23-11-2012, 01:02 PM
Essentials of Thermoelectric (TE) Cooling
Thermoelectric Cooling.ppt (Size: 2.43 MB / Downloads: 88)
Why are TE Coolers Used for Cooling?
No moving parts make them very reliable; approximately 105 hrs of operation at 100 degrees Celsius, longer for lower temps (Goldsmid,1986).
Ideal when precise temperature control is required.
Ability to lower temperature below ambient.
Heat transport controlled by current input.
Able to operate in any orientation.
Compact size make them useful for applications where size or weight is a constraint.
Ability to alternate between heating and cooling.
Excellent cooling alternative to vapor compression coolers for systems that are sensitive to mechanical vibration.
Disadvantages
Able to dissipate limited amount of heat flux.
Lower coefficient of performance than vapor-compression systems.
Relegated to low heat flux applications.
More total heat to remove than without a TEC.
What are Some Applications?
Cooling:
Electronic enclosures
Laser diodes
Laboratory instruments
Temperature baths
Refrigerators
Telecommunications equipment
Temperature control in missiles and space systems
Heat transport ranges vary from a few milliwatts to several thousand watts, however, since the efficiency of TE devices are low, smaller heat transfer applications are more practical.
Basic Principles
A typical thermoelectric cooling component is shown on the next slide. Bismuth telluride (a semiconductor), is sandwiched between two conductors, usually copper. A semiconductor (called a pellet) is used because they can be optimized for pumping heat and because the type of charge carriers within them can be chosen. The semiconductor in this examples N type (doped with electrons) therefore, the electrons move towards the positive end of the battery.
The semiconductor is soldered to two conductive materials, like copper. When the voltage is applied heat is transported in the direction of current flow.
Method of Heat Transport
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.
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.