  Thermoelectric generators are particularly suitable to investigate the
irreversible processes which govern the coupled transport of matter and heat in
solid-state systems. We study the efficiency at maximum power in the strong
coupling regime, where the thermal flux is proportionnal to the electrical
current inside the generator. We demonstrate that depending on the source of
irreversibility we obtain either the Curzon-Ahlborn efficiency for external
dissipation or a universal efficiency at maximum power for internal
dissipation. A continuous change between these two extremes is evidenced.
Effects of dissymetry of thermal contact conductance are also investigated.
