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Technical Information
Click on the section of interest:
Frequently Asked Questions on
Thermoelectrics
1. How does a thermoelectric module work?
Thermoelectric modules are solid-state heat pumps that operate on the
Peltier effect (see definitions). A thermoelectric module consists of an
array of p- and n- type semiconductor elements heavily doped with
electrical carriers. The array of elements is soldered so that it is
electrically connected in series and thermally connected in parallel. This
array is then affixed to two ceramic substrates, one on each side of the
elements (Figure 1). Let's examine how the heat transfer occurs as
electrons flow through one pair of n- and p- type elements (often referred
to as a "couple") within the thermoelectric module:
Electrons can travel freely in the copper conductors but not so freely in
the semiconductor. As the electrons leave the copper and enter the
hot-side of the p-type, they must fill a "hole" in order to move
through the p-type. When the electrons fill a hole, they drop down to a
lower energy level and release heat in the process. Essentially the holes
in the p-type are moving from the cold side to the hot side. Then, as the
electrons move from the p-type into the copper conductor on the cold side,
the electrons are bumped back to a higher energy level and absorb heat in
the process. Next, the electrons move freely through the copper until they
reach the cold side of the n-type semiconductor. When the electrons move
into the n-type, they must bump up an energy level in order to move
through the semiconductor. Heat is absorbed when this occurs. Finally,
when the electrons leave the hot-side of the n-type, they can move freely
in the copper. They drop down to a lower energy level and release heat in
the process.
In summary, heat is always absorbed at the cold side of the n- and p- type
elements. The electrical charge carriers (holes in the p-type; electrons
in the n-type) always travel from the cold side to the hot side, and heat
is always released at the hot side of thermoelectric element. The heat
pumping capacity of a module is proportional to the current and is
dependent on the element geometry, number of couples, and material
properties.
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2. What is the mathematical equation for describing the
operation of a thermoelectric module?
| Figure 2 represents a thermoelectric couple. It
shows some terms used in the mathematical equation: |
| L = element height |
A = cross-sectional area |
Qc = heat load |
| Tc = cold-side temperature |
Th = hot-side temperature |
I = applied current |
| Additionally, there is the following: |
| S = Seebeck coefficient |
R = electrical resistivity |
K = thermal conductivity |
| V = voltage |
N = number of couples |
|
| Here are the basic equations: Qc = 2 * N* [S * I *
Tc -1/2 * I^2 * R * L/A K * A/L * (Th Tc)] V = 2 * N * [S *
(Th Tc) + I * R * L/A] |
These equations are very simplified and are meant to show the basic idea
behind the calculations that are involved. The actual differential
equations do not have a closed-form solution because S, R, and K are
temperature dependent. Unfortunately, assuming constant properties can
lead to significant errors.
TE Technology uses special, proprietary modeling software which takes into
account the temperature dependency of the thermoelectric material
properties as well as all the relevant design aspects of the overall
system. The software uses material property data from actual test results
on thermoelectric modules, so it yields highly accurate results. When we
build a custom cooler for your application, that high accuracy means you
generally only need one prototype to verify cooling performance.
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3. What are the advantages of a thermoelectric unit
over a compressor?
Thermoelectric modules have no moving parts and do not require the use of
chlorofluorocarbons. Therefore they are inherently reliable and are
virtually maintenance free. They can be operated in any orientation and
are ideal for cooling devices that may be sensitive to mechanical
vibration. Their compact size also makes them ideal for applications that
are size or weight limited where even the smallest compressor would have
excess capacity. Their ability to heat and cool lends them to applications
where both heating and cooling is necessary or where precise temperature
control is critical.
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4. What industries does thermoelectrics serve?
Thermoelectric coolers are used for the most demanding industries such as
medical, laboratory, aerospace, semiconductor, telecom, industrial, and
consumer. Uses range from simple food and beverage coolers for an
afternoon picnic to extremely sophisticated temperature control systems in
missiles and space vehicles.
A thermoelectric cooler permits lowering the temperature of an object
below ambient as well as stabilizing the temperature of objects above
ambient temperatures. A thermoelectric cooler is different from a heat
sink because it provides active cooling unlike a heat sink which provides
only passive cooling.
Thermoelectric coolers can be used for applications that require heat
removal ranging from milli-watts up to several thousand watts. However,
there is a general axiom in thermoelectrics: the smaller the better. A
thermoelectric cooler makes the most sense when used in applications where
even the smallest vapor compressor system would provide much more cooling
than necessary. Consequently, a thermoelectric cooler can provide a
solution that is smaller, weighs less, and is more reliable than a
comparatively small compressor system.
However, the trend in recent years has been for larger and larger
thermoelectric systems. As power supplies become less expensive this has
driven the cost of a complete thermoelectric system (cooler, power supply,
and temperature controller) lower so higher power systems are now more
marketable. Systems with capacities in the 200-400 watt range are becoming
more common, although they are still not nearly as common as smaller
systems where the cooling capacity is below 100 watts.
Large thermoelectric systems in the kilowatt range have been built for
specialized applications such as cooling within submarines and railroad
cars or cooling process baths in specialized areas such as in
semiconductor manufacturing. In cases where thermoelectric coolers are
used for such large applications there generally has been a good reason
why a vapor compressor system has not been used (for example, vibration
needs to minimized, precision temperature control is required). In which
case, the extra cost and higher power consumption of the thermoelectric
cooler can be justified.
Typical applications for thermoelectric coolers include:
Laser diodes
Laboratory instruments
Temperature baths
Electronic enclosures
Refrigerators
Telecommunications equipment
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5. What is the efficiency of a thermoelectric module?
Technically, the word efficiency relates to the ratio of the amount of
work one gets out of a machine to the amount of power input. In heat
pumping applications, this term is rarely used because it is possible to
remove more heat than the amount of power input it takes to move that
heat. In which case, the "efficiency" would be greater than
unity (implying a perpetual-motion machine!). For thermoelectric modules,
it is standard to use the term "coefficient of performance"
rather than "efficiency." The coefficient of performance (COP)
is the amount of heat pumped divided by the amount of supplied electrical
power.
The COP depends on the heat load, input power, and the required
temperature differential. Typically, the COP is between 0.3 and 0.7 for
single-stage applications. However, COPs greater than 1.0 can be achieved
especially when the module is pumping against a positive temperature
difference (that is, when the module is removing heat from an object that
is warmer than the ambient). Figure 3 shows a normalized graph of COP
versus I/Imax (the ratio of input current to the module's Imax
specification). Each line corresponds with a constant DT/DTmax (the ratio
of the required temperature difference to the module's DTmax
specification).
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6. I want to make my own cooling assembly. How do I
select the right module for my system?
Other module software programs we've seen base performance and
recommendations on certain assumptions that can lead to significant
errors. Our module selector program doesn't make any assumptions about
your system design ‹recommendations are based on the modules' operating
temperatures, heat load, and DTmax. This makes for a more accurate
selection process since you know what assumptions are being made. Be aware
that proper module selection is an iterative process that does take time
and research. If you do not want to spend the time and expense of
selecting your own module, designing your own system, having the necessary
skilled labor to assemble it, etc. then we have a highly recommended
alternative: standard (or custom) cooling assemblies. All of the hard work
is taken out when you purchase an assembly from TE Technology. You can use
our assembly selector program at www.tetech.com/design.
However, if you are certain that you want to make your own cooling
assembly, here is a brief description of what's involved:
First you must define your operating temperatures and how much heat you
need to remove. Based on these parameters the Module Selector program will
help you select a module for lowest power consumption, for smallest size,
or a combination of the two. (Again, see www.tetech.com/module
for further information.)
Next, you analyze your thermal system based on the size and operating
voltage and current for the selected module. In this step, you are making
sure the operating temperatures and heat load you used to select the
module are realistic. If the analysis shows that your numbers were
realistic, then you are finished. Otherwise, you must enter a new heat
load and operating temperatures and iterate the process until the module
you select meets your final requirements.
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7. How reliable are thermoelectric systems?
Thermoelectric systems are highly reliable provided they are installed and
used in an appropriate manner. The specific reliability of thermoelectric
coolers tends to be difficult to define because failure rates are highly
dependent upon the particular application. Thermoelectric modules that are
at steady state (constant power, heat load, temperature, etc.) can have
mean time between failures (MTBFs) in excess of 200,000 hours. However,
applications involving thermal cycling show significantly worse MTBFs,
especially when TE coolers are cycled up to a high temperature. With
thermal cycling, a more appropriate measure of reliability isn't time but
rather number of cycles.
All materials expand or contract as they are heated or cooled. Different
materials will expand at different rates. The rate of expansion is given
by the material property called the coefficient of thermal expansion (CTE).
This means that as the cold side of a module gets colder, it will shrink,
and as the hot side gets hotter, it will expand. This flexes the
thermoelectric elements and their solder junctions. Furthermore, because
the module is constructed of several different materials, there is added
stress simply because the materials themselves are expanding/contracting
at different rates. After repeated thermal cycling the solder junctions
within the module fatigue, and the electrical resistance increases.
Cooling performance is reduced, and eventually the module becomes
inoperable. The "failure point" is thus a function of operating
temperature, the amount of temperature cycling, and how much degradation
the particular system can tolerate before performance becomes
unacceptable. All thermoelectric modules (regardless of manufacturer)
experience the same stresses of operation, but how they tolerate these
stresses is a question of build quality‹selecting a manufacturer with
good, strong solder junctions is a must! (Of course, we take special care
in ensuring that our modules have the highest quality solder junctions.)
A similar phenomenon occurs when a module is soldered or epoxied to a heat
sink. The "zero-tension" point (that is, the point where there
is no internal stress resulting from mismatches in CTE) will freeze
between the ceramic substrate and the heat sink when the solder or epoxy
becomes rigid at some temperature which is typically different from the
operating temperature. In other words, the module is pre-stressed when the
module and solder cool back down to room temperature (assuming the module
is soldered to a heat sink).
As the assembly is thermally cycled, not only does the module itself
undergo fatigue stress, the bond line between the module and heat sink is
also stressed. Again, different materials will expand at different rates.
The heat sink, the solder (or epoxy), and the module will expand
differently. This can be particularly troublesome because the bond could
potentially fail at local spots. The module could overheat at these local
spots which would exacerbate the problem. This is why we do not recommend
soldering (or epoxy-ing) the module to its heat sink. If you do solder (or
epoxy) the modules, we recommend that you thermal cycle the compete
assembly to make sure you get adequate lifetimes.
TE Technology does not publish thermoelectric cooler reliability data for
general use. Reliability data is only valid for the conditions under which
a test was conducted, and it is not necessarily applicable to other
configurations. There are numerous application parameters and conditions
that will affect reliability; cooler assembly, mounting methods, power
supply and temperature control systems and techniques, temperature
profiles are just a few factors that can combine to produce failure rates
ranging from extremely low to very high. The "failure point" is
always a function of operating temperatures, the amount of temperature
cycling, and how much degradation the particular system can tolerate
before performance is unacceptable. It is specific to each application.
There can also be tradeoffs between a cooler¹s thermal performance, the
cost to manufacture the cooler, and the reliability with respect to
thermal cycling or other factors. For example, our line of standard
cooling assemblies is optimized for our typical customer ‹these
customers are not using the system under repeated thermal cycling and thus
so not want to pay (in cost or performance) for a cooler optimized for
thermal cycling.
Please feel free to contact us if your application involves thermal
cycling. Perhaps we can provide non-proprietary test results that to some
extent might be applicable; if not we can help you with a testing program
so you have the data for making a determination of how suitable the
cooling system will be in your application. To assess the true
reliability, we recommend that all cooling systems be tested in their
actual application.
Below are just a few comments that address general trends with respect to
reliability:
a) Thermoelectric modules exhibit relatively high mechanical strength in
compression mode but comparatively low tensile and shear strength.
Consequently, a TE module should not be used to support weight that would
subject it to tension or shear stress in particular. Furthermore, in
applications where shock and vibration will be present, a thermoelectric
module should be clamped between two plates as opposed to using solder or
epoxy to secure the module to its heat sink. When properly mounted,
thermoelectric modules have successfully met the shock and vibration
requirements of aerospace, military, and similar environments. In
addition, our potting provides increased mechanical strength. In fact, our
potting was originally developed to allow modules to survive ballistic
missile launch stresses.
Similarly, when using multiple modules in an assembly they should share
common heights to within 0.025 mm. Otherwise, uneven clamping forces could
crack a module.
b) Moisture should not be allowed to enter the inside of a thermoelectric
module in order to prevent both a reduction in cooling performance and the
possible corrosion of module materials.
c) An application that will involve large temperature changes or thermal
cycling can induce thermal fatigue stress. Again, thermoelectric modules
should not be installed using solder or epoxy. Such mounting methods can
cause stress concentrations because of the various mismatches in
coefficients of thermal expansion. We strongly recommend that modules be
mounted by clamping (compression) and using thermal grease or a flexible
mounting material such as graphite foil as the interface between module
and plate. In any case, rigid mounting is not recommended at all for
modules larger than approximately 15mm square.
To minimize the impact of thermal cycling, minimize the temperature range
of the cycle and minimize the number of thermal cycles. If thermal cycling
is a must, you should choose a physically small module with a large pellet
footprint. (The pellet is the thermoelectric element used in the module.
In the module part number, the second number defines the width of each
pellet, in mm, which in turn determines the pellet footprint.) In summary,
the smaller the module size the more reliable it tends to be, and the
larger the pellet footprint, the more reliable it tends to be. Also,
modules can also be customized to better handle thermal cycling if
required.
d) Temperature control methods also have an impact on thermoelectric
module reliability. Linear or pulse-width-modulated (frequency at least
400 Hz) control should always be chosen over ON/OFF control to ensure
better reliability. The ON/OFF type of controller basically causes thermal
cycling and so should be avoided.
e) Exposure to high temperatures should be minimized as much as possible
to extend reliability. Standard modules are rated for a maximum of 80 °C.
High-temperature modules are rated for 150 °C. There are also 200 °C
modules. However, these temperature limits are somewhat arbitrary. All
modules, regardless of manufacturer, will be affected by operation at high
temperatures. Some, of course, are more resistant to changes than others
though.
The module is constructed with nickel-plated copper conductors to
electrically connect the thermoelectric pellets to each other. The nickel
plating serves as a diffusion barrier to the copper. The copper has a
tendency to diffuse into the thermoelectric material. The copper would
then degrade the performance of the material. Unfortunately, the nickel is
not a perfect barrier, and copper atoms will still diffuse albeit at a
much lower rate than if there were no nickel barrier at all. The rate of
diffusion typically increases exponentially with temperature. So, the
higher the operating temperature, the more quickly will diffusion occur
and the corresponding degradation in performance. However, in particular
with the 80 °C module, at 85 °C, solder constituents can begin migrating
along cleavage plains of the thermoelectric material due to a theorized
minor eutectic reaction. This leads to a mechanically weak solder joint
and physical expansion of the pellet.
The temperature ratings for the modules are derived from their
construction technique. The 80 °C module uses solder that melts at 140 °C.
It has excellent electrical contacts. The 150 °C module uses solder that
melts at 183 °C. It has an extra nickel diffusion barrier at the ends of
each pellet, so there is an increase in electrical resistance. The 200 °C
module also has two nickel barriers, but it uses a non-eutectic solder
with a liquidus point at 222 °C and solidus point at 183 °C. As such,
extreme caution should be taken when using the 200 °C module at
temperatures above 183 °C.
f) Additional information can be found by downloading publications
concerning reliability here.
g) Not all thermoelectric modules are made with the same quality!
Different manufacturers have different techniques, and we have seen widely
varying quality when comparing modules of equivalent size and capacity
from a variety of manufacturers. Improper soldering, improper
metallization of the ceramics, and improper nickel plating are just a few
of the potential problems that can reduce reliability. Be careful when
selecting your module vendor!
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8. Does TE Technology do contract manufacturing?
TE Technology does contract manufacturing for companies who have an
existing thermoelectric design and would like to find a company to
manufacture their part. We have in house state of the art machining
capabilities along with a complete environmental control test department.
When companies add up the costs of the thermoelectric engineers, assembly
workers, inventory, and manufacturing floor space along with the costs of
designing, maintaining, and calibrating the required thermoelectric test
equipment, they find this is more expensive than the raw materials
themselves. Through outsourcing, these customers reduce their overhead
expenses while benefiting from our consistently excellent build quality.
No matter how small or big your production levels are, if you would like
to explore this option please send us the specifications of your
thermoelectric cooling assembly with the quantities you require, and we
will be happy to quote.
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9. Should I use thermoelectrics for a heater?
If you are strictly heating, then a resistive heater will most likely be a
less expensive initial investment than a thermoelectric heater. However, a
thermoelectric heater can have a higher COP (see definitions for COP) than
a resistive heater, so in the long-term, a thermoelectric heater might be
less expensive than a resistive heater. As it is with cooling, the heating
COP is dependent on the temperature difference. That is, the COP decreases
(assuming constant power input) as the temperature difference increases.
This needs to be kept in mind when evaluating whether a thermoelectric
heater would be appropriate.
Also, a thermoelectric heater will typically provide more responsive
temperature control since it can, if necessary, provide active cooling as
well. Please see question 5 though for comments regarding temperature
cycling.
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10. How big or small can a thermoelectric cooler be?
There are practical limits to the individual sizes of a module or cooling
assembly. Micro-modules, for example, are more expensive to produce
because they are less suitable for automated processing. For larger
modules, coefficients of thermal expansion and costs tend to limit
thermoelectric modules to within a certain physical footprint.
For cooling assemblies, the minimum size might be limited by the minimum
requirements needed to provide sufficient heat sinking. The maximum size
is limited by the requirements of the mounting plates. If the plates get
too large, then it becomes too difficult to maintain sufficient surface
flatness. Generally, when more cooling capacity is required than what the
typically largest size cooler can provide, multiple coolers are used
rather than using one giant cooler. Approximately speaking, the largest
individual cooler has a footprint of approximately 254 mm x 177 mm, such
as our standard CP-218. There are always exceptions though; these are just
general guidelines.
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11. What is the best way to power a thermoelectric
cooler?
a) Ideally, thermoelectric coolers should operate on purely direct current
for the best performance. However, a ripple factor of 10% will only result
in 1% degradation in temperature difference. Most power supplies have
better filtering than that, so ripple is not likely to be a concern.
b) Care should be taken not to overpower the cooler. Overpowering the
cooler could lead to inadvertently exceeding the temperature ratings and
causing damage to the cooler.
c) The input power for maximum efficiency of a cooler does not correspond
to its maximum operating voltage and current for (Vmax and Imax). When
maximum efficiency is desired, the applied power is typically 1/3 to 2/3
of its true maximum power rating.
d) If a temperature controller is used, it should be of the linear type or
the pulse-width-modulated (frequency at least 400 Hz) type to minimize any
detrimental effects of temperature cycling.
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12. How precisely can a thermoelectric cooler maintain
temperature?
There are many factors that contribute to or detract from the overall
system stability. However, a thermoelectric cooler can provide a very high
degree of temperature stability because the amount of cooling it provides
is proportional to the applied current. One of our customers has reported
stability to within +/-0.0003 °C. Achieving that level of stability
requires considerable effort though. The answer to this question is
ultimately a function of the (1) the controller and its resolution, (2)
the response time of the specific cooling assembly, and (3) the response
time of the object being cooled.
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13. What temperature ranges can a thermoelectric
cooler achieve?
The vast majority of applications involve temperature differences of less
than 60 °C across the TE module, and less than 45°C from the cooled
object to ambient. One application involved cooling down to 145 K.
However, that requires a very special effort to achieve that. In any case,
the temperature range will depend on a variety of factors, principally on
the number of stages. By stacking modules one on top of another, each
module, or stage, acts like an electronic heat sink for the module above
it. As the number of stages increases, the achievable temperature
difference also increases, but the heat pumping capacity decreases.
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14. What ambient temperature environments do
thermoelectric coolers withstand?
The maximum ambient temperature will depend on the desired reliability,
the heat sink, how much heat is being dissipated, and the temperature
rating for the module or other system components (such as fans and
insulating materials). See question 5 for further details.
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15. How do I determine if thermoelectric cooling is
best for my application?
Thermoelectric cooling is ideal for very small cooling systems.
Thermoelectrics are also ideal when both heating and cooling is needed and
when precision temperature control is required. Thermoelectric systems are
also ideal for aerospace applications because the cooler can be mounted in
any orientation and still function properly. However, as the heat load, in
particular, increases, the advantages that thermoelectric cooling offer in
comparison to compressor systems diminishes. When evaluating on the basis
of heat load alone, a compressor system will likely be more cost effective
when the heat load is greater than approximately 250 W.
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16. Why should I have TE Technology manufacture a
system for my application?
TE Technology has technical expertise in all relevant disciplines
applicable to thermoelectrics. Over forty years of thermoelectric
experience go into every product. In addition, we have specialized test
equipment unique to the thermoelectric industry that enables quick
(inexpensive) and accurate test results on 100% of our products (click
here for more information). We provide reliable, durable,
cost-effective systems, and we provide them on-time. Our large inventory,
state of art machining and vast global resources provide added versatility
from prototype to production manufacturing.
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17. What type of testing does TE Technology recommend?
TE Technology recommends that all products be tested under
"worst-case" conditions in their actual or simulated
application. We want our customers to feel comfortable that the cooling
system will meet all of their suitability and reliability requirements.
While we cannot tell our customers whether certain products may be
suitable or reliable for their specific requirements, we can and do test
products and collect data so customers can make informed decisions. TE
Technology possesses extensive testing equipment including:
temperature-controlled chambers; high humidity enclosures; thermal cycling
equipment; temperature measurement equipment; and, thermoelectric testers.
TE Technology offers its valuable testing services so your company does
not have to "reinvent the wheel". Further, we can assist our
customers in designing customized testing experiments for the products.
Just give us a call, and we will be happy to discuss our various testing
services and costs.
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18. What kind of over temperature protection do I
need?
If a cooling assembly is being purchased, we also recommend that
over/under-temperature protection be utilized to minimize potential damage
to the coolers during operation. This can happen if the liquid (in a
liquid cooler) is allowed to freeze, or if the cooling medium (air,
liquid, etc.) is reduced and the cooler becomes overheated. Some customers
use our standard temperature controllers, such as the TC-24-25 series,
which have over-temperature protection circuitry that may reduce the
likelihood of such situations occurring. Other customers choose to
incorporate this protective circuitry into the power supply. Of course, we
at TE Technology are happy to assist our customers in choosing the type of
protection which may be most effective for their systems. Please note that
TE Technology coolers will not be equipped with over/under temperature
protection, unless otherwise specified. If it is not specified, it is the
customer's responsibility to provide this protection, or to request that
over/under temperature protection be included. We have designed and
integrated many of these safeguards into the products at our facility.
Simply contact us to discuss your options.
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19. How do Pulse-Width Modulated (PWM) controllers
such as the TC-24-25 and TC-24-10 operate?
The TC-24-25 and TC-24-10 series controllers will operate with input
voltages of 12-24 volts (a minimum of 12 V input is needed to operate the
on-board microprocessor). These controllers are pulse-width modulated (PWM)
devices, meaning that the power is switched either fully "ON" or
fully "OFF" hundreds of times per second. Varying the ratio of
"ON" time to "OFF" time then regulates the amount of
cooling. Said a different way, the output of the controller will be a
square wave with the duty cycle (the "ON" portion of the
waveform) being varied as necessary to achieve the desired cooling in the
thermoelectric device. The "ON" and "OFF" pulses occur
so rapidly that the module is not thermally cycled with each pulse. Thus,
these controllers do not degrade the reliability of a module from thermal
cycling in the same way that a thermostatic or "ON-OFF"
controller would.
The input voltage to the temperature controller will define the output
voltage when the controller is full "ON"--there is not a linear
output voltage that increases as more cooling is required. One should
therefore choose an input voltage that is no greater than the Vmax of the
cooling assembly or thermoelectric module(s). If you are making your own
cooling system from thermoelectric modules, the maximum operating voltage
(the controller's input voltage) is usually no more than 75% of module's
Vmax. Of course, if you wire multiple modules in series or in a
series-parallel combination, Vmax of the module system will be the Vmax of
each module multiplied by the number of modules in series. In this case,
the input voltage is generally no more than 75% of the module system.
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20. What are some considerations for using a liquid
chiller?
TE Technology's standard liquid coolers have been designed for cooling
water and inert gasses. This style of exchanger is ideal for low cost and
high performance. It allows for a larger number of flow passages than
could otherwise be obtained with exchangers that use a single serpentine
tube pressed into a plate.
There are some special considerations when using this style of exchanger.
Any fluid you use in the coolers will be in contact with anodized
aluminum, copper, and the epoxy that is used to bond in the copper tubes.
Certain fluids, additives, and corrosion inhibitors will erode the epoxy
and corrode the metal surfaces. Therefore, if you plan to use any other
fluids and/or additives, you should thoroughly test the unit under actual
operating conditions and temperatures before designing it into your
product to make sure it will not be damaged. It should be noted that
corrosion of the metal surfaces can be detrimental not only to heat
transfer but also to other components in the system. For example, cooling
saltwater in a marine aquarium may cause copper to be introduced into the
water. This might harm or even kill the fish, so this type of liquid
cooler is not recommended for this application. In any case, you should
test the cooler to verify its suitability for the application.
On a related note, the standard liquid coolers are pressure tested to 410
kPa (60 psi). However, it is recommended that operating pressures not
exceed 205 kPa (30 psi). This should be kept in mind should you
inadvertently cool to below the freeze point of the water since water will
expand as it freezes and this can potentially break epoxy joints or burst
the copper tubing itself. You might also need to consider shipping and
storage temperatures. If the cooler is not drained prior to storage or
shipping, freezing could occur and damage could result. Again though, if
you use an additive to depress the standard freeze point of water (or some
other liquid), the additive should be tested for compatibility.
Thermal cycling can also potentially cause problems with the exchanger (as
well as with the thermoelectric modules, which is addressed in a separate
FAQ). The aluminum, epoxy, and copper all have different coefficients of
thermal expansion. Consequently, rapid changes in temperature can induce a
thermal fatigue stress that can result in leaks.
TE Technology can replace the standard liquid exchanger in the cooling
assembly with a liquid exchanger in which the liquid would be in contact
with only one material. We can offer exchangers that have a single-piece
stainless steel serpentine tube pressed into an aluminum plate. These
exchangers can be attached to some of our standard cold plates,
effectively turning them into a liquid chiller. Also, as a custom device,
the epoxy-bonded copper tubes in our standard liquid exchanger can be
replaced with welded-on aluminum end caps and thread-in fittings for the
fluid inlet and outlet. This technique removes epoxy compatibility issues
and thermal cycling issues from the exchanger considerations. TE
Technology has also manufactured folded-fin liquid exchangers and liquid
exchangers machined from a solid block of material such as stainless steel
or copper. If you are interested in custom devices please contact the
factory.
Lastly, the standard performance rating for the liquid chillers is based
on the assumption that water is flowing at 1.6 L/min (25 gph). Performance
will change if a different fluid is used and/or a different flow rate is
used. Consult with TE Technology, and we can determine the performance
under different operating conditions for you.
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21. What is the manufacturing test process for all
cooling assemblies at TE Technology?
TE Technology performs numerous tests at the component and system level to
ensure the quality and consistency of the thermoelectric cooling systems
we manufacture. Each step is a link in a chain of quality that has been
developed from years of experience in making tens of thousands of cooling
assemblies.
The process starts by testing 100% of the thermoelectric (TE) modules for
their thermoelectric properties. Each module is tested on our own,
custom-made thermoelectric testing system. This system measures the
thermoelectric material properties: electrical resistivity, thermal
conductivity, Seebeck coefficient, and figure of merit. These measurements
ensure that the semiconductors used in the modules provide consistent
thermal and electrical properties when used in a cooling assembly. The
system also checks the AC-resistance of the entire module. This check is
important as it confirms that the solder connections within a module are
not damaged. For example, a typical 127 couple module contains 254
thermoelectric elements and 508 solder junctions. If any one of these
solder junctions breaks then the entire module will be useless.
Furthermore, if more than one module is wired in series then all of the
modules wired in that series will be useless, too. It is important to
remember that having a "dead" module in the system is much worse
than if it were not there at all. Not only will the dead modules fail to
provide any useful cooling, they will also provide a path for heat leakage
from the hot side of the cooling assembly back to the cold side.
Next, the components of the cooling assembly are checked to make sure they
have the physical characteristics necessary to effectively transfer heat
from the cold sink, through the TE module, and then into the heat sink. To
accomplish this the physical parameters of the heat exchangers and the TE
modules are inspected. The surfaces of the heat exchangers are measured
for flatness and surface finish in the areas that contact the TE modules.
If more than one module will be used in a cooling assembly, the module
heights are matched so no more than 0.025mm of height variation exists
between them. The modules are also checked to ensure the ceramic
substrates are flat and parallel within specification.
Thus far in the process the components have been inspected to ensure all
of the components are of sufficient quality to be used in the assembly.
However, this alone does not guarantee that a good cooling assembly will
result. There are still many problems that can arise during the assembly
process. The three main concerns and their test solutions are as follows:
- One or more of the TE modules is inadvertently placed upside down in
the cooler: TE modules invariably have the wires connected to the hot
side of the module. Without powering the module this is the only way
you can tell the hot side from the cold side of a module. When modules
are being wired into a harness it is possible to inadvertently flip a
module so that it heats instead of cools. This becomes easier to do if
the module is sealed with an epoxy encapsulent and as it gets thinner
and it's thickness approaches that of the lead wire thickness.
Therefore, as the assembly is made the modules are placed on the heat
sink and briefly powered with a low current. The assembler then
verifies that the cooling sides of the modules are all in the correct
position by touching each module and making sure it is operating in
the cooling mode and not in the heating mode.
- A TE module's wire has shorted to the heat sink or cold sink: If an
excess ball of solder or a strand of wire contacts the heat sink or
cold sink, the voltage supplied to the thermoelectrics can be shorted
to the metal surfaces of the cooler thus causing a potentially
dangerous condition for anyone touching the device while it is
powered. TE Technology verifies there are no electrical short circuits
by measuring the high-potential (hi-pot) resistance between the
module's wiring and the exposed metal surfaces.
- Inadequate thermal interfaces: Consider a typical cooling assembly
where the cold sink, TE modules, and heat sink are all clamped
together using screws. These screws are designed to apply the
necessary compressive forces on the components. The screws are torqued
to a specific level that, in turn, translates to a specific
compressive force on the module allowing intimate thermal contact
between the heat sinks and the TE modules. If there is a burr in any
of the tapped holes, if there is a deformed thread on the screw, if
the screw is too long or the tapped hole too short the torque will not
translate into the proper compression force. If there is a spec of
dirt or piece of hair hidden by the thermal grease the thermal
interface will be ruined. Inspection for this problem is nearly
impossible; especially since most often a vapor-sealing gasket
surrounds the perimeter of the modules. TE Technology has developed a
unique thermal junction quality test to combat this problem. Using the
aforementioned thermoelectric test equipment, a small current is
applied to the thermoelectric modules and a temperature difference
between the heat sink and the cold sink is created. Then, the current
is switched off and the temperature difference is allowed to decay.
The TE modules act as small power generators during the decay, so by
monitoring the corresponding rate of voltage decay the quality of the
thermal interfaces within the assembly can be measured. The AC
resistance of the cooler is also checked to make sure the solder
junctions within the modules have not been damaged during the assembly
process.
These tests take only minutes to complete and are done on 100% of the
assemblies made at TE Technology. Because the thermal interface test is so
fast it costs much less then a full performance test, which is the only
other way to verify the thermal junctions in an assembly.
In summary, the following tests are performed for every assembly:
- The semiconductor properties are checked for every module.
- The AC resistance is checked on every to make sure the solder
connections within the modules are not damaged.
- The physical dimensions and finishes are checked for all components.
- The modules are checked for proper wiring polarity/orientation
during assembly.
- The high-potential (hi-pot) resistance between the module's wiring
and the exposed metal surfaces is tested to verify there are no
electrical short circuits.
- The thermal interfaces are verified so proper heat transfer is
guaranteed.
- The AC resistance of each completed assembly is checked to verify
the solder connections within the modules have not been damaged during
assembly.
Thus, by following this chain of steps TE Technology can ensure consistent
performance for every cooling assembly we make.
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Terms and Definitions
| Ambient Temperature: |
Temperature of the air or environment surrounding a
thermoelectric cooling system; sometimes called room temperature. |
| Active Heat Load: |
The amount of heat being generated by something regardless of
whether a temperature difference exists. For example, this could
be the waste heat from a powered electronic device. Typically,
this is the input power of the device (voltage * current) minus
any output power. Another example is the heat produced by an
exothermic chemical reaction. See also "Passive Heat
Load." |
| AC-Resistance (ACR): |
The electrical resistance of a thermoelectric module. The
"AC" refers to alternating current and serves as a
reminder that measuring with a typical ohm-meter (which uses a DC
signal) will yield erroneous results. Actually, even an AC
ohm-meter can also yield erroneous results (although not as severe
errors compared with typical ohm-meters). Therefore, TE Technology
uses specially designed test equipment to accurately measure this
parameter. |
| Btu (British Thermal Unit): |
The amount of heat required to raise one pound of water by one
degree Fahrenheit at a standard temperature of 39.2 °F and at one
atmosphere pressure. 1 Btu = 1055 J. |
| CFM (Cubic Feet per Minute): |
The volumetric flow rate of a gas, typically air, expressed in
the English system of units. This generally refers to the amount
of air passing through the fins of a forced convection heat sink. |
| COP (Coefficient of Performance): |
COP is the ratio of the heat removed (or added, in the case of
heating) divided by the input power. |
| DTmax: |
The maximum obtainable temperature difference between the cold
and hot side of a thermoelectric module when Imax is applied and
there is no heat load applied to the module. This parameter is
measured at a hot-side temperature 300 K. In reality, it is
virtually impossible to remove all sources of heat in order to
achieve the true DTmax. Therefore, the number only serves as a
standardized indicator of the cooling capability of a
thermoelectric module. |
| Electrical Resistivity: |
Electrical resistivity relates the amount of current an object
will transmit through its volume caused by a voltage difference
across that volume. Typical unit is ohm * m. Electrical
resistivity is an intrinsic property of a material. When
multiplied by the length of an object and divided by the cross
sectional area of an object it yields the electrical resistance of
the object. |
| Heat Pumping: |
The amount of heat that a thermoelectric device is capable of
removing or "pumping" at a given set of operating
parameters. |
| Heat Sink/Cold Sink: |
A heat sink is a device that is attached to the hot side of
thermoelectric module. It is used to facilitate the transfer of
heat from the hot side of the module to the ambient. A cold sink
is attached to the cold of the module. It is used to facilitate
heat transfer from whatever is being cooled (liquid, gas, solid
object) to the cold side of the module. The most common heat sink
(or cold sink) is an aluminum plate that has fins attached to it.
A fan is used to move ambient air through the heat sink to pick up
heat from the module. Another style uses a plate with tubing
embedded in it. A liquid is sent through the tubing to pick up
heat from the module. |
| Imax: |
The current that produces DTmax when the hot-side of the
thermoelectric module is held at 300 K. |
| Passive Heat Load: |
The heat transferred by virtue of a temperature difference. For
example, this is the heat that enters through insulated cabinet
walls when the cabinet is colder than the ambient temperature.
Another example is the heat from solar radiation. |
| Peltier Effect: |
The phenomenon whereby the passage of an electrical current
through a junction consisting of two dissimilar metals results in
a cooling effect. When the direction of current flow is reversed
heating will occur. |
| Qmax: |
The amount of heat that a TE cooler can remove at a zero degree
temperature difference at a hot-side temperature of 300 K. |
| Seebeck Coefficient: |
The Seebeck Coefficient is a measure of the electrical voltage
potential that exists in an electrical conductor whose ends are
maintained at two different temperatures and current is not
flowing. It is an intrinsic property and has units of V/K.
Thermocouples used for temperature measurement utilize this
principle. |
| Specific Heat: |
The amount of thermal energy required to raise the temperature
of a given amount of a given substance by one degree. Typical
units are J/kg/K. |
| Thermal Coefficient of Expansion: |
A measure of the dimensional change of a material due to a
change in its temperature. Common measurement units include
centimeter per centimeter per degree Celsius and inch per inch per
degree Fahrenheit. |
| Thermal Conductivity: |
Thermal conductivity relates the amount of heat an object will
transmit through its volume when a temperature difference is
imposed across that volume. It is an intrinsic property and
typical units include W/m/K and Btu/h/ft/°F. When multiplied by
the cross sectional area of an object and divided by the length of
an object it yields the thermal conductance of the object. |
| Thermal Interface: |
A physical interface between two objects through which heat is
conducted. In the case of thermoelectrics, this refers to the
physical connection the module has with the heat sink/cold sink.
Usually, thermal grease is used between the module and heat sink.
Sometimes it might be solder. Other times, it might be a thermally
conductive pad. |
| Thermal Resistance: |
A measure relating a temperature rise per unit of applied heat.
All mediums through which heat is conducted have an associated
thermal resistance. Common thermal resistances are heat sink
resistance and thermal interface resistance. Thermoelectric
coolers perform better with better, meaning lower thermal
resistance, heat sinks. |
| Thermoelectric Module: |
A semiconductor-based electronic component that functions as a
small heat pump. By applying a low voltage DC power source to a TE
module, heat will be moved through the module from one side to the
other. Therefore, one side will be cooled while the opposite side
will be heated. Consequently, a TE module can be used for both
heating and cooling. |
| Thomson Coefficient: |
The Thomson Coefficient is a measure of the change in heat
content within an electrical conductor whose ends are at two
different temperatures and in which current is flowing. When
electrical carriers flow from the cold to the hot end, they must
absorb heat in keeping with equilibrium with the temperature. When
electrical carriers flow from the hot end to the cold end, they
will release heat in keeping with temperature equilibrium.
Usually, the Thomson effect is intrinsic to the material. However,
the Thomson effect can also be extrinsically applied to a
conductor by varying the material type along the length of the
conductor. This can actually improve the cooling performance as
compared to the usual isotropic material. The Thomson effect is
really more complex than that described above. It is difficult to
put into words what the mathematics accurately describe. |
| Vmax: |
The voltage that is produced at DTmax when Imax is applied and
the hot-side temperature of the thermoelectric module is at 300 K. |
| Figure-of-Merit (Z) |
The Z is a direct measure of the cooling performance of a
thermoelectric module. Z = S^2/R/K where S is the Seebeck
Coefficient, R is electrical resistivity and K is the thermal
conductivity of the thermoelectric material. Z is temperature
dependent though, so, when comparing one module to another, they
must be based on the same hot-side temperatures. |
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