The global generating capacity of wind farms has been overestimated and the world may not have access to as much wind power as thought, U.S. researchers say. "People have often thought there's no upper bound for wind power -- that it's one of the most scalable power sources," Harvard University applied physicist David Keith says. The thought is based on the belief gusts and breezes aren't likely to "run out" on a global scale in the way oil wells might run dry, he said in a Harvard release Monday. But an atmospheric modeling study, published in the journal Environmental Research Letters, suggests a law of diminishing returns when it comes to the largest of wind farms. Every wind turbine creates a downwind "shadow" in which the air has been slowed by drag on the turbine's blades, so turbines have to be spaced far enough apart to reduce the effect of these wind shadows. But as wind farms grow larger, Keith said, they start to interact and regional-scale wind patterns matter more. The generating capacity of very large wind power installations, larger than around 40 square miles, may top out between 0.5 and 1 watts per square meter, much lower than previous estimates that ignored the turbines' slowing effect on the wind, the study researchers said. "One of the inherent challenges of wind energy is that as soon as you start to develop wind farms and harvest the resource, you change the resource, making it difficult to assess what's really available," study co-author Amanda S. Adams of the University of North Carolina at Charlotte said. That means wind power's ability to contribute to global energy needs may need to be reanalyzed, Keith said. "It is still one of the most scalable renewables but our research suggests that we will need to pay attention to its limits and climatic impacts if we try to scale it beyond a few terawatts," he said.