The advantages of the thermally coupled approach
A team from the Helmholtz Center Berlin, the University of Ulm and the University of Heidelberg has now investigated how sunlight can be used to generate hydrogen at the South Pole and which method is most promising. Their conclusion: In extremely cold regions, it can be significantly more efficient to attach the PV modules directly to the electrolyser, i.e. to couple them thermally. Because the waste heat from the PV modules increases the efficiency of the electrolysis in this environment. The results of this study, now in Energy and environmental sciences, are also relevant for other cold regions of the world, such as Alaska, Canada and high mountain regions. In these places, solar hydrogen could replace fossil fuels like oil and gasoline.
When environmental physicist Kira Rehfeld from Heidelberg University visited Antarctica for her research, she was impressed by the intense light there. “It’s always bright in summer. This solar radiation could actually be used to supply the research infrastructure with energy,” she observes. However, generators, motors and heating in these remote regions have so far mostly been powered by fossil fuels that are delivered by ship, such as crude oil or gasoline, which are causing global warming. In addition to the associated high economic costs, pollution from the smallest of leaks is also a major problem that threatens the particularly sensitive ecosystem.
However, fossil fuels could be replaced by hydrogen, a versatile energy medium that can also be stored extremely well at low temperatures. “Our idea was therefore to use solar modules to produce climate-neutral hydrogen on site in the Antarctic summer by splitting water into hydrogen and oxygen through electrolysis,” said May, then a postdoc at the Helmholtz Center Berlin Institute for Solar Fuels. Rehfeld and May applied for funding from the Volkswagen Foundation to investigate whether sunlight can also be used to generate hydrogen at sub-zero temperatures and which process is best suited for this. Low temperatures can significantly reduce the efficiency of the electrolysis, although cold actually increases the efficiency of most solar panels.
May and his HZB colleague Moritz Kölbach have now empirically compared two different approaches: a conventional structure in which the photovoltaic module is thermally and physically separated from the electrolysis tank, and a newer, thermally coupled structure in which the photovoltaic module is in close contact with the wall of the electrolysis tank, which promotes heat diffusion. To simulate the Antarctic conditions, Kölbach procured a freezer, cut a hole in the door, installed a quartz window, and lit the inside of the cabinet with simulated sunlight. He filled the electrolysis container with 30 percent sulfuric acid (also called battery acid), which has a freezing point of -35 degrees Celsius and conducts electricity well.
Then Kölbach set up the test cells and carried out the series of measurements. During operation, it turned out that the cell with the thermally coupled PV modules produced comparatively more hydrogen, since the illuminated PV modules emit their waste heat directly to the electrolyser. “We were even able to increase efficiency by having the electrolyser additionally thermally insulated. As a result, the electrolyte temperature climbed from -20 to +13.5 degrees Celsius during the lighting, ”says Kölbach.
The results of this study confirm that thermally coupled systems are potentially more efficient than thermally decoupled ones. However, it remains to be seen whether these advantages can be used economically. “That’s why we want to test prototypes under realistic conditions in the next phase. It will certainly be exciting and we are currently looking for partners for this, ”says May.
Not only at the South Pole, but also in other extremely cold and sparsely populated regions of the world, locally generated solar hydrogen could be an option to replace fossil fuels and eliminate the associated environmental pollution and CO2 emissions. This could include the high Alps, Canada and Alaska, the Andes, and other mountain regions such as the Himalayas.
“Perhaps solar-generated hydrogen will initially be economical in such remote regions of the world,” May recalled the triumph of photovoltaics, which first supplied satellites in space with electricity around 60 years ago.
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Moritz Kölbach et al., Efficiency gains for thermally coupled solar hydrogen generation in extreme cold, Energy and environmental sciences (2021). DOI: 10.1039 / D1EE00650A
Provided by the Helmholtz Association of German Research Centers
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