[Minneapolis/St. Paul Business Journal] Target Corp. is spending millions of dollars to cuts its energy costs and secure a greener image in the process. The Minneapolis-based retailer has installed solar panels on the roofs of four of its California retail stores and plans to install similar systems at 14 more locations later this year. The solar-panel systems will generate about 20 percent of those stores’ annual energy requirements. Target joins a growing number of retailers and other major corporations that are adopting alternative-energy sources. Rival discount retailer Wal-Mart Stores Inc. also has adopted renewable energy sources at some of its stores, though it only has dabbled in solar power.