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 f ree 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 imp lication is that the radiative power spectral density within a thermophotov oltaic 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 tran sferred 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 larg est contribution to the transmission of radiation. The black body, the obje ct, and the gap region are all modeled as lossless dielectrics.