  By combining first-principles simulations including an on-site Coulomb
repulsion term and Boltzmann theory, we demonstrate how the interplay of
quantum confinement and epitaxial strain allows to selectively design $n$- and
$p$-type thermoelectric response in (LaNiO$_3$)$_3$/(LaAlO$_3$)$_1(001)$
superlattices. In particular, varying strain from $-4.9$ to $+2.9\%$ tunes the
Ni orbital polarization at the interfaces from $-6$ to $+3\%$. This is caused
by an electron redistribution among Ni $3d_{x^2-y^2}$- and $3d_{z^2}$-derived
quantum well states which respond differently to strain. Owing to this charge
transfer, the position of emerging cross-plane transport resonances can be
tuned relative to the Fermi energy. Already for moderate values of $1.5$ and
$2.8\%$ compressive strain, the cross-plane Seebeck coefficient reaches $\sim
-60$ and $+100$ $\mu$V/K around room temperature, respectively. This provides a
novel mechanism to tailor thermoelectric materials.
