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