, 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

This work investigated experimentally the photothermal conversion efficiency (PTE) of 10 gold nanofluids in a cylindrical tube under natural solar irradiation conditions, and compared with a 11 developed 3-dimensional numerical model. The PTE of gold nanofluids was found to be much higher 12 than that of pure water, and increased non-linearly with particle concentration, reaching 76% at a 13 concentration of 5.8 ppm. Significant non-uniform temperature distribution was identified both 14 experimentally and numerically, and a large uncertainty can be caused in the PTE calculation by using 15 only one temperature measurement. A mathematical model was also developed to calculate the 16 absorption efficiency without knowing the temperature field, which can be used to predict the 17 theoretical PTE for nanofluids based on their optical properties only. 18 19

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