Characterization of a nanofluid volumetric solar absorber / steam generator

Solar thermal energy has shown remarkable growth in recent years - incorporating many new technologies into new applications [1]. Nanofluids - suspensions of nanoparticles in conventional fluids - have shown promise to make efficient volumetric-absorption solar collectors [2-4]. It has also been shown that concentrated light energy can efficiently cause localized phase change in a nanofluid [5]. These findings indicate that it may be advantageous to create a 'direct, volumetric nanofluid steam generator'. That is, a solar collector design which could minimize the number of energy transfer steps, and thus minimize losses in converting sunlight (via thermal energy) to electricity. To study this, we use a testing apparatus where concentrated laser light at 532 nm - a wavelength very near the solar spectrum peak - is incident on a highly absorbing sample. The highly absorbing samples compared in this study are black dyes, black painted surfaces, and silver nanofluids - with de-ionized water as a base fluid. Each of these samples converts light energy to heat - to varying degrees - in a localized region. This region is monitored simultaneously with a digital camera and an infrared camera. The resulting observed temperature profile and bubble dynamics are compared for these fluids. For pure water with a black backing, some very high temperatures (>300°C) are observed with a laser input of ∼75 W/cm². Using a similar absorption potential, we observed higher temperatures in the nanofluids when compared to black dyes. A simplified boiling heat transfer analysis based on these results is also presented. We also noticed differences in bubble size and growth rates for the different samples. Overall, this study represents a proof-of-concept test for a novel volumetric, direct steam generator. These results of this test indicate that it may be possible to efficiently generate steam directly in a controlled, localized volume - i.e. without heating up passive system components.