Freshwater is a scarce resource. Although 70% of the Earth is covered by water, freshwater that we drink and use only makes up 3% of it. Worse still, two-thirds of the world’s freshwater is not available to us as it is trapped in glaciers or other inaccessible sources. Some experts predict that by 2025, two-thirds of the world’s population may suffer from water shortage[1]. To overcome water shortage, we can recycle waste water or desalinate seawater, but traditionally, such methods entail costly large-scale facilities. Then there is the solar still – a condensation trap that heats up waste water or seawater with solar energy, and the evaporated freshwater is cooled and collected. Meanwhile, Dr Yuen-hong Tsang, Associate Professor, Department of Applied Physics observed the ubiquitous use of polyurethane foam as a packaging material without any incentive to recycle them properly. He then led a research team in developing a method to upcycle black packaging foam by using it as a solar absorber in solar stills. The foam is treated to enhance water evaporation and is highly promising both as a way to reduce waste and as a method to generate freshwater.
Conventional solar steam generation
Recycling waste water and desalination of seawater have been expensive choices to generate freshwater, as they only make sense when operating in large scale. “Conventional solar stills require high concentration of solar energy by up to 1,000 times its normal intensity to generate steam. To concentrate sunlight, optical devices such as Fresnel lenses or parabolic reflectors must be precisely polished, and much labour and cost are invested to make those devices useful. Besides, conventional systems aren’t always efficient because of the high optical loss and surface heat loss,” explained Dr Tsang. Therefore, the team is more interested in novel and low cost setups to generate water vapour, such as the solar still, which does not require complicated or precise optical engineering.
Heat localization to generate more water vapour
The solar still works by capturing and isolating the solar heat on the surface while keeping the remaining body of water relatively cool. Dr Tsang said, “If you put a glass of water under the sun, the whole glass of water is heated up. By placing the black foam on top, more evaporation will occur. It is because, firstly, the foam absorbs more heat due to its dark colour; secondly, only the water on the surface is heated up because the foam essentially insulates the surface from the bulk of the water and traps heat.” To even further encourage evaporation, the team treated the polyurethane foam with dopamine, which increases its wettability. The foam attracts more water and thus, wicks more moisture onto the surface than its untreated counterpart. Experiments showed that the use of dopamine-treated black foam increases water evaporation on the surface by more than 3.5 times.
Case in point: village house in Hong Kong
A typical village house in Hong Kong has a roof of about 700 square feet. Assuming that the same performance is achieved, 60% of the evaporated water is collected from the roof and there are 5.22 hours of sunlight per day, setting up a solar still with black upcycled foam on the roof can regenerate 169 litres of freshwater from waste water each day. Such volume is enough to meet the basic food and hygiene needs of eight adults. Better still, the foam would isolate and trap heat, so that the interior of the house will be cooler.
In December 2018, the Waste Sponge as Solar Absorber for Vapour Generation won a silver medal in the 1st Asia Exhibition of Inventions Hong Kong.
