The air duct 20 is for air-cooling the cell assembly 10 and includes a duct made of an aluminum material and having an air passage for moving a cooling air. An outer surface of the air duct 20 oriented toward the cell assembly 10 is disposed in contact with the edge portion of the cell assembly 10. At this time, it is preferable that a heat transfer material made of a thermal interface material (TIM) having high thermal conductivity is interposed between the edge portion of the cell assembly 10 and the air duct 20.

A truss structure 21 formed by repeatedly arranging frames to form a substantially triangular shape is provided inside the duct of the air duct 20 to improve the cooling performance and provide sufficient mechanical rigidity for protecting the cell assembly 10 from an external impact or the like.

In addition, a plurality of uneven patterns are formed on the surface of the truss structure 21 of the air duct 20. The uneven patterns improve the cooling performance by increasing the internal surface area of the air duct 20.

The air duct 20 is provided on both sides of the cell assembly 10, respectively, and is disposed so that the longitudinal direction thereof is extended identical to the longitudinal direction of the cell assembly 10.