Very large radiative transfer over small distances from a black body for thermophotovoltaic applications

The;maximum amount of radiated heat intensity which can be transferred from a black body of refractive index n(BB) to an object of refractive index n(OBJ) located a short distance-away is shown to be n(smaller)(2) times the free space Planck distribution, where n(smaller) is the smaller of n(BB) and n(OBJ), and where n(BB) and n(OBJ) are assumed greater than unity, The implication is that the radiative power spectral density within a thermophotovoltaic cell could be designed to be much greater than the free space Planck distribution. The maximum radiative intensity transferred occurs when the index of the black body matches that of the object at wavelengths where the Planck distribution is sizeable. A simple expression is found for the transferred radiative intensity as a function of the refractive indices of, and the distance separating, the black body and the object. This expression is interpreted in terms of the specific black body modes which are evanescent in the space between the black body and the object and which make the largest contribution to the transmission of radiation. The black body, the object, and the gap region are all modeled as lossless dielectrics.