German Researchers Learn How To Store Solar Energy Chemically – CleanTechnica

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Just when you think there is nothing new under the sun, along comes new technology that upends traditional wisdom. Enough solar energy falls on the Earth every day to meet the energy needs of all humanity for more than a year. It’s free — all we have to do is harvest it. 30 years ago, solar panels could only convert less than 2 percent of sunlight into electricity. Today, the upper limit for most commercial grade solar panels is ten times that or more. So we have a pretty good handle on how to harvest the energy the sun sends us; what we need now are better ways to store the energy we get from the sun so it can be used later.

Most electricity is made by using steam that turns massive generators. The steam can come from heat created by burning coal or methane or through nuclear fission. In a traditional electrical grid, the amount of electricity can be varied to meet demand by adding or subtracting generators, but the output of those generators seldom varies. The output from solar panels and wind turbines varies constantly, and that variability is the primary challenge to relying exclusively on renewables. Something is needed to soak up excess electricity until it is needed. Most of the time, that something is a lithium-ion battery. They work fine for periods of two to four hours, but for longer term storage, other strategies are needed. Pumped hydro works well but needs a lot of space and can be quite expensive. Stacking and unstacking massive blocks of concrete can also be an effective way of converting kinetic energy into potential energy.

Researchers at Johannes Gutenberg University Mainz (JGU) and the University of Siegen in Germany have developed a novel way of storing solar energy for weeks or even months. Instead of using sunlight to create electricity, they use it to store heat in chemical bonds in specialized molecules known as photoswitches. The operative factor here is that about 50 percent of all the energy consumed by human society is in the form of heat. It warms our buildings and it also supplies the process heat that is vital to many industrial processes. The findings of the researchers are contained in a press release published on October 29, 2024.

It begins by saying that, according to the International Energy Agency, approximately 50 percent of global final energy consumption is dedicated to heating, yet the utilization of solar power in this sector remains relatively low compared to fossil energy sources. An inherent problem limiting the widespread usage of solar energy is the intermittency of its direct availability, but now a promising new solution has emerged in the form of molecular solar energy storage systems.

Conventional thermal energy storage strategies store the energy for short periods, often in the form of hot water. In contrast, molecular solar energy storage systems store solar energy in the form of chemical bonds, allowing it to be preserved for several weeks or even months. These specialized molecules — or photoswitches — absorb solar energy and release it later as heat on demand. A key challenge for current photoswitches is the trade-off between energy storage capacity and efficient absorption of solar light. That conflict has resulted in limited overall performance. To overcome that issue, research teams at Johannes Gutenberg University Mainz (JGU) and the University of Siegen present a novel approach in a collaborative study.

Decoupling The Absorption And Storage Of Solar Energy

The novel class of photoswitches was first introduced by group of researchers under the direction of Professor Heiko Ihmels at the University of Siegen, which successfully demonstrating exceptional energy storage potential comparable to conventional lithium-ion batteries. However, their functionality was initially limited to activation by UV light, which constitutes only a small portion of the solar spectrum. The research teams at Mainz and Siegen then devised an indirect light harvesting method, which is comparable to the function of the light harvesting complex in photosynthesis. This incorporates a second compound, a so-called sensitizer, which exhibits excellent absorption properties of visible light. “In this approach, the sensitizer absorbs light and subsequently transfers energy to the photoswitch, which cannot be directly excited under these conditions,” explained Professor Christoph Kerzig of the JGU Department of Chemistry.

This new strategy has increased solar energy storage efficiency by more than one order of magnitude, representing a major step forward for the energy conversion research community. The potential applications of these systems include household heating solutions and large-scale energy storage, which make possible a promising path towards sustainable energy management.

The Mainz-based team of researchers led by Professor Christoph Kerzig and PhD student Till Zähringer conducted detailed spectroscopic analyses to explore the complex system. Those analyses were essential to understanding the underlying mechanism. Each reaction step was carefully examined by the first author, Till Zähringer, which resulted in a thorough understanding of how the system operates. “By doing so, we could not only push the light harvesting limit substantially but also improve the conversion efficiency of light to stored chemical energy,” explained Zähringer.

Under operational conditions, each absorbed photon can trigger a chemical bond formation process, which is rarely observed in photochemical reactions due to several energy loss channels. The scientists successfully validated the robustness and practicality of the system by cycling between the energy storage state and the energy release state multiple times employing solar light, which highlighted its potential for real-world applications. The results of the research were published in Angewandte Chemie, the journal of the German Chemical Society, in September and has been classified as a Hot Paper due to the number of exceptional evaluations from scientific reviewers.

The Takeaway

There are companies like Electrified Thermal Solutions that are working on ways to use renewable energy to heat modified fire bricks to provide process heat. Our good friend Mark Z. Jacobson at Stanford is also pursuing fire bricks as a source of process heat. Both are taking a good idea and making it better. But the German research may do them one better because it converts sunlight into chemical bonds that can create heat when needed but do not need to be kept hot in the meantime.

In other words, storing solar energy in chemical form takes one step out of the process by doing directly what today is done indirectly, and as our readers know, every step increases losses. The more steps and conversions there are, the more those losses add up. For maximum efficiency, simplerer is betterer. That’s what the researchers in Germany are offering the world — using sunlight more efficiently to meet the needs of society with more bang for each photon received from the sun.