1. Introduction
Sustainable Energy Development Strategies typically involve three major technological changes: energy savings on the demand side [1,2], efficiency improvements in the energy production [3,4], and replacement of fossil fuels by various sources of renewable energy [5,6]. Consequently, large-scale renewable energy implementation plans must include strategies for integrating renewable sources in coherent energy systems influenced by energy savings and efficiency measures [7–10].
First, the major challenge is to expand the amount of renewable energy in the supply system. Renewable energy is considered an important resource in many countries around the world [11–18], but as illustrated in Fig. 1, on a global scale less than 15% of primary energy supply is renewable energy, and the major part is hydro power and wood fuels in developing countries. Renewable sources, such as wind and solar, only constitute a very small share of the total supply. However, the potential is substantial. And in some regions and countries, the share of renewable