Water is vital to the survival of life. However, water scarcity has become a major problem in modern society. Today, one-fifth of the world’s population lives in water-deficient areas, especially in areas where there is no electricity.
For people in
such areas, access to clean drinking water is often a difficult task.
Therefore, they urgently need an efficient, low-cost, sustainable, and easily
accessible technologies and devices to generate clean water. Solar energy is
one of the most abundant and widespread resources on earth. Solar-powered water
purification technology is simple and efficient to obtain clean drinking water
from non-drinkable water sources such as lake water, sewage or seawater.
Today, a team
led by Prof. Shu-Hong Yu from the University of Science and Technology of China
(USTC) reports an efficient and sustainable biomimetic hierarchical solar steam
generator (HSSG) based on bacterial cellulose (BC) nanocomposites. This HSSG is
fabricated through a one-step aerosol-assisted biosynthesis process.
The designed
microbial synthesis process is successfully combined with the deposition of
nanomaterials, and a sophisticated biomimetic hierarchical structure is
constructed simply and efficiently. The hierarchical structure of this HSSG
contains three continuous layers with different functions, including
light-absorbing layer of carbon nanotubes/BC, thermal insulation layer of glass
bubbles /BC and wood substrate for supporting and water transporting.
In HSSG,
three-dimensional (3D) cellulose nanofiber network of BC hydrogel significantly
reduced the energy consumption to convert the liquid water into vapor and
accelerate the vaporization of water. Owing to the hierarchical structure
design and reduced vaporization enthalpy of nanocomposites of HSSG, a high
evaporation rate of 2.9 kg m-2 h-1 and solar-to-vapor efficiency of 80% can be
achieved.
In this HSSG,
the hierarchical structure nanocomposites grow on the wood substrate and are
tightly combined with the wood substrate through BC network of nanofibers. BC
nanofibers crosslink with the cellulose of wood forming infiltrating layer in
wood, which acts as a strong binder between wood and BC nanocomposite layers.
This structure
ensures the fast water transportation from wood to the BC nanocomposite layers
and makes them firmly attached to wood substrate, which provides the structural
foundation of thermal insulation and water transportation. Glass bubbles are
microscale hollow glass spheres, which provides the structural foundation of
thermal insulation and water transportation.
On the top of
the device, the carbon nanotubes and BC nanocomposite layer have sophisticated
interlaced structure where carbon nanotubes and cellulose nanofibers form
double-network of nanofibers. In this double-network, carbon nanotubes function
as highly effective solar light absorber and BC nanofibers are used to
transport water and reduce the energy consumption of evaporation.
This
multilayered structure of wood, glass bubbles /BC and carbon nanotubes/BC is
designed to achieve fast water transportation, thermal management, effective
light absorption and reduced vaporization energy consumption. Moreover, to
systematically investigate the relation between evaporation rate, energy
efficiency and energy consumption of evaporation, the team provides a novel
two-dimension chart with guide lines showing different enthalpy of
vaporization. This theoretical analysis method shows potential for analyzing
the contributions of different functional parts in solar steam generator
devices for evaporation rate.
Comparing with
other technology of solar-powered water purification, HSSG have great advantage
on evaporation rate, energy efficiency, sustainability and cost, which make it
a promising technology for future water purification.