Research presented in the Advanced Materials Interfaces journal demonstrates an efficient system of solar steam generation that utilizes a solar energy receiver of carbon black (CB)-based composite nanofiber networks with a specially designed, integrated 2D water path.
Study: Scalable Carbon Black Enhanced Nanofiber Network Films for High-Efficiency Solar Steam Generation. Image Credit: Eaum M/Shutterstock.com
There are reported to be over 20,000 desalination plants in operation globally, with many countries and localities turning to this process of removing salt and other impurities from saline and other water sources as a potential means of ensuring water security amid growing climate concerns.
Two common types of desalination processes are employed: processes involving the use of membranes and nanofiltration, and processes that make use of thermal techniques.
Interfacial solar steam generation devices have seen great interest in recent years, offering a novel approach to thermal desalination.
Solar energy-based steam generation has traditionally involved the heating of bulk liquid – a process that necessitates the use of expensive high optical concentrations or highly specialized vacuum conditions to ensure that saline or otherwise unpotable water is evaporated effectively. These approaches also involve a degree of heat loss to the external environment, adversely affecting their energy efficiency.
New research presented in the Advanced Materials Interfaces journal has showcased an efficient system of solar steam generation that utilizes an innovative solar energy receiver of carbon black (CB)-based composite nanofiber networks with a specially designed, integrated 2D water path.
This solar energy receiver employs a two-layer structure that has been fabricated via the electrospinning of a carbon black particle suspension in cellulose acetate (CA) which is then placed onto a porous polyethylene terephthalate (PET) substrate.
This novel device offers excellent efficiency because the upper layer of CA/CB nanofiber is able to accommodate both light absorption and water evaporation, while the lower—and most notably hydrophilic—PET layer functions as a 2D water path, allowing water to be supplied and evaporated effectively while heat loss is minimized.
Instead of heating a bulk water supply, interfacial solar steam generation technology is localized on a thin layer of water. For this process to work effectively, it is necessary that the solar energy receiver used be as efficient as possible, that heat loss is kept to a minimum, and that an adequate supply of water is available to be evaporated.
The researchers’ work aims to tackle the first of these considerations—a robust solar energy receiver—by using advanced carbon-based nanomaterials to develop an efficient solar energy receiver. Carbon black represented an ideal candidate for this purpose, due to its potential to act as a natural light absorber and its capacity for cost-effective production at industrial scales.
Fresnel Solar Steam Generator - System Overview
Video Credit: Industrial Solar/Youtube.com
In order to minimize thermal energy loss and further improve the efficiency of the process, the researchers aimed to avoid any direct contact between the bulk water source and the solar evaporator. Instead, they employed a supporting substrate material of polystyrene (PS) foam to afford the device improved heat insulation while ensuring that this had a sufficiently low mass density to allow it to float naturally on the surface of the water source.
It was also important that the material connecting the bulk water source to the solar receiver was porous and hydrophilic enough to transport sufficient water to the solar receiver to be evaporated.
Because the researchers were able to develop a device with an integrated 2D water path of microporous hydrophilic polyester (PET), this significantly improved the efficiency of water supply to the solar receiver.
The device was tested to evaluate its applicability in different regions. This was done by testing its capacity to accommodate aqueous solutions of NaCl. Three different concentrations based on real-world scenarios were used in desalination tests: 1.4 wt% - equivalent to the North Sea, 3.5 wt% - average salinity, and 4.1 wt% - equivalent to the Red Sea.
Concentrations of Na+ in the water evaporated from the aqueous solutions displayed decreases by at least three orders of magnitude – confidently below minimum safe salinity levels defined by World Health Organization.
Upon testing, the researchers discovered that the solar evaporator’s unique structure afforded this a high evaporation rate of 1.48 kg m–2 h–1 and an excellent solar energy conversion efficiency of 98.6% under 1 kW m−2 (1-sun) illumination.
Perhaps most notably, the simple device structure, low cost of raw materials, and ease of production via highly scalable processes offer the potential for these devices to be applied quickly and easily across a range of plants and locations.
Membrane-based desalination processes continue to dominate the sector, accounting for almost all of the currently installed desalination capacity. In 2017 it was reported that membrane technology accounted for 95.6% of annual contracted capacity while thermal processes accounted for just 4.4%.
With over 300 million people across 150 countries reported to rely on desalinated water for some or all of their daily needs, this novel carbon-black-based process based on sustainable energy could offer a more efficient, more environmentally-friendly, and potentially more effective means of meeting global desalination needs.
Zhang, R., Zhou, Y., Xiang, B., Zeng, X., Luo, Y., Meng, X., Tang, S., Scalable Carbon Black Enhanced Nanofiber Network Films for High-Efficiency Solar Steam Generation. Adv. Mater. Interfaces 2021, 2101160. https://onlinelibrary.wiley.com/doi/10.1002/admi.202101160
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