To overcome the limitations of conventional solar thermal collectors (high conductive and convective thermal resistances between the absorber and the fluid), a promising technology is represented by direct absorption solar collectors working with nanofluids, where the incoming solar irradiance is absorbed directly within the volume of fluid. The main issue hindering the diffusion of such technology is related to its reliability, since nanofluids can lose their chemical stability due to nanoparticles sedimentation. Thus, the present work aims at investigating the stability and absorption capability of two nanofluids made of Single-Wall-Carbon-NanoHorns in a volumetric solar receiver. The present investigation covers the study of material compatibility, the laboratory measurements of nanofluid absorbance and the field simultaneous measurement of thermal and optical efficiency. Since the final performance of direct absorption solar collectors strongly depends on the nanofluid stability, the double efficiency measurement allows to better verify any possible instability effect. Furthermore, field measurements during nanofluid circulation are rare in the literature. The efficiency of the volumetric solar collector is between 88 % and 92 % at null reduced temperature difference. Finally, tests are performed at high flow rate leading to an evident performance degradation, due to nanoparticles deposition, that can be reversed with sonication.