Tyagi theoretically investigated the performance of a
direct absorption solar collector(DAC) exploiting aluminum-water
nanofluid as the absorbing medium. Fig. 1 shows the schematic of
a nanofluid-based DAC of their study with glass surface on the top
and completely isolated at the bottom side. They supposed a
steady-state two dimensional model for heat transfer.By using the
following equation,the collector efficiency is obtained: