Over 14% Solar to Hydrogen Energy Conversion Efficiency by Water Splitting using Conventional Concentrated Photovoltaic and Electrochemical Cell: Concentrated Photovoltaic Electrochemical Cell (CPEC)
Katsushi Fujii a, Behgol Bagheri a, Masakazu Sugiyama b, Tohma Watanabe b, Yuya Taki b, Kentaroh Watanabe c, Yoshiaki Nakano c, Shinichiro Nakamura d
a School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo, 113-8656
b RIKEN Research Cluster for Innovation, Nakamura Lab., 2-1 Hirosawa, Wako, Saitama, 351-0198
Oral, Katsushi Fujii, presentation 024
Publication date: 31st March 2013

              Solar to chemical energy storage is one of important technologies to establish stand-alone renewable energy system. Especially, the conversion from solar to hydrogen energies using water splitting is interested in due to the high feasibleness. Therefore, high conversion efficiency from solar to hydrogen energies using water splitting (StH) was reported since 2000 [1,2].

Considering the system totally, solar energy is one of energy sources like wind and water powers. This shows that not only solar but also all of the energy sources would be better to use for the energy storage. Thus, the devices for solar to electricity and for electricity to chemical energy are better to be separated [3]. In this report, the conversion efficiency of StH using direct connection of conventional concentrated photovoltaic (CPV) and polymer electrolyte electrochemical cell (PEEC) are discussed in order to apply the storage system of the stand-alone renewable energy system.

The solar and electrochemical cells used in the experiments were 10 x 10 mm square of CPV with a GaInP/InGaAs/Ge three-tandem structure and 40 mm2area of PEEC. The source of the water splitting was pure water. The experiments were performed at room temperature without temperature control.

The conversion efficiency of the direct connection of one-CPV and one-PEEC under solar simulator with 10.12 SUN was 12.2% evaluated by the operation current. The major energy loss of the conversion was CPV, that is, the maximum conversion efficiency of the solar cell (Pmax) was 20.1%. The other loss was voltage mismatch between the voltage of maximum conversion efficiency of CPV (Vmax = 2.12 [V]) and the operation voltage of PEEC by CPV (Vop = 1.56 [V]). In order to minimize the voltage mismatch, the direct connection of two-CPV and three-PEEC under solar simulator with 10.95 SUN was demonstrated. The conversion efficiency was 15.3% with the Pmax of 18.2%. The Vmax and Vop were 4.41 V and 4.67 V, respectively. The conversion efficiency was improved with the reduction of the voltage mismatch.

We also evaluated the conversion efficiency of StH using two-CPV and three-PEEC system with 100-times solar concentrator under direct normal irradiation (DNI) of 936 W/m2. The conversion efficiency of StH was 14.5% with Pmax of 27.6%. From this value, the concentrated photovoltaic electrochemical cell (CPEC) system with the direct connection of conventional CPV and PEEC has a potential for the energy storage of stand-alone renewable energy system.

Fig. 1 Current-voltage relationship of two-CPV and three-PEEC combination for the energy conversion from solar to hydrogen with splitting water.
[1] Licht S.; Wang B.; Mukerji S.; Soga T.; Umeno M.; Tributsch H. Efficient Solar Water Splitting, Exemplified by RuO2-Catalyzed AlGaAs/Si Photoelectrolysis, J. Phys. Chem. B 2000, 104, 8920-8924. [2] Peharz G.; Dimrorth F.; Wittstadt U. Solar Hydrogen Production by Water Splitting with a Conversion Efficiency of 18%, Int. J. Hydrogen Energy 2007, 32, 3248-3252. [3] Fujii K.; Nakamura S.; Watanabe K.; Bagheri B.; Sugiyama M.; Nakano Y. Over 12% Light to Hydrogen Energy Conversion Efficiency of Hydrogen Generation from Water: New System Concept, Concentrated Photovoltaic Electrochemical Cell (CPEC), Mater. Res. Soc. Symp. Proc. 2013, 1491, DOI: 10.1557/opl.2012.1739.
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