric—nematic phase change, guest—host type of liquid crystal has a good chance of being used for large-area optical switching. This type of liquid crystal switches by a field induced phase change. It exhibits a continuous gray scale operating from 20—120 V AC with a contrast ratio of 5 : 1 and uses no polarizers. The upper temperature stability is 100°C. Guest—host liquid crystal devices are being developed by Asulab in Switzerland for eyeglasses and car mirrors. A car mirror based on this technology can switch visible reflectance R "0.48—0.12 with 7 switching times of 15 and 360 ms for on and off, respectively [40]. This has been possible with the development of improved electrochemical and UV stability of the liquid crystals.
4. 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 lm long) suspended in an organic fluid or gel. This layer is laminated or filled
SOLMAT 1809 BRR SAVITHA RAVI
218
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
Fig. 5. Schematic of the construction of a dispersed particle glazing. The characteristics of the dispersed layer are shown to the left, in both the field-off (absorbing) and field-on (transmitting) conditions.
Fig. 6. Spectral transmittance of a dispersed particle glazing made by Hankuk Glass.
between two electrical conductors. In the off condition the particles are random and light absorbing. When the electric field is applied, the particles align with the field, causing transmission to increase. A schematic of the device is shown in Fig. 5. Typical transmission ranges are 20—60%, 10—50%, 0.1—10%, with switching speeds of
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
219
100—200 ms. The voltage required for the device depends on thickness and ranges from 0—20 to above 150 V AC. The development of suspended particle glazing has been slowed by a number of technological problems including long-term stability, cyclic durability, particle settling and agglomeration, and gap spacing control for larger glazing. In recent years, many of these problems have been reduced or controlled [41—43]. Commercial development of these devices for goggles, eye glasses and windows is being done chiefly by Research Frontiers, Inc. (RFI) (Plainview, NY) and its licensees. Recent activities have been directed at encapsulating the particles in a polymer sheet. A group of 50 RFI windows (0.46 in]0.46 in) have been installed as a demonstration in the Japan Steel Works Building (Fuchua, Japan). Gloverbel, S.A. (Brussells, Belgium) is developing automotive mirrors and visors. Commercial groups working on this technology are Hankuk Glass Industries (Korea), St. Gobain (Paris, France) for architectural glass and Sanyo Electric for flat display panels [44] Fig. 6. Another type of DPS window is being developed by Toyota Labs. By modification of the particles. several colors (green, blue, red, purple) can be achieved.
5. 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 lighting and cooling. The prospects are great with many major companies involved in the development of switchable glazing worldwide. Also, the scientific and technological challenges of the development of these complex glazing are large, creating a specialized area of research. There are several promising devices for window glazing. For the electrochromic layer, tungsten trioxide, nickel oxide, iridium oxide and viologens are the most developed for devices. In the future, the developments will lie in the designing of the appropriate ionic and electronic materials to make devices durable over many years and cycles. Dispersed liquid crystal devices have a very good future but are restricted by three characteristics: (1) the unpowered state is diffuse, (2) haze remains in the activated (transparent) state, and (3) UV stability is poor. The development in dispersed particle devices is moving forward with a number of commercial activities and demonstration windows. These devices are colored in the off-state and become transparent in the on-state. With further improvements in processing and the development of a sheet form of this material, these devices could see widespread use. Durability is of importance for all smart window devices, and an expected lifetime of at least 5—15 years must be assured. The great challenge is to make these technologies, through advanced deposition and fabrication techniques, economical in the 100—250 US $/m2 price range. But still, switchable glazings are emerging in a few products. It is expected their application will greatly redefine our concept of the window.
SOLMAT 1809 BRR SAVITHA RAVI
220
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
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.
References [1] C.M. Lampert, Proc. of Windows Innovations Conf. Toronto, Canada, June 1995, p. 348. [2] C.M. Lampert Y. Ma, Fenestration 2000, Part 111, Advanced Glazing Materials, Study, (1992), ETSU report, S-1215, UK Dept. of Energy. [3] C.M. Lampert, C.G. Granqvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, SPIE Optical Engineering Press, Bellingham, WA, 1990. [4] J.L. Warner, M.S. Reilly, S.E. Selkowitz, D.K. Arasteh, G.D. Ander, Proc. ACEEE Summer Study on Energy Efficiency in Buildings, 30 August—5 September, 1992. [5] S.E. Selkowitz, M. Rubin, E.S. Lee, R. Sullivan, Proc. SPIE 2255 (1994) 226. [6] R. Sullivan, E.S. Lee, K. Papamichael, M. Rubin, S.E. Selkowitz, Proc. SPIE 2255 (1994) 443 . [7] R. Sullivan, P. Lyons, I. Heimanen, I. Anderson, H. Simmler, Proc. of Windows Innovations ’95, Toronto, Canada, June 1995. [8] N. Ozer, C.M. Lampert, Proc. Am. Opt. Soc. Tech. Digest 17 (1995) 344. [9] N.R. Lynam, in: M.K. Carpenter, D.A. Corrigan (Eds.), Proc. Electrochromic Materials, The Electrochem. Society, vol 90, No. 2, 1990, p. 201. [10] H. Kaneko, K. Miyake, Appl. Phys. Lett. 49 (2) (1988) 112. [11] C.G. Granqvist, Appl. Phys. A 57 (1993) 19. [12] C.M. Lampert, Solar Energy Mater. 11 (1984) 1. [13] T. Oi, Ann. Rev. Mater. Sci. 16 (1986) 185. [14] A. Donnadieu, Mater. Sci. Eng. B 3 (1989) 185. [15] C.G. Granqvist, Solid State Ionics 70/71 (1994) 678. [16] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [17] C.M. Lampert, Thin Solid Films 236 (1993) 6. [18] C.B. Greenberg, Thin Solid Films 251 (1994) 81. [19] C.M. Lampert, Proc. MRS-Japan, April 1996. [20] S.F. Cogan, E.J. Anderson, T.D. Plante, R.D. Rauh, Proc. SPIE 562 (1985) 23. [21] R.B. Goldner, G. Seward, K. Wong, T. Hass, R. Foley, R. Chapman, S. Schultz, Solar Energy Mater. 19 (1989) 17. [22] J.S.E.M. Svensson, C.G. Granqvist, Solar Energy Mater. 12 (1985) 391. [23] T. Onishi, S. Yokoishi, Y. Nonobe, Japan Patent Application, 62-290896, 17 December, 1987. [24] S.C. Yang, in: C.M. Lampert, C.G. Granqvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE, Bellingham, WA, 1990, p. 335. [25] R. Jiang, S. Dong, J. Chem, Soc. Faraday Trans. 85 (7) (1989) 1585. [26] C.M. Lampert, IEA SH&C Task 18 report, LBL-36847, June 1994. [27] H. Wittkopf, Flachglas AG, private communication. [28] J. Nagai, G. McMeeking, T. Seike, Y. Noutomi, Smart electrochromic glazing, Glazing Today, December 1994, p. 34. [29] C.M. Lampert (Ed.), Glazing Materials for Solar and Architectural Applications, IEA SH&C Task 10C, LBL-34436, September 1994. [30] A. Pennisi, C.M. Lampert, Proc. SPIE 1016 (1988) 131. [31] Y.-P. Ma, P.C. Yu, C.M. Lampert, Proc. SPIE 1526 (1991) 93. [32] C.M. Lampert, S.J. Visco, M.M. Doeff, Y.P. Ma, Y. He, J.-C. Giron, Solar Energy Mater. and Solar Cells 33 (1994) 91.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221 [33] [34] [35] [36] [37]
[38] [39] [40]
[41] [42] [43] [44]
221
C.M. Lampert, V.-V. Troung, J. Nagai, M.C. Hutchins, IEA SH&C Task 10, report, LBL-29632, 1991. A.W. Czandema, C.M. Lampert, SERI (NREL) Report, TP-255-3537, July 1990. P.S.J. Drzaic, Appl. Phys. 60 (1986) 2142. J.L. Fergason, SID Digest 85 (1985) 68. G.P. Montgomery Jr., in: C.M. Lampert, C.G. Granqvist (Eds.), Large-area chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE, Bellingham, WA, 1990, p. 577. P.G. Clark, Liquid crystal glazing, Proc. SAE, Paper 950046, February 1995. P. van Konynenburg, S. Marsland, S. McCoy, Solar Energy Mater. 19 (1989) 27. N. Busturk, J. Grupp, in: C.M. Lampert, C.G. Granqvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE, Bellingham, WA, 1990, p. 557. R.L. Thompson, R.L. Saxe, U.S. Patent, 4,078,856 (1978). G. Beni, H.G. Craighead, S. Hackwood, Appl. Phys. Lett. 39 (1981) 195. R.L. Saxe, U.S. Patent, 4,772,103, 20 September, 1988. K. Schachter, Popular Sci. (1989).
Smart switchable glazing for solar energy and daylight control Carl M. Lampert* Star Science, 2384 Stanford Place, Santa Clara, CA 95051-1530, USA Received 11 November 1996; received in revised form 6 June 1997
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. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Smart windows; Solar energy control
1. 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
* E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 2 7 9 - 1
SOLMAT 1809 BRR SAVITHA RAVI
208
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
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—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—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.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
209
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 In O : Sn (also known as ITO) and SnO : F. The 2 3 2 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 )/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.
2. 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 transmission of ¹ "0.65—0.50 and fully colored transmittance of ¹ "0.25—0.10. 7 7
SOLMAT 1809 BRR SAVITHA RAVI
210
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
Lower levels of transmittance have been noted below 1% but only in specialized devices. The shading coefficient (SC) is about SC"0.67—0.60 for the bleached condition, and SC"0.30—0.18 for the fully colored condition. Electrochromic materials change their optical properties due to the action of an electric field and can be changed back to the original state by a field reversal. There are two major categories of electrochromic materials: transition metal oxides including intercalated compounds, and organic compounds (including polymers). The electrochromic effect occurs in inorganic compounds by dual injection (cathodic) or ejection (anodic) of ions (M`) and electrons (e~). A typical reaction for a cathodic coloring material using lithium as a coloration ion is: WO (colorless)#yLi`#ye~ % Li WO (blue) 3 y 3 A typical complimentary anodic reaction is: Li V O (light yellow)!yLi`!ye~ % V O (blue) y 2 5 2 5 These reactions show, that by using two different materials, one layer that colors upon intercalation and one that colors on deintercalation, both sides of the device can color at the same time giving greater optical density. Depending on the electrochromic other coloration ions can be used, such as H`, Na` and Ag`. The inorganic materials that have gained the most research interest are various forms of WO , NiO, MoO , 3 3 and IrO . These compounds, among other transition metal oxides, are the subject of x several reviews [12—18]. An electrochromic device must use an ion-containing material (electrolyte) in close proximity to the electrochromic layer as well as transparent layers for setting up a distributed electric field. Devices are designed in such a way that they shuttle ions back and forth into the electrochromic layer with applied potential. An electrochromic window can be fabricated from five (or less) layers consisting of two transparent conductors, electrolyte or ion conductor, counter electrode, and electrochromic layer. A schematic of the device structure is shown in Fig. 1.
Fig. 1. Schematic of an electrochromic glazing. Film thickness is not to scale.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
211
Depending on the components used in devices, some of the layers can be combined serving dual functions. Some devices use even more layers depending on design. Ion conductors and solid electrolytes also require further development for this technology. The most promising ion conductors are certain immobile solvent polymer systems, ionic glasses and open channel metal oxide structures such as perovskites [19]. Typical electrochromic device structures developed by industry and university groups are given in Table 1. Certain forms of crystalline WO can have substantial near-infrared modulation. 3 This material has the potential to control part of the infrared portion of the solar spectrum. The more common amorphous form, used by most developers, has only a little effect in the near-infrared. Crystalline WO is being studied by several 3 investigators [20—22]. Peak near-infrared switching levels from 20% to greater than 75% has been achieved with crystalline tungsten oxide. The reflectance properties of tungsten oxide films produced so far seem to lie far from the theoretically limiting behavior. Organic electrochromics are based on the viologens, anthraquinones, diphthalocyanines, and tetrathiafulvalenes. With organic compounds, coloration is Table 1 Electrochromic device structures Electrochromic IC/Electrolyte
Ion storage
Maker
Application
Prussian blue NiO y CeO x MeO x
Central glass Asahi St. Gobain Pilkington Flachglas EIC Uppsala U. St. Gobain Tufts LBNL! LBNL! NREL
Glazing Building glazing Auto glazing Building/auto Glazing Building glazing Building glazing Auto glazing Building glazing Building glazing Building glazing Building glazing
Gentex Donnelly Nikon Schott Tovota Dornier PPG EIC LBNL!
Auto mirrors Auto/truck mirrors Auto mirrors Auto mirrors Auto glazing Aircraft glazing Aircraft glazing Spacecraft glazing Building glazing
Lithium systems a-Li WO x 3 a-Li WO x 3 a-Li WO x 3 a-Li WO x 3
LiClO #PC 4 Metal oxide LI-PEO Li-polymer
a-Li WO x 3 a-Li WO x 3 a-Li WO x 3 c-Li WO x 3 a-Li WO x 3 a-Li WO x 3 a-Li WO x 3
Li-B-SiO glass Li CrO x 4 PPG-LiClO -MMA Li V O 4 y 2 5 Li-PEO NiO y LiNbO Li CoO 3 x 2 Modified a-PEO Ion storage polymer Li-a-PEO Li Ni O x y Li-polymer LiV O 2 5
Proton systems Viologen a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3 a-H WO x 3
PMMA#Organic Ta O 2 5 Ta O 2 5 SiO /metal 2 Polymer Polymer Poly-AMPS Ta O 2 5 a-PEO copolymer
None NiO Ir Sn O : F x y 2 WO 3 Polyaniline Polyaniline Cu grid a-IrO x Redox polymer
!Information about the LBNL devices can be obtained from the author.
SOLMAT 1809 BRR SAVITHA RAVI
212
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
achieved by an oxidation—reduction reaction, which may be coupled to a chemical reaction. The viologens are the most studied and used commercially of the organic electrochromics. Originally, organic electrochromics tended to suffer from problems with secondary reactions during switching, but recently more stable organic systems have been developed. Both the Donnelly and Gentex have commercially developed organic electrochromic materials for automotive mirrors. There are efforts at Sony and Sharp in Japan to develop a multicolor flat panel display using organic electrochromics. New techniques for the electrodeposition of polymer electrochromics from solution have been reported by Toyota (Japan) [23]. Uniform films of polythiophene, polyaniline and polypyrrole have been deposited by a oxidation polymerization technique. Polyaniline is one of the favored organic polymer electrochromics [24,25]. Electrochromic products currently in the marketplace are automotive and truck mirrors, and sunglasses. The mirrors are designed to automatically regulate glare in response to incident light levels. Both the Gentex Co. and their licensees and Donnelly Co. have commercially developed organic electrochromic materials for automotive mirrors. The truck mirror commercialized by Donnelly is based on inorganic H WO /Ta O /NiO . Mirrors are the most commercially developed electrochromic y x 2 5 x product to date. Gentex has produced over 1.5 million mirrors over the last few years. Prototype electrochromic glazing panels are being tested for automobile sun-roofs and visors. St. Gobain Glass is developing electrochromic sunroofs for French automakers. Donnelly has developed a sunroof for the Ford Motor Co. This is an ideal large-area entry market because of the smaller size and shorter lifetime required of the window compared to building glazings, although the upper temperature limits are higher (90—120°C). Several international research groups are investigating electrochromic materials and devices for building windows [26]. They are listed in Tables 2 and 3. Many of these are organized under government funded programs. Also, many of the participants have other parallel chromogenic projects. One of the bigger multinational projects have is the Joule 11 Commission of the European Communities (CEC) project. Pilkington PLC (UK) has a multilayer project electrochromic glazing under the JOULE 11 program. This project involves several organizations, including Flachglas (Germany) and Davionics AS (Denmark), Oxford Brookes University and the University of Southampton. The Oxford group is known for their work on electrochromic nickel oxide. Flachglas has recently shown a prototype switchable glazing of 0.80 in]1.2 in. For their window they obtained a range of ¹ "65—20% and 7 ¹ "45—10% [27]. The glazing takes a few minutes to change color and to bleach. 4 Flachglas has also made prototype sunroofs for cars (46 cm]78 cm) with a ¹ of 4 14%. Under this project, laminated glazing have been developed based on an Li WO /Li-polymer/metal oxide structure. Other participants in CEC projects are x 3 the Granqvist group at the University of Uppsala (Sweden) working with Coat AB (Sweden) and Leybold AG (Germany). In France, St. Gobain, Corning Europe, and C.S.T.B. and the University of Domaine are working with electrochromic devices testing and components. In Italy, there are several groups involved in electrochromic devices testing and development. In Japan, Asahi Glass has been steadily developing prototype electrochromic windows (0.4 in]0.6 in and 0.6 in]0.8 in) based on
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
213
Table 2 World research in electrochromic devices North America: Donnelly Corp (Holland, MI, Tucson, AZ) EIC Labs (Norwood, MA)/OCLI/Andersen Windows Tufts Univ. (Medford, MA) Gentex (Zeeland, MI) LOF Glass (Toledo, OH) 3M (St. Paul, MN) University of Moncton (Canada) Institute Nat. de la Rech. Sci. (Varennes, Canada) Lawrence Berkeley Nat. Lab (LBNL) (Berkeley, CA)
(Santa Rosa, CA) NREL (Golden, CO) SAGE (Piscataway, NJ) Southwall Tech. (Palo Alto, CA) Enermodel Engin. Ltd. (Waterloo, Canada) Ashwin-Ushas (Freehold, NJ) Gumbs Assoc. (E. Brunswick, NJ)
South America: Univ. of Campinas (Campinas, Brazil) Univ. of Sao Palo (San Carlos, Brazil)
Univ. Nacional de Eng. (Limam Peru)
Europe: Univ. of Uppsala (Uppsala, Sweden) Schott Glass (Mainz, Germany) Pilkington (Lathom, UK)/Flachglas AG (Gelsenkirchen, Germany) St. Gobain Recherche (Aubervilliers France) Univ. of Lanquedoc (Montpellier, France) P. and M. Curie Univ. (Paris, France) Fraunhofer Inst. (Freiburg, Germany) Imperial College (London, England) Leybold AG (Aahen, Germany) Latvian State Univ. (Latvia) Dornier (Friedrichshafen, Germany) University of Rome (Rome, Italy) Univ. of Catania (Catania, Italy) Oxford Brookes Univ. (Oxford, UK) Univ. of Bordeaux (Bordeaux. France) Elf Aqutaine (Lacq, France) SIV (San Salvo, Italy) Corning Europe (Avon, France) Univ. of Southamption (Southampton, UK) CRF Fiat (Orbassano, Italy) Davionics AS (Odense, Denmark) Coat AB (Gothenburg, Sweden) Norwegian Institute of Tech. (Trondheim) Conphoebus (Catania, Italy) ENEA (Rome, Italy) Isoclima (Este, Italy) Nat. Inst. of Chem. (Ljubljana, Slovenia) C.S.T.B. (Grenoble, France) Domaine Univ. (Grenoble, France) Japan and Asia: Asahi Glass (Yokohama) Central Glass (Tokyo) Toyota-Cars (Nagakute) Shizuoka Univ. Tokyo Metro. Univ. (Tokyo) Takai-Rika-Cars Carmate-Cars
NIRIN-(Nagoya) Tokyo Univ. (Tokyo) Ichikoh-Cars (Tokyo) Nikon-Cars/Eyeglasses (Tokyo) Nagoya Univ. (Nagoya) Murakami Kaimeido-Cars Chiba Univ. (Chiba)
Li WO /metal oxide/NiO for testing and evaluation. Part of Asahi’s work has been x 3 x funded by the Japanese Government, Sunshine project, which is part of MITI. The project goals are to develop a glazing by Spring 1996, with 50% visible change, 10 year projected lifetime and cycle lifetime [28]. For this glazing the Asahi group has obtained optical properties of ¹ "73—18% and ¹ "55—11% withstanding 25—60°C 7 4
SOLMAT 1809 BRR SAVITHA RAVI
214
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
Table 3 World research in field-effect chromogenic devices North America: G.M. Research Labs (Warren, MI)-PDLC 3M (Minneapolis, MN)-PDLC Raychem/Taliq (Sunnyvale, CA)-PDLC Kent State Univ. (Kent, OH )-PDLC Research Frontiers (Woodbury, NY)-DPS Litton Systems (Canada)-DPS Meyer and Lonberg Assoc. (Brookline, MA)-Liquid crystals Europe: Gloverbel (Brussels, Belgium)-DPS Saint Gobain Rech. (Paris, France)-PDLC Pilkington (Lathom, UK)-PDLC Asulab-Liquid Crystals-(Neuchatel, Switzerland) Isoclima (Este, Italy)-PDLC Japan and Asia: Asahi Glass (Yokohama, Japan)-PDLC Nippon Sheet Glass (Itami City, Japan)-PDLC, DPS Hankuk Glass (Korea)-DPS Japan Steel Co. (Fuchan, Japan)-DPS Toyota Research (PDLC, DPS)
long-term cycling. The transmission properties are shown in Fig. 2. About two hundred (0.4 in]0.4 in size) of an early prototype electrochromic window have been installed in the Seto Bridge Museum (Kojima, Okayama-Pref., Japan). These prototypes were evaluated in the IEA SH&C Task 10 activities [29]. Another Japanese Government funded project at NIRIN is on basic electrochromic materials for devices including characterization. Under the 1995-96 U.S. DOE (Department of Energy) Electrochromics Initiative, both Donnelly Corp., and OCLI (Santa Rosa, CA) in partnership with EIC Labs (Norwood, MA) and Anderson Windows (St. Paul, MN) have contracts to develop large-area electrochromic glazing. Also, under the U.S. Dept. of Commerce, NIST grant, the partnership of 3M (St. Paul, MN) and SAGE Corp. (Piscataway, NJ) was awarded a sizable grant to develop electrochromics on plastics. We have at LBNL developed devices and components for laminated devices based on WO , NiO, NbO 3 x and Li NiO [30—32]. Another development at LBNL has been on polymer ion x storage layers for lithium and proton storage [33]. The development of testing and evaluation procedures have been done through IEA (The International Energy Agency) [34] and at NREL (Golden, CO). Another approach is the development of low-cost switching devices based on plastic substrates. Southwall Technologies (Palo Alto, CA) has produced electrochromic electrodes of tungsten oxide/ITO on polyester. Over the next few years we expect to see further development in this field.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
215
Fig. 2. Spectral transmittance for an Asahi electrochromic window. Both the bleached (upper curve) and colored (lower curve) conditions are shown. ¹ "72—20%, ¹ "53—10%, switching at !0.7 to 2 V. 7 4
3. 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 the overall optical properties of the window. Open circuit memory is not possible with liquid crystals. Dispersed liquid crystals are the only liquid crystal type used for commercial glazing. The largest size of any chromogenic glazing in the market place is the dispersed liquid crystal glazing. This glazing is currently produced by 3M. Also, Raychem (Sunnyvale, CA) (formerly under Taliq), along with other licensees including Isoclima (Italy), St. Gobain and Nippon Sheet Glass (Japan), produce a glazing called NCAP for products known as “Varilite”, “Umu” (Japan) and “Vision Panel” for specialty automotive and building applications. The guest—host liquid crystals are other choices for large-area glazing. Of the dispersed liquid crystals, there are two types the polymer dispersed liquid crystal (PDCL) and encapsulated liquid crystal (NCAP-nematic curvilinear aligned phase) which consists of nematic liquid crystals distributed in microcavities [35,36]. PDCL and NCAP materials have very similar characteristics but are defined in the patent literature differently according
SOLMAT 1809 BRR SAVITHA RAVI
216
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
to preparation procedure. NCAP films are formed from an emulsion and PDCL films are formed from an isotropic solution which phase separates during curing. 3M with General Motors Research Laboratory and Kent State University have developed PDLC technology for automotive glazing [37]. The 3M product is used in specialty architectural windows by Viracon and Marvin Windows. Large-area NCAP glazing have been fabricated in 1 in]2.5 in sheets. With the liquid crystals are encapsulated within an index matched polymer matrix. The composite polymer is fabricated between two sheets of ITO coated polyester (PET) that serves as electrodes. A schematic of the construction and operation is shown in Fig. 3. The device can also be fabricated between one sheet of glass and one sheet of plastic or two sheets of conductive glass. The switching effect of this device spans the entire solar spectrum, up to the absorption edge of glass. In the off-state, the device appears translucent white. Since the off-state in these devices is diffusely transmitting, the device has application for privacy and security. When an electric field is applied, the liquid crystal droplets align with the field and the device becomes transparent. Typically these devices operate between 60—120 V AC, (potentially lower in the future). Their power consumption is less than 20 W/m2 but require continuous power to be clear [38]. In general, compared to electrochromics, the power consumption is higher for liquid crystals because of the need for continuous power in the activated state. The typical integrated hemispherical visible transmission values for a 3MPDLC device are ¹ (off-on)"0.49—0.80. The shading coefficient changes by 7 SC"0.63—0.79 [43]. Pleochroic dyes can be added to darken the device in the off-state film. The spectral properties of a Raychem dyed film are shown in Fig. 4. These films were evaluated in IEA SH&C Task 10 activities [39]. Dispersed liquid crystal devices have a very good future but are restricted by three characteristics: the unpowered state is diffuse, haze remains in the activated (transparent) state and UV
Fig. 3. Schematic of the construction of a dispersed liquid crystal glazing. Film thickness is not to scale. To the left are shown are the of (diffusing) and on (specular) states.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
217
Fig. 4. Spectral transmittance of a dyed dispersed liquid crystal glazing made by Raychem. The two transmission states are shown.
stability is poor. Also, for widespread use of NCAP and PDLC devices the cost will have to decline. Another possibility for a glazing is the guest—host liquid crystal. The guest—host liquid crystal uses dichroic dye molecules mixed with liquid crystals. The guest—host type is being developed mainly for display applications, but has been considered for optical shutter applications. The cholesteric—nematic phase change, guest—host type of liquid crystal has a good chance of being used for large-area optical switching. This type of liquid crystal switches by a field induced phase change. It exhibits a continuous gray scale operating from 20—120 V AC with a contrast ratio of 5 : 1 and uses no polarizers. The upper temperature stability is 100°C. Guest—host liquid crystal devices are being developed by Asulab in Switzerland for eyeglasses and car mirrors. A car mirror based on this technology can switch visible reflectance R "0.48—0.12 with 7 switching times of 15 and 360 ms for on and off, respectively [40]. This has been possible with the development of improved electrochemical and UV stability of the liquid crystals.
4. 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 lm long) suspended in an organic fluid or gel. This layer is laminated or filled
SOLMAT 1809 BRR SAVITHA RAVI
218
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
Fig. 5. Schematic of the construction of a dispersed particle glazing. The characteristics of the dispersed layer are shown to the left, in both the field-off (absorbing) and field-on (transmitting) conditions.
Fig. 6. Spectral transmittance of a dispersed particle glazing made by Hankuk Glass.
between two electrical conductors. In the off condition the particles are random and light absorbing. When the electric field is applied, the particles align with the field, causing transmission to increase. A schematic of the device is shown in Fig. 5. Typical transmission ranges are 20—60%, 10—50%, 0.1—10%, with switching speeds of
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
219
100—200 ms. The voltage required for the device depends on thickness and ranges from 0—20 to above 150 V AC. The development of suspended particle glazing has been slowed by a number of technological problems including long-term stability, cyclic durability, particle settling and agglomeration, and gap spacing control for larger glazing. In recent years, many of these problems have been reduced or controlled [41—43]. Commercial development of these devices for goggles, eye glasses and windows is being done chiefly by Research Frontiers, Inc. (RFI) (Plainview, NY) and its licensees. Recent activities have been directed at encapsulating the particles in a polymer sheet. A group of 50 RFI windows (0.46 in]0.46 in) have been installed as a demonstration in the Japan Steel Works Building (Fuchua, Japan). Gloverbel, S.A. (Brussells, Belgium) is developing automotive mirrors and visors. Commercial groups working on this technology are Hankuk Glass Industries (Korea), St. Gobain (Paris, France) for architectural glass and Sanyo Electric for flat display panels [44] Fig. 6. Another type of DPS window is being developed by Toyota Labs. By modification of the particles. several colors (green, blue, red, purple) can be achieved.
5. 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 lighting and cooling. The prospects are great with many major companies involved in the development of switchable glazing worldwide. Also, the scientific and technological challenges of the development of these complex glazing are large, creating a specialized area of research. There are several promising devices for window glazing. For the electrochromic layer, tungsten trioxide, nickel oxide, iridium oxide and viologens are the most developed for devices. In the future, the developments will lie in the designing of the appropriate ionic and electronic materials to make devices durable over many years and cycles. Dispersed liquid crystal devices have a very good future but are restricted by three characteristics: (1) the unpowered state is diffuse, (2) haze remains in the activated (transparent) state, and (3) UV stability is poor. The development in dispersed particle devices is moving forward with a number of commercial activities and demonstration windows. These devices are colored in the off-state and become transparent in the on-state. With further improvements in processing and the development of a sheet form of this material, these devices could see widespread use. Durability is of importance for all smart window devices, and an expected lifetime of at least 5—15 years must be assured. The great challenge is to make these technologies, through advanced deposition and fabrication techniques, economical in the 100—250 US $/m2 price range. But still, switchable glazings are emerging in a few products. It is expected their application will greatly redefine our concept of the window.
SOLMAT 1809 BRR SAVITHA RAVI
220
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221
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.
References [1] C.M. Lampert, Proc. of Windows Innovations Conf. Toronto, Canada, June 1995, p. 348. [2] C.M. Lampert Y. Ma, Fenestration 2000, Part 111, Advanced Glazing Materials, Study, (1992), ETSU report, S-1215, UK Dept. of Energy. [3] C.M. Lampert, C.G. Granqvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, SPIE Optical Engineering Press, Bellingham, WA, 1990. [4] J.L. Warner, M.S. Reilly, S.E. Selkowitz, D.K. Arasteh, G.D. Ander, Proc. ACEEE Summer Study on Energy Efficiency in Buildings, 30 August—5 September, 1992. [5] S.E. Selkowitz, M. Rubin, E.S. Lee, R. Sullivan, Proc. SPIE 2255 (1994) 226. [6] R. Sullivan, E.S. Lee, K. Papamichael, M. Rubin, S.E. Selkowitz, Proc. SPIE 2255 (1994) 443 . [7] R. Sullivan, P. Lyons, I. Heimanen, I. Anderson, H. Simmler, Proc. of Windows Innovations ’95, Toronto, Canada, June 1995. [8] N. Ozer, C.M. Lampert, Proc. Am. Opt. Soc. Tech. Digest 17 (1995) 344. [9] N.R. Lynam, in: M.K. Carpenter, D.A. Corrigan (Eds.), Proc. Electrochromic Materials, The Electrochem. Society, vol 90, No. 2, 1990, p. 201. [10] H. Kaneko, K. Miyake, Appl. Phys. Lett. 49 (2) (1988) 112. [11] C.G. Granqvist, Appl. Phys. A 57 (1993) 19. [12] C.M. Lampert, Solar Energy Mater. 11 (1984) 1. [13] T. Oi, Ann. Rev. Mater. Sci. 16 (1986) 185. [14] A. Donnadieu, Mater. Sci. Eng. B 3 (1989) 185. [15] C.G. Granqvist, Solid State Ionics 70/71 (1994) 678. [16] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [17] C.M. Lampert, Thin Solid Films 236 (1993) 6. [18] C.B. Greenberg, Thin Solid Films 251 (1994) 81. [19] C.M. Lampert, Proc. MRS-Japan, April 1996. [20] S.F. Cogan, E.J. Anderson, T.D. Plante, R.D. Rauh, Proc. SPIE 562 (1985) 23. [21] R.B. Goldner, G. Seward, K. Wong, T. Hass, R. Foley, R. Chapman, S. Schultz, Solar Energy Mater. 19 (1989) 17. [22] J.S.E.M. Svensson, C.G. Granqvist, Solar Energy Mater. 12 (1985) 391. [23] T. Onishi, S. Yokoishi, Y. Nonobe, Japan Patent Application, 62-290896, 17 December, 1987. [24] S.C. Yang, in: C.M. Lampert, C.G. Granqvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE, Bellingham, WA, 1990, p. 335. [25] R. Jiang, S. Dong, J. Chem, Soc. Faraday Trans. 85 (7) (1989) 1585. [26] C.M. Lampert, IEA SH&C Task 18 report, LBL-36847, June 1994. [27] H. Wittkopf, Flachglas AG, private communication. [28] J. Nagai, G. McMeeking, T. Seike, Y. Noutomi, Smart electrochromic glazing, Glazing Today, December 1994, p. 34. [29] C.M. Lampert (Ed.), Glazing Materials for Solar and Architectural Applications, IEA SH&C Task 10C, LBL-34436, September 1994. [30] A. Pennisi, C.M. Lampert, Proc. SPIE 1016 (1988) 131. [31] Y.-P. Ma, P.C. Yu, C.M. Lampert, Proc. SPIE 1526 (1991) 93. [32] C.M. Lampert, S.J. Visco, M.M. Doeff, Y.P. Ma, Y. He, J.-C. Giron, Solar Energy Mater. and Solar Cells 33 (1994) 91.
SOLMAT 1809 BRR SAVITHA RAVI
C.M. Lampert/Solar Energy Materials and Solar Cells 52 (1998) 207—221 [33] [34] [35] [36] [37]
[38] [39] [40]
[41] [42] [43] [44]
221
C.M. Lampert, V.-V. Troung, J. Nagai, M.C. Hutchins, IEA SH&C Task 10, report, LBL-29632, 1991. A.W. Czandema, C.M. Lampert, SERI (NREL) Report, TP-255-3537, July 1990. P.S.J. Drzaic, Appl. Phys. 60 (1986) 2142. J.L. Fergason, SID Digest 85 (1985) 68. G.P. Montgomery Jr., in: C.M. Lampert, C.G. Granqvist (Eds.), Large-area chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE, Bellingham, WA, 1990, p. 577. P.G. Clark, Liquid crystal glazing, Proc. SAE, Paper 950046, February 1995. P. van Konynenburg, S. Marsland, S. McCoy, Solar Energy Mater. 19 (1989) 27. N. Busturk, J. Grupp, in: C.M. Lampert, C.G. Granqvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE, Bellingham, WA, 1990, p. 557. R.L. Thompson, R.L. Saxe, U.S. Patent, 4,078,856 (1978). G. Beni, H.G. Craighead, S. Hackwood, Appl. Phys. Lett. 39 (1981) 195. R.L. Saxe, U.S. Patent, 4,772,103, 20 September, 1988. K. Schachter, Popular Sci. (1989).