, 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.
Vaporisation (evaporation and boiling) through direct volumetric solar collectors has recently drawn significant attention. Many studies suggested plasmonic nanoparticles, such as gold nanoparticles, to significantly enhance the photo-thermal conversion efficiency. However, there is still a lack of comparative studies of the feasibility of using gold nanoparticles for solar applications. This study performed well-controlled experiments for two different categorised particles, i.e., gold and carbon black suspended in water, and assessed their performance in terms of evaporation rate, materials cost and energy consumption. The results show that gold nanofluids are not feasible for solar evaporation applications, where the cost of producing 1g/s vapour is ~300 folds higher than that produced by carbon black nanofluids. This infeasibility is mainly due to the high cost and the low absorbance of gold comparing to carbon black nanoparticles. Moreover, this work reveals that the higher the nanoparticles concentration and/or the incident solar radiation is, more energy is trapped in a small volume of the nanofluid near the interface, resulting in a higher temperature near the interface and a higher evaporation rate. Future optimization of the system should consider concentrating more solar energy at the surface to allow the maximum amount of solar is used for evaporation.
Steam generation of nanofluid under solar radiation has attracted intensive attention from researchers. Due to strong absorption of solar energy, nanoparticle-based solar vapor generation is promising in desalination, sterilization and producing steam for electricity generation.Steam generation for different concentrations of gold nanoparticle dispersions under focused sunlight of 5 sun and 10 sun were performed in this paper. A numerical model combining radiative heat transfer, moisture transport, and laminar flow was built to investigate the temperature profile, evaporation rate above the surface and radiative intensity distribution inside the nanofluid. We found that localized energy trapping at the surface of nanofluid was responsible for the fast vapor generation. To convert more solar radiative energy into latent heat of water (i.e., to vaporize water) at the surface, a new method was proposed to optimize the range of nanofluid concentration and optical depth for solar vapor generation design.
This is a repository copy of CFD analysis of a nanofluid-based microchannel heat sink.
Nanoparticle-based volumetric solar absorption has been shown to be an effective technique to realize efficient solar harvesting. However, most of such systems under study are 8 stationary and cannot realize solar energy transport, which limits their potential applications to a 9 large extent. A novel idea of using directive absorptive nanofluids in oscillating heat pipes (OHP) 10 is investigated in this work, which would achieve efficient solar energy capture and transportation 11 simultaneously without the use of any additional pumping power. The influence of a variety of parameters such as nanoparticle type, nanoparticle concentration, nanofluids filling ratio and solar radiation intensity on the performance of OHPs are investigated. There exists an optimal filling ratio of the nanofluid for the OHP (i.e., 83%), under which single direction circulation of the working fluid is observed, where the thermal resistance of the OHP reaches the minimum. The OHP reaches an extremely high thermal conductivity, i.e., 6000 W m K , when filled with 3.0 wt% MWCNT nanofluid. The maximum energy conversion efficiency has been observed as much as 92% for the current experimental settings. It is found that strong absorption of solar energy, efficient vapor generation inside the OHP and proper configuration of the OHP should be responsible for the efficient operation of this system.
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