As energy is converted, the total amount of energy is preserved, but some energy is converted into a form like thermal energy which causes gradual lack of energy we use. Also, fossil fuels can be depleted due to nonrenewable and limited resources, and cause environmental pollution problems such as global warming. Therefore, the development or use of new energy sources (new energy) or renewable energy is very important. Moreover, when using natural energy like sun or wind, it is important to increase energy efficiency and so much development has been made.