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 that are heavily doped with electrical carriers. The elements are arranged into array that is electrically connected in series but thermally connected in parallel. This array is then affixed to two ceramic substrates, one on each side of the elements (see figure below). Let’s examine how the heat transfer occurs as electrons flow through one pair of p- and n-type elements (often referred to as a “couple”) within the thermoelectric module:
The p-type semiconductor is doped with certain atoms that have fewer electrons than necessary to complete the atomic bonds within the crystal lattice. When a voltage is applied, there is a tendency for conduction electrons to complete the atomic bonds. When conduction electrons do this, they leave “holes” which essentially are atoms within the crystal lattice that now have local positive charges. Electrons are then continually dropping in and being bumped out of the holes and moving on to the next available hole. In effect, it is the holes that are acting as the electrical carriers.
Now, electrons move much more easily in the copper conductors but not so easily in the semiconductors. When electrons leave the p-type and enter into the copper on the cold-side, holes are created in the p-type as the electrons jump out to a higher energy level to match the energy level of the electrons already moving in the copper. The extra energy to create these holes comes by absorbing heat. Meanwhile, the newly created holes travel downwards to the copper on the hot side. Electrons from the hot-side copper move into the p-type and drop into the holes, releasing the excess energy in the form of heat.
The n-type semiconductor is doped with atoms that provide more electrons than necessary to complete the atomic bonds within the crystal lattice. When a voltage is applied, these extra electrons are easily moved into the conduction band. However, additional energy is required to get the n-type electrons to match the energy level of the incoming electrons from the cold-side copper. The extra energy comes by absorbing heat. Finally, when the electrons leave the hot-side of the n-type, they once again can move freely in the copper. They drop down to a lower energy level, and release heat in the process.
The above explanation is imprecise as it does not cover all the details, but it serves to explain in words what are otherwise very complex physical interactions. The main point is that heat is always absorbed at the cold side of the n- and p- type elements, 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.
The advantages of a thermoelectric unit.
The use of thermoelectric modules often provides solutions, and in some cases the ONLY solution, to many difficult thermal management problems where a low to moderate amount of heat must be handled. While no one cooling method is ideal in all respects and the use of thermoelectric modules will not be suitable for every application, TE coolers will often provide substantial advantages over alternative technologies. Some of the more significant features of thermoelectric modules include:
No Moving Parts: A TE module works electrically without any moving parts so they are virtually maintenance free.
Small Size and Weight: The overall thermoelectric cooling system is much smaller and lighter than a comparable mechanical system. In addition, a variety of standard and special sizes and configurations are available to meet strict application requirements.
Ability to Cool Below Ambient: Unlike a conventional heat sink whose temperature necessarily must rise above ambient, a TE cooler attached to that same heat sink has the ability to reduce the temperature below the ambient value.
Ability to Heat and Cool With the Same module: Thermoelectric coolers will either heat or cool depending upon the polarity of the applied DC power. This feature eliminates the necessity of providing separate heating and cooling functions within a given system.
Precise Temperature Control: With an appropriate closed-loop temperature control circuit, TE coolers can control temperatures to better than +/- 0.1°C.
High Reliability: Thermoelectric modules exhibit very high reliability due to their solid state construction. Although reliability is somewhat application dependent, the life of typical TE coolers is greater than 200,000 hours.
Electrically “Quiet” Operation: Unlike a mechanical refrigeration system, TE modules generate virtually no electrical noise and can be used in conjunction with sensitive electronic sensors. They are also acoustically silent.
Operation in any Orientation: TEs can be used in any orientation and in zero gravity environments. Thus they are popular in many aerospace applications.
Convenient Power Supply: TE modules operate directly from a DC power source. Modules having a wide range of input voltages and currents are available. Pulse Width Modulation (PWM) may be used in many applications
Spot Cooling: With a TE cooler it is possible to cool one specific component or area only, thereby often making it unnecessary to cool an entire package or enclosure.
Ability to Generate Electrical Power: When used “in reverse” by applying a temperature differential across the faces of a TE cooler, it is possible to generate a small amount of DC power.
Environmentally Friendly: Conventional refrigeration systems can not be fabricated without using chlorofluorocarbons or other chemicals that may be harmful to the environment. Thermoelectric devices do not use or generate gases of any kind.
3. Industries that thermoelectrics serve.
- Black box cooling
- CCD (Charged Couple Devices)
- CID (Charge Induced Devices)
- Cold chambers
- Cold plates
- Compact heat exchangers
- Constant temperature baths
- Dew point hygrometers
- Electronics package cooling
- Electrophoresis cell coolers
- Environmental analyzers
- Heat density measurement
- Ice point references
- Immersion coolers
- Integrated circuit cooling
- Inertial guidance systems
- Infrared calibration sources and black body references
- Infrared detectors
- Infrared seeking missiles
- Laser collimators
- Laser diode coolers
- Long lasting cooling devices
- Low noise amplifiers
- Microprocessor cooling
- Microtome stage coolers
- NEMA enclosures
- Night vision equipment
- Parametric amplifiers
- Photomultiplier tube housing
- Power generators (small)
- Precision device cooling (lasers and microprocessors)
- Refrigerators and on-board refrigeration systems (aircraft, automobile, boat, hotel, insulin, portable/picnic, pharmaceutical, RV)
- Restaurant portion dispenser
- Self-scanned arrays systems
- Semiconductor wafer probes
- Stir coolers
- Thermal viewers and weapons sights
- Thermal cycling devices (DNA and blood analyzers)
- Thermostat calibrating baths
- Tissue preparation and storage
- Vidicon tube coolers
- Wafer thermal characterization
- Water and beverage coolers
- Wet process temperature controller
- Wine cabinets
4.Efficiency of a thermoelectric module.
Efficiency relates to the amount of energy produced from 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 the result is heat pumping.
For thermoelectric modules, 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 certain 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 thermoelectric modules.