Burning Crystals can use different energies. They are just power batteries. During the TBC period, they were fueled with fel, simple as that. Because Blizzard never updated the visuals from Silvermoon, we still see the fel-green evil-eyed version of the crystals.
During the What’s in the Box? quest, Rommath says: “This Korune device is similar to the fel crystals we once used to enslave demonic energies.”
Also, this TBC era video talks about how they used demonic power in the lack of the Sunwell to maintain Silvermoon.
