The deterioration of environment is a clear warning that the present realization of human progress has its limitations. The emerging international environmental consciousness was formulated in a concept of a sustainable human progress. The sustainable human progress is defined as: " to ensure that it sustainable development) meets the needs of the present without compromising the ability of future generations to meet their own needs"l . A new challenge has emerged at the end of the 20th century that represents a search for and a utilization of new and sustainable energy sources.
The urge of this challenge is underlined by limited resources of the fossil fuels on the Earth and increasing demand for energy production. This is the reason why the attention is turning to the renewable energy sources. Energy is an essence of any human activity. When we are interested in how he human civilization has been producing and using energy, we can describe it in terms of an energy system. The main characteristics of the energy system are: the population, the total consumption of energy, and the sources and forms of energy that people use.
The energy system at the beginning of the 21st century is characterised by six billion people that live on the Earth and the total energy consumption of approximately 1. 3 x 1010 kW. World Commission on Environment and Development (WCED), Our Common Future, Oxford/New York: Oxford University Press (1987). 1. 2 Primary energy sources Figure 1. 1 presents an overview of the present primary energy sources 2. The primary energy sources can be divided in two groups. The first group includes those energy sources that will be exhausted by exploiting them.
These energy sources are called the depleting energy sources and they are the fossil fuels and nuclear energy. The fossil fuels and nuclear power are the main source of energy in todays energy system and they supply 78% of the energy demand. Under the assumption that the population of mankind does not change drastically and it consumes energy at the urrent level, the fossil fuel reserves will be exhausted within 320 years and the nuclear energy within 260 years.