  We evaluate theoretically the thermoelectric efficiency of the layered
perovskite Sr$_2$Nb$_2$O$_7$ via calculations of the electronic structure and
transport coefficients within density-functional theory and Bloch-Boltzmann
relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$,
computed with energy-, chemical potential- and temperature-dependent relaxation
times, has one component increasing monotonically from around 0.4 at room
temperature to 2.4 at 1250 K at an optimal carrier density around
2$\times$10$^{20}$ cm$^{-3}$, while the other components are small. The Seebeck
coefficient is about 250 to 300 $\mu$V/K at optimal doping, and reaches 800
$\mu$V/K at lower doping. We provide a {\tt python} code implementing various
approximations to the energy-dependent relaxation time transport, which can be
used to address different systems with an appropriate choice of material
parameters.
