  We show that the maximum work that can be obtained from the thermal radiation
emitted between two planar sources in the near-field regime is much larger than
that corresponding to the blackbody limit. This quantity as well as an upper
bound for the efficiency of the process are computed from the formulation of
thermodynamics in the near-field regime. The case when the difference of
temperatures of the hot source and the environment is small, relevant for
energy harvesting, is studied in detail. We also show that thermal radiation
energy conversion can be more efficient in the near-field regime. These results
open new possibilities for the design of energy converters that can be used to
harvest energy from sources of moderate temperature at the nanoscale.
