Designing Nanomaterial-infused Working Fluids for Energy Efficiency Improvement in Microstructured Copper Heat Absorbers

Passive solar-thermal systems depend on efficient heat transfer to keep absorber temperatures low. Nanofluid working media and microstructured copper interfaces can intensify boiling and rewetting using passive modifications. Integrated, stability-screened comparisons that combine nanofluids with fabricated microstructures under unified boundary conditions remain limited. This study quantifies the coupled influence of nanofluid formulation and copper surface topology on passive phase-change devices. Graphene, CuO, Al₂O₃, and TiO₂ nanofluids (0.05–0.30 wt%) were prepared and characterized for dispersion size, zeta potential, UV–Vis retention, viscosity, and thermal conductivity, and were tested in circular and flat-plate wicked heat pipes and a wickless thermosyphon with plain, hemispherical, and micropillar evaporators from 20–100 W at 0°–90° with triplicate repeats (k=2). Micropillar evaporators exhibit low resistance; the maximum reduction was 48% compared with plain copper, indicating enhanced capillary replenishment and wetted area. Graphene provides the strongest fluid effect; at 100 W on micropillars in the circular device, the thermal resistance fell from 0.210 to 0.168 °C/W, supporting coupled conductivity and interfacial conditioning. These quantified gains support absorber-interface redesign for higher energy efficiency and reduced orientation sensitivity. Future work targets long-cycle durability, fouling, and outdoor exposure of nanomaterial suspensions.