An energy storage system of the present disclosure includes: at least one battery module including a plurality of battery cells, and a cooling fan for forming an air flow to the inside of the battery module, wherein the battery module includes: a plurality of battery cells disposed to be spaced apart from each other in a different direction; a first frame which is in contact with a lower portion of each of the plurality of battery cells, and fixes the disposition of the plurality of battery cells; and a second frame which is in contact with an upper portion of the plurality of battery cells, disposed to be spaced apart from the first frame, and fixes the disposition of the plurality of battery cells, wherein the cooling fan forms an air flow into a space separated between each of the plurality of battery cells.