Selected problems of classification of energy sources - What are renewable energy sources?

The contribution is dedicated to study of issues related to the classification of energy sources from the point of significance and interpretation accuracy. There are some aspects described and documented by examples that show the ambiguity of currently most used methods of dividing an energy source or their classification respectively to generally exerted categories and identifications, where relevance and exactness are not clear, but it depends on specific conditions. However, such conditions are generally not included in the classifications. For analysis, mostly physical, technical, environmental, regional and time aspects were taken into account. The increasing complexity of energy mixes, technologies, and legislative environment naturally leads to the need of perceiving broader aspects of reviewing and evaluation of individual sources. The correct interpretation is a prerequisite for communication between specialists, public and mainly in the field of legislation, which enables influencing of the implementation of individual sources and technologies in practice, mainly by regulation and support from decision-making and control bodies, from regional up to the global level. The findings point out to the fact that the road to orientation within the issue leads through identification and evaluation of representation of typical characteristics of individual sources, which, in a particular context lead to the classification of a source into a particular category. However, such position may be changed in time and place.