High temperature optical properties of nanoparticle suspensions for direct solar absorption

Abstract

Novel approaches for solar energy conversion are increasingly garnering research and commercial interest, particularly as hybrid thermal and electrical energy sources. Additionally, the need for systems capable of producing thermal energy at temperatures up to 300°C is growing as a means to provide process heat to industry and distributed generation for small communities. One such concept under rapid development since 2008 is the use of nanoparticles suspended in a heat transfer fluid that is directly exposed to incoming solar irradiance. Such collectors, often referred to as direct absorption solar collectors or volumetric solar collectors, have primarily been investigated (experimentally and numerically) at low temperatures due to the challenge of creating long-term stable suspensions at temperatures above 100°C. Working fluids with boiling points well above 100°C are not well investigated for nanoparticle dispersion. The likely reason for this gap is that many high temperature fluids are non-polar, which makes the stability problem even more challenging. Additionally, most surfactants work best in water and break down above 100oC. Thus, even though many solar collectors operate above 100oC and many applications require >100oC heat, only a limited number of investigations have measured the optical properties of direct absorber liquids after exposure to these temperatures. To our knowledge no study has investigated nanofluid optical properties above 200°C. This represents critical missing data in the field, since high temperatures can affect particle stability, particle morphology, and plasmon resonance all which can lead to spectral changes in transmittance while changes in the fluid optical properties can occur at temperature. Additionally, the large coefficient of thermal expansion associated with common heat transfer fluids can result in significant changes to the working volume fraction leading to reduction in overall absorption magnitude. In this study, the optical properties of selected fluids in the solar spectrum were measured from room temperature up to 300°C. Glycol, silicon, hydrocarbon, diphenyl-oxide/biphenyl based fluids with nanotubes and indium tin oxide nanoparticles suspended with various surface treatments. As such, this paper provides a vital set of optical property data to enable further development of promising candidates for broadband spectrally selective nanofluid solar absorbers.