The present invention relates to a heat accumulator (1) comprising a heat exchange chamber (2) having a first opening (3) configured to pass through a working fluid and a second opening (5) configured to pass through a working fluid, and a plurality of heat storage elements (4) filled in the heat exchange chamber (2). At least one cord (6) extends between the heat storage elements (4). The present invention also relates to an apparatus and a method of forming a heat accumulator (1) .