Radiation energy transfer is modeled as the enthalpy flux of photons across the boundary of a thermodynamic system. It is proved that this energy transfer process can be treated as heat transfer. Compression work must be applied to the system to push the photons out. The energy transfer rate and maximum conversion efficiency computed from the model are identical to those determined from the Stefan-Boltzmann law and the Carnot efficiency for blackbody radiation. (C) 2008 Elsevier Ltd. All rights reserved.