Investigating the use of silver nanofluids for solar collectors connected to a thermal storage system

Abstract

This work focuses on a potential decarbonising solution for heating and hot water, namely enhanced volumetric based solar thermal capture with silver nanofluids combined with thermal storage. This was shown to be a feasible solution for UK homes < median demand of 12,000 kW h y^-1, > average roof area of 15 m² , space for a thermal store and utilising an enhanced efficiency collector > 70 % efficiency. Morphologically distinct nanofluids were synthesised with different optical properties, including for the first time experimentally, a novel broadband silver absorber produced by combining three silver nanofluids. The morphology was assessed using transmission electron microscopy (TEM), the potential absorption efficiency estimated using UV-vis and UV-vis-IR spectrophotometry and the performance measured in static testing utilising a solar simulator. To assess the nanofluids suitability for the application, one potential stabilisation strategy, namely SiO₂ coating was evaluated. The stability of the nanofluids, colloidal (storage in the dark at 4 °C ≤ 603 days), elevated temperature (70 °C), under simulated sunlight and natural sunlight was measured. The stability of the novel broadband absorber under flow conditions was also evaluated. The uncoated silver nanofluids showed a measured efficiency in the static solar simulator of ≈ 84 % for the novel silver broadband absorber and ≈ 85 % for the less dilute silver nanoprisms similar to the estimations from UV-vis spectrophotometry (< 10 % difference). In contrast the SiO₂ coated silver nanoprisms had a measured performance of 72 % – 77 % lower than the estimations obtained from the UV-vis spectrophotometry. The stability testing showed the nanofluids to be morphologically unstable and in the case of the silver nanoprisms unstable to aggregation in the longer term natural sunlight tests. Coating with SiO₂ reduced the morphological stability questioning the use of this stabilisation strategy. Although this work has highlighted the importance of application relevant stability testing it is currently not possible to recommend the use of these silver nanofluids for solar and elevated temperature applications. 

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Identifiers

ORCID for Harriet Jill Kimpton: orcid.org/0000-0002-3219-217X

ORCID for Xunli Zhang: orcid.org/0000-0002-4375-1571

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