The paper is devoted to the topic of direct absorption solar collectors (DASCs). The main focus is on comparing two kinds of working fluids that can be used in DASCs, namely carbon black nanofluids and biodegradable coffee colloids. At first, the fluids were tested by exposing them to artificial light (indoor experiments) and real solar irradiation (field experiments). The indoor experiments used a system of relatively large geometry, so most of the heat absorption occurred close to the

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... ed surface. This resulted in a similar performance by both fluids. The field experiments involved a smaller system, and the light was concentrated by a solar collector. Here the nanofluid outperformed the coffee colloids. Next, the process was analysed using a theoretical analysis that gave good correspondence with the experiments. Finally, we extended the theoretical analysis to a DASC with a flowing fluid. The model was validated against results from the literature, but it also supported our experimental findings.