<p>Increasing the thermal performance of PVT systems through cooling channels is a crucial concern to improve the efficiency of the PV panels by lowering their temperature. Recent studies on PVT systems’ cooling channels demonstrate notable improvements in both the design and number of the cooling channels. However, research on dimensional optimization of cooling channels to enhance flow and heat transfer characteristics with various subchannel dimensions and Re numbers is still lacking. In this study, PVT system models with four novel cooling subchannels are generated to investigate how subchannel dimensions affect the temperature distribution and the average temperature of the PV module, velocity distribution in the subchannels, pressure drop through the channels, and the figure of merit of the cooling channels with respect to varying Re numbers. Temperature distribution results indicate that the average temperature of the PV module decreases with increasing subchannel dimensions and increasing Re number. Therefore, the lowest average PV module temperature is obtained as 32.63&#xa0;°C for the subchannel dimension of 20 × 20 mm^{<CitationRef CitationID="CR2">2</CitationRef>} as Case (4) which is 8% lower than the literature value indicating higher thermal performance for the PV modules. As a result, subchannel dimensions of the PVT system are optimized at 20 × 20 mm^{<CitationRef CitationID="CR2">2</CitationRef>} for Re number of 2275 in terms of the lowest PV module temperature and the lowest pressure drop.</p>