  Substitution of Cu for Ni in the Kondo lattice system CeNi$_2$Al$_3$ results
in a simultaneous optimization of the three interdependent thermoelectric
parameters: thermoelectric power, electrical and thermal conductivities, where
the electronic change in conduction band induced by the extra electron of Cu is
shown to be crucial. The obtained thermoelectric figure of merit $zT$ amounts
to 0.125 at around 100 K, comparable to the best values known for Kondo
compounds. The realization of ideal thermoelectric optimization in
Ce(Ni$_{1-x}$Cu$_x$)$_2$Al$_3$ indicates that proper electronic tuning of Kondo
compounds is a promising approach to efficient thermoelectric materials for
cryogenic application.
