A schematic diagram of a two-stage TEC system driven by two-stage TEG is shown in Fig. 1. The system consists of
an irreversible two-stage TEG and an irreversible two-stage TEC in series with internal and external irreversibilities. The
internal irreversibilities are caused by Joulean loss and heat conduction loss through the semiconductor between the hot
and cold junctions. The external irreversibilities (not considered in Ref. [13]) are caused by thermal resistance of four heat
exchangers, as expressed by zigzag between the TEGs (TECs) and their heat reservoirs. The direct current power source of
the TEC is the output current generated by the TEG.