|TestEquity - Thermoelectric Heating and Cooling|
About Thermoelectric Cooling and Heating
Thermoelectric modules are solid-state devices (no moving parts) that convert electrical energy into a temperature gradient, known as the "Peltier effect".
With no moving parts, thermoelectric modules are rugged, reliable and quiet heat pumps. 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 DC source, one side of the module will be made cold while the other is made hot. See below 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 within a tenth of one degree.
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.
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 to 15°C above ambient.
A typical thermoelectric module is composed of two ceramic substrates that serve as a foundation and electrical insulation for P-type and N-type Bismuth Telluride dice that are connected electrically in series and thermally in parallel between the ceramics. The ceramics also serve as insulation between the modules internal electrical elements and a heat sink that must be in contact with the hot side as well as an object against the cold side surface. An electrically conductive material, usually copper pads attached to the ceramics, maintain electrical connections inside the module. Solder is most commonly used at the connection joints to enhance the electrical connections and hold the module together.
Most modules have and even number of P-type and N-type dice and one of each sharing an electrical interconnection is known as, "a couple." The above module would be described as an 11-couple module.
While both P-type and N-type materials are alloys of Bismuth and Tellurium, both have different free electron densities at the same temperature. P-type dice are composed of material having a deficiency of electrons while N-type has an excess of electrons. As current (Amperage) flows up and down through the module it attempts to establish a new equilibrium within the materials. The current treats the P-type material as a hot junction needing to be cooled and the N-type as a cold junction needing to be heated. Since the material is actually at the same temperature, the result is that the hot side becomes hotter while the cold side becomes colder. The direction of the current will determine if a particular die will cool down or heat up. In short, reversing the polarity will switch the hot and cold sides.
TestEquity Model TEC1 is the world's first commercially available Thermoelectric Temperature Chamber. It was designed for engineering testing of pacemakers and defibrillators over a relatively narrow range of temperature. Thermoelectric devices are ideally suited for this application, where just a little bit of cooling is required. The resulting design is quieter, more reliable, more compact, and weighs less than conventional refrigeration systems which use a compressor.
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