Nanofluids for advanced thermal management: A multi-criteria review of performance, cost, and environmental impacts

Nanofluids—engineered suspensions of nanoparticles in conventional fluids—have emerged as next-generation coolants due to their exceptional thermal properties. This review synthesizes recent progress in nanofluid research with a multi-criteria evaluation of thermophysical performance, cost-efficiency, and environmental impacts. Carbon-based nanofluids, particularly graphene and multi-walled carbon nanotubes (MWCNTs), dominate current research (36%) owing to their ultrahigh thermal conductivities (∼4000–6000 W/m·K) and favorable dispersion stability. In contrast, metal-based (28%) and metal oxide-based (24%) nanofluids offer a trade-off between performance and affordability, with Al 2 O 3 (44%) emerging as the most studied oxide. Hybrid nanofluids, although representing only 12% of studies, demonstrate synergistic thermal enhancements—boosting thermal conductivity up to 42.7% and reducing peak battery temperatures by as much as 26.4% in advanced battery thermal management systems. Despite their performance gains, nanofluids face economic and operational challenges. Costs range widely—from $180 for 4L of 2% Al 2 O 3 nanofluid to $85,000 for gold-based formulations. Viscosity models show that even small increases in nanoparticle concentration significantly elevate flow resistance, demanding greater pumping power. Environmental concerns remain significant, as certain nanoparticles commonly used in thermal applications—such as Al 2 O 3 and Ni—have extremely low reference dose thresholds (e.g., <0.01 mg/kg/day), indicating that even minimal exposure could pose toxicological risks to human health and the environment. A nanoparticle scoring framework integrating thermal performance, cost, and environmental impact identifies graphene and low-grade MWCNTs as optimal additives. This review concludes that while nanofluids hold transformative potential for energy, electronics, and transportation sectors, further efforts are required to enhance their economic viability, environmental safety, and long-term stability for widespread industrial adoption. • Introduces multi-criteria nanoparticle scoring for optimized selection. • Proposes novel index combining cost, toxicity, and conductivity. • Establishes graphene as optimal through normalized performance analysis. • Integrates environmental toxicity into nanofluid performance assessment. • Bridges nanoscale materials research with system-level BTMS design.