have a large heat storage capacity, which would help to keep the sink temperature
low relative to the junction temperature. This latter solution could be achieved using
a phase change material (PCM).
PCMs have long been identified as candidates for thermal storage systems, due
to the high energy densities (MJ/m3). A further advantage of PCMs is that heat
transfer normally takes place at a constant temperature (the transition temperature).
This is appropriate for thermoelectric cooling, and refrigeration units, especially
those used for food and medicine storage which require precise temperature control.
The principle of this technique is that as the temperature rises due to dissipated heat
energy, the PCM absorbs energy, first as sensible heat, and then as latent heat when
the phase change temperature is reached. At this stage, the temperature remains constant
until the phase change is complete.
PCMs are available with a large range of phase change temperatures, and thus
may be utilised on both the cold and hot junctions of a TEC and for a range of
applications and environments. By selecting a PCM with suitable transient temperature
and large storage capacity, the temperature difference across the thermoelectric
module may be maintained at a low value, thus improving the performance of the
device.
The heat transfer between thermoelectric cells and heat sink devices may be further
improved by the use of heat pipes. Heat pipes [6] are devices with a very high
thermal conductivity and typically consist of a sealed tube with an internal wick (see
Fig. 4). The pipe is charged with a refrigerant, such as water, ethanol or methanol
and when heat is applied to one end of the pipe (evaporator), the liquid refrigerant
evaporates and the resulting vapour travels to the cold end (condenser), where it
condenses into liquid while dissipating heat. The liquid refrigerant is then returned
to the evaporator via the wick. Heat pipes could be used on both sides of thermoelec