BRIDGEPORT, CONN. — How can it be a power plant if there’s no smokestack? Ironically, Bridgeport’s huge 14.9-megawatt zero-emission fuel cell park, the largest in North America, is already partly built but was just announced last Friday in a gritty industrial city whose skyline is still dominated by the red-and-white-striped stack of an ancient but still operating coal plant (one of the state’s “filthy five”).