Solar cell turns raindrops into energy

Chinese researchers are developing an all-weather solar cell that generates energy not only from sunlight but from raindrops, too. Their innovation means that rainy regions could one day produce their own clean energy. Elke Bunge reports on the rain-or-shine solar panel.

Solar Decathlon 2011

Chinese scientists are developing a solar panel that can generate electricity come rain or shine. (Image credit: Stefano Paltera/U.S. Department of Energy Solar Decathlon)

It’s a made-in-China idea: an environmentally friendly source of energy that can be used in a variety of weather conditions. This is not the case with present-day solar panels, which are only effective on sunny days. When the sun disappears behind a cloud or at night, the system stops working and no electricity is produced.

An effective system able to generate electricity from a variety of renewable sources of energy does not yet exist. But a team of Chinese researchers are hoping to change this, as Qunwei Tang and his colleagues describe in the international edition of the journal Angewandte Chemie.

The team, which consists of scientists from Ocean University of China in Qingdao in eastern China and researchers from Yunnan Normal University Kunming in the south of the country, have developed a solar cell that can convert both sun and rain into energy.

Graphene-coated solar cells

To convert solar energy into electricity, the researchers used what is known as a dye solar cell, which was developed and patented by Michael Grätzel at the École Polytechnique Fédérale de Lausanne in the early 1990s.

Unlike conventional solar cells, which use a semi-conductor material, dye solar cells use chlorophyll or other organic dyes to absorb light. These types of solar cells are then coated with a wafer-thin transparent film made of graphene, a carbon structure made of two-dimensional, honey-comb latticed carbon atoms.

Graphene, which consists of only one atomic layer, is especially conductive, allowing electrons to flow freely across its surface. When this material comes in contact with rainwater, the electrons on the surface bind to the positive ions in the rainwater.

The dissociated ions are the result of the low salt content of natural water. Positive ions such as sodium, calcium and ammonium are attracted to the electrons on the grapheme layer and settle on the surface. A capacitor with a potential difference is then formed at the interface between the graphene and saline water and is sufficient enough to generate a current and subsequent voltage.

Still in its infancy

Despite its promise, the idea is still in its infancy: the solar cells currently being developed by the researchers only convert 6.5 per cent of usable solar energy into electricity. Solar collectors currently on the market have an efficiency of 22 per cent.

And instead of natural raindrops, the researchers are using an artificially produced saline solution for now. “Frankly speaking, there are still some unresolved points. For example, how we can generate electricity from rain with lower ion concentrations. It will still take some time before it can be realised on a practical level,” says Qunwei Tang.

But if and when they succeed, their solar cells could fill the energy gap between sunny and less sunny areas and provide regions with heavy rainfall an environmentally friendly form of electricity.

Emerging economies embracing renewables

It’s precisely efforts like this that are paving the way to sustainable energy systems. And while it might be surprising to some that this innovation was developed in the world’s largest coal producer, it reveals China’s lesser “green face”, says John Mathews, professor of management at Macquarie University in Sydney, Australia. In 2013, the share of electricity from renewable energy was at 20 per cent and it’s rising each year.

In 2015, global investments in renewable energy reached a record USD 286 billion. It was also the first time that investments in developing countries and emerging economies overtook those in developed countries.

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