EC (electronically commutated) fans are considered to be the most energy efficient fans on the market. These fans allow for the efficiency of a DC fan without the hassle of needing a AC to DC power supply.
EC fans have a flexible peak operating range unlike traditional AC fans, which have a very specific range (often disrupted by minor fluctuations in voltage). This means that if speed is slightly increased or decreased, output and energy usage will remain relatively linear. A similar AC fan in that environment would either drastically drop airflow output or drastically increase energy usage. EC motors can also have windings made in both directions that would allow the reversal of polarity to reverse blade spin direction (similar to DC fans); this is a feature that not all EC fans have, but can be added during manufacturing.