Scientists are studying some of the research behind a growing technology in the global drinking water crisis.
A viable but promising solution to the world’s water scarcity problems could be water purification through direct solar steam generation technology. But while researchers are still on track to practically apply this technology, the goal is still a long way off. A new study of Elsevier’s Solar Energy Materials and Solar Cells takes us a part of this tremendous research journey, which includes strategies for designing devices to optimize the process of creating steam.
Without drinking water there is no life. However, nearly 1.1 billion people worldwide have no access to fresh water and another 2.4 billion people have diseases caused by drinking water. In fact, science has provided advanced methods of water treatment, such as membrane distillation and reverse osmosis, which are often implemented in developing countries because of their high cost and low productivity.
Newborn technology shows promise as an alternative to these regions of the world: direct solar steam generation (DSSG). DSSG is the collection of heat from the sun to convert water into steam, thereby desalinating it or removing other soluble impurities. The steam is cooled and collected as clean water for use.
This is a simple technology, but it is introducing a key step, the evaporation, the marketing roadblocks. With existing technology, evaporation performance has exceeded the theoretical limit. However, this is not enough for practical implementation. Measures to improve the design of the device have been taken to minimize the loss of solar heat before reaching high water, recycle latent water, also absorb and harness the surrounding energy, etc. to improve the evaporation performance beyond the theoretical limit. to make this technology feasible.
In a new article published in the journal Solar Energy Materials and Solar Cells, Professor Lei Miao has reviewed Xiaojiang Mu, Yufei Gu, and Jianhua Zhou of Electronic Technology at Guilin University in China to overcome this theoretical limit over the past two years. “Our goal is to summarize the story of the development of new evaporation strategies, highlight current shortcomings and challenges, and design directions for future research to accelerate the practical application of DSSG purification technology,” says Professor Miao.
The pioneering strategy that begins this evolutionary saga is the volumetric system, which uses a suspension of noble metals or carbon nanoparticles instead of bulk heating to absorb solar energy, transfer heat to the water surrounding these particles, and generate steam. While this increases the energy absorbed by the system, there is a large loss of heat.
To address this problem, a “direct contact type” system was developed, in which water is covered by a double-layer structure composed of pores of different sizes. The upper layer with larger pores serves as a means of heat absorption and vapor escape, and the lower layer with smaller pores is used to transport water from the vessel to the upper layer. In this system, the contact between the heated top layer and the water is concentrated, and heat loss is reduced by 15%.
The “2D water path” or “indirect contact type” system came up, which reduced further heat loss, avoiding contact between the solar energy absorber and the water vessels. This paved the way for the development of a “1D waterway” system that is inspired by the natural process of water transport based on capillary actions in plants. This system shows an impressive evaporation rate of 4.11 kg m-2h-1, almost three times the theoretical limit, and a heat loss of only 7%.
The injection control technique was then followed, as controlling the water sprayed on the solar energy absorber allows it to absorb similar to the soil. This results in an evaporation rate of 2.4 kg m-2h-1 from solar energy to water vapor with an efficiency of 99%.
In parallel, strategies are being developed to obtain additional energy from the environment or from high water itself and to recover the latent heat of high-temperature steam to improve the rate of evaporation. Techniques for reducing the energy required for evaporation are also being developed, such as hydrogels and light-absorbing aerogels, a polyurethane sponge with carbon black nanoparticles, and carbon-coated wood (CD) to hold solar energy and water. evaporate.
Many other such design strategies exist and others will come. Many important issues — such as condensate water collection, material durability, and stability in outdoor applications in changing wind and weather conditions — remain unaddressed.
However, the pace of work on this technology allows us to look ahead. “The path to practical implementation of the DSSG is fraught with problems,” says Professor Miao. “But given its advantages, it will provide one of the most tremendous solutions to our drinking water shortage problem.”
Reference: Xiaojiang Mu, Yufei Gu, Pengfei Wang, Anyun Wei, Yongzhi Tian, Jianhua Zhou, Yulian Chen, Jiahong Zhang, Zhiqiang Sun, Jing Liu, Lixian Sun, Sakae, “Strategies to break the theoretical limit of evaporation in the creation of direct solar steam” . Tanemura and Lei Miao, October 19, 2020, Solar Energy Materials and Solar Cells.
DOI: 10.1016 / j.solmat.2020.110842
Funding: China National Key Research and Development Program, Guangxi Natural Science Foundation of China, Guangxi Scientific Research and Technology Development Program.