As solar thermal energy systems are an important pillow toward green energy production, the enhancement of their thermophysical properties using nanofluids is a highly relevant topic. However, when nanofluids are designed by the addition of nanoparticles (NPs), their colloidal stability is frequently impaired during high-temperature processing, a phenomenon related to particle size, morphology, and concentration. In this work, we synthesized nanofluids composed of ligand-free colloidal CuNPs dispersed in ethylene glycol by continuous-flow, picosecond-pulsed laser ablation in liquids synthesis, yielding monomodal-CuNPs with mean diameters of 2.5 and 4.8 nm. The nanofluids’ thermal conductivity (knf) was measured using a guarded-hot-plate method in the temperature range from 298 to 318 K. We observed a nanoparticle surface area-dependent enhancement of the knf up to 30 % at ultra-low volume concentration of 20 ppm. This corresponds to 30 times higher concentration-normalized knf in comparison to the state-of-the-art, while the resulting nanofluids retain their rheological properties. The findings are matched with Yu–Choi’s theoretical model calculations, indicating that heat transfer at the nanoparticle-solvent interface is driven by an interfacial layer of solvent molecules. These findings highlight the suitability of laser-fabricated ligand-free CuNPs as additives for heat transfer fluids, maximizing performance in mid-temperature heat transfer applications like solar thermal collectors.