Photo: A
solution to the growing clean drinking water availability problem is direct
solar steam generation technology, which can remove harmful soluble pollutants
from water.
Courtesy: Lei Miao from SIT
Scientists
review some of the research behind a technology that could cauterize a growing
global drinking water crisis.
A nascent
but promising solution to the world’s water scarcity problems could be water
purification via the direct solar steam generation technology. But while
researchers are well on the path to making this technology practically
applicable, the finish line remains a ways away. A new study in Elsevier’s
Solar Energy Materials and Solar Cells takes us through a part of this
incredible research journey, which involves device design strategies to
optimize the steam generation process.
Without
drinkable water there is no life. Yet, nearly 1.1 billion people worldwide lack
access to fresh water and another 2.4 billion suffer from diseases borne by
unclean drinking water. This is because while science has yielded advanced
water treatment methods such as membrane distillation and reverse osmosis,
these are often difficult to implement in developing countries owing to their
high cost and low productivity.
A more
nascent technology shows promise as an alternative for such regions of the
world: direct solar steam generation (DSSG). DSSG involves harvesting the heat
from the sun to convert water into vapor, thereby desalinating it or ridding it
of other soluble impurities. The vapor is then cooled and collected as clean
water for use.
This is a
simple technology, but a key step, evaporation, is presenting roadblocks for
its commercialization. With existing technology, evaporation performance has
hit the theoretical limit. However, this is not sufficient for practical
implementation. Measures to improve device design to minimize solar heat loss
before it reaches bulk water, recycle latent heat in the water, absorb and
utilize energy from the surroundings as well, and so on, have been taken to
improve the evaporation performance beyond the theoretical limit and make this
technology viable.
In a new
paper published in Solar Energy Materials and Solar Cells, Professor Lei Miao
from Shibaura Institute of Technology, Japan, along with colleagues Xiaojiang
Mu, Yufei Gu, and Jianhua Zhou from Guilin University of Electronic Technology,
China, review strategies formulated in the last two years to surpass this
theoretical limit. “Our aim is to summarize the story of the development of new
evaporation strategies, point out current deficiencies and challenges, and
layout future research directions to hasten the practical application of the
DSSG purification technology,†says Prof. Miao.
A
pioneering strategy with which this evolutionary saga begins is the volumetric
system, which, in lieu of bulk heating, uses a suspension of noble metals or
carbon nanoparticles to absorb the sun’s energy, transfer heat to the water
surrounding these particles, and generate steam. While this increases the
absorbed energy of the system, there is much heat loss.
To address
this issue, the “direct contact type†system was developed, in which a
double-layer structure with pores of different sizes covers the bulk water. The
top layer with larger pores serves as a heat absorber and vapor escape route
and the bottom layer with smaller pores is used to transport water up from the
bulk to the top layer. In this system, the contact between the heated top layer
and the water is concentrated, and heat loss is reduced to about 15%.
The “2D
water path†or “indirect contact type†system came next, which further lowered
heat loss by avoiding contact between the solar energy absorber and bulk water.
This paved the way for the eventual development of the “1D water path†system,
which is inspired by the natural capillary-action-based water transport process
in plants. This system displays an impressive evaporation rate of 4.11 kg
m-2h-1, nearly thrice the theoretical limit, along with a heat loss of only 7%.
This was
followed by the injection-control technique in which the controlled sprinkling
of water as rain on the solar energy absorber allows its absorption in a manner
mimicking that in soil. This results in an evaporation rate of 2.4 kg m-2h-1
with a conversion efficiency of 99% from solar energy to water vapor.
Parallelly,
strategies to gain additional energy from the environment or from the bulk
water itself, and recover the latent heat from high-temperature steam, have
been under development to improve the evaporation rate. Techniques to reduce
the energy required for evaporation in the first place are also being developed,
such as hydratable and light-absorbing aerogels, polyurethane sponge with
carbon black nanoparticles, and carbon dot (CD) coated wood to hold the sun’s
energy and the water to be evaporated.
Several
other such design strategies exist and several more are to come. Many pertinent
issues—like the collection of the condensed water, durability of the materials,
and stability during outdoor applications under fluctuating wind and weather
conditions, remain to be addressed.
Yet
the pace at which work on this technology is progressing makes it one to look
forward to. “The path to the practical implementation of DSSG is riddled with
problems,†says Prof. Miao. “But given its advantages, there is a chance that
it will be one of the frontrunning solutions to our growing drinking water
scarcity problem.â€