Single-Wall-Carbon-Nanohorns (SWCNHs) based nanofluids have been proven to be promising media for the direct absorption of solar radiation due to their favourable optical properties and potential low cost. Still, their stability in real working conditions is an open issue because they have been studied mainly under stagnant conditions while limited information is available on the performance of these nanofluids during circulation in real systems. In the present work, the optical behaviour of SWCNHs based nanofluids has been investigated with the aim of detecting possible effects of circulation and exposure to radiation, avoiding other effects such as thermal instability. An ad-hoc experimental apparatus has been realized to check the stability of the circulating fluids in-situ using a novel approach based on the use of pyranometers. Three suspensions are tested, two are surfactant-stabilized and one is based on pre-oxidised and functionalised SWCNHs. Efficiency values higher than 90 % were measured for more than 65 hours of circulation. The effects of fluid circulation and exposure to solar radiation have been addressed, finding that the absorption efficiency decreases during tests due to the degradation of the optical properties and the nanofluid circulation is the main responsible for such degradation.