A solar power generation system includes a solar receiver disposed on a tower that receives radiant heat reflected from a field of solar collectors. The solar receiver includes an evaporator having a plurality of vertically oriented tubes to form a panel for receiving a fluid, such as water and/or steam, wherein the tubes have a rifled internal surface. The fluid within the tubes has a mass flow greater than 0.2x106 lb/hr/ft3 at a pressure in the range of 100-2850 psia, wherein radiant heat fluxes on the outside of the tubes exceed 185,00 btu/hr/ft2.