  We analyse the process of conversion of near-field thermal radiation into
usable work by considering the radiation emitted between two planar sources
supporting surface phonon-polaritons. The maximum work flux that can be
extracted from the radiation is obtained taking into account that the spectral
flux of modes is mainly dominated by these surface modes. The thermodynamic
efficiencies are discussed and an upper bound for the first law efficiency is
obtained for this process.
