I didn't mean we could start in the near future, just that they don't contradict our current understanding of physics the way, say, FTL travel would. Even if sending the energy back proves to be impossible to do efficiently enough to be worth it, you could probably just use the it there. As far as resources go, the solar system is filled with them. We don't really need Mercury anyway. The purpose doesn't need to be anything beyond meeting the needs of an expanding population. For a space faring civilization, anything planetside is inefficient because gravity. But yeah, it's possible that the answer to the question is that buried in the nitty gritty details of how it would actually be built, there lie challenges that render it completely either impossible or pointlessly inefficient. But that would be curious itself, since it doesn't look like that from here.