From sunshine to fuel – solar thermal fuel

Background: The need for new energy ways

Solar thermal fuel might just be what we needed. In accordance with the Paris Climate Agreement, the 196 nations involved have agreed to reduce global warming to below 2 ° C1. At the same time, energy requirements are expected to double over the next 40 years2. In order to meet these challenges, a lot of research has been carried out in recent decades, including in the field of solar energy, with the number of systems installed worldwide increasing from 6 gigawatts (GW) to 402 GW between 2007 and 20173. At the same time, hydrocarbons are still the main source of energy worldwide. This can mainly be explained by the fact, that they are relatively cheap to break down, can be used directly and can be used for a long time. Due to further development of batteries, hydrocarbons are being replaced more and more in certain areas, as can be seen, in the field of electromobility. Since the sun does not always shine, the big problem is that there is currently no cheap and long-term efficient way to store this energy, as the current batteries do not meet the necessary requirements for capacity and recyclability. However, this could change in the foreseeable future.

Innovation: Solar thermal fuel as energy storage

Swedish scientists and the researcher Dr. Kasper Moth-Poulsen from Chalmers University of Technology in Sweden have developed a special liquid called “solar thermal fuel”, which is able to store energy for up to 18 years. As an alternative to batteries, solar thermal fuels are able to store solar energy over a long period of time and release it again when required. Unlike batteries, which emit electrical energy, this technology emits energy in the form of heat when activated by a catalytic converter. In the course of the last year, four papers were published that deal with this fascinating new approach. The latter was published in the prestigious Energy & Environmental Science magazine4.

The solar energy system called MOST (Molecular Solar Thermal Energy Storage) requires no external input other than solar radiation and works in a closed circuit5. The fuel is charged with the help of sun rays. The solar collector is concave and has a transparent tube in the middle, which the fuel flows through. The collector determines the course of the sun and focuses the sun’s rays, so that they hit the pipe. Some photochromic alternatives have been identified as fuel. Photochromic substances have the ability, due to solar radiation, to enable a reversible conversion of the starting material, whereby its physical properties change. One of the most promising fuels is a norbornadiene molecule, which consists of carbon, hydrogen and nitrogen atoms. Norbornadiene are bicyclic hydrocarbons which, when exposed to sunlight result in rearrangement of the compounds to form a valence isomer, the so-called quadricyclane6. When exposed to sunlight, double connections break open and become single connections. The chemical remains the same, but the physical properties change.

The resulting energetically charged version of the molecule is stable and has strong connections between which the energy is held and stored. This is also the reason why the energy loss when cooling to room temperature is minimal and no insulation is necessary7. The fuel can then be stored locally in order to continue to circulate in the circuit or, if necessary, be taken from the system. If the energy is required, the fuel is filtered through a cobalt-based catalyst, causing a reaction that heats the fuel by 63 ° C within a few minutes. This means that at a room temperature of 20 ° C a temperature of 83 ° C can be achieved, which can be used to heat houses and provide hot water, among other things. During this process, the molecules return to their original shape and can be used for another run. With the fuel used, it is possible, according to the researchers, to store 250 watt hours of energy per kg. This would be twice the energy that Tesla’s Powerwall battery can store. The scientists are also positive about long-term use. The fuel could be loaded and unloaded without any significant loss.

Potential: More usages of solar thermal fuel

The possibilities of the technology are far from exhausted. The researchers assume that temperature increases of 110 ° C are possible, which makes it possible to generate electrical energy. It is expected that the technology will be ready for the market in around 10 years9. This technology makes solar energy transportable and a sensible alternative to fossil fuels. By storing and transporting solar energy, energy needs can be served anywhere in the world. It is important that the energy is not generated centrally and then distributed around the world, since this favors concentrations of power as currently prevailing with fossil fuels. It is to be hoped, that this system will arouse international interest and be implemented on a decentralized basis in order to take an important step towards a democratic and environmentally friendly energy supply10.

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