The U.S. Department of Energy says wind energy prices have hit an all-time low and the cost of installing turbines has fallen 20 to 40 percent in the past five years.  The agency’s new report says last year’s wind-power price contracts with utilities averaged under two-and-a-half cents per kilowatt hour, down from seven cents in 2009.  That could mean a resurgence of interest and investment in wind farms.  The Energy Department report also says wind turbines are changing, allowing them to produce more power at lower wind speeds.