Coal and petroleum are depleting fossil fuels. On the other hand solar power, wind power, wave power, hydraulic power, biomass energy  (bioenergy) and so on are the energies obtained from the phenomena repeatedly occurring in the natural environment of the earth itself. These are collectively called renewable energy. Renewable energies are "nearly infinite," but most of them are said to be not economical. However, these energies are regaining attention from the perspective of preventing global warming. In addition, these are expected as alternatives to nuclear power that does not emit carbon dioxide. However, they are disadvantageous in that they are not economical and difficult to be supplied on a large scale.

This renewable energy can also be said earth friendly energy. In June 1992, the United Nations Environment Development Conference (Earth Summit) was held in Rio de Janeiro, Brazil. Then, it was decided to treat it as a treaty to solve environmental problems on a global scale while leaving it ambiguous measures. Regarding energy, the emission level of global warming gas (carbon dioxide, etc.) will be back to the 1990 level by 2000. However, this is what is required of developed countries such as Europe, America and Japan. Even if the treaty is ratified in countries such as former Soviet Union, Eastern Europe now in the process of economic transition and developing countries, these decisions need to be carried out in response to financial and technical assistance from the developed countries. The cause of acid rain is sulfur oxides and nitrogen oxides. The fossil fuels (coal, oil, natural gas, etc.) contain sulfur and nitrogen as well as carbon, the source of carbon dioxide that is the main cause of global warming. Therefore expectations for the energy that can replace fossil fuels are increasing. In other words, it is an expectation for the energy kind to the earth. Nuclear power is an energy that can replace fossil fuel in that sense. However, nuclear power is not included in the energy kind to the earth because there is a risk of environmental destruction by the nuclear. Typical energies are renewable ones like solar energy, wind energy and natural energy. However, these have the drawback because they cannot be replaced in large quantities with fossil fuels in terms of economic efficiency. Not being energy sources, saving energy, using energy efficiently and reducing energy consumption are positioned as the most earth-friendly energies. Natural gas, a fossil fuel, is relatively clean (compared to coal and oil) and the shift to natural gas use will be further accelerated.