Solar panels are a wonderful way to capture the sun’s energy for electric power. Another way to use solar energy, fast-gaining traction around the world, is to drive chemical reactions with it and create fuels. The technology, called artificial photosynthesis, can be used to split water into hydrogen for fuel, or to make liquid fuels from carbon dioxide and water.
Researchers in Germany and Canada have now taken a big step towards making such solar fuels affordable and commonplace. In a study published in the journal Joule, they report a detailed design for panel-like reactors that could produce hydrogen on rooftops.
The photoreactors are low cost, have a relatively simple design, and could be easily manufactured via mass fabrication techniques using commonly known plastics, say the researchers. And unlike other systems made so far, the design offers high energy efficiency without the need to track the sun. This leads to a compact system that is easier to maintain and operate, making it promising for small rooftop applications.
Hydrogen is extracted today mostly by from natural gas, producing carbon dioxide in the process. Artificial photosynthesis is a green route to make sustainable hydrogen for cars and trucks, and for home heating. It involves using special catalysts that absorb sunlight and trigger water-splitting reactions.
Such photoreactors have shown promise in small-scale laboratory setups and rooftop demonstrations. Now researchers are trying to increase the efficiency and lower the cost of solar hydrogen production. Many larger systems rely on techniques to concentrate solar power using large mirrors and parabolic dishes, or on ways to track the sun during the day. These add complexity and cost.
“In the ideal case, the photoreactor should guide the incident sunlight to the photocatalyst with hardly any losses, no matter where it comes from or where the sun is currently positioned,” said Paul Kant of the Institute for Micro Process Engineering at the Karlsruhe Institute of Technology in Germany, who led the new work.
Kant and his colleagues created such a photoreactor using optical modeling and computer-aided design. The reactor is composed of hundreds of parallel V-shaped reaction channels, each with a tube-shaped cavity at the bottom. The inside surfaces of the channel and the tube are coated with a mirrored reflective material. The V shape captures light coming in from different directions and funnels it into the tube-like cavity that contains water and the photocatalyst.
The channel arrays could be 3D-printed with commodity plastic materials and coated with a thin aluminum layer for reflectivity. Including a catalyst worth US$ 1 million per ton, the researchers estimate that the material cost of the photoreactor system would be about $22 per square meter.
Right now, the system exhibits an efficiency of 5.8 percent, which is high for the materials used, the team writes. They are now working on developing better photocatalysts and optimizing the reactor to boost efficiency. Studying real-world aging of the plastics and optical coatings, as well as addressing challenges such as dust accumulation channels will also be important, they say.
“Low-cost and high-efficiency photoreactors are the essential keys to the success of solar-driven photosynthesis,” they write, and for that “the introduced photoreactor design and approach is a significant step.”
Source: Paul Kant et al. Low-cost photoreactors for highly photon/energy-efficient solar-driven synthesis. Joule, 2023.
Image: Karlsruhe Institute of Technology (KIT)
