In this work, the thermal performance of a conventional tubeless collector is improved by inserting porous substrates at the inner side of the collector absorber plate. The porous substrates improve the convective heat transfer coefficient between the absorber plate and the fluid. This improvement is investigated numerically and its effects on the efficiency and the useful gain of the collector are evaluated. It is found that inserting the porous substrate may raise the collector efficiency by 3–32% especially at high values of the overall heat loss coefficient.