The comparison of colloidal, optical, and solar collection characteristics between Fe2O3 and Fe3O4 nanofluids operated in an evacuated tubular volumetric absorption solar collector

The iron oxide nanoparticles as low-cost, eco-friendly, and efficient nanomaterials exhibit interesting optical properties which make them valuable for nanofluids-based volumetric absorption solar collectors. The first aim of this study is to experimentally compare the colloidal stability and optical properties of Fe2O3 and Fe3O4 nanofluids in three volume fractions of 0.005, 0.01, and 0.02%. To the following, it is intended to examine the energy conversion efficiency of these nanofluids in an evacuated tubular volumetric absorption solar collector according to the ASHREA standard. Based on the results, the size and morphology of the iron oxide nanoparticles were nearly similar together, but when dispersed in water they exhibited different colloidal and optical properties. For example, the Fe2O3 nanofluids showed considerable spectral extinction coefficients in the visible range, while the Fe3O4 nanofluids presented limited extinction coefficients in the ultraviolet region with the lower values. At penetration depth of 4 cm, the 0.02%-Fe2O3 nanofluid resulted in a 100% solar weighted absorption fraction in comparison to 83.2% of the Fe3O4 nanofluid with the same volume fraction. According to the collector results, the Fe2O3 nanofluids displayed greater enhancement in the solar to thermal energy conversion than the Fe3O4 nanofluids. The maximum efficiency was 73% at 0.02%-Fe2O3 nanofluid and 51% at 0.02%-Fe3O4 nanofluid.