Entropy density and entropy flux in near-field thermal radiation
  We propose a method to evaluate the entropy density and entropy flux in a vacuum gap between two half-spaces that takes into account influence of near-field effects, i.e., interference, diffraction, and tunneling of waves. The method developed is used to determine the maximum work that can be extracted through near-field radiative transfer between two half-spaces at different temperatures. 
Arvind Narayanaswamy and Yi Zheng
cond-mat.mes-hall cond-mat.stat-mech physics.class-ph