A production tax credit for the wind power industry expires Dec. 31, but it already has taken a toll on Hall County’s ZF Wind Power, a $98 million plant that opened last year amid international fanfare.“The timeline to install new turbines is about six months and so ... you can do the math,” said Elizabeth Umberson, president of 1925 New Harvest Drive, off Calvary Church Road.The approaching deadline has had a “chilling” effect on orders, causing them to drop dramatically.“We were supposed to be at full production levels, with a staff of 250, and we’re at 160 today,” Umberson said. “And we’ve capped that.”The plant’s opening in Sept. 27, 2011, drew a crowd of more than 200 people, with company representatives from Germany and a variety of state and local dignitaries, including Gov. Nathan Deal, in attendance.Also speaking at the event was Johnny Hoy Henriksen, senior vice president with Vestas, which receives ZF gearboxes at its Colorado wind turbine plant.The expiring tax credit gives wind manufacturers 2.2 cents for every kilowatt hour produced, said Jennette Gayer of Atlanta-based Environment Georgia.“It assumes that wind (energy) is going to be a little bit more expensive, so to help encourage manufacturers to build turbines and feed into the (electricity) grid, it gives a small incentive per kilowatt hour to do so,” she said.“It’s an unfortunate situation for a couple types of renewables.