Introduction

The whole world has been focusing on energy related research for a quite while, because it is widely acknowledged that the conventional energy source on the earth will be used up sooner or later. Another issue closely related to energy usage is so-called global warming, which is attributed to the emission of carbon dioxide when using the conventional energies (fossil oils and coals). To address these problems, one approach is to increase energy efficiency, so that the conventional energy could be used for a longer time, while the energy consumption can be largely reduced. However, this is not the fundamental solution to solve the problems. Therefore, searching for new energy becomes more important. New energy is generally known as renewable energy, sustainable energy, clean energy, alternative energy and so on, which is also called. Renewable energy includes solar energy, wind energy, geothermal energy and so on. One of the problems that we encounter with renewable energy is that they are not continuously available. For example, solar energy is only available during the day (sun-shining) time, while electricity is required in the evening and at night. As a result, it is desired to store the renewable energy when it is unavailable but is required.