tally-friendly”, as they do not employ working fluids that are harmful to the environment.
The fact that they are solid-state devices with no moving parts, gives them an
extremely long operational life, provided that they are installed correctly and operated
within their recommended range of operating temperature. In addition, Peltier
coolers have the ability to operate in conjunction with proportional control systems,
which enables them to maintain enclosures at a steady temperature, rather than fluctuating
over a small range, as is the case with vapour compression systems.
Operation of Peltier coolers is based on the Peltier effect, i.e., when a DC current
passes through a cell made of a pair of n- and p-type semiconductor materials, one
of the junctions will be cooled whilst the other is heated depending on the direction
of the current. The amount of heat removed by the warm side corresponds to the
cooling effect and the electrical power input. A single stage thermoelectric module
(size 55 mm´55 mm) can achieve a temperature difference up to 70°C, or can transfer
heat at a rate of 125 W [5]. For higher temperature differences, multi-stage
(cascade) thermoelectric modules are used.
Any thermoelectric device has a given operating temperature range beyond which
its operation may cease. For this reason, all thermoelectric coolers (TECs) require
heat sinks in order to dissipate the energy generated or absorbed at the two junctions.
A Peltier cooling unit is therefore composed of three basic components, the Peltier
module, the heat dissipater at the hot side of the module and the cooling component
at the cold side of the module, as shown in Fig. 2. The body to be cooled may be
in direct contact with the cold ceramic plate of the module, or may be cooled through
a heat exchanger using either a liquid jacket or forced air. The waste heat from the
hot side may also be dissipated to the ambient either by natural or forced air, or
removed using a liquid jacket heat exchanger. Fig. 3 shows various arrangements
for heat transfer from the two junctions.
Design or selection of a heat sink is crucial to the overall operation of a thermoelectric
system. The heat sink should be designed to minimise the thermal resistance.
This could be achieved by maximising the exposed surface area or by using heat
pipes to enhance the heat transfer. Alternatively, the heat sink could be designed to