issue: October 2003 APPLIANCE Magazine
Engineering Heated Glass
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
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
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 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.
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).
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
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
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
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).
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.
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
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
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
(CLICK to see larger
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).
5. Standard Heating Element.
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.
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 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.
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