  We study quantum transport through two-terminal nanoscale devices in contact
with two particle reservoirs at different temperatures and chemical potentials.
We discuss the general expressions controlling the electric charge current,
heat currents and the efficiency of energy transmutation in steady conditions
in the linear regime.
  With focus in the parameter domain where the electron system acts as a
power-generator, we elaborate workable expressions for optimal efficiency and
thermoelectric pameters of nanoscale devices. The general concepts are set at
work in the paradigmatic cases of Lorentzian resonances and antiresonances, and
the encompassing Fano transmission function: the treatments are fully analytic,
in terms of the trigamma functions and Bernoulli numbers. From the general
curves here reported describing transport through the above model transmission
functions, useful guidelines for optimal efficiency and thermopower can be
inferred for engineering nanoscale devices in energy regions where they show
similar transmission functions.
