Abstract
Abstract This paper details an investigation into the synthesis and temperature stability testing of silver-nanofluids, aimed at producing three silver-based nanofluids with distinctive morphologies and absorption characteristics suitable for enhanced efficiency volumetric solar-thermal collectors, which could be combined with a suitable thermal storage system to provide low-carbon heating and hot-water. When combined the three silver-based nanofluids were designed to give spectrally broadband absorption of the incident solar radiation in the 300–1300 nm range. The starting point was a previously developed synthesis producing triangular silver-nanoparticles with a strong absorbance in the 850–950 nm range (Nanofluid 1). The effect of changing various reagents in the synthesis was then investigated. Increasing the silver nitrate concentration and changing the silver to reducing agent ratio produced Nanofluid 2 (strong absorbance in 650–750 nm range), containing smaller more rounded triangular nanoparticles. For Nanofluid 3, a two-step synthesis had to be adopted, with a seed nanofluid made initially by lowering the concentration of reducing agent (sodium borohydride) and oxidizing agent (hydrogen peroxide). Additional silver nitrate and reducing agent were added, giving a more concentrated nanofluid, containing small silver nanodiscs, with a 450 nm absorption maximum. These three nanofluids were combined together to give a broadband absorber and for the first time, the stability to a temperature of 70 °C measured using UV–vis–IR spectroscopy. Results indicate that although a broadband absorber based on silver can be designed it is not suitable for use without an appropriate stabilization strategy, due to a lack of spectral stability with temperature.
Highlights
There is an unmet need for low carbon heating and hot water [1]
Further understanding of the synthesis was required to achieve this. The performance of this broadband absorber and stability with simulated sunlight exposure has been published elsewhere [20] this paper focuses on the recipe development of the individual nanofluids and subsequent stability to temperature measurements, complementing and extending this previous publication
By varying the amount and timing of the reagents added during the synthesis of silver nanofluids it allowed for the development of recipes to produce 3 different morphologies of silver nanoparticles, which gave 3 distinct absorption profiles covering a broad range of wavelengths
Summary
There is an unmet need for low carbon heating and hot water [1]. Widespread heat pump use could decarbonise domestic heating but may destabilise the electricity grid [2]. An alternative approach is to use solar thermal collectors coupled with inter-seasonal storage [3]. To facilitate this it would be useful to improve the efficiencies of current solar thermal collectors. A volumetric absorber consists of a flowing working fluid, protected by a suitable glazed or glass front face. This working fluid absorbs the solar radiation directly and there is no need for a conventional selective surface absorber.
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