INTRODUCTION :
With the intensification of energy
crisis all over the world, all the countries are looking for the way to solve
this serious problem. One way is to search the new energy and take advantage of
the renewable energy. Another way is to exploit the new energy-saving
technologies to reduce energy consumption, and improve utilization efficiency
of energy. Solar energy is the most direct, common, and clean energy on our
planet we have already found until now. Total solar energy absorbed by the
Earth is about 3,850,000 exajoules (EJ) in one year, which is even twice as
much as all the non-renewable resources on the earth found and used by human
being, including coal, oil, natural gas, and uranium etc. 