, Steam generation in a nanoparticle-based solar receiver, Nano Energy, http://dx.doi.org/10. 1016/j.nanoen.2016.08.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Steam production is essential for a wide range of applications, and currently there is still strong debate if steam could be generated on top of heated nanoparticles in a solar receiver. We performed steam generation experiments for different concentrations of gold nanoparticles dispersions in a cylindrical receiver under focused natural sunlight of 220 Suns. Combined with mathematical modelling, it is found that steam generation is mainly caused by localized boiling and vaporization in the superheated region due to highly non-uniform temperature and radiation energy distribution, albeit the bulk fluid is still subcooled. Such a phenomenon can be well explained by the classical heat transfer theory, and the hypothesized 'nanobubble', i.e., steam produced around the heated nanoparticles, is unlikely to occur under normal solar concentrations.In the future solar receiver design, more solar energy should be focused and trapped at the superheated region while minimizing the temperature rise of the bulk fluid. Graphical abstract

Direct absorption nanofluid has been introduced to as an effective alternative to increase the solar thermal conversion efficiency. Hybrid nanofluids were also recently proposed to broaden the absorption spectrum, however, a comparative assessment of the performance of commonly used nanomaterials for solar energy harness is still lacking. This study performed a well-controlled experiment for three different categorised particles, i.e., gold, copper, carbon black nanofluids and their hybrids, and assessed their performance in terms of photothermal conversion efficiency (PTE), specific absorption rate (SAR) and materials cost. Contrary to previously reported, the PTE was not increased by blending different nanofluids with different absorbance peaks, mainly due to the dilution of nanoparticle concentration. Though having high SAR, the high cost of gold prevents its practical use, and carbon black appears to be more feasible. The theoretical PTE can be well predicted by the optical properties of the nanofluids used.

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