I doubt that energy alone will be the future currency. If you are really optimistic in natural resources then try water or clean air. With the world getting polluted day by day it's not unknown to the many that clean air, clean water etc. are getting more scarce.
While energy is mandatory for survival, water and air are more important, an di would bet my money rather on those then on energy, which we can create/produce more easily.
Actually, the percentage of water that we, humans, consume is just a 1% of the total water in the Earth. It'll not get scarce unless the whole ocean and other water bodies becomes polluted. While I doubt that it'll happen, shortly after that incident, humans would die as our environment is already dying at that point.
I don't think energy would be that scarce that it can be traded into other goods or become a currency. A lot of things can produce energy. All of them are natural, wind, water, thermal, geothermal, etc. The supplies of those things are infinite and the only reason why would those be scarce would be because the whole world is already dying.