Solar water battery energy storage

16 September 2016
Posted by Chris Smith.

A new concept in energy storage based on water and capable of soaking up power from solar cells has been developed by scientists in Korea. Photovoltaic solar panels on the roof of a house near Boston Massachusetts

Energy arrives on the Earth's surface from the Sun at the rate of 173,000 terawatts. This is thousands of times larger than current human global energy consumption, which averages 13 terawatts, making a compelling case for solar as a renewable energy source.

But the problem is that, quite literally, night follows day meaning that a major barrier to the wider scale adoption of photovoltaic generation is how to store the energy in a safe and efficient manner.

One popular strategy is to use the electricity to split water into hydrogen and oxygen. The former can be stored and then used later to release the embodied energy. But, as anyone who has heard of the Hindenburg knows, storing hydrogen is impractical and potentially unsafe.

Now, writing in Scientific Reports, a team from Korea has come up with what they dub a "solar water battery" which, they say, surmounts the hydrogen storage problem and instead locks away the energy inside the liquid.

The system consists of three linked electrodes immersed in water. Two are open to the air while the third is behind a permeable membrane and oxygen is actively excluded.

The first electrode is made from titanium dioxide and receives incoming sunlight. This releases electrons, which are tapped off for storage in the third electrode, and triggers the conversion of surrounding water molecules into a chemical form called hydroxyl radicals. These react together to form water and oxygen, which is released into the air.

The third electrode, where the energy is stored, is made of tungsten. The arriving electrons temporarily convert the tungsten atoms into a "reduced" oxidation state.

To discharge the cell, the electrons are tapped off from the tungsten electrode, which returns to its original oxidation state in the process, and sent around a circuit to do useful work. 

From there they pass to the second electrode, made of platinum, which uses oxygen from the air to produce water again.

The cell operates at about 0.6 volts and, following a one hour blast of sunlight, to charge it, hung on to 99% of the stored energy for ten hours.

The great virtue of the system is that it produces only oxygen and water and could easily be added to existing solar systems to create a self-contained energy capture and storage solution.

The team also speculate that it may even be able to help break down pollutants that can be added to the water in the cell where the hydroxyl radicals will destroy them.

However, the amount of energy that can be stored by the cell is low at just 10mAh per gram, which is hundreds of times less than a conventional battery. But, as the researchers point out, their solar water battery is a proof of principle rather than a market-ready product.

"There is considerable scope for improvement by adapting the numerous materials and technologies developed for solar water splitting which have proven to be an efficient means of solar energy conversion," the team say.

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