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How energy is created from salt water?

Electrical energy can be generated naturally when fresh water comes into contact with salt water through a membrane during

By Wahid Bhat
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Water crisis due to climate change: More serious than previously thought

Electrical energy can be generated naturally when fresh water comes into contact with salt water through a membrane during what is known as osmosis. The concept is relatively simple: a semipermeable membrane separates two liquids with different salt concentrations, and salt ions pass through the membrane until both concentrations equilibrate.

The innovation is based on a membrane barely three atoms thick that separates the two fluids, and the study was published in the journal Nature.

The researchers, led by Shunichi Fukuzumi of Osaka University, published a paper on the new method of photocatalytic production of hydrogen peroxide in a recent issue of Nature Communications.

Scientists have used sunlight to convert seawater (H2O) into hydrogen peroxide (H2O2), which can then be used in fuel cells to generate electricity. It is the first photocatalytic method of H2O2 production that achieves an efficiency high enough for the H2O2 to be used in a fuel cell.

The invention may have a great future, since each membrane, with a standard size of one square meter, would be capable of generating enough light to power 50,000 energy-saving light bulbs. The process is called osmosis and the membrane is three atoms thick.

"Any time there is an imbalance situation, such as two tanks of water with different concentrations of salt, there is often an opportunity to convert this thermodynamic energy into electricity," says first author Makusu Tsutsui.

With a single nanopore 20 nm in size, the device achieved a maximum power efficiency of 400 kW/m2. However, the researchers found that adding too many nanopores to the membrane actually reduced the energy that could be extracted. The optimal pore configuration, nanopores 100 nm in size arranged in a grid one micrometre apart, produced an osmotic power density of 100 W/m2.

Generate electricity only with salt water? 

That is what a Colombian renewable energy company, E-Dina, has achieved, which has developed a portable lantern called Water Light, which is capable of producing electricity from seawater for 45 days and can also recharge the cell phone battery.

The lamp has been developed together with the Wunderman Thompson agency, also from Colombia, and constitutes a reliable alternative for lighting in communities that lack electricity.

These communities are generally based on the sea, which is the most abundant battery of energy in the world. Or, at least, this was demonstrated by a Colombian company that transformed salt water, which represents 70% of the Earth, into energy through a flashlight. 

"We have been working on this device for six years to bring light to these marginalized areas, but we wanted it to be clean energy, especially since fossil fuels pollute a lot and it is estimated that they will run out in the next 52 years," says Juan Felipe Avendaño, manager by E-Dina.

WaterLigh, as the flashlight is called, was developed by the Colombian company E-Dina, in cooperation with Wunderman Thompson Colombia. The first time it was used was six years ago. Back then they only managed to make two of the led bulbs flicker. 

“Just that it turned on with water was a big surprise for us, so we continue working on how the device could be improved,” says Avendaño. After several trials, trials and errors, they managed to get all the light bulbs to light up and stay lit for a long time.

How is light generated? 

Avendaño explains that what is done is to harness the power of ionization, a process by which saltwater electrolytes react with the aluminum inside the device, transforming the magnesium inside into electrical energy. 

“Each lantern, which weighs about 2 kilos, has a cylindrical wooden outer casing, a circuit at its base, and a perforated lid at the top that allows water to flow into the device while the hydrogen created during the burning process ionization escapes”, he clarifies.

When you put salt in water, the water molecules separate sodium and chlorine ions so they float freely, increasing conductivity. These ions are what carry electricity through the water with an electrical current. In short, salt water (water + sodium chloride) can help produce electricity.

Once the salt particles evaporate, each device can be vacated for reuse. "The water can then be reused for other activities," says Avendaño. To fill each one of them and generate light for 45 days, 500 milliliters of seawater is required or, failing that, urine, due to the large amount of nitrogen, phosphorus, and potassium it contains. WaterLigh is designed to run for 5,600 hours, which equates to over 230 days, depending on how often it is used.


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