Effect of V-shaped rib roughness on the performance of wnanofluid-based direct absorption solar collectors

Solar heating systems, which used nanofluid-based direct absorption solar collectors, have been recently introduced as the more efficient solar thermal systems because of the volumetric absorption of solar radiation within the direct absorption solar collectors. In this paper, a new direct absorption solar collector is constructed with the V-shaped ribs on its bottom surface in order to study the outdoor thermal performance of the conventional and ribbed collector, experimentally. Using graphene oxide nanofluid as the working fluid, the energy and exergy efficiencies of the conventional and ribbed direct absorption solar collectors are obtained by varying the nanofluid volume fraction, flow rate and the rib geometry (forward and backward rib). Measuring the spectral transmittance of the nanofluid samples shows that the absorbed energy fraction (F) for collector channel depth of 1 cm is 50.9%, 59.82%, 64.5% and 65.3% for nanofluid samples of 125 ppm, 250 ppm, 500 ppm and 1000 ppm, respectively. In volumetric flow rate of 15 L h(-1), the zero-loss efficiency for BR-DASC and FRDASC is increased up to about 16.6% and 11.7%, compared to DASC. These values attain 21% and 13.4% when the volumetric flow rate increases from 15 to 45 L h(-1). The optimum volume fraction of graphene oxide nanofluid is 500 ppm in conventional and forward-ribbed direct absorption solar collectors in terms of both energy and exergy efficiencies. The higher friction factors, as a result of ribs, lead to lower exergy efficiency of the ribbed direct absorption solar collector in comparison with the conventional one.