As the problem of the low and intermittent energy density of solar energy is solved with changed solar energy into hydrogen energy, it is important for advancing of energy conversion efficiency in a system that the matching of the highest working condition of the energy conversion equipment. In this paper, it required that the characteristic of solar generator and the condition searched for improvement of energy conversion efficiency from solar energy to hydrogen energy when the system operated all day long.

The cost of liquid hydrogen from water electrolysis is estimated assuming the electricity is produced from photovoltaic power plant located in an overseas desert. The assumed system consists of the plants of solar cells, storage batteries, seawater desalination, water electrolysis, hydrogen liquefaction, liquid hydrogen holders, and ocean cryo-tankers. The calculated CIF price of liquid hydrogen is compared with that of the fossil fuels. Effect of carbon tax on the break-even point of the price of hydrogen is briefly discussed.

Electrolysis of water has been performed for hydrogen production for a long time and the technology could be matured. However, at the request of clean energy system, more efficient and economical larger scale electrolyzer are required. This report includes recent topics on research and development of electrolysis by various methods.