In the present invention, the temperature of a battery container is controlled using the thermosiphon principle. To this end, heat exchange between circulation air and a battery is performed by a container air conditioner, and heat of the circulation air is exchanged with the environment outside the container by the thermosiphon principle as well as by the container air conditioner. To this end, the energy storage system comprises: a container having a battery and an air conditioner therein; a partition wall disposed between the battery and the air conditioner; and an upper space and a lower space provided above and below the partition wall, respectively, whereby circulation air introduced from the air conditioner through the lower space cools the battery and is introduced again into the air conditioner through the upper space, and heat is circulated by thermosiphons installed in the upper space and on the outer top of the container, respectively. A device employing the thermosiphon principle includes a thermosiphon indoor unit installed in the upper space and a thermosiphon outdoor unit installed on the outer top of the container.