Toward to Real Hydrogen Storage System from the 24.4% Solar to Hydrogen Energy Conversion Efficiency using Concentrated Solar Cell and Electrochemical Cell

Katsushi Fujii^a^,^b^,^c, Akihiro Nakamura^b, Kayo Koike^b, Masakazu Sugiyama^b, Yoshiaki Nakano^b, Shinichiro Nakamura^c

^aThe University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyūshū-shi, Japan

^bRIKEN - Japan, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan

Materials for Sustainable Development Conference (MATSUS)
Proceedings of September Meeting 2016 (NFM16)

Berlin, Germany, 2016 September 5th - 13th

Organizers: Marin Alexe, Enrique Cánovas, Celso de Mello Donega, Ivan Infante, Thomas Kirchartz, Maksym Kovalenko, Federico Rosei, Lukas Schmidt-Mende, Laurens Siebbeles, Peter Strasser, Teodor K Todorov, Roel van de Krol and Ulrike Woggon

Oral, Katsushi Fujii, presentation 025
Publication date: 14th June 2016

In order to realize the renewable energy as a real energy source, we need to achieve high efficient and large amount energy storage technique due to the supressing of nature energy fluctuation. The 24.4% solar to hydrogen (StH) energy conversion using the combination of concentrated photovoltaics (CPV) and water splitting electrochemical cell encourages the energy storage technique [1,2].

Analysing the energy loss of the StH conversion, the efficiency determining parameters are divided into 4-parts, that is, 1) maximum power point of solar to electricity energy conversion efficiency by solar cell (SC), 2) power difference between the maximum power of SC and the power of electrochemical cell (EC) operation, 3) voltage difference of the hydrogen formation Gibbs energy (1.23 V) and EC operation, and 4) the conversion ratio from current to hydrogen (Faradic efficiency). From the analysis, it is clarified that the lowest efficiency is the energy conversion of SC, but the operation power matching of SC and EC is also important from the system point of view and is controllable with changing the system configuration.

The demonstration of 24.4% StH energy conversion was performed under almost ideal and optimized condition. The operation power matching and Faradic efficiency were almost 100% and the efficiency of voltage difference for hydrogen generation was over 80%. Applying this technology to the realization of StH conversion, the operation power matching is much more difficult and important. That is, the rated output voltage of commercial SC panel is different for each and the operating voltage of EC is also different for each due to its difference of cell stacking number. In addition, the output power of SC is fluctuated by changing weather condition and time but the fluctuation affects the EC operational efficiency and may damage EC in some extreme cases. This shows that stable SC output voltage and the voltage matching of SC and EC are required at the direct connection of SC and EC. Otherwise, high performance variable input DC/DC converter is required for the high StH conversion.

[1] A. Nakamura et al., Appl. Phys. Express 8 (2015) 107101.

[2] K. Fujii et al., Int. J. Hydrogen Energy 38 (2013) 14424.

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