More seriously, space is pretty cold, and will consume large amounts of radiated heat. The problem, of course, it that the amount you can radiate thermally at, say, 150°C is pretty limited. According to the Stephan-Bolzmann equation, it's about 1800W for a perfect black body. For 5GW, that would take a square radiator 1.7km wide, always concealed from sunlight. Realistically, much larger as the temperature would drop as the coolant flows along.