Circular Economy: Solar Stills Go Bananas

Singapore has a bit of a problem with its organic waste. The country generates twenty thousand tons of fruit waste annually, mainly from the juice industry. Turning waste into a resource is one of the pillars of the circular economy. And that is what Edison Ang, a young researcher at the University of Singapore, thought. Surely a new application could be found for those mango peelings and banana skins that used to end up as compost at best and in landfills at worst. And it seems to have succeeded. The secret lies in MXeno, a revolutionary nanomaterial that has been used in a pioneering way in solar desalination plants, i.e., by passive evaporation.  

What is MXene?

Before continuing, a brief explanation of this fascinating material. MXene is a type of material that has gained much attention in recent years for its unique properties and potential applications. It belongs to a class of two-dimensional materials, which means it is extremely thin, just a few atoms thick.

MXene combines transition metals, such as titanium or molybdenum, and carbon. Its structure is arranged in layers, resembling a stack of sheets. These layers can be separated, allowing scientists to work with individual sheets or combine them to create thicker materials. The result is a material with high conductivity, flexibility, and strength.

One of the key characteristics of MXene is its excellent conductivity. It can efficiently conduct heat and electricity, making it useful for various electronic and energy-related applications. It is also quite strong and flexible, which adds to its versatility.

MXene’s unique properties make it suitable for a wide range of applications. It can be used in energy storage devices such as batteries and supercapacitors, where its conductivity and high surface area contribute to improved performance. In fact, MXene-based batteries could be charged in just a few seconds. They are also being studied for use in flexible electronics, sensors, biomedical applications and, as we will see in this article, solar stills.

A MXene-based solar still

Due to its conductivity, MXene offers an extraordinary ability to convert sunlight into heat. In other words, it accelerates the evaporation of water and multiplies the effect of solar radiation. The problem with MXene is that, for now, it is expensive and complex to produce. That is where the new process for harnessing organic waste comes in. Ang and his team have applied a two-stage carbonization process that makes it possible to manufacture a highly efficient solar absorber.

Thus, the new MXene based on fruit residues is cheaper than existing commercial alternatives since one of the reactive sources needed to manufacture it is obtained at practically zero cost as it is present in the organic matter.

In raw numbers, the MXene obtained from this waste has a light-to-heat conversion efficiency of 90 %. When it comes to producing drinking water, this translates into a 50 % increase in the amount produced compared to solar evaporators on the market.

In addition, after tests with the initial prototype, the researchers found that the purity of the water produced meets the strict World Health Organization standards for drinking water. In other words, the system can produce water that is fit for human consumption.

As a low-cost passive solar evaporator, it could be produced on a large scale for use in remote areas with no potable water supply and regions affected by natural disasters such as hurricanes or earthquakes. And all this using renewable and sustainable energy.

Other types of solar desalination technologies

While in large-scale desalination reverse osmosis is the most common and efficient technique, passive solar desalination plants can be a handy tool for small consumers and emergencies. Apart from the MXeno solution, we have previously covered some solar desalinators with very innovative materials and designs. Here are some examples:

  • Using hydrogels such as this gel-polymer hybrid, which multiplies the absorption of sunlight and, at the same time, has hydrophilic properties to absorb the water vapor generated. The new system can purify up to 25 liters of water per square meter per day.
  • The MIT’s solar desalination plant, which uses everyday materials to produce a convection circulation that pushes salt particles to the bottom and makes near-surface water potable.
  • The portable solar desalinator designed by a University of Texas researcher using a sheet of paper impregnated with a carbon solution. This black material enhances the absorption of sunlight and makes it possible to produce up to 2.2 liters of water per hour.

Of course, another way to harness sunlight to carry out desalination is to use photovoltaic panels to generate the necessary electricity. For example, a PV-powered desalination plant capable of producing drinking water for thirty-five thousand people per day was installed in Kenya in 2018.

If, apart from solar desalination plants, you would like to know more about desalination technologies and other scientific advances, please subscribe to our newsletter at the bottom of this page.        



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