Thermoelectric modules are solid-state devices (no moving parts) that convert electrical energy into a temperature gradient, known as the "Peltier effect" or convert thermal energy from a temperature gradient into electrical energy, the "Seebeck effect." Thermoelectric modules used as TE generators or TEGs are rather inefficient and little power is produced. Typical applications of this type include NASA supplying power to spacecraft with a radioisotope thermoelectric generator or (RTG) and electronic equipment along fuel pipelines where fuel may be burned off. TE modules may also be used as thermocouples for temperature measurement. This discussion will focus on the use of thermoelectric modules TEMs for cooling TECs and for temperature stabilization.
With no moving parts, thermoelectric modules are rugged, reliable and quiet heat pumps, typically 1.5 inches (40 x 40mm) square or smaller and approximately ¼ inch (4 mm) thick. The industry standard mean time between failures is around 200,000 hours or over 20 years for modules left in the cooling mode. When the appropriate power is applied, from a battery or other DC source, one side of the module will be made cold while the other is made hot. Click here to see how they work. Interestingly, if the polarity or current flow through the module is revered the cold side will become the hot side and vice versa. This allows TE modules to be used for heating, cooling and temperature stabilization.
Since TE modules are electrical in nature, in a closed-loop system with an appropriate temperature sensor and controller, TE modules can easily maintain temperatures that vary by less than one degree Celsius. Simpler on - off control can also be produced with a thermostat.
Because the cold side of the module contracts while the hot side expands modules with a footprint larger than 1.5 - 2 inches square usually suffer from thermally induced stresses, at the electrical connection points inside the module causing a short, so they are not common. Long, thin modules want to bow for the same reason and are also rare. Larger areas than an individual module can maintain are cooled or have the temperature controlled by using multiple modules.
We know from the second law of thermodynamics that heat will move to a cooler area. Essentially, the module will absorb heat on the "cold side" and eject it out the "hot side" to a heat sink. The addition of a heat sink to a module creates a thermoelectric device or TED. In addition to the heat being removed from the object being cooled, the heat sink must be capable of dissipating the electrical power applied to the module, which also exits through the modules hot side.
As any Electrical Engineer will tell you the resistive or "Joule heat" created is proportional to the square of the current applied (I2 R). This is NOT the case with thermoelectric modules. The heat created is actually proportional to the current (amperes x volts) because of the flow of current is working in two directions (the Peltier effect). Therefore, the total heat ejected by the module is the sum of the current times the voltage plus the heat being pumped through the cold side.
To understand the capabilities of a thermoelectric module, and related assembly, it is necessary to understand what TE module specifications represent and their implications. The four standard specifications for a module are 1.) The heat pumping capacity or Qmax in watts 2.) The maximum achievable difference in temperature between the hot and cold sides of the module known as the Delta Tmax or DTmax, usually represented in degrees Celsius 3.) The maximum (optimal) input current in amps or Imax 4.) The maximum input voltage or Vmax when the current input is optimal (Imax).
As a practical matter it is only possible reach either heat pumping capacity in watts or to obtain the maximum temperature differential in degrees. In other words, the DTmax is the maximum temperature difference between the hot and cold side of the module when optimal power is applied and there is no heat load (Q=0). As a thermal load Q is added, the difference in temperature between the two surfaces will decrease until the heat pumping capacity or Qmax value is achieved and there is no net cooling (DT=0). Since your application will likely require net cooling of an object with a thermal mass, the actual heat pumped or Q will be less than Qmax and the actual difference in temperature will be less than the DTmax.
Curves may be produced to show the relationship between power applied to a module and net cooling. From our module specifications page you may see the curves for our most popular modules by clicking on the appropriate part number in the first column.
After learning what power is required for an appropriate module to reach the desired level of cooling it is necessary to focus on the assembly required, specifically heat sink selection, in order to allow the module to maintain the desired results.
The actual temperature achieved, with a given level of cooling or DT on the module, in an assembly is derived by subtracting the temperature of the cold side Tc from the temperature of the hot side Th. (The advanced user should be aware that hot and cold side temperatures are expressed in degrees Kelvin when used in equations.) Naturally, the cooler the hot side of the module, the cooler the cold side will be. Many people not familiar with thermoelectrics assume that the temperature of the hot side will be the same as the ambient temperature. This is probably not the case. As mentioned earlier, as soon as power is applied to the module the hot side of the module will begin ejecting this as heat to the heat sink causing it to rise in temperature. The ability of the heat sink to dissipate this heat as well as the heat being pumped through the cold side will determine the actual operating temperature of the hot side thus, the cold side.
This brings us to the importance of selecting an appropriate heat sink. In general, the better (the lower the thermal resistance of) the heat sink the easier it is to keep the hot side temperature from increasing. Liquid heat sinks typically have the lowest thermal resistance however they are relatively expensive and plumbing is required. The use of a liquid heat sink assumes that a "house water supply" or chiller is available to cool the water or liquid being circulated through the heat sink. The most common type of heat sink used in thermoelectric applications is made from a thermally conductive material like aluminum or copper and has fins that are perpendicular to a base.
A typical extruded heat sink profile
It is recommended that you select the largest (greatest surface area) heat sink that you can accommodate. In general, to reduce the thermal resistance of a heat sink by 50% it is necessary to increase it's volume by 400%.
In most TE applications that our modules will be appropriate for, a heat sink alone will not be able to remove a sufficient amount of heat by natural convection keep the hot side at an acceptably low temperature. In order to help the heat sink remove heat on and around the heat sink fins, a fan or blower must be attached which forces ambient temperature air over the fins and exhausts the heat to ambient. This is known as a forced convection heat sink. Even with a forced convection heat sink it is common that the hot side will stabilize at 10 - 15°C above ambient.
When installing TE modules in an assembly it is most common to compress or "clamp" them between a heat sink and something to be cooled. The object cooled can be a block of metal creating a cold plate, another forced convection heat sink making an air-to-air exchanger, a liquid heat sink forming a liquid-to-air exchanger, a probe for a water cooler or just about anything else with a flat surface.
Thermoelectric modules are operated from a DC (Direct Current) power source rather than AC (Alternating Current) sources such as most homes have available. DC power supplies, AC/DC converters, batteries and battery chargers (without too much AC ripple) can all be used as sources of power. When precise temperature control is required the power supply must be adjustable so that as information is returned from a temperature controller, corrections to input power can be made.
With these basics guidelines to work with you should be able to evaluate the use of thermoelectric modules as a possible solution to your thermal management challenge. If you would like to learn more, we highly recommend:
Additional information that may be important to you may include: