Two of the world’s leading oil producers have almost overnight joined some of the biggest players in wind power in the United States, accelerating a trend of large corporations investing in the rapidly growing alternative-energy field. Shell is one of the nation’s top five generators of wind power, while BP‘s Alternative Energy group – launched 16 months ago – aims to develop projects that produce 550 megawatts of electricity this year, one-sixth of the projected US wind energy output in 2007.