SOLAR ENERGY CONVERSION PRINCIPLE BASICS

The fossil fuels such as coal, oil, and natural gas, which maintain our industrialized world, will be surely running out sometime in the twenty-first century. Moreover, burning such fossil fuels causes the global air pollution problems due to the production of CO2, SO2, and NOx, leading to global warming and acid rain problems.

The development of alternative clean energy resources is, therefore, one of the most urgent subjects with which contemporary scientists have to struggle. Among them, utilization of solar energy seems to be the most promising and potential, and an important subject that a number of researchers in the world are now studying.

A variety of ways for utilizing solar energy are known, for example, thermal energy by heat collectors, electrical energy by silicon solar cells, and chemical energy by photosynthesis, where the latter is referred to as the conversion of solar energy into chemical energy.

The significant problems common to all these are the low cost performance, the low energy conversion efficiency, and/or the lack of persistence. In order to replace solar energy for fossil energy on an economical basis, it is absolutely necessary to overcome these problems as soon as possible.

Here, we focus on the solar energy conversion processes which involve chemical reactions in any way. The best known example of such a case is the plant photosynthesis; that is, plants can produce chemical energy in the form of a carbohydrate from CO2 and H2O with the help of light.

Although the conversion efficiency is not high, the plant photosynthesis contains many important processes which we have to mimic in order to develop an artificial photosynthesis with a higher energy conversion efficiency. It is, therefore, indispensable to know the mechanism of plant photosynthesis.

Next, we give several examples of artificial photosynthesis using heterogeneous catalytic systems and wet type solar cells using semi- conductor photoelectrodes. This research field was boosted by Fujishima and Honda who found that slightly reduced TiO2, a semiconducting oxide, can decompose water into hydrogen and oxygen under illumination of near ultraviolet light.

Again, the problem was its low conversion efficiency. We overview the principle of the process and the recent progress in this field.