Cleaning water using steam

Materials World magazine
,
3 Jun 2020

A solar-powered steam generator made from natural materials could make it easier and more affordable to purify and desalinate water in undeveloped rural areas. Idha Valeur asks how it works.  

A three-ingredient aerogel made of 90% air, 10% cellulose, plus a polymeric dye, can make water drinkable in areas where clean water is lacking, say researchers in Sweden.

Simone Fabiano, Head of the Organic Nanoelectronics Group at Linköping University, says, ‘The cellulose works as a scaffold and provides mechanical stability to the aerogel, while the polymeric dye absorbs the sunlight and converts this energy into heat. 

‘A 2mm-thick sample is sufficient to absorb over 99% of the energy in the solar spectrum. If the aerogel is [left to] float on top of a water bucket, the water will be sucked and confined into the tiny, micron-size, void spaces of the aerogel.’ 

He continues, ‘The heat produced by the polymeric dye then gets transferred to the water, which heats up and vaporises. The purified water is then condensed and collected. One square metre of aerogel produces something like 1.5 litres of fresh water per hour.’

Fabiano explains that the choice of cellulose as the scaffold material is because it is readily available, cheap and also biocompatible and biodegradable. He adds that common thermal distillation technologies consume a large amount of energy from fossil fuels to heat the water into steam. ‘We estimate the cost could be as small as a few cents of €/m2, which should allow for a large distribution in rural and under-developed areas. It would require a simple greenhouse for the collection of the purified water. Considering the low cost, we hope this could reach those who are in need,’ Fabiano says. 

The generator can desalinate seawater over many cycles. However, over time, salt crystals formed upon water evaporation will build up inside the aerogel and ‘clot the porous structure responsible for water transport, thus slightly reducing the water evaporation rate by about 4% in 24 hours. However, the aerogel can be washed by using the same seawater for a few minutes and the efficiency is fully recovered’, Fabiano describes. 

The team has applied for funding and the aim is to get a prototype onto the market in three to five years.
 ‘So far, we have only worked with cm2 samples in a lab setting. The next step would be to go on a larger scale and investigate how the efficiency changes with size,’ he notes.‘We would also need to investigate how other contaminants besides salt affect the solar steam generator efficiency.’