Energy is limited and production of energy has costs and side effects. Sure, over time we can increase our production of energy, but again - with costs and side effects depending on how we choose to do it. And at any point in time our access to energy is limited and we need to make choices about how that access is distributed.
Right now we use markets to do that. But we've clearly seen that markets, with out intentional, ethical and moral guidance will often make insane choices.
And note, I say this as someone who believes post-scarcity (with in wise limits) for many of the basics is more or less with in our technological reach. IE. We can reach a point of post-scarcity for food, housing, clothing, energy where everyone has what they need and a good chunk of what they want as long as we don't let anyone hoard and insane amount of these resources or burn them away in frivilous uses.
Human ingenuity applied to the abundant energy available from both short and long term effects of the sun give me great hope. The barrier is basically politics.