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Posted: Jun 06, 2017
The hidden talent of mushrooms for solar steam generation
(Nanowerk Spotlight) Steam is important for desalination, hygiene systems, and sterilization; and in remote areas where the sun is the only source of energy, being able to generate steam with solar energy could be very useful.
"There exists a contradiction in almost all previous designs of solar steam generation," Jia Zhu, a professor at Nanjing University, explains to Nanowerk. "The efficient water supply and suppressed conductive heat loss couldn't be realized simultaneously. In most if not all of the current designs, the absorbers are always in direct contact with bulk water, i.e. 'direct bulk water contact', to ensure that the absorbed solar energy can be efficiently transferred to bulk water to generate vapors."
However, even with advanced heat localization designs with all the other parasitic heat loss minimized, bulk water itself – with thermal conductivity around 0.5W/mK – becomes an intrinsic and dominant thermal conduction path.
A significant portion of the absorbed energy will unavoidably dissipate through bulk water, evidenced by undesirable temperature elevation of bulk water. That's the reason for all of these 'direct bulk water contact' designs, high efficiency can only be achieved for rather limited water quantity with increased solar irradiation (using optical concentrators) and thermal insulation, as the heat loss through bulk water can only be minimized under these rather strict conditions.
"For the same reason, the efficiency of solar steam generation will decrease dramatically with increased water quantity," Zhu continues. "In our research, we have found that the mushroom structure can surprisingly overcome this problem. The stipe of the mushroom can serve as efficient water supply path, meanwhile, due to the extreme small ratio of the areas of fibrous stipe and black pileus, only little heat (useless heat loss) conducted into water."
How to suppress convection and radiation losses is another challenging task. Here, the umbrella-shaped pileus with a large surface-to-projected area ratio provides a large surface area for evaporation, decreasing its temperature under sunlight, so that minimizing the loss from radiation and convection.
In this work, for the first time, researchers utilize living organisms – mushrooms – to generate steam under sunlight. It turns out that the micro- and macrostructures of mushrooms possess all the needed characteristics for a good solar steam-generation device: high solar absorption; efficient water supply and vapor escape; and good thermal management. Interestingly, a mushroom is an unlikely candidate as it typically lives in the shadow, i.e. it doesn't get to see sunlight that much.
The mushroom maintains its hydrophilicity before and after carbonization because of its components, which include carbohydrates and proteins; the nitrogen functional groups exist even after carbonization.
The scientists attributed mushrooms' capability of high-efficiency solar steam generation to their unique natural structures, including their umbrella-shaped black pileus, porous context, and fibrous stipe with a small cross section.
First, the umbrella-shaped black pileus can absorb a huge amount of solar energy. Second, the hydrophilic fibrous stipe working as efficient water supply path can pump water into the mushroom context by capillary force. Third, the porous context not only acts as a bridge to pump the water further into the top pileus but also provides sufficient vapor channels.
"What’s more" as Zhu points out, "the geometry of mushrooms is naturally optimized for minimizing all three components of heat loss, which include conduction, convection and radiation. The ratio of the areas of fibrous stipes and black pilei is so small that only little heat (useless heat loss) conducted into water. In addition, the umbrella-shaped pileus with a large surface-to-projected area ratio not only provides a large surface area for evaporation but also minimizes the loss from radiation and convection."
To fabricate their solar steam generation device, the team carbonized shiitake mushrooms. Although the entire structure of the mushroom shrinks by about 30% during this process, the carbonized mushroom maintains a porosity similar to that of the natural one (∼90%). However, the surface roughness of the pileus increases, which is beneficial for light absorption. Due to the reduced reflection, this results in a dramatically increased absorption of solar energy – 96% after carbonization versus 79% of the natural mushroom.
Mushroom-based solar steam generation. (a) Schematic of a mushroom-based solar steam generation device and its heat behavior. (b) Physical picture of a shiitake mushroom. (c) Physical picture of a mushroom after carbonization. SEM images of the context of natural (d) and carbonized (e) mushroom. (click on image to enlarge) (Image: Jia Zhu group, Nanjing University)
These findings not only revealed the hidden talent of mushrooms as low-cost materials for solar steam generation but also provided inspiration for the future development of high-performance solar thermal conversion devices.
"We can further optimize larger artificial devices by learning form the mushroom structure, realizing high conversion efficiency," Zhu notes.
Since mushrooms themselves as devices are relative small, in order to enable large-scale applications, it is necessary to design larger mushroom-like artificial structures.
"With high energy transfer efficiency demonstrated through various materials and structures development, we will need to focus on how to apply this high efficiency interfacial solar steam generations to a variety of applications from desalination, purification, sterilization and electricity generations," Zhu concludes. "That requires a fundamental understanding of nanoscale heat transfer and photon management, as well as the development of scalable processes, low cost materials, more practical and portable designs."