The work in Ref. [13] was based on non-equilibrium thermodynamics, i.e. the external heat transfers among the hot
and cold junctions of the two-stage TEG and the two-stage TEC and their respective reservoirs were not considered. That
is, the model is an exo-reversible model. A further step beyond Ref. [13] made in this paper is to build a combined device
model of two-stage thermoelectric cooler (TEC) driven by two-stage thermoelectric generator with external heat transfer
and internal irreversibility by using the combination of finite time thermodynamics and non-equilibrium thermodynamics.
For a fixed total thermal conductance of four heat exchangers, the allocations of the thermal conductance among the four
heat exchangers are optimized for maximizing the cooling capacity and the coefficient of performance (COP) of the combined
thermoelectric device. For the fixed total number of thermoelectric thermocouples of the combined device, the allocations of
the number of thermoelectric thermocouples among the two thermoelectric generators and the two thermoelectric coolers
are also optimized for maximizing cooling capacity and COP, respectively.