Evaluation of the thermal performance of various nanofluids used to harvest solar energy

The use of alternative energy sources is nearly the only way to meet the energy needs of people worldwide caused by the exhaustion of natural fuel sources. Currently, solar energy is the most affordable and environmentally sustainable energy resource. However, the efficiency of the existing solar collectors is below 50%. There are many technological approaches to increase the heat transfer efficiency of solar collectors, which imply the enhancement of solar radiation uptake. The efficiency of solar collectors was also found dependent upon the properties of the working liquid. Nanofluids are used to improve the thermal properties of solar collectors as they are characterized by good radiation absorption performance. This paper examines the thermal performance of different nanofluids used to collect solar energy at the volumetric flow rate ranging from 50 to 100 ml/h using a GALMET heat exchanger. The examined nanofluids are aqueous solutions composed of Al2O3, SiO2, TiO2, and graphene nanoparticles (0.500…2wt.%). With these nanofluids, direct absorption solar collectors (DASCs) demonstrated efficiency improvements of up to 5%. The effect of flow hydrodynamics on heat transfer capacity and thermal efficiency of solar collectors has been studied. The thermal performances of nanofluids were as follows: graphene-based, 10.755 kJ/m; TiO2-based, 9.414 kJ/m2; SiO2-based, 7.467 kJ/m2; and Al2O3-based, 5.714 kJ/m2. The efficiencies of solar collectors using these nanofluids as a working fluid were 64%, 59%, 57%, and 52%, respectively. The multi-factor cluster analysis revealed that graphene nanofluid was the most efficient choice.