issue: October 2003 APPLIANCE Magazine Engineering Heated Glass Next-Generation Heated Glass Products	Share  Printable format  Email this Article  Search
by Peter Gerhardinger, vice president of Technology, Engineered Glass Products, LLC Heated Glass has been used in commercial refrigeration doors for many years. Recent advances in glass coatings, electronic control technology, and interconnect technology can significantly improve the energy efficiency and safety of these products.

To make a heater out of a piece of glass requires that the surface be coated with a material that will conduct electricity, as obviously, the glass is an insulator. An example of one of the most common techniques, used by millions and existing for nearly 40 years, is the defogger in the rear window of an automobile.

This technology lends itself very well to low-voltage, 12-V systems and is reasonably simple to implement. It has several shortcomings, however, most notably that lines are visible and interfere with vision through the glass. Principally because of this, they are limited to the rear windows of vehicles.

Figure 1. Standard IG Configuration

An alternative approach to heated glass is the use of transparent thin-film conductors. Certain metal oxides can be applied to the surface of glass, resulting in a thin film that conducts electricity, but does not interfere with vision through the glass. Another property of these metal oxides is that they reflect heat. They convert the glass surface from a heat-absorbing (high-emissivity) material to a heat-reflecting (low-emissivity or Low E) material. In the infrared (IR), all glass is opaque, meaning that heat transfer takes place by absorption and emission, not direct transmission. This is an important property that influences the efficiency of many types of glass heaters, which will be discussed later in the text.

These coatings are known as Transparent Conductive Oxides (TCOs). There are actually several different types of TCO materials; the three most common of which are Fluorine-doped Tin Oxide (SnO2:F), Indium-Tin Oxide (ITO), and thin stacks of oxides and metallic silver. All three of these materials are conductive, heat reflecting, and transparent. However, the only one that is robust and suited for a variety of industrial uses is tin oxide. ITO is used almost exclusively in the electronic display industry due to its high cost, and the silver-based material is used in the window industry as a Low E coating for sealed double-pane windows. The silver-based materials are not intrinsically stable, and can only be used in a hermetically sealed assembly.

The glass industry is responsible for the rapid growth and current large-quantity, low-cost production of both tin oxide and silver materials, as they both compete in the Low E window market and are produced by the millions of square feet every year. The manufacturing processes for the materials are quite different from each other. The tin oxide is deposited in a thermal process during the manufacture of the glass itself. The silver materials are deposited onto cut-sheets of glass in a high-volume vacuum deposition process. Economically, the market pricing is roughly similar for both technologies, and they have similar properties for Low E windows. The tin oxide is more tolerant of manufacturing processes, but the silver materials have a performance advantage.

The focus of this article will be on the tin oxide products available from the glass manufacturer.

Properties of Fluorine-Doped Tin Oxide

Tin oxide coatings for Low E window use are available for a reasonable cost, in enormous quantities, on different thicknesses of clear float glass, ranging from 2.3-mm window glass to 6-mm architectural glass.

Table 1. Relationship between Sheet Resistance, Emissivity, and IR Reflectance

The Low E glass products have an electrical property described as the sheet resistance, measured in ohms per square (Ω/sq), where the square is dimensionless. This planar property comes from the semiconductor industry and is described as resistivity multiplied by thickness.

There is an inverse relationship between the sheet resistance and the heat reflection. The lower the sheet resistance, the higher the percentage of IR reflection. There are some practical limits to both the sheet resistance and the IR reflection, due principally to the structure of the coating, as it gets too thick, becomes hazy, and scatters light. This is undesirable in a window. For Low E windows, the performance attribute requires that the IR reflectance be as high as possible, which is the same as having the emittance as low as possible.

Table 1 shows the relationship between sheet resistance, emissivity, and IR reflectance. The products shown in Table 1 are not all that are available, but they represent the range of values available. Note that the only ones that are used as Low E coatings are the AFG and PNA Products. The other products are produced in lower volumes for specialty applications and are considerably more expensive.

Most of the development work done so far has been on products utilizing the Pilkington TEC 15, which is very cost effective and is the most uniform coating available. It is also the most conductive of the common Low E products.

Making Electrical Connection to TCO-Coated Glass

In order to make an electrical connection to electrically conductive coated glass, it is common to deposit "bus bars" along two opposed edges of a square or rectangular plate. Bus bars function to distribute the current uniformly across the thin film, and must be low resistance, durable, easy to make connection to, and well adhering. Most commonly, a wire or clip is soldered to the bus bar, but in some applications, a spring clip arrangement can be used.

The most common bus bar material is printed and fired silver frit. Silver frit materials are screen printed onto the glass prior to any heat treating and are fired into the coated surface. The conductivity is adequate, and they exhibit good adhesion. The soldering operation requires special solder and a special technique to avoid overheating the frit. Other disadvantages are the cost of the silver frit (about U.S. $0.25 per lineal ft) and the capital cost and complexity of screen printing.

An alternative approach has been developed using thermally deposited copper. Patents have been filed on this process, but basically copper is deposited onto coated glass, which is already heat treated in a high-speed proprietary process.

• The advantages of the copper bus bars include:
• Material cost about 1/10 that of silver frit
• Deposition rates as high as 400 in per min
• Adhesion greater than 25-lb of pull strength
• Conductivity comparable to silver frit
• Low capital cost
• Applied after glass tempering/processing
• Excellent solderability with common electronic solder

This copper bus bar process is available for license for applications including heated glass, switchable glass, and thin film photovoltaic solar cell TCO glass.

All of the heated glass products described herein utilize the copper bus bar material. The only limitation is temperature. The copper material is good to approximately 400¼F (204¼C), which is also the practical in-service temperature limit for soda-lime float glass. For an extremely high-temperature application, such as with coated quartz or ceramic materials, silver bus bars are required, as they can withstand temperatures of up to 1,000¼F (538¼C).

Electrically Heated Products

Knowing the sheet resistance, desired size of the glass, and the desired power dissipation, the requirements for the electrical connection and control circuit can be calculated. Although there are a number of powering options, both a.c. and d.c. at low and high voltages, there are some limitations to the use of d.c., so the focus is primarily on a.c.-powered devices.

In many cases, the power requirements of a given application can be satisfied by choosing a standard coated glass product. An example will best illustrate this method. Given the following information:

Glass Size: 30 in by 60 in (12.5 ft²)
Aspect Ratio: 2:1
Power Density: 8 W/ft²
Supply Voltage: 120 V a.c.

the calculated required resistance is 144 Ω. This, however, is the terminal resistance (as measured at the leads). To determine the sheet resistance, the aspect ratio is used. With the bus bars on the short dimension, the coating sheet resistance must equal:

144 • (2 / 1) = 288 Ω/sq

With the bus bars on the long dimension, the coating sheet resistance must equal:

144 • (1 / 2) = 72 Ω/sq

The commercially available 70 Ω/sq products can be readily used in this application, and indeed, that is why that coating value is made and sold. Similarly, for the same glass size, powered by 230 V a.c. (as in Europe), the required sheet resistance is 250 Ω/sq. European freezer doors are the target market for the 250 Ω/sq products.

The calculations presented are the basis for all heated glass articles. The relationship between power and temperature is a function of the glass construction and the environment in which it is used. It is obvious that more power will lead to higher temperatures, but the heat transfer to the surroundings ultimately determines the actual temperature of the glass. To determine the power requirements for different applications, it is common to use a thermal resistance model to predict the temperatures. Actual prototypes can be constructed and tested, or, in the event that an electronic control circuit is used, a trimmer adjustment can be included to vary the power.

The Rationale for Electronic Control

The freezer door example just presented functions without any type of control circuit. It can be simply connected across the power line, as the electrical properties of the TCO coating have been optimized for that application. Similarly, the low-temperature burner product does not need an electronic controller. By using a 250 Ω/sq coating on a square plate measuring about 4.5 by 4.5 in, a resistance of 250 Ω is obtained, which, when connected across 120 V, dissipates just under 60 W. This not only provides sufficient energy to maintain the desired temperature, but is low enough to self-limit the temperature to a value below which there is no risk of thermal stress breakage.

In most instances, however, a good match like this cannot be obtained. In these cases, an electronic controller can be programmed to provide the proper power level using low-cost, readily available glass products. Also, the use of an electronic control circuit can provide additional functionality, such as variable power control and safety, such as glass breakage detection. Finally, in certain applications, the use of a low-resistance coating (which is a true Low E coating) can improve the efficiency of the product, but will require the use of a control circuit to match the electrical characteristics of the coating to the available voltage.

The criteria established for the heated freezer door example are also applicable to architectural (window) applications of heated glass.

A freezer door is a double- or triple-pane insulating glass unit, with a Low E coating on one surface and a TCO coating on another surface. These coatings are on the surfaces facing each other, with the heated surface being the inboard surface of the outermost lite of glass—the one facing the store (see Figure 2).

Figure 2. High-Performance IG Configuration

The purpose of heating this glass is to ensure that there is no condensation formed on the outer surface, which impedes the ability to see the merchandise in the freezer. Without heat, there is a high probability of condensation forming, as the temperature in the freezer cabinet can be as low as -20°F, which will drag the outer glass surface temperature down to 55°F. If the store is warm and/or humid, condensation will form on the outer surface, as the glass temperature is below the dew point.

The actual temperature of the outer surface will be a function of the U-value of the insulating glass unit. For a triple-pane unit, with one Low E and one high-resistance TCO coating, the U-value will be 0.30 BTU/(hr • ft²• °F). Unheated, the outer surface temperature will be 54.8°F when the freezer is -20°F and the store is 70°F. It takes about 8 W/ft² to heat the outer surface to 70°F. A typical door measuring 30 by 60 in (12.5 ft²) amounts to a total dissipation of 100 W per door. Since these doors run continuously, that totals 876 kWh of electricity per year. At $0.08 per kWh, each door consumes $70.00 worth of electricity per year. It is not uncommon for a supermarket to have more than 100 freezer doors.

Now the improvement from substituting a Low E coating for the high-resistance TCO (see Figure 2) is considered. The U-value improves from 0.30 to 0.24 BTU/(hr • ft² • °F), meaning that the unheated outer glass temperature will be 57.6°F, warmer than before. To raise that temperature to 70°F requires 5.97 or 6 W/ft². With the same 12.5-ft² door, the total power is now 75 W—a 25-percent improvement in efficiency. Over 1 year, the energy consumption is 657 kWh, which, at the same cost basis, would cost only $52.56—a savings of $17.44.

Since a low-resistance TCO cannot be directly connected to the power line, a simple switching circuit is used that turns a triac on and off at a preprogrammed rate. This circuit can also be made adjustable, can sense problems such as broken glass and shut itself down, and can be made small enough to sandwich into the frame.

The way this circuit functions is to control the duty cycle, or the percentage of time that the heat is on. Even though the low resistance load may draw much more power than is desired, by switching it on and off very rapidly, essentially any desired power level can be "dialed in." The programming to control the triac firing is a simple algorithm in a programmable chip and can be implemented as a fixed-duty cycle, or controlled from a potentiometer.

To prove that the energy savings allowed by controlling the power applied to a low-resistance unit using this method would in fact cause a proportional decrease in measured energy usage, each type of load was connected through a watt-hour meter. After 30 days, the meter on the low resistance, triac-controlled circuit did show that this configuration consumed 25-percent less energy than the high-resistance unit connected directly across the line.

Note that triac control is now being used to control electric stove burners. The older bimetallic control also switched, but did so on such a long time cycle (several times per minute), that the temperature swings were very large, making control at low temperatures very difficult.

The heated freezer circuit with fixed duty cycle is less than $10.00, and 12.5 ft² of Low E glass costs about $5.00 less than custom TCO glass. That means the door is $5.00 more expensive to build, yet it saves $17.44 per year in operating cost. This is a 4-month payback. The added benefit is enhanced safety. If the heated glass breaks, the circuit will shut down all the power, eliminating a potential shock hazard from exposed conductors.

Heated Glass Shelves

Figure 3. Heated Shelf and Controller

One new product that has come out of these technologies is a heated glass shelf for foodservice appliances (U.S. Patent No. 6,111,224 Hatco Corporation; see Figure 3). This shelf is built with a proprietary edge material, is differentially heated, and includes a controller to allow the user to determine the temperature.

In this case, the TCO coated glass is on the bottom, and the top piece is clear glass, with both lites being fully tempered for safety. By putting the power into the bottom pane (which is separated from the top pane by 3/8 in (10 mm), a higher temperature is achieved on the bottom than on the top (see Figure 4). This is desirable, as these heated glass shelves are typically used in the center position of a multi-shelf cabinet, where the base is typically a heated stainless steel piece, and the top has ribbon heaters and lights.

The differential heat means that the hotter bottom lite will radiate heat downward to the foods placed on the base, and the top lite will run somewhat cooler to avoid damaging food containers and causing a possible burn hazard.

Figure 4. Heated Shelf Glass Surface Temperatures @ 50-Percent Power (CLICK to see larger image)

In practice, the differential can be greater than 100°F, with the maximum temperature limit of the bottom pane being 350°F. A typical deli cabinet may require up to 450 W/ft² to achieve these temperatures, making this a much higher power application than freezer doors or windows. This requires more attention to wire size, heat sinking of the triac, conductivity of bus bars, etc.

Because of the uniformity of the TCO coating, the heated glass provides a much more uniform source of heat than a standard heating element (see Figures 5 and 6).

Figure 5. Standard Heating Element.		Figure 6. Heated Glass Shelf

From a merchandising standpoint, glass shelves improve the visibility of the food items, are more sanitary, and can be made in a much smaller cross section. This has become a new concept in hot food merchandising.

Low Temperature Burner/Warmer

Another new application of thin film glass heaters is the heat source for low-temperature burners and warmers. A glass-based heater replaces an embedded coil, providing a cost savings, a more uniform radiation pattern, and better control at low temperatures. In some designs, less wattage is required to achieve the same cooking surface temperature.

For low-temperature operation, a serpentine coil in an insulating form is a low-efficiency radiator, as the coil is operating at a low temperature, and the radiation of energy to the ceramic top is low. Using a glass-based resistance heater, the radiation transfer is quite efficient, as the glass blackbody surface is disposed parallel to the ceramic surface, which is also a blackbody.

In this instance, a high-resistance coating has been used, with a controller to allow for variable power. This approach results in a self-limiting temperature, as less than 100 W are dissipated, so there is no need for a high-temperature thermal limiter.

Summary

Thin film based heaters in general are very reliable, as the energy is not concentrated in a small area with corresponding high temperatures, but is evenly diffused over a large area. This means that there are no stressed areas that can fail due to high-temperature swings, high-power densities, and other stressful conditions.

The uniform radiation, especially at moderate temperatures, is one of the strengths of this technology, often requiring less overall energy for equivalent performance.

Using common tempered soda-lime glass, the upper temperature limit of the heated glass is about 350°F (176°C), due to the thermal stress and coefficient of thermal expansion of that glass type. Also under development are coatings on borosilicate glass and different ceramic materials to explore higher temperature applications.

The author wishes to acknowledge Dillon Ashton, chief engineer, Preferred Power Inc., Toledo, OH, U.S., for his assistance with this paper.