The specific energy of aluminum is 31 megajoules per kilogram (MJ/kg), but from the wikipedia article on the element, the most efficient current smelters require 52-56 MJ/kg to convert aluminum oxide to aluminum metal.

The only way to use aluminum as an energy transport mechanism would be to have the energy source (eg, your 'energy island') used to do the smelting to make the aluminum, transport the aluminum, then 'burn' it to generate the power needed on the mainland.

BUT!!!!

Since the difference in energy required for production to energy released is 52/31 at best, that represents nearly a 40% loss in energy conversion.

And that's assuming the 31 MJ/kg as 100% burning efficiency. Never gonna happen.

No energy conversion process is 100% efficient.

And then there's the energy costs of transporting the aluminum and aluminum oxide back and forth.

As Scotty on Star Trek would say, "Ya canna beat the law's of physics, cap'n!!!"

Brian