Light-induced energy conversion in liquid nanoparticle suspensions

Liquid nanoparticle suspensions, popularly called “nanofluids” in the heat-transfer literature, have been widely investigated for their interesting thermal conduction and convection properties (see, e.g., [1–6]). The radiative properties of nanofluids, however, have been much less studied [7]. It is readily apparent to anyone who has observed a prepared nanofluid, though that nanofluids strongly scatter and absorb visible light. This observation motivated the concept that nanofluids can be used as direct volumetric absorbers for solar radiation [8–16], leading potentially to more efficient solar energy conversion that may be termed volumetric photothermal energy conversion. The basic idea is schematically presented in Figure 5.1. A flowing nanofluid is exposed to incident light, which can be concentrated or not, through a transparent cover. Absorption of light within the nanofluid, largely by the nanoparticles, leads to a temperature rise in the nanofluid that can then be exploited as thermal energy. Continuing research demonstrates that not only can sensible nanofluid temperature increases occur, but also boiling can take place around the nanoparticles. This gives rise to the possibility that direct-absorption nanofluids, like those under investigation here, can be utilized for concentrating-solar, direct-steam generation, a promising direction for concentrating solar power technology [11,15,17–20].