Decarbonizing the heat for residential use by absorbing the solar energy through carbon nanofluids

Over recent years, low and medium temperature (< 200 °C) solar thermal collectors have proved to be a reliable solution to supply heat and decarbonize the thermal needs in residential and industrial sectors. The possible market for solar thermal collectors is still huge, but it is important to improve the technology and reduce the costs, in order to make it a competitive alternative to fossil-fuel based systems. In conventional solar thermal collectors, the solar irradiance heats up an absorber surface with a selective coating and part of the heat is transferred to the working fluid. Conductive and convective thermal resistances between the absorber and the fluid make the effectiveness of the solar-to-thermal energy conversion quite limited because of the high heat losses from the absorber surface to the surroundings. To overcome these limitations, direct absorption solar collectors (DASCs) have recently emerged as a promising technology where the incoming solar irradiance is absorbed directly within the volume of the working fluid. In such way, DASCs can benefit from the absence of the absorber surface, which represents the main cost item in conventional solar collectors. Since most of the employed working fluids (i.e. water or ethylene glycol) display low absorption coefficients, the addition of small concentrations of carbon nanoparticles can greatly enhance their optical properties and improve the efficiency of the solar collector. The main issue hindering the diffusion of DASC technology is related to its reliability, since nanofluids can lose their chemical stability due to nanoparticles agglomeration and sedimentation. Thus, the present work aims at investigating the stability and the absorption capability of two nanofluids made of Single-Wall-Carbon-NanoHorns (SWCNHs) in a volumetric solar receiver. The effect of circulation, working temperature and nanoparticles concentration on the stability of the nanofluids is discussed. Although the higher-concentration nanofluid leads to 5% increase in the thermal efficiency of the DASC, it has proven to be less stable compared to the low-concentration nanofluid. With the tested nanofluids, the thermal efficiency of the volumetric solar collector is between 88% and 92% at ambient temperature.