After fossil fuels depletion, only natural energy such as solar and wind can be utilized, and then the conversion
techniqu
es from such fluctuated energy to secondary energy are necessary as energy storage. Thus, the hydrogen
produ
ction techniques are fast being studied. In the field on energy conversion of solar energy, the photovoltaic
cells, photo−catalysts, and solarheat techniques are presently developing as t
he power generation systems. To