The Sun is the limit. Even if there were another source that could compare in abundance, we still couldn't use more energy than we get from the Sun. The limit is not how much energy we can get, but how much energy Earth can get rid of (via radiation). We can use other energy sources, but every joule of heat we generate has to be balanced by a joule of sunlight we reject. We can either harness the power of the Sun, or reject it.