Direct absorption solar collectors (DASC) use nanofluids for the simultaneous absorption and transport of solar heat. The volumetric absorption of solar radiation reduces the collector's thermal loss to the environment. As shown in the literature, the thermal efficiency of lab-scale prototypes was better than that of solar collectors with surface absorption. However, these nanofluids are expensive and contain toxic nanoparticles of metals and carbon. There are no recorded field tests of a full-scale DASC prototype without solar concentration. This paper demonstrates how to produce an aqueous nanofluid using an inexpensive organic coffee powder. The size of the particles in the nanofluid is about 100 nm, and the extinction coefficient can be set up to 700 m^-1. We present the design of a field-scale DASC prototype with a surface area of 1.6 m². The design is obtained using a numerical model of the DASC. We describe the results of a field study of the prototype in the climate conditions of Western Norway. The best thermal efficiency of the DASC prototype was ~85% which was higher than the efficiency of a commercial flat-plate collector operating under similar conditions.