Abstract
A proper definition of sustainability should consider two key elements: space and time. These two elements have not been considered in commonly recognized definitions for this term. The present work examines in details sustainability of underground mineral and fuel resources for the current century in our world. Under these boundary conditions, availability of underground resources in this century is considered to be secured. In addition, the electricity output potentials in Japan, estimated by considering scenarios relating to not only social restrictions but also business profits for solar photovoltaics power, wind power, hydropower, geothermal power and bio-power, were comprehensively reviewed and examined. From the above mentioned energy sources, only solar photovoltaics power and wind power are expected to grow quantitatively in the years to come. However, from the viewpoint of level land area and primary energy consumption per unit level land area, power cost, area required to install power facilities, power of solar energy, quality of electric power, etc. it is essentially difficult to consider both of solar photovoltaics power and wind power as the key power source at the present stage . The best way may be to utilize them as a locally generated, locally used electricity supply systems that is combined with rechargeable batteries.
