  By combining ab initio simulations including an on-site Coulomb repulsion
term and Boltzmann theory, we explore the thermoelectric properties of
(LaNiO$_3$)$_n$/(LaAlO$_3$)$_n$(001) superlattices ($n=1,3$) and identify a
strong dependence on confinement, spacer thickness, and epitaxial strain. While
the system with $n=3$ shows modest values of the Seebeck coefficient and power
factor, the simultaneous reduction of the LaNiO$_3$ region and the LaAlO$_3$
spacer thickness to single layers results in a strong enhancement, in
particular of the in-plane values. This effect can be further tuned by using
epitaxial strain as control parameter: Under tensile strain corresponding to
the lateral lattice constant of SrTiO$_3$ we predict in- and cross-plane
Seebeck coefficients of $\pm 600$ $\mu$V/K and an in-plane power factor of $11$
$\mu$W/K$^2$cm for an estimated relaxation time of $\tau = 4$ fs around room
temperature. These values are comparable to some of the best performing oxide
systems such as La-doped SrTiO$_3$ or layered cobaltates and are associated
with the opening of a small gap ($0.29$ eV) induced by the concomitant effect
of octahedral tilting and Ni-site disproportionation. This establishes oxide
superlattices at the verge of a metal-to-insulator transition driven by
confinement and strain as promising candidates for thermoelectric materials.
