issue: July 2003 APPLIANCE Magazine
Next-Generation Heated Glass Products
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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
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.
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.
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.
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
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
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.
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
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
- 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).
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
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.
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
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
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.
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.
4. Heated Shelf Glass Surface Temperatures @ 50-Percent Power
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
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).
Standard Heating Element.
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.
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
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
The uniform radiation, especially at moderate temperatures, is one of the
strengths of this technology, often requiring less overall energy for equivalent
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.
Gerhardinger is vice president of Technology, Engineered Glass Products,
a position he has held for 5 years. Prior to that, Mr. Gerhardinger
spent 20 years at Pilkington LOF in various technical and managerial
assignments. He has an undergraduate degree in Electrical Engineering
from the University of Toledo and holds five patents.