I’ve known for years that solar concentrators do a good job at harnessing heat. You may also use them to generate electricity, but it’s not their best job possible. A recent article published in the journal Science shows how solar concentrators can be used to directly extract hydrogen from water, without any middlemen such as photovoltaic panels and electricity.
The authors use a catalyst based on cerium, an element that’s almost as abundant as copper and that can be recycled once it’s used.
A focusing lens directs sunlight through a transparent quartz windows into a reaction chamber. This chamber is made of mirrors and is designed for an almost total internal reflection, making the part that absorbs light act like an almost perfect black body. “The selected dimensions ensure multiple internal reflections and efficient capture of incoming solar energy; the apparent absorptivity exceeds 0.94, approaching the ideal blackbody limit,” the authors claim.
The final temperature of the heat absorber is somewhere between 1400 and 1600 degrees Celsius, and it takes about 10 to 12 minutes for it to reach that spot. The high temperatures cause a chemical change in the cylinder of porous cerium dioxide.
At that temperature, the cerium dioxide loses one of its two oxygen molecules. Then, the authors flow an inert gas over the catalytic cylinder and a steady flow of oxygen can be detected exiting the device. If they pumped in carbon dioxide, the system would produce carbon monoxide.
After some 200 cycles, the cerium dioxide suffered a drop in performance, associated with the rearrangement of its internal structure due to repeated heating. Through this process, somewhat larger particles are formed and the process starts performing again, to about 400 cycles.
“The solar-to-fuel energy conversion efficiency obtained in this work for CO2 dissociation is about two orders of magnitude greater than that observed with state-of-the-art photocatalytic approaches,” the authors state. “The gravimetric hydrogen production rate exceeds that of other solar-driven thermochemical processes by more than an order of magnitude.”
Besides the obvious advantage of being able to generate hydrogen from water, the system has a couple of drawbacks. First, it needs a steady supply of inert gas and second, the water or carbon dioxide that are fed into it have to be kept pure, because otherwise they would cause other chemicals to build up on the cerium dioxide and wreck it.
The system’s ability to also dissociate carbon dioxide can be another green benefit that can be used by harvesting solar power. Despite its drawbacks, one such hydrogen harvesting device could ensure continued research in this area.