Thermoelectric Technology

Thermoelectric cooling, also called "the Peltier Effect,"
is a solid-state method of heat transfer through
dissimilar semiconductor materials. To understand it,
one should know how thermoelectric cooling systems
differ from their conventional counterparts.

 

Both systems obey the laws of thermodynamics.
So it follows that thermoelectric cooling has much in
common with conventional refrigeration methods only the actual system for cooling is different.

Perhaps the best way to show the difference in the two refrigeration systems is to describe the systems themselves.

In a conventional refrigeration system, the main working parts are the evaporator, condenser, and compressor. The evaporator surface is where the liquid refrigerant boils, changes to vapor and absorbs heat energy. The compressor circulates the refrigerant and applies enough pressure to increase the temperature of the refrigerant above ambient level. The condenser helps discharge the absorbed heat into surrounding room air.


Thermoelectric refrigeration
replaces the three main working parts with:
a cold junction, a heat sink and a DC power source.
The refrigerant in both liquid and vapor form is replaced by two dissimilar conductors. The cold junction (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from one semiconductor to another, instead of energy absorption by the refrigerant as it changes from liquid to vapor. The compressor is replaced by a DC power source which pumps the electrons from one semiconductor to another. A heat sink replaces the conventional condenser fins, discharging the accumulated heat energy from the system.

The difference between two refrigeration methods, then, is that a thermoelectric cooling system refrigerates without use of mechanical devices, except perhaps in the auxiliary sense, and without refrigerant.

Thermoelectric (Def): Stated as simply as possible, in a thermoelectric cooler, semiconductor materials with dissimilar characteristics are connected electrically in series and thermally in parallel, so that two junctions are created.

Semiconductors
The semiconductor materials are N and P type, and are so named because either they have more electrons than necessary to complete a perfect molecular lattice structure (N-type) or not enough electrons to complete a lattice structure (P-type). The extra electrons in the N-type material and the holes left in the P-type material are called "carriers" and they are the agents that move the heat energy from the cold to the hot junction. Heat absorbed at the cold junction is pumped to the hot junction at a rate proportional to carrier current passing through the circuit and the number of couples.

Good thermoelectric semiconductor materials such as bismuth telluride greatly impede conventional heat conduction from hot to cold areas, yet provide an easy flow for the carriers. In addition, these materials have carriers with a capacity for transferring more heat.

 


Heat Sinks
The design of the heat exchanger is a very important aspect of a good thermoelectric system.

The upper part of the diagram above illustrates the steady-state temperature profile across a typical thermoelectric device from the load side to the ambient.

 


The total steady-state heat which must be rejected by the heat sink to the ambient may be expressed as follows:

 

If the heat sink cannot reject enough Qs from the system, the systemís temperature will rise and the cold junction temperature will increase. If the thermoelectric current is increased to maintain the load temperature, the COP (Coefficient of Performance) tends to decrease. Thus, a good heat sink contributes to improved COP.

Energy may be transferred to or from the thermoelectric system by three basic modes: conduction, convection, and radiation. The values of Qc and QI may be easily estimated; their total, along with the power input, gives Qs, the energy the hot junction heat sink must dissipate.

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