Deep eutectic solvent (DES) based nanofluids have gained ample attention owing to their extraordinary thermophysical properties such as wide temperature range and thermal stability. While poor static stability of DES based nanofluids heavily hinders their practical application due to the incompatibility with dispersant. Herin, a novel ZnO nanofluids using ethylene glycol and potassium acetate DES for solar thermal utilization was developed. With the aim at addressing the poor stability, a one-step in situ synthesis involving microwave-induced dehydration was employed to prepare self-dispersing ZnO nanoparticles without external dispersants. Thermophysical properties and photothermal performance of nanofluids with varying mass fractions (0.5-5 wt.%) were systematically investigated. Results indicate that ZnO inclusion significantly improves thermal conductivity and photothermal conversion. Specifically, the 5 wt.% sample exhibited a 12% increase in thermal conductivity at 65℃ compared to the base fluid, while the 0.5 wt.% sample demonstrated optimal photothermal response under low light intensity. Additionally, the fluids displayed anomalously enhanced specific heat capacity (up to 14.6%), attributed to the formation of ordered interfacial liquid layers on the high-surface-area ZnO nanoparticles through electrostatic interactions and hydrogen bond rearrangement, offering dual advantages in heat transfer and storage, while maintaining dispersion stability for approximately two weeks, which thus presents a low-cost, stable, and environmentally friendly strategy for developing heat transfer fluids suitable for medium-to-high temperature solar collection systems.