Energy and exergy analysis and optimization of low-flux direct absorption solar collectors (DASCs): Balancing power- and temperature-gain

The objective of this work is to determine the balance between power gain and temperature gain in nanofluid-based direct absorption solar collectors (DASCs). The radiative transfer and energy conservation equations were numerically coupled and solved. It was observed that, despite the low exergy efficiency in low-flux DASCs, the design point at which exergy efficiency is maximized provides a compromise between power- and temperature-gain. The trade-off between power gain and temperature gain was investigated where it was observed that a collector efficiency of up to 55% can be obtained without sacrificing temperature gain. Beyond that, the trade-off between power- and temperature-gain dominates. Moreover, by expanding the parameter space, some seemingly contradictory results reported in the literature have been examined and justified regarding the effects of collector length on energy efficiency and flow velocity on exergy efficiency. Finally, the effect of nanofluid thermal conductivity on collector performance was found to be a function of the dominant solar radiation absorption mechanism. Comparing different base fluids, it was found that for water the energy efficiency was less sensitive to changes in nanofluid thermal conductivity when compared to oil.