  Using the merger of local density approximation and dynamical mean field
theory, we show how electronic correlations increase the thermopower of
Na$_{0.7}$CoO$_2$ by 200%. The newly revealed mechanism is an asymmetric shift
of (quasi) electrons and holes away from the Fermi level, concurrent with an
asymmetry of the respective (group) velocities. Exploiting this effect in
bandstructure and correlation engineering may lead to a substantial increase of
the thermoelectric figure of merit.
