This makes them ineffective as a general energy storage medium for passenger vehicles. This material serves as a heat storage medium. Hydrogen is being developed as an electrical energy storage medium. They were essentially a personal storage medium. Time Machine places strict requirements on the backup storage medium. This records data on a removable storage medium. Time Machine is able to do rotating backups on more than one storage medium. Solar radiation heats the water, which acts as a thermal storage medium. Digital sound files can be stored on any computer storage medium. The PDP-1 uses punched paper tape as its primary storage medium. It is also working on graphene as an energy storage medium. It was the first console to use Blu-ray Disc as its primary storage medium. It specifies how bits are used to encode information in a digital storage medium. Logical filesystems require data and metadata to be stored on the data storage medium. It uses CD-ROM as its storage medium.