Photo-electrochemical charging of redox couples for conversion and storage of solar energy

Storage capacity

in new solar cells

Can you store solar energy and retrieve it again when you need it? And can you do it in a way that is not only inexpensive, but also energy-efficient and environmentally responsible? Researchers have developed a new technology for charging flow batteries with solar power. It looks as though this could be very significant for the supply of electricity to private consumers in the future.

A small electrode made of glass and haematite is one of the key ingredients in a new type of solar cell that not only converts solar energy into electricity, but also stores it.

A new type of solar cell converts light into chemical energy and stores it in tanks containing liquids so it can be taken out again when required. The idea is simple, but it has wide-ranging potential. Researchers are testing a new method for charging flow batteries with solar power.

There is much to indicate that a technology capable of storing energy from the sun and subsequently converting it into electricity can be a reality. The key to its success is a new technology for charging flow batteries with solar power, developed by researchers at Aarhus University in collaboration with a group at the University of Porto. In the coming years, they will improve the technology in international collaboration, making it cheaper and more efficient.

“Storing solar energy currently involves such large costs that it’s far from being profitable. We’ll change the way we use energy and therefore work with a new technology that can charge flow batteries with solar energy. Flow batteries are interesting because they make it possible to store and convert energy from sustainable sources. The question is just how efficiently it can be done. We’re looking at some specific photoelectrodes and liquids that seem promising,” says Associate Professor Anders Bentien.

A flow battery basically works the same way as a fuel cell, but with the difference that the electricity is stored in liquids as opposed to hydrogen and oxygen in fuel cells.

Solar energy stored in two liquids
Today, solar cells immediately convert the energy available from sunlight into electricity. The disadvantage is that solar energy varies depending on the time of day and the weather conditions. It has to be used while it is there – or else it goes to waste. And this is precisely the problem the researchers are trying to solve with the new type of solar cells which can charge a flow battery directly.

They are using a technology based on photoelectrochemical principles to store the energy in two liquids. These liquids can subsequently be pumped into a flow battery where the energy is converted into electricity. The technology works according to the same principle as fuel cells where hydrogen and oxygen are produced photoelectrochemically from water, and can subsequently be converted into electricity in a fuel cell.

The advantage of using liquids is that it makes it possible to store electricity much more efficiently in the flow battery. The technology for solar charging is also both simple and inexpensive because the flow battery’s photoelectrode is made of ordinary glass, and the solar-active material simply consists of a thin layer of haematite – also known as rust.

The initial laboratory experiments have demonstrated that the principle of charging flow batteries directly with solar power works, but it is still too early to say anything about the degree of efficiency the researchers can achieve.

The perfect match
The key issue in relation to efficiency – and the really great challenge faced by the researchers – is to identify a liquid that matches a specific photoelectrode in order to optimise the charging of flow batteries with solar power. The researchers have a handful of different photoelectrode materials in their sights, and even more liquids that look interesting.
“Until now, we’ve shown that it’s possible to directly charge flow batteries with solar power, and the major challenge in terms of research is to identify the optimal match between the liquid and the material for the photoelectrode. We’d like to reach a solution that provides the greatest effect and the lowest energy loss, and that is both cheap to produce and environmentally responsible at the same time,” says Associate Professor Bentien.

For the time being, it appears that a special combination of water, sulphuric acid and organic molecules could be the answer to this challenge.

During the course of the next four years, the researchers in the project will spend time studying how to make the technology for charging flow batteries with solar power as efficient as possible in the laboratory.