Like an 'artificial tree': Solar reactor converts water into renewable hydrogen, oxygen, and heat

The system can power around 1.5 hydrogen fuel cell vehicles driving an average annual distance.
Deena Theresa
A pilot-scale solar reactor.
A pilot-scale solar reactor.


At first sight, the parabolic dish looks like any other telecommunications infrastructure. But the one on the EPFL campus is one of its kind - it doubles up as an artificial tree. A reactor above the dish uses sunlight to convert water into renewable hydrogen, oxygen, and heat.

The dish would be the first system-level demonstration of solar hydrogen generation. 

"Unlike typical lab-scale demonstrations, it includes all auxiliary devices and components, so it gives us a better idea of the energy efficiency you can expect once you consider the complete system, and not just the device itself," Sophia Haussener, head of the Laboratory of Renewable Energy Science and Engineering in the School of Engineering, said in a statement.

The work is based on preliminary research that demonstrated the concept on the laboratory scale, using LRESE’s high-flux solar simulator. Now the team has published the results of their "scaled-up, efficient, and multi-product process" under real-world conditions in Nature Energy.

"With an output power of over 2 kilowatts, we’ve cracked the 1-kilowatt ceiling for our pilot reactor while maintaining record-high efficiency for this large scale. The hydrogen production rate achieved in this work represents a encouraging step towards the commercial realization of this technology," said Haussener.

The system can be used to power hydrogen fuel cells and provide residential and commercial central heating

The dish concentrates the sun's rays, after which water is pumped into its focus spot, which has an integrated photoelectrochemical reactor. Within this reactor, photoelectrochemical cells utilize solar energy to electrolyze or split water molecules into hydrogen and oxygen. 

Now, heat is also generated, but it is passed through a heat exchanger, thereby allowing it to be harnessed instead o released it as a system loss.

The oxygen molecules released by the photoelectrolysis reaction are also recovered and used.

"Oxygen is often perceived as a waste product, but in this case, it can also be harnessed – for example for medical applications," Haussener said.

The system is already being used commercially. LRESE-spinoff SoHHytec SA is working with a Swiss-based metal production facility to build a demonstration plant at the multi-100-kilowatt scale that will produce hydrogen for "metal annealing processes, oxygen for nearby hospitals, and heat for the factory’s hot-water needs".

The system can also be used to power hydrogen fuel cells and provide residential and commercial central heating.

According to the release, the EPFL campus system can power around 1.5 hydrogen fuel cell vehicles driving an average annual distance; or meet up to half the electricity demand and more than half of the annual heat demand of a typical four-person Swiss household at an output level of about half a kilogram of solar hydrogen per day.

Study Abstract:

The production of synthetic fuels and chemicals from solar energy and abundant reagents offers a promising pathway to a sustainable fuel economy and chemical industry. For the production of hydrogen, photoelectrochemical or integrated photovoltaic and electrolysis devices have demonstrated outstanding performance at the lab scale, but there remains a lack of larger-scale on-sun demonstrations (>100 W). Here we present the successful scaling of a thermally integrated photoelectrochemical device—utilizing concentrated solar irradiation—to a kW-scale pilot plant capable of co-generation of hydrogen and heat. A solar-to-hydrogen device-level efficiency of greater than 20% at an H2 production rate of >2.0 kW (>0.8 g min−1) is achieved. A validated model-based optimization highlights the dominant energetic losses and predicts straightforward strategies to improve the system-level efficiency of >5.5% towards the device-level efficiency. We identify solutions to the key technological challenges, control and operation strategies and discuss the future outlook of this emerging technology.

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