Unlike convectional heat engines or compression refrigerators,
TE devices are solid-state; they contain neither moving parts nor
refrigerants [9]. Therefore, the whole system can be simplified and
operated over an extended period of time without maintenance
[10]. TE devices can produce energy without using fossil fuel and can
thus reduce greenhouse gas emissions. However, the energy
conversion efficiency of TE devices is lower than those of convectional
heat engines or refrigerators [11]. The efficiency of TEGs and
the coefficient of performance (COP) of TECs are functions of not
only the figure of merit (ZT) but also the temperature difference
across the devices [12]. ZT is the performance index of a thermoelectric
material. Its value is relatively low (about 1.0) for the best
existing commercial TE cooling modules whereas that for conventional
air-conditioning system is about 4.0 [13]. Consequently,
a strategy for improving the performance of TE devices is needed.