Bulgaria has one of Europe's richest renewable energy resource bases. This paper presents an assessment of Bulgarian renewable sources of energy as a stage for increasing the share of these resources in the country's total energy balance, to the end of the improvement of the environment, stimulation of industrial development and creation of new jobs. It considers the theoretical (total) resource potential for solar, wind, both large and small hydro, geothermal, and biomass and waste sources. The paper presents the present status of renewable energy technologies and their installed capacities. The opportunities and constraints for development and use of renewable energy are considered.

The photovoltaic (PV) generators are a major element of PV power systems, characterized by a low conversion ratio. This accounts for the necessity to study the current-voltage and power-voltage characteristics of PV sources in order to obtain maximum power output when there are changes both in the modules themselves and in the environment where they operate. In this paper the classical and the modified single-diode models have been used to model the electrical characteristics of PV cells and modules. Mathcad software package has been used for the modelling. The impact of solar radiation, temperature and shading both on electrical characteristics and on the PV cells and modules efficiency has been studied.