Kenji Kaneko, Nikkei BP CleanTech Institute – The University of Tokyo and the University of Miyazaki (UOM) announced that researchers of the universities have made a system capable of water electrolysis using electricity generated by solar cells and succeeded in storing 24.4% of sunlight energy as hydrogen.
In August 2015, an Australian research group achieved a sunlight-to-hydrogen energy conversion efficiency of 22.4% by using a similar principle. This time, the Japanese researchers realized an efficiency higher than that and achieved the world’s highest-class efficiency. The energy conversion efficiency of a conventional hydrogen generation method using a photocatalyst is 10% or less, they said.
The latest result was achieved by a research group led by Kensuke Nishioka and Yasuyuki Ota (associate professor and specially appointed assistant professor, respectively, of the Faculty of Engineering, UOM) and Masakazu Sugiyama (The University of Tokyo). It was published on the Sept 16, 2015 issue of Applied Physics Express, which is an international journal of the Japan Society of Applied Physics (JSAP), and selected as a spotlighted thesis.
The research group reduced energy loss and improved sunlight-to-hydrogen energy conversion efficiency by using a “concentrating solar cell,” which generates electricity with a strong light concentrated by a lens, and making improvements to the method to electrically connect to the water electrolysis unit.
Conventionally, in such a research, mimic sunlight is used. However, this time, real sunlight was used.
In the latest research, Sumitomo Electric Industries Ltd’s concentrating solar cell with a new optical design was mounted on THK Co Ltd’s sun-tracking mounting system at UOM, which is an R&D base for concentrating solar cells. As a result, a power generation efficiency of 31% was achieved under the sunshine conditions of Miyazaki.
When the efficiency of electricity-to-hydrogen energy transfer in water electrolysis (80%) is taken into consideration, the sunlight-to-hydrogen energy conversion efficiency reaches 24-25%.
The solar cell and water electrolysis unit used for this system are commercially available, and, thus, the system can be realized with existing technologies. Concentrating solar cells are more expensive than normal solar cells. But, with their high power generation efficiencies, they can reduce power generation costs in high-illuminance regions outside Japan and are expected to reduce hydrogen cost to US$4/kg, which is a goal of the US Department of Energy (DOE).
Main image: The concentrating solar power generation system installed at the University of Miyazaki (source: The University of Miyazaki)