Vijayanandan, A.S.Kandath Valappil, R.S.Mohan Balakrishnan, R.M.2026-02-052020Sustainable Energy Technologies and Assessments, 2020, 37, , pp. -22131388https://doi.org/10.1016/j.seta.2019.100598https://idr.nitk.ac.in/handle/123456789/24065An attempt has been made to compare the optical properties of cobalt oxide (Co<inf>3</inf>O<inf>4</inf>) nanoparticles using experimental values and theoretical predictions. Optical transmittance of the nanoparticles obtained was higher than 65% in 550–850 nm containing visible spectrum and the experimental results were in accordance with the predictive datum. The absorption coefficient peak observed is close to the predictive value and is present in the visible region of the light. In addition, there was an excellent agreement between theoretical and experimental results in extinction coefficient and refractive index. Besides, this work proposes and validates a novel idea of using Co<inf>3</inf>O<inf>4</inf> nanofluids to enhance solar thermal conversion efficiency. Co<inf>3</inf>O<inf>4</inf> nanofluids synthesized using endophytic fungus Aspergillus nidulans isolated from a medicinal plant, Nothapodytes foetida has been used to illustrate the energy storage capacity of nanofluids. Experimental results reveal that Co<inf>3</inf>O<inf>4</inf> nanofluids have good specific absorption rate (SAR) and better photo-thermal conversion efficiency than water. Nanofluid exhibited a greater temperature gradient than pure water, which is desired. Thus the good absorption ability of Co<inf>3</inf>O<inf>4</inf> nanofluids for solar energy indicated that it is suitable for direct absorption solar thermal energy systems. © 2019AspergillusCobalt compoundsConversion efficiencyEnergy storageNanoparticlesRefractive indexSolar energySolar heatingWater absorptionAbsorption co-efficientEnergy storage capacityExtinction coefficientsPhoto-thermal conversionsSpecific absorption rateNanofluidicsabsorption coefficientextinction coefficientfungusnanomaterialoptical propertyrefractive indexsolar powerEmericella nidulansFungiNothapodytes foetida