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To improve the efficiency of total conversion of solar thermal collectors, the conversion of solar energy into thermal energy has to be improved, for example by improving the solar absorption.

The typical solar collectors use black absorbers in the outer surface of tubes. Among the limitations of this configuration is the fact that at high levels of solar radiation a significant temperature difference between the absorber layer and the fluid is established. This difference leads to emissive losses and therefore to a reduction of the conversion efficiency. To overcome this problem, a valid alternative under study is the use of a solar receiver based on a volumetric absorber, or on a system in which the radiation is absorbed directly by the fluid that delivers the heat energy. In this topic, a recent development in solar collectors is the use of nanofluids as light absorbers. In fact, the improvement in heat transfer in solar collectors is a critical point for the enhancement of the overall system efficiency and compactness and the use of solid additives of nanometric size in base fluids is an innovative technique, which is designed also to improve the overall transfer of heat.

In recent years the research group of ICMATE, in collaboration with the INO and ITC CNR Institutes and with Carbonium Srl, has developed fluids containing single walled carbon nanohorns. These consist of a single graphene sheet wrapped in a irregular tubule with a variable diameter (generally 2-5 nm and a length of 30-50 nm), that typically arranges to form nearly spherical aggregates classified into three types called dahlia, bud and seed. One of these nanostructures, dahlia, is shown here to the left as an example.
Stable dispersions of these nanostructures were then obtained both in water and in glycol and optical measurements have shown that such fluids absorb the entire spectrum of solar radiation even at very low concentrations (0.005 wt%).

    nanofluids volumetric solar absorption single walled carbon nanohorn optical properties

    • E. Sani, S. Barison, C. Pagura, L. Mercatelli, P. Sansoni, D. Fontani, D. Jafrancesco, F. Francini
      Carbon nanohorns-based nanofluids as direct sunlight absorbers
      Optics Express, 2010, 18 (5), 5179-5187
    • L. Mercatelli, E. Sani, G. Zaccanti, F. Martelli, P. Di Ninni, S. Barison, C. Pagura, F. Agresti, D. Jafrancesco
      Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers
      Nanoscale Research Letters, 2011, 6, 282
    • E. Sani, L. Mercatelli, S. Barison, C. Pagura, F. Agresti, L. Colla, P. Sansoni
      Potential of carbon nanohorn-based suspensions for solar thermal collectors
      Solar Energy Materials and Solar Cells, 2011, 95, 2994-3000
    • E. Sani, P. D. Ninni, L. Colla, S. Barison, F. Agresti
      Optical Properties of Mixed Nanofluids Containing Carbon Nanohorns and Silver Nanoparticles for Solar Energy Applications
      Journal of Nanoscience and Nanotechnology (2015) 15(5) 3568-3573
    • L. Fedele, L. Colla, S. Bobbo, S. Barison, F. Agresti
      Experimental stability analysis of different water-based nanofluids
      Nanoscale research letters, 2011, 6, 1

    • Accordo di Programma CNR-Ministero dello Sviluppo Economico per la Ricerca di Sistema Elettrico

    • CNR ITC
    • CNR INO
    • University of Padova