Magnetic Photothermal Nanofluids with Excellent Reusability for Direct Absorption Solar Collectors

Solar thermal collector and heat exchanger are two indispensable components in the practical application of nanofluids in photothermal technology. There is inevitable heat loss produced by the heat exchanger. Here, we propose a new concept “magnetic photothermal nanofluids”, which not only can avoid the heat transfer process of the heat exchanger but also can use the base liquid of nanofluids directly. The magnetic cobalt nanoparticles (NPs) embedded in nanoporous carbon (Co@NC) are prepared via carbonization of zeolitic imidazolate framework-67 (ZIF-67). The obtained Co@NC retains the morphology of the original ZIF-67, and the magnetic Co nanoparticles distribute evenly in the nanoporous carbon. Co@NC has an excellent magnetic property and broad absorption in the visible and infrared regions. The maximum photothermal conversion efficiency has been achieved by 100 ppm of Co@NC/EG nanofluids at carbonization temperature of 900 °C, which is 99.6% at 60 s. This magnetic photothermal nanofluid can be reused at least 60 successive cycles without significant loss of photothermal conversion efficiency. This study paves a new avenue for direct use of the base liquid of nanofluids in the solar thermal conversion technique.