The temperature stability and development of a broadband silver nanofluid for solar thermal applications

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.