Solar steam generation by heat localization

Abstract

Currently, steam generation using solar energy is based on heating bulk liquid to high temperatures. This approach requires either costly high optical concentrations leading to heat loss by the hot bulk liquid and heated surfaces or vacuum. New solar receiver concepts such as porous volumetric receivers or nanofluids have been proposed to decrease these losses. Here we report development of an approach and corresponding material structure for solar steam generation while maintaining low optical concentration and keeping the bulk liquid at low temperature with no vacuum. We achieve solar thermal efficiency up to 85% at only 10 kW m À 2 . This high performance results from four structure characteristics: absorbing in the solar spectrum, thermally insulating, hydrophilic and interconnected pores. The structure concentrates thermal energy and fluid flow where needed for phase change and minimizes dissipated energy. This new structure provides a novel approach to harvesting solar energy for a broad range of phase-change applications.

Cold Vapor Generation beyond the Input Solar Energy Limit

“…However, conventional techniques for generating solar vapor typically rely on costly and cumbersome optical concentration systems to enable bulk heating of a liquid, resulting in relatively low efficiencies (e.g., 30–40%) due to heat absorption throughout the entire liquid volume that is not directly translated into vapor production. Recently, various advanced and expensive metallic plasmonic5, 6, 7, 8, 9, 10, 11, 12, 13 and carbon‐based nanomaterials14, 15, 16, 17, 18, 19, 20, 21, 22, 23 have been explored for use in solar vapor/steam generation. However, the vaporization efficiencies of these reported structures are still relatively low under 1 sun illumination (e.g., from 48%10 to 83%21).…”

“…However, the widely used definition of the solar energy conversion efficiency (e.g., refs. 9, 10, 11, 12, 13, 16, 18, 19, 20, 21, 26, 30) is obviously not applicable in this transient process since there is no solar input. Therefore, the physical picture of the solar‐driven evaporation should be revisited.…”

Tuning Transpiration by Interfacial Solar Absorber‐Leaf Engineering

“…In recent years, interfacial solar steam/vapor generation is attracting a lot of attention for achieving high energy transfer efficiency. And various optical and thermal designs at the solar absorber–water interface for potential applications in water purification,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 seawater desalination,24, 25, 26, 27 and power generation28, 29 appear.…”

“…Transpiration‐induced mass changes as a function of time for the three samples are recorded with dark transpiration totally subtracted, as shown in Figure 3 a. In analogy to the interfacial solar vapor generation process, the transpiration efficiency (η) of plant can be defined as10, 15 ${\eta }_{\mathrm{Transp}}=\frac{\dot{m}{h}_{\mathrm{LV}}}{q_{\mathrm{solar}}}$where $\dot{m}$ is the transpiration rate, h LV is the latent enthalpy of the liquid–vapor phase change, and q solar is the solar irradiation (≈300 W m −2 ). Figure 3b shows the measured transpiration rate ($\dot{m}$) and efficiency (η Transp ) for three states of the same one leaf.…”

“…On the basis of global water cycle, moderate global temperature, humidity, and precipitation may be attainable through the tunable plant transpiration. Moreover, photothermal conversion shows the potential as the regulator of the nature, besides the emerging applications, such as radiative cooling,42, 43, 44, 45 thermophotovoltaics,46, 47, 48, 49 water purification,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 seawater desalination,24, 25, 26, 27 and power generation 28, 29…”