Time: Great big stores cost a fortune to heat and leave a huge carbon footprint, so both economic and environmental self-interest argues for innovation. Walmart has put windmills in a few of its parking lots; Target has plants on some of its roofs to harvest rainwater and cool the stores in summer. Now Ikea, the world’s favorite Swedish home furnisher, is trying to give America a gentle shove into using renewable resources. It is working with the U.S. Department of Energy’s National Renewable …