  A transverse thermoelectric generator for magnetic-field-free and
high-density power generation utilizing the anomalous Nernst effect is
constructed and its performance is characterized. By alternately stacking two
different permanent magnets with the large coercivity and anomalous Nernst
coefficients of opposite sign, transverse thermoelectric voltage and power can
be generated in the absence of external magnetic fields and enhanced owing to a
thermopile structure without useless electrode layers. In the
permanent-magnet-based stack, the magnetic attractive force enables easy
construction of the thermopile structure with a high fill factor. In this
study, we construct a bulk module consisting of twelve pairs of SmCo$_5$- and
Nd$_2$Fe$_{14}$B-type permanent magnets with respectively having the positive
and negative anomalous Nernst coefficients, whose fill factor reaches ~80%,
whereas that of conventional thermoelectric modules based on the Seebeck effect
is typically 30-60%. We demonstrate magnetic-field-free anomalous Nernst power
generation up to 177 $\mu$W at a temperature difference of 75 K around room
temperature, which corresponds to the largest anomalous Nernst power density of
65 $\mu$W/cm$^2$. The presented module structure concept will provide a design
guideline for high-performance transverse thermoelectric power generation.
