With intermittent sources like wind and solar becoming more common, energy storage is increasingly seen as crucial for greening the grid.Photo: mike_tnIn just about every story on renewable energy, there's a familiar cast of characters: green power developers, utilities, and sundry state and federal regulators. But there's one key player that often lurks in the background -- the grid operator. In the Golden State, most of the power grid is controlled by the California Independent System Operator. Based in a suburb of Sacramento, Cal ISO, as it's known, essentially ensures that electricity supply and demand stay in balance to prevent …