Characterization of light-induced, volumetric steam generation in nanofluids

Solar thermal energy has shown remarkable growth in recent years [1]. At the same time, research has shown that nanofluids - suspensions of nanoparticles in conventional fluids - can potentially enhance the efficiency of these solar thermal systems [2-7]. The fundamental advantage of this type of collector is that it can be designed to minimize the number of energy transfer steps, thereby reducing losses in converting sunlight (via thermal energy) to electricity. To investigate the inherent advantages of an optically tailored nanofluid system, we have experimentally tested the feasibility of volumetrically absorbing, direct steam collection mediums. In this study, we use a transparent testing apparatus where concentrated laser light at 532 nm (a wavelength very near the solar spectrum peak) is incident on optically absorbing mediums (samples). The samples compared in this study are black dyes, black painted surfaces, and nanofluids - with de-ionized water as a base fluid for each. Each of these converts laser light energy to heat in a localized region. The heated region is monitored with a digital camera and an infrared camera, simultaneously. Resulting temperature profiles and bubble dynamics are compared for the different samples. For pure water with a black backing, high temperatures (>300 °C) are observed with a laser input of ∼287 W/cm ². For the same laser input, nanofluids show lower peak temperatures as compared to water with a black backing, but up to 50% more vapor generation. Vapor bubbles are seen to be larger in pure water as compared to nanofluids or black dyes. These results indicate volumetric absorbers (notably nanofluids) provide a more effective coupling between light energy and liquid-to-vapor phase change than surface absorbers. That is, our results indicate that a well-designed volumetric absorbing, direct steam nanofluid collector can potentially enhance the efficiency of the light-to-steam conversion in a solar thermal system.