[trhway]

>Unfortunately it's too slow.

well, it depends on the target temperature you want to maintain. Radiative power grows with the forth power of temperature (https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law). So that red glow of Merlin engines mentioned by the other comenter radiates so intensively that any other method of heat transfer can hardly come close. Another example - just 5700K of the Sun surface is enough to radiate away all the power the Sun's body generates.