What are these grilles in the passenger cabin of this 787?
The answers to this question indicate that these are the exhaust vents for the cabin air circulation. I think they appear to be much too large for just that purpose. I don't have a technical reference for this, but I believe those vents also prevent the cabin floor buckling in the event of a sudden pressurization failure.

The cargo hold is beneath the passenger cabin, and both are pressurized because the cylindrical hull can resist pressure better than the flat cabin floor. If the cabin suddenly loses pressure the air in the hold could buckle the floor upward, or if the hold loses pressure the air in the cabin could buckle the floor downward. The floor must be generously vented to prevent this.

US patents # 6129312 and # 5137231 claim systems for combining the cabin air circulation vents with the depressurization relief vents. From the '9312 patent:


  An air return grille mounted below the passenger cabin sidewall
  comprises openings to allow return airflow from the air conditioning
  system. The grille openings are covered by a single thin baffle with
  cutouts within its periphery that slip over and are held in place by
  fasteners on the molded grille. During normal operation, the thin
  baffle restricts flow through a small opening in the grille. During a
  sudden decompression of the cargo compartment, the baffle is dislodged
  from the fasteners. This allows the airflow to pass through all of the
  molded grille openings in order to achieve rapid pressure
  equalization.