The earth is around 71 percent water. An overwhelming 97 percent of that water is found in the oceans, which means that we only have 3 percent in the form of fresh water – and much of it is frozen in the form of glaciers. That leaves only 0.3 percent of that freshwater on the surface in lakes, marshes, sources and our most important sources of drinking water, rivers and streams.
Despite the famous blue appearance of our planet from space, thirsty aliens would be disappointed. Drinkable water is actually quite scarce.
As if that does not sound disturbing, the little water we have also threatened by climate change, urbanization, pollution and a world population that continues to expand. More than 2 billion people live in regions where their only source of drinking water is polluted. Pathogenic microbes in the water can cause cholera, diarrhea, dysentery, polio and typhus, which can be fatal in areas without access to vaccines or medical treatment.
Destroying seawater is a possible solution, and one approach includes porous materials that absorb water that evaporates when heated by solar energy. The problem with most existing evaporators on solar energy is that they are difficult to scale up for larger populations. The performance decreases with the size, because less water vapor can escape from materials with small pores and thick boundaries – but there is a way to overcome this.
Salt
Researcher Xi Shen of the Hong Kong Polytechnic University wanted to find a way to improve these types of systems. He and his team have now created an airgel that is much more efficient in turning fresh water than earlier desalination methods.
“The most important factors that determine the evaporation performance of porous evaporators are heat room, water transport and vapor transport,” said Shen in a study recently published in ACS Energy Letters. “Important progress has been made in the structural design of evaporators to realize very efficient thermal localization and water transport.”
Solar radiation is the only energy used to evaporate the water, and therefore many attempts have been made to develop what photothermal materials are called. When sunlight touches these types of materials, they absorb light and convert it into heat energy, which can be used to accelerate evaporation. Photothermal materials can be made from substances, including polymers, metals, alloys, ceramics or cement. Hydrogels are used to successfully disinfect and descend water successfully, but they are polymers that are designed to retain water, which has a negative influence on efficiency and stability, in contrast to aerogels that are made of polymers who retain air. This is why Shen and his team decided to make a photothermal airgel.
