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Synthetic molecule is capable of storing solar energy

27 November 2012

Researchers have developed a system which can store solar energy in chemical bonds, based on a synthetic molecule that is changed by sunlight.

The three structural forms of the molecule fulvalene diruthenium. On the left the low-energy form, in the centre a transition state and on the right the high-energy form. Picture: Karl Börjesson

The molecule can be transported and stored for several years and then used to generate heat on demand.

Many researchers believe that using the sun as the energy source offers the best opportunities for developing a sustainable energy system. One challenge in this area is to find efficient storage methods for saving the captured energy and transporting it to other locations.

Researchers at Chalmers University of Technology in Sweden have made progress in developing an “all-in-one” system for the capture, storage and use of solar energy. The method is known as the thermochemical process. It attracted a lot of interest during the 1980s, but researchers at the time were unable to resolve the issue.

Two years ago, a group of American researchers demonstrated that the method is theoretically possible. Chalmers researchers Kasper Moth-Poulsen and Karl Börjesson, working with researchers from UC Berkeley in California, have now progressed from theory to practical devices.

The research team has created a demonstration unit made up of three components: a solar collector, a catalytic reactor and a heat exchanger. The three components are connected in a micro fluidic system. The designed molecule, fulvalene diruthenium, which has high energy density, is the heart of the system.

The molecule’s structure is changed in the solar collector with the aid of sunlight. The catalyst enables the molecular to regain its original state, releasing heat in the process.  The molecule can be transported and stored for several years without loss, as all heat is bound in a stable chemical compound. This means that the system can be reused many times over.

The Chalmers researchers have therefore been able to prove that the theory works in practice. The ongoing challenges revolve around making the system more efficient, with greater increases in temperature. Another difficulty with solar energy is adapting the technology to large-scale application and integrating it into existing energy production.

“What we have done now is to build the first demonstration unit”, says Kasper Moth-Poulsen.  “The next step will be to improve the materials and processes involved in solar energy storage to make it cheaper and more efficient”.

”If we can develop better and cheaper materials, in principle there is nothing to prevent us applying the concept on a larger scale. We hope that we can help develop systems which fit into an energy system where use of renewable energy is expanding and there is an increasing need for energy storage”. 

One benefit of the new method is that it provides a holistic solution for solar energy. Many other solar energy systems require several different technologies to be interlinked for the energy to reach the end user.

“In our system, energy storage is built in from the start, which means that solar energy can be easily stored until needed, for example at night or in the winter”.  “In the long term, our technology can be considered an alternative to thermal solar energy storage in water”, says Kasper Moth-Poulsen.


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