If you can't beat 'em, join 'em. In the past five years, the renewable energy sector has had to watch billions of dollars flow into midstream oil and gas, thanks to a decades-old tax break and the arrival of shale. But in the last year, some enterprising power companies have come up with a scheme of their own. The "yieldco," as it's informally known, offers a new home for renewable energy power plants, and to investors, it looks a lot like the midstream sector: pure catnip. Now, the question facing renewables advocates is this: How far can the yieldco go?