Solar-powered innovation has shown year-long stability with zero utility energy costs, thanks to a new type of photothermal material
Desalination has always been an energy-hungry way of turning salt water into fresh water, making it largely the preserve of wealthy countries with abundant fossil fuel reserves.
Yet, an outdoor demonstration prototype in China has managed to exhibit year-long stability with zero utility energy costs, thanks to a new type of photothermal material.
The researchers developed an innovative method to weave nanoparticles into a three-dimensional photothermal evaporation material, significantly boosting the efficiency of converting solar energy to drive desalination.
Experiments showed that the structure achieved a solar absorption rate of as much as 90.2 per cent, while cutting the energy needed to evaporate the same volume of seawater by 45.7 per cent.
At a small trial site, the device was successfully used for desalination, helping to irrigate 5 square metres (nearly 54 square feet) of farmland for a full growth cycle using only natural sunlight and requiring no external power grid infrastructure.
Based on a projected two years of operation, the team noted that the cost of producing water would fall below that of bottled water and that the economic advantage “would become even more pronounced if the system were scaled up or used over the long term”.
Jointly conducted by researchers from the Beijing-based Institute of Process Engineering (IPE) at the Chinese Academy of Sciences and Shenzhen University, the study was published in the journal Advanced Materials on June 21.
Desalination has traditionally been energy-intensive, either consuming vast amounts of electricity or depending on expansive and expensive infrastructure.
Its dominant technology today is reverse osmosis, also known as membrane desalination, and it uses electricity to pump seawater through large pipes and into thin membranes that allow water molecules to pass while blocking salt.
Since desalination began to be used on a large scale in the 1950s, the Gulf Arab states – beset by water shortages but rich in energy resources and wealth – have been among the pioneers in the field.
Gulf countries produce roughly 40 per cent of the world’s desalinated water and operate more than 400 desalination plants along their coasts, according to an Al Jazeera report this year.
Solar evaporation is seen as a promising green alternative because it uses photothermal materials to convert sunlight into heat, driving seawater to evaporate before the vapour is condensed into freshwater.
But putting that into practice has always been challenging.
The most effective photothermal materials are usually ultrafine powder nanoparticles. However, the trouble comes when those tiny particles have to be turned into a usable evaporation device: they tend to clump together like flour, which blocks the pathways for water vapour.
In addition, many of the organic polymer substrates used to hold the particles together lose performance after prolonged exposure to intense sunlight, much like plastic ageing and cracking in the sun.
For their study, the team drew inspiration from buttons. They turned the nanoparticles into individual “buttons” and used polymer “threads” to stitch them firmly together, preventing clumping while building a robust three-dimensional framework.
To do this, they first prepared hollow nanospheres with multiple shell layers. They then used a solvent to allow polymer chains to thread precisely through the tiny pores in the shells, like sewing thread through fabric.
After cooling, the polymer chains acted like locks, binding billions of these nanospheres together into a three-dimensional “nanoforest”.
Subsequent tests showed that the new structure was highly robust and durable.
The team placed the composite material in seawater and stirred it continuously at 450 revolutions per minute for 30 days to simulate harsh marine conditions. Under the microscope, they found that virtually no nanoparticles had detached.
The structure also caused sunlight to bounce and scatter repeatedly inside the material, pushing its solar absorption rate up to 90.2 per cent.
The researchers then built an 0.75-square-metre trial system. Solar panels powered a fan that actively carried the water vapour produced during evaporation into a condensation module, where fresh water was recovered.
Under natural sunlight, the device produced more than 20 litres (about 5.3 US liquid gallons) of fresh water per day – enough to meet the basic drinking needs of about 10 people.
The water met World Health Organisation drinking-water standards, according to an IPE press release.
The water was then used to irrigate a 5-square-metre test plot, supporting the full growth cycles of spinach, corn and Chinese cabbage. Meanwhile, the photothermal material itself showed excellent durability, maintaining its performance with year-long stability.
“The team is now working to improve condensation efficiency and reduce system costs, with the aim of scaling up the technology for use in water-scarce coastal areas, islands and remote regions,” the IPE said.
Source: https://tinyurl.com/534mhwpr
