Glass is a challenging material for designers to work with, but for applications with see-through requirements, glass is typically the material of choice. The challenge of designing with glass can become more pronounced in situations where the glass must be electrically heated – resistive heating elements must be applied to, or within, the glass without significantly interfering with its transparency. Fortunately, a number of suppliers offer solutions for such cases.

Electrically heated glass can be used in several ways. In the food service industry, the material is used for transparent food displays that keep food hot and still visible without drying it out. A heated glass shelf can keep food items warm while providing the ability to see the food items on the shelf below.

At the supermarket, heated glass provides just enough heat to eliminate condensation and fogging and allow consumers to view the chilled and frozen goods in refrigerated display cases.

A towel warmer from Thermique Technologies features glass coated with metal oxide that will heat the glass up to 150 DegF. Thermique is a subsidiary of Engineered Glass Products.

Heated glass radiators and towel warmers can keep rooms cozy and towels toasty, while heated mirrors in a steamy bath can remove or eliminate condensation.

Heated glass can also be used to improve visibility and operation of liquid crystal displays (LCDs) in certain applications. When used for heating LCDs, the heated glass keeps the displays at a functioning temperature in cold environments. Gas pump displays, kiosk displays, and some portable electronic device displays used outdoors rely on LCD heaters to maintain their legibility.

Other outdoor applications include security and traffic cameras, where the heaters prevent lens fogging.

Display cases, towel warmers, and LCDs are just some of the better-known uses for electrically heated glass. Other applications are less traditional. One company, Chicago-based Engineered Glass Products (EGP) is looking at supplying zoos with display cases that will allow visitors to look at their favorite reptiles while keeping the case at the appropriate temperature for the cold-blooded creatures. Another company, Anthony International of Sylmar, Calif., created the display case that enclosed the 5,300-year-old prehistoric man found frozen in the Italian Alps in 1991. When the body went on display, a freezer case was needed to preserve the body, and the glass had to be clear so the body could be seen.

While heated glass has many applications, it does have its limitations. One concept that has been put dreamed up is a glass toaster, which isn't actually feasible. To toast bread, a toaster chars the carbon in the bread, and it takes a high blackbody temperature to do that.  To perform this task, a toaster, with its glowing red wires consuming about 750 to 800 W, gets hotter, and hotter faster, than glass can get, says George Usinowicz, an application expert for Thermique Technologies, a subsidiary of EGP. Thermique Technologies recently released a new, freestanding glass towel warmer.

Enlarge this picture Fig. 1.  Infrared analysis from Engineered Glass Products compares the uniform heat distribution across the surface of the glass (top), as compared to a traditional heating element (bottom).

The limitations on heat output limits the range of applications for heated glass, but use of the material is growing nonetheless in those areas where it is suitable. EGP, one of several companies making heated glass materials, targets its products for towel warmers, food display cases and architectural windows. Saint Gobain Glass of Germany makes heated glass for residential space-heating radiators, towel warmers and other products. Minco of Minneapolis, makes Thermal-Clear, a heated glass product used primarily for LCD heaters. Anthony International uses heated glass primarily for commercial glass refrigerator and freezer doors, as does Schott Termofrost, Arriva, Sweden.

Electrically heated glass maintains a steady and consistent temperature across the surface of the glass, especially when compared to traditional heating elements (see Fig. 1.). Heat radiates off of the glass toward the object to be warmed. The transparent heated glass has a wide temperature range that it can produce. Some heated glass manufacturers specify their products to warm to 350 DegF to 400 DegF. EGP’s heated glass products can reach 350 DegF and can be incorporated into the smooth surface of a cooktop for use as a burner to boil water and other stovetop cooking and warming applications. Beyond that, as temperatures near 500 DegF, there can be concerns about glass breakage as well as thermal expansion issues.

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The ability to produce various temperatures is important as each application has its own temperature requirements, says Paul Artwohl, vice president of engineering for Anthony International. A defogging application requires less heat than a food warming application.  Freezer cabinets operate at temperatures as low as -20 DegF, which can cool the surface temperature of the inside glass down to below 0 DegF. Condensation on the glass doors will form when the moisture laden warmer air of the store comes in contact with the colder glass surface, intersecting at the dew point. The dew point refers to the temperature at which water vapor in air at a given relative humidity will condense. For anti-fogging applications, it is only necessary to heat the glass just enough to keep it above the dew point to prevent condensation, Artwohl says. Alternatively, the surface can be coated with anti-fog coatings than can contain combinations of hydroscopic and hydrophilic properties.

In many instances, a closed-loop control system is used to respond to changing conditions. EGP, Anthony International and others offer electronic controllers for their products. Schott Termofrost, for instance, offers the Eco-S system that works with its tin oxide-coated heated glass doors. With these systems, sensors monitor ambient temperature and humidity and electronically control the glass temperature to keep it at 1 DegF above dew point.

A closed loop control can save energy in two ways. It avoids heating the glass any more than necessary, and minimizes the heat load that heated glass can introduce into a refrigerated space, according to Artwohl. 

Condensation would typically form when these doors are opened to the warmer store temperature, but these Termofrost doors from Schott clear the moisture.

Heated glass not only works on large-scale applications such as refrigerator and freezer display cases, but also on smaller applications such as LCD heaters. In LCDs that are used outside, the need to heat LCDs are critical to the operation of the device. Typically, LCD’s are rated to 32 DegF, although some higher-priced, extended range LCDs are rated down to below 0 DegF. With a standard LCD, if the temperature drops just a couple degrees below freezing, say to 30 DegF, the liquid crystals can become sluggish and even freeze, says Brian Williams, Minco’s Global Heaters Product Marketing Manager. By adding 1 W to 2 W per sq. in., the heaters can keep LCDs operating in temperatures as low as –67 DegF. 

Of course, not all devices that are used outside require an LCD heater. Cell phones, for example, are often used outside in cold weather, but rarely use an LCD heater because of space and cost issues. Instead, those applications typically use waste heat generated by the chips on the PCBs to keep the crystals warm and eliminate condensation.

And, LCDs heaters are not just for use in outside applications. There are indoor applications, including LCDs in use on industrial freezers and semiconductor applications in which the manufacturing process itself gets really cold, Williams says. Another application that he has seen is a ruggedized computer that went into a petrochemical facility. In this case, the computers needed to be washed down every night and the heater helped dry the display.

Thermal-Clear heaters from Minco deliver uniform watt density across the entire heater area.

In all of these applications, suppliers of electrically heated glass take great pains to make sure that the units are safe. This ranges from making sure that a towel warmer won’t get hot enough to burn a human hand, to insulating the coatings or the resistive wires to negate any electrical shock hazard.

When thinking of utilizing heated glass in an application, it’s important to first understand how it is made. Glass, of course, is an insulator, so it must be modified with conductive material to make it a resistive heater. There are a number of different ways to accomplish this in terms of materials, configuration, and construction. Each of which has their own set of advantages and disadvantages.

There are two common methods for achieving this:

• The application of fine, resistive wire to the glass.
• The use of metal oxides, infused into the glass, or applied to the surface as a coating or film.

A simple version of a resistive wire can be found on the rear window defroster in automobiles. In fact, electrically heated glass was first developed in World War II to prevent aircraft windshields from frosting over and obscuring visibility. Minco takes this wire approach, but unlike the unsightly wire pattern on the back window of a car, the company uses very thin diameter wires that are far less conspicuous. The typical wires is about 0.001 in., but they can be as small as 0.0008 in.

The Hatco cabinet keeps food hot with a warming shelf that allows the food to stay hot while being in full view of customers.

The wires are placed in a grid-like pattern and can match the shape of the glass. Typically, Minco tries to create a product that has 50 percent space with wires and 50 percent space with open area. The spacing of the wires can also be varied to target specific areas of the glass such as around the edges where mounting brackets are located. These brackets act like a heat sink, says Williams. By varying the density of the wire pattern across the plane of the glass, extra heat can be funneled to the edges to make up for this heat loss, a process that Williams calls profiling. Minco’s heaters can get as hot as 248 DegF.

Williams says that the grid pattern can also be designed to consider the size of the LCD, where the information will be displayed and if there might be any cold spots. For instance, in addition to cold spots from mounting brackets, some electronics might use a cooling fan, which may cool one section of the LCD but not another section.

The heater wires are encapsulated between two pieces of transparent material in a laminate “sandwich” construction, which protects them from damage or abrasion, permitting a rugged application. The time it takes to reach specified temperatures can be as short as 5 seconds in some applications.

Williams says one of the benefits of a wire element system as compared to using metal oxide coated glass, is that the wire systems have better resistance tolerances. The heaters have resistance tolerances that start at +/- 10 percent, and can be built down to +/- 2 percent. By contrast, Williams says that heaters that use metal oxide to make the glass conductive might have resistance tolerances of up to 20 to 25 percent, depending on the uniformity of the coating on the glass. This is especially important in a battery-powered operation, he says. With a consistent tolerance, the effect of the LCD heater on the life of the battery can be estimated with some degree of accuracy.

SGG Thermovit Elegance, Diamant model glass radiator is transparent.

A downside of a wire-element based system is that it can interfere with light transmission and the ability to read the display. Heating glass with a wire system has a transparency of around 80 percent, while metal oxide systems can be 90 percent or more transparent. The wires can obscure the display, especially in LCD units with small pixels, unless they are very accurately positioned.

The other approach, using metal oxide conductors as a film or coating, typically makes for better visibility. Light transmission rates stay pretty constant, even when higher temperatures are required, says Usinowicz. He says it isn’t the amount of coating that affect heat generation, but the formulation of the coating.

The conductive material used can vary in terms of the type of metal oxide used, as well as the construction of the glass component. The two most prevalent transparent conductive oxides used in thin-film coatings are fluorine-doped tin oxide, and Indium tin oxide. Tin oxide is robust and suited for a variety of uses, indium tin oxide is a good material to use especially in the electronic display industry, but is more costly. 

The metal oxides on the glass can be vapor deposited, also called sputtering, spray coated, or infused within the glass itself. Sputtering is a process in which the coating material is bombarded with negative ions under high-voltage accelerations. The material ejects atoms and molecules from the target material, which are propelled against the glass substrate where they bond.

Enlarge this picture An exploded view of where the heater is placed in an LCD. Photo: Minco.

Infusing the glass is done with a pyrolitic deposition, a process used by companies such as Anthony International and EGP. Pyrolytic coating is a thin-film coating that is applied at high temperatures and sprayed onto the glass surface during the float glass, or molten, stage of glass making. This is the most durable way to make a glass conductive and helps ensure even distribution of the metal oxide throughout the glass material.

Artwohl says that the metal oxide coating can be custom formulated to get the optimal resistance and emissivity levels desired for the application. In one formulation, the glass can deliver just enough heat to prevent condensation. Other applications call for multiple layers of coatings that can break down the light and block the ultraviolet and infrared rays while allowing the visible light to pass through. This is important because it is the UV and IR light that can spoil food, and especially milk and other dairy products.

In terms of tuning the glass to the appropriate temperature, Usinowicz says that different sheet resistances are used.  In a freezer door, it is not uncommon to use 5 W to 8 W per sq. ft. to dissipate condensation. But, he says, in a food service application where a plate needs to be kept warm and the glass is in contact with the bottom of a plate, 20 W per sq. ft., might be required.  For a towel warmer that reaches 150 to 170 DegF, 30 W per sq. ft. could be needed.

Another method of using metal oxide is through application of the material as a film. One such company that does that is Saint Gobain Glass in Germany. The company’s Thermovit heatable laminated glass is a sandwich type construction. It comprises two or more sheets of glass interlaid with one or more films of polyvinyl butyral (PVB). It includes two 6 mm leaves of laminated safety glass, sandwiching a 1 mm layer of PBV in the middle.  A conductive coating is applied to the inside surface of one of the glass panels. The laminated version is limited to 149 DegF, a non-laminated, single pane version could be allowed to reach 248 DegF.

An LCD heater outfitted with a wire resistive heater from Minco.

Whether coating or film, an electrical connection must be made to the conductive glass. EGP, Anthony International and other companies that use this technology place buss bars along two opposing edges. These serve to distribute the electrical current across the film and give consistent coverage through the glass.

The distance between the buss bars, which is determined by the length of the glass, can create different amounts of available power requirements, says Usinowicz. For instance, if there are two pieces of glass, a 12 in. x 36 in. and a 12 in. x 24 in, and they both use the same coating formulation, the larger glass would require 3 amps, and deliver 142 W per sq. ft. and reach 192 DegF. The smaller pane with the closer buss bars requires 4.6 amps, can go to 338 W per square foot, and reach 305 DegF. So the smaller pane can generate more watts per square foot and reach higher temperatures. Of course, different metal oxide coatings can be selected, and the same coating wouldn’t necessarily be used on both pieces of glass, says Usinowicz. The goal is to use the least amount of power, in amps, to get the required power density, or watts per square foot.

Some systems require transformers to step down the voltage. The transformers are individually wound and sized according to the specifications for glass temperature, size and other factors. For instance, the same transformer couldn’t be used on the 12 in. x 36 in. glass as compared to the 12 in. x 24 in. pane.

Other units use line voltage as would be found in any U.S. home. EGP’s heated glass works with a GFCI outlet using 110 VAC or 240 VAC, says Fred Fowler, president of EGP.

Minco Thermal-Clear heaters can be used in both the heads up display and the display in the portable electronic device.

While heated glass is already established as a material for use in many applications, research is continuing. Companies that supply these materials are looking at different glass formulations, as well as ceramic materials. Other companies are developing glass doors with coatings that capture heat from light and do not require a heated system.

The future, however, might be in the form of transparent, carbon nanotube sheets that are applied to glass. Researchers at the University of Texas – Dallas and at the Fraunhofer Technologies Institute in Stuttgart, Germany, are independently working on such technology. The researchers say that carbon nanotube sheets are more mechanically robust as compared to oxide films such as indium tin oxide and can offer substantially greater sheet resistance.

Carbon nanotubes have been described as minute bits of string and trillions of the nanotube strings must be assembled to make a macroscopic product such as a sheet. Until recently, nanotube sheets were made in a week-long process akin to paper making, by filtering solutions of nanotubes and then peeling the nanotubes off the filter once dry. In Dallas, Prof. Ray Baughman is overseeing the work developing carbon nanotube sheets that speeds up production to about seven meters per minute. The researchers can now create forests of nanotubes, which are teased from the forest sidewall onto adhesive tape and drawn into a continuous sheet. In order to create a denser material, researchers can place a nanotube sheet onto a substrate such as glass and immerse in an ethanol solution. The ethanol evaporates when the substrate is withdrawn from the solution and the result is      a sheet with a density of roughly 0.5 g/cm3. The researchers were able to create a 5-cm, 1-meter long, 50-nm thick, transparent carbon nanotube sheets. 

High transparency was achieved by the densified nanotube sheets in combination with usable electrical conductive substrates, according to Baughman.  This combination is needed for such applications as displays, video recorders, solar cells and solid state lighting. The research obtained a sheet resistivity of 700 ohms per square in the draw direction and 10 to 20 times higher than that in the orthogonal direction.

At Fraunhofer, Ivica Kolaric, Department Manager of the Energy Efficient Mechatronic System, is overseeing the carbon nanotube project. There, carbon nanotubes sheets are spray coated onto the glass. Researchers in Germany have created carbon nanotube sheets with a heating power of >2,500 W per square meter. Much of their applications are in the automotive industry, but other applications might be nearing fruition.

Kolaric says that the key to using carbon nanotubes is in the dispersion. If the nanotubes are not dispersed well — and nanotubes tend to clump together — then hot and cold spots can occur. Kolaric says the nanotubes come in powdered form and are then dispersed in solvent or water. The liquid material is spray coated onto the glass and as the liquid dries, the nanotubes form a conducting network inside the lacquer. By passing a current through this network, the nanotube sheet heats and passes that heat into the glass.

The ability to spray coat the carbon nanotube material is one way to make the process less expensive to use, and, Kolaric feels, more attractive to manufacturers. He also says that that carbon nanotube sheets also use less voltage as compared to a metal oxide coated pane of glass. In a de-icing application, the metal oxide glass needed about 30 to 40 volts to clear the ice, while the nanotube unit needed 3 volts.

Challenges, however, still need to be overcome. The wet processes can affect transparency, creating a haze, but Kolaric feels that these issues are being overcome.

While carbon nanotube technology might represent the future, electrically heated glass is being sold now. Whether using resistive wires or metal oxide coatings and films, applications for heated glass are growing. The versatility of being able to tune the coating formulation, or resistive wire grid pattern, and reach the temperature, power and size requried, are making this old material new for use in a growing array of applications. 

For more information, email:
Anthony International: susanw@anthonydoors.com
Engineered Glass Products: info@egpglass.com
Minco: Dave.Hans@minco.com
Schott: tim.dye@us.schott.com
Saint Gobain Glass: christian.willers@saint-gobain-glass.com