184 Energy Optimization in Process Systems 5.4. FLUX OF CLASSICAL EXERGY Consider now the flow of black photons. The total time derivative of the exergy of enclosed volume B = 1 - T 0 T 4aVT 3 T + a 4 (3T 4 - 4T 3 T 0 + T 0 ) V 3 (5.57) does not represent the exergy flux. The situation is similar to that for energy at flow when the time derivative of the enclosed energy is not the energy flux because the latter incorporates the work against the pressure forces. As the energy flux is the product of the enthalpy density and the volume flux, the flux of radiation exergy B f satisfies the general thermodynamic formula B f B + (P - P 0 ) V Using this formula and Equation (5.48) one obtains for radiation fluid B f B + (P - P 0 ) V = {h v - h v0 - T 0 (s v - s v0 )} V (5.58) where subscript v refers to the respective quantity per unit volume (density).