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How
do thermoelectric coolers (TECs) work?
What do the thermoelectric parameters Imax, Vmax,
dTmax and Qmax mean?
What simple test can I use to verify that my TEC
is operating correctly?
What does a thermoelectric system offer over a
compressor?
What is the best way to power and control a TEC?
How big can you make a thermoelectric cooler in
length and width? What limits the size?
What is the efficiency of a thermoelectric
cooler?
Thermoelectric coolers are best mounted using
what method?
About Marlow Industries
If a
standard thermoelectric cooler does not fit our application, can Marlow provide
a custom TEC?
What is the maximum operating or non-operating temperature that
Marlow coolers can withstand?
What quality system is Marlow operating under?
Is Marlow exploring new ways to improve TE performance and
efficiency?
Do you have any stocking distributors?
What does your nomenclature -01, -02, -03, and AC, BC, AB mean?
Do you accept credit cards?
How
do thermoelectric coolers (TECs) work?
Thermoelectric
coolers are solid state heat pumps that operate on the Peltier effect, the
theory that there is a heating or cooling effect when electric current passes
through two conductors. A voltage applied to the free ends of two dissimilar
materials creates a temperature difference. With this temperature difference,
Peltier cooling will cause heat to move from one end to the other. A typical
thermoelectric cooler will consist of an array of p- and n- type semiconductor
elements that act as the two dissimilar conductors. The array of elements is
soldered between two ceramic plates, electrically in series and thermally in
parallel. As a dc current passes through one or more pairs of elements from n-
to p-, there is a decrease in temperature at the junction ("cold
side") resulting in the absorption of heat from the environment. The heat
is carried through the cooler by electron transport and released on the opposite
("hot") side as the electrons move from a high to low energy state.
The heat pumping capacity of a cooler is proportional to the current and the
number of pairs of n- and p- type elements (or couples).
FIGURE 1
- Diagram of a typical thermoelectric cooler
N and P
type semiconductors (usually Bismuth Telluride) are the preferred materials used
to achieve the Peltier effect because they can be easily optimized for pumping
heat and due to the ability to control the type of charge carrier within the
conductor.
FIGURE 2
Figure
2 illustrates an "N-type" semiconductor element utilized to facilitate
the Peltier effect. In this example, negatively charged electrons are repelled
by the negative pole and attracted to the positive pole of the DC power source.
This forces electron flow in a clockwise direction through the
"N-type" material. Heat is absorbed at the bottom junction and
actively transferred to the top junction by the electrons as they flow through
the semiconductor element. This is known as electron transport.
As current
flows through a material, heat is generated. Thermoelectric material is no
different. There is a point where the heat generated internally offsets the TECs
ability to pump heat. Each TEC has a limit on how much heat that it can pump.
This limit is referred to as Qmax. The current associated with Qmax is referred
to as Imax. The corresponding voltage across the coolers is referred to as Vmax.
If a TEC is completely insulated and isolated from the environment and running
at Imax, it will produce its maximum temperature difference, dTmax. At this
point it will also be pumping no heat whatsoever. As heat is applied to the cold
side of the TEC, the temperature differential is suppressed. Effectively, one
trades temperature differential for heat pumping. As such, if the temperature
differential is 0, the corresponding heat load is Qmax. The coefficient of
performance (COP) is defined as the amount of heat pumping one gets for each
unit of electrical power supplied.
Measuring
electrical resistance is a good way to check the health of a TEC. Marlow
recommends making an AC resistance measurement using a digital impedance meter
or LCR. Using a standard ohm meter with a dc input signal will not yield
accurate readings due to the fact that the dc voltage applied across the TEC
will generate a temperature change and variations in resistance readings.
Nominal AC resistance specifications for Marlow coolers are available upon
request.
Thermoelectric
coolers are solid state heat pumps which have no moving parts and do not require
the use of harmful CFCs. Because they have no moving parts they are inherently
reliable and require little or no maintenance. They are ideal for cooling
devices that may be sensitive to mechanical vibration. Their compact size makes
them well suited for applications that are space or weight limited such as
portable or airborne equipment. The ability to use TECs to heat as well as cool
is great for applications requiring temperature stabilization of a device over a
wide ambient temperature range such as laser diodes.
Thermoelectric
coolers require smooth dc current for optimum operation. A ripple factor of less
than 10% will result in less than 1% degradation in delta T. Voltage or current
limiting should be used in order to ensure that Imax for the TEC is not
exceeded. A bipolar power supply is required for those applications requiring
both heating and cooling. Pulse width modulation may be used at frequencies
above 1 kHz. Linear proportional, PI or PID control can also be used. Marlow
does not recommend ON/OFF control. While this is the simplest control technique,
temperature cycling within the TEC as power is cycled from full ON to full OFF
may result in premature failure.
About 40mm
in length and/or width is the practical size limit for TECs. One side of the TEC
contracts as it cools and the other side expands as it heats. This stresses the
elements and solder joints. Since thermal expansion occurs on an inch per inch
basis, the larger the cooler gets, the greater the stress becomes on the
elements around the perimeter of the cooler. In cases where the required heat
pumping capacity exceeds that which could be provided by one TEC, additional
TECs can be used side-by-side.
Technically,
the word efficiency relates to the amount of energy one gets out of a machine
versus how much energy one puts into it. In heat pumping applications, this term
is rarely used because the energy-in is very different from the service
provided. We supply electrical energy to a TEC, but we get heat pumping. For
TECs, it is standard to use "coefficient of performance" not
efficiency. The coefficient of performance (COP) is the amount of heat pumping
divided by the amount of supplied electrical power. In other words, COP tells
you how many units of heat pumping you will get for each unit of electrical
power you supply. It is possible, in special situations, to pump more watts of
heat than the watts of electrical power input. COP depends on the application,
heat pumped, and temperature differential required. Typically, the coefficient
of performance, heat pumped then divided by input power, is between 0.4 and 0.7
for single stage applications. However, higher COPs can be achieved with
optimized custom TECs.
The three
mounting methods are soldering, epoxy, or compression. Each method has
advantages and disadvantages based on the application and sizes. See
our design guide for further details.
Marlow has
an experienced engineering staff that is dedicated to working with customers on
custom applications. Marlow uses internally developed software to optimize new
TEC and thermoelectric subassembly designs to the customer's requirements
providing the maximum performance.
Marlow's
standard MI series products can withstand operating and non-operating
temperatures of 85 degrees C, and short term exposure temperatures of less than
125 degrees C. Custom and DuraTEC series can withstand operating and
non-operating temperatures of 120 degrees C and short-term exposure to
temperatures of less than 170 degrees C.
Marlow
Industries is set up in accordance with MIL-I-45208 which includes calibration
back to the National Institute of Standards and Technology per MIL-C-45662. We
are also in compliance with ISO-9001 2000. Marlow Industries is a recipient of the 1991 Malcolm Baldrige National Quality
Award.
Marlow
Industries has an entire department dedicated to materials research and TE
performance improvements.
Marlow
Industries has some sales representatives in foreign nations but they do not
stock. In the U.S. we sell and ship from the Dallas, Texas location. In the
United Kingdom, we have an office that is primarily sales for the UK and Europe.
We do offer worldwide technical assistance from Dallas. Click
here to see our worldwide distributor information.
-01 means
both sides of the TEC are metallized -02 means the bottom (hot) side is
metallized -03 means neither side is metallized (bare ceramic on both sides) -AC
means Alumina ceramics -BC means Beryllium ceramics -AB means there is a mix of
Alumina and Beryllia on top or bottom ceramics (or within the stages of a
multi-stage thermoelectric cooler)
Yes, we
now accept VISA and Master Card.
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