As energy is converted, the total amount of energy is preserved, but there is a growing lack of energy that we use because some energy is converted to forms like thermal energy. Fossil fuels can be depleted due to nonrenewable and limited reserves, and they can cause environmental pollution problems such as global warming. So the development and use of new energy resources or renewable energy are very important. In addition, when using natural energy such as the sun and wind, it is important to increase energy efficiency, and so much development is being made.